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Abstract: TEMPO was selected in 2012 by NASA as the first Earth Venture Instrument, for launch between 2018 and 2021. It will measure atmospheric pollution for greater North America from space using ultraviolet and visible spectroscopy. TEMPO observes from Mexico City, Cuba, and the Bahamas to the Canadian oil sands, and from the Atlantic to the Pacific, hourly and at high spatial resolution (~2.1 km N/S×4.4 km E/W at 36.5°N, 100°W). TEMPO provides a tropospheric measurement suite that includes the key elements of tropospheric air pollution chemistry, as well as contributing to carbon cycle knowledge. Measurements are made hourly from geostationary (GEO) orbit, to capture the high variability present in the diurnal cycle of emissions and chemistry that are unobservable from current low-Earth orbit (LEO) satellites that measure once per day. The small product spatial footprint resolves pollution sources at sub-urban scale. Together, this temporal and spatial resolution improves emission inventories, monitors population exposure, and enables effective emission-control strategies.
TEMPO takes advantage of a commercial GEO host spacecraft to provide a modest cost mission that measures the spectra required to retrieve ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), formaldehyde (H2CO), glyoxal (C2H2O2), bromine monoxide (BrO), IO (iodine monoxide), water vapor, aerosols, cloud parameters, ultraviolet radiation, and foliage properties. TEMPO thus measures the major elements, directly or by proxy, in the tropospheric O3 chemistry cycle. Multi-spectral observations provide sensitivity to O3 in the lowermost troposphere, substantially reducing uncertainty in air quality predictions. TEMPO quantifies and tracks the evolution of aerosol loading. It provides these near-real-time air quality products that will be made publicly available. TEMPO will launch at a prime time to be the North American component of the global geostationary constellation of pollution monitoring together with the European Sentinel-4 (S4) and Korean Geostationary Environment Monitoring Spectrometer (GEMS) instruments.
Abstract: The chemical species emitted by forests create complex atmospheric oxidation chemistry and influence global atmospheric oxidation capacity and climate. The Southern Oxidant and Aerosol Study (SOAS) provided an opportunity to test the oxidation chemistry in a forest where isoprene is the dominant biogenic volatile organic compound. Hydroxyl (OH) and hydroperoxyl (HO2) radicals were two of the hundreds of atmospheric chemical species measured, as was OH reactivity (the inverse of the OH lifetime). OH was measured by laser-induced fluorescence (LIF) and by taking the difference in signals without and with an OH scavenger that was added just outside the instrument's pinhole inlet. To test whether the chemistry at SOAS can be simulated by current model mechanisms, OH and HO2 were evaluated with a box model using two chemical mechanisms: Master Chemical Mechanism, version 3.2 (MCMv3.2), augmented with explicit isoprene chemistry and MCMv3.3.1. Measured and modeled OH peak at about 106 cm-3 and agree well within combined uncertainties. Measured and modeled HO2 peak at about 27 pptv and also agree well within combined uncertainties. Median OH reactivity cycled between about 11 s-1 at dawn and about 26 s-1 during midafternoon. A good test of the oxidation chemistry is the balance between OH production and loss rates using measurements; this balance was observed to within uncertainties. These SOAS results provide strong evidence that the current isoprene mechanisms are consistent with measured OH and HO2 and, thus, capture significant aspects of the atmospheric oxidation chemistry in this isoprene-rich forest.
Abstract: Concentrations of atmospheric trace species in the United States have changed dramatically over the past several decades in response to pollution control strategies, shifts in domestic energy policy and economics, and economic development (and resulting emission changes) elsewhere in the world. Reliable projections of the future atmosphere require models to not only accurately describe current atmospheric concentrations; but to do so by representing chemical, physical and biological processes with conceptual and quantitative fidelity. Only through incorporation of the processes controlling emissions and chemical mechanisms that represent the key transformations among reactive molecules can models reliably project the impacts of future policy, energy, and climate scenarios. Efforts to properly identify and implement the fundamental and controlling mechanisms in atmospheric models benefit from intensive observation periods (IOPs), during which co-located measurements of diverse, speciated chemicals in both the gas and condensed phases are obtained. The Southeast Atmosphere Studies (SAS, including SENEX, SOAS, NOMADSS and SEAC4RS) conducted during the summer of 2013, provided an unprecedented opportunity for the atmospheric modeling community to come together to evaluate, diagnose, and improve the representation of fundamental climate and air quality processes in models of varying temporal and spatial scales.
In the summer of 2015 a group of experimentalists and modelers convened at the NOAA Geophysical Fluid Dynamics Laboratory (GFDL), to discuss use of SAS observations to evaluate, diagnose, and improve air quality and climate modeling over the Southeast US. The effort focused primarily on model representation of fundamental atmospheric processes that are essential to the formation of ozone, secondary organic aerosols (SOA) and other trace species in the troposphere over the Southeastern US, with the ultimate goal of understanding the radiative impacts of these species in the Southeast and elsewhere. The workshop addressed questions surrounding four key themes: gas phase chemistry, aerosol chemistry, regional climate and chemistry interactions, and natural and anthropogenic emissions. This paper serves as a summary from this meeting and more importantly, as guidance for future modeling efforts.
Abstract: Retrievals of tropospheric NO2 columns from UV–visible observations of reflected sunlight require a priori vertical profiles to account for the variation in sensitivity of the observations to NO2 at different altitudes. These profiles vary in space and time but are usually approximated using models that do not resolve the full details of this variation. Currently, no operational retrieval simulates these a priori profiles at both high spatial and high temporal resolution. Here we examine the additional benefits of daily variations in a priori profiles for retrievals already simulating a priori NO2 profiles at sufficiently high spatial resolution to identify variations of NO2 within urban plumes. We show the effects of introducing daily variation into a priori profiles can be as large as 40 % and 3 × 1015 molec. cm-2 for an individual day and lead to corrections as large as -13 % for a monthly average in a case study of Atlanta, GA, USA. Additionally, we show that NOx emissions estimated from space-based remote sensing using daily, high-spatial-resolution a priori profiles are ∼ 100 % greater than those of a retrieval using spatially coarse a priori profiles, and 26-40 % less than those of a retrieval using monthly averaged high-spatial-resolution profiles.
Abstract: Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NOx ≡ NO + NO2) and biogenic isoprene. Model estimates of surface ozone concentrations tend to be biased high in the region and this is of concern for designing effective emission control strategies to meet air quality standards. We use detailed chemical observations from the SEAC4RS aircraft campaign in August and September 2013, interpreted with the GEOS-Chem chemical transport model at 0.25° × 0.3125° horizontal resolution, to better understand the factors controlling surface ozone in the Southeast US. We find that the National Emission Inventory (NEI) for NOx from the US Environmental Protection Agency (EPA) is too high. This finding is based on SEAC4RS observations of NOx and its oxidation products, surface network observations of nitrate wet deposition fluxes, and OMI satellite observations of tropospheric NO2 columns. Our results indicate that NEI NOx emissions from mobile and industrial sources must be reduced by 30–60 %, dependent on the assumption of the contribution by soil NOx emissions. Upper-tropospheric NO2 from lightning makes a large contribution to satellite observations of tropospheric NO2 that must be accounted for when using these data to estimate surface NOx emissions. We find that only half of isoprene oxidation proceeds by the high-NOx pathway to produce ozone; this fraction is only moderately sensitive to changes in NOx emissions because isoprene and NOx emissions are spatially segregated. GEOS-Chem with reduced NOx emissions provides an unbiased simulation of ozone observations from the aircraft and reproduces the observed ozone production efficiency in the boundary layer as derived from a regression of ozone and NOx oxidation products. However, the model is still biased high by 6 ± 14 ppb relative to observed surface ozone in the Southeast US. Ozonesondes launched during midday hours show a 7 ppb ozone decrease from 1.5 km to the surface that GEOS-Chem does not capture. This bias may reflect a combination of excessive vertical mixing and net ozone production in the model boundary layer.
Abstract: The majority of anthropogenic CO2 emissions are attributable to urban areas. While the emissions from urban electricity generation often occur in locations remote from consumption, many of the other emissions occur within the city limits. Evaluating the effectiveness of strategies for controlling these emissions depends on our ability to observe urban CO2 emissions and attribute them to specific activities. Cost-effective strategies for doing so have yet to be described. Here we characterize the ability of a prototype measurement network, modeled after the Berkeley Atmospheric CO2 Observation Network (BEACO2N) in California's Bay Area, in combination with an inverse model based on the coupled Weather Research and Forecasting/Stochastic Time-Inverted Lagrangian Transport (WRF-STILT) to improve our understanding of urban emissions. The pseudo-measurement network includes 34 sites at roughly 2 km spacing covering an area of roughly 400 km2. The model uses an hourly 1 × 1 km2 emission inventory and 1 × 1 km2 meteorological calculations. We perform an ensemble of Bayesian atmospheric inversions to sample the combined effects of uncertainties of the pseudo-measurements and the model. We vary the estimates of the combined uncertainty of the pseudo-observations and model over a range of 20 to 0.005 ppm and vary the number of sites from 1 to 34. We use these inversions to develop statistical models that estimate the efficacy of the combined model-observing system in reducing uncertainty in CO2 emissions. We examine uncertainty in estimated CO2 fluxes on the urban scale, as well as for sources embedded within the city such as a line source (e.g., a highway) or a point source (e.g., emissions from the stacks of small industrial facilities). Using our inversion framework, we find that a dense network with moderate precision is the preferred setup for estimating area, line, and point sources from a combined uncertainty and cost perspective. The dense network considered here (modeled after the BEACO2N network with an assumed mismatch error of 1 ppm at an hourly temporal resolution) could estimate weekly CO2 emissions from an urban region with less than 5 % error, given our characterization of the combined observation and model uncertainty.
Abstract: With the majority of the world population residing in urban areas, attempts to monitor and mitigate greenhouse gas emissions must necessarily center on cities. However, existing carbon dioxide observation networks are ill-equipped to resolve the specific intra-city emission phenomena targeted by regulation. Here we describe the design and implementation of the BErkeley Atmospheric CO2 Observation Network (BEACO2N), a distributed CO2 monitoring instrument that utilizes low-cost technology to achieve unprecedented spatial density throughout and around the city of Oakland, California. We characterize the network in terms of four performance parameters — cost, reliability, precision, and systematic uncertainty — and find the BEACO2N approach to be sufficiently cost-effective and reliable while nonetheless providing high-quality atmospheric observations. First results from the initial installation successfully capture hourly, daily, and seasonal CO2 signals relevant to urban environments on spatial scales that cannot be accurately represented by atmospheric transport models alone, demonstrating the utility of high-resolution surface networks in urban greenhouse gas monitoring efforts.
Abstract: Observations of trace gases from space based instruments offer the opportunity to constrain chemical and weather forecast and reanalysis models using the tools of data assimilation. To date, attempts at assimilation of nitrogen dioxide (NO2) satellite remote sensing have focused on updating emissions and concentrations. These initial efforts evaluated updates at length scales of ~ 100 km using once a day measurements from satellites with ground pixels of 13 km × 24 km or larger. In the boundary layer, NO2 has a lifetime on the order of five hours and corresponding 1/e concentration variations near urban and point sources occur on spatial scales on the order of 50–75 km. Accurate observations and modeling of these variations require spatial resolution of order 4 km. In addition, because of the short lifetime, NO2 variations are more strongly coupled to short time scale meteorological parameters than longer lived chemicals such as CO or CO2. In the next few years, we anticipate the launch of several instruments with ~ 3 km spatial resolution. In addition, some of these instruments will be in geostationary orbits and thus have hourly revisit times. In anticipation of these instruments, we investigate the potential of high space and time resolution column measurements to serve as constraints on urban NOx emissions using a geostationary observation simulator coupled to a data assimilation system. We find that constraints on emissions are strongest in regions with high emissions and are most effective when coupled to hourly assimilation of meteorological observations. We find that errors in the meteorological fields result in unrecoverable biases in the updated emissions confirming a conjecture that simultaneous meteorology and chemical assimilation is essential to accurate description of the emissions and chemistry.
Abstract: Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO3) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than three decades, during which time a large body of research has emerged from laboratory, field and modeling studies. NO3-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms and organic aerosol yields for NO3-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO3 radical, the difficulty of characterizing the spatial distributions of BVOC and NO3 within the poorly mixed nocturnal atmosphere and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry-climate models.
This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first section summarizes the current literature on NO3-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO3-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.
Abstract: The lifetime of nitrogen oxides (NOx) affects the concentration and distribution of NOx and the spatial patterns of nitrogen deposition. Despite its importance, the lifetime of NOx is poorly constrained in rural and remote continental regions. We use measurements from a site in central Alabama during the Southern Oxidant and Aerosol Study (SOAS) in summer 2013 to provide new insights into the chemistry of NOx and NOx reservoirs. We find that the lifetime of NOx during the daytime is controlled primarily by the production and loss of alkyl and multifunctional nitrates (ΣANs). During SOAS, ΣAN production was rapid, averaging 90 ppt h-1 during the day, and occurred predominantly during isoprene oxidation. Analysis of the ΣAN and HNO3 budgets indicate that ΣANs have an average lifetime of under 2 h, and that approximately 45 % of the ΣANs produced at this site are rapidly hydrolyzed to produce nitric acid. We find that ΣAN hydrolysis is the largest source of HNO3 and the primary pathway to permanent removal of NOx from the boundary layer in this location. Using these new constraints on the fate of ΣANs, we find that the NOx lifetime is 11 ± 5 h under typical midday conditions. The lifetime is extended by storage of NOx in temporary reservoirs, including acyl peroxy nitrates and ΣANs.
Abstract: Formation of organic nitrates (RONO2) during oxidation of biogenic volatile organic compounds (BVOCs: isoprene, monoterpenes) is a significant loss pathway for atmospheric nitrogen oxide radicals (NOx), but the chemistry of RONO2 formation and degradation remains uncertain. Here we implement a new BVOC oxidation mechanism (including updated isoprene chemistry, new monoterpene chemistry, and particle uptake of RONO2) in the GEOS-Chem global chemical transport model with ∼25 × 25 km2 resolution over North America. We evaluate the model using aircraft (SEAC4RS) and ground-based (SOAS) observations of NOx, BVOCs, and RONO2 from the Southeast US in summer 2013. The updated simulation successfully reproduces the concentrations of individual gas- and particle-phase RONO2 species measured during the campaigns. Gas-phase isoprene nitrates account for 25-50 % of observed RONO2 in surface air, and we find that another 10 % is contributed by gas-phase monoterpene nitrates. Observations in the free troposphere show an important contribution from long-lived nitrates derived from anthropogenic VOCs. During both campaigns, at least 10 % of observed boundary layer RONO2 were in the particle phase. We find that aerosol uptake followed by hydrolysis to HNO3 accounts for 60 % of simulated gas-phase RONO2 loss in the boundary layer. Other losses are 20 % by photolysis to recycle NOx and 15 % by dry deposition. RONO2 production accounts for 20 % of the net regional NOx sink in the Southeast US in summer, limited by the spatial segregation between BVOC and NOx emissions. This segregation implies that RONO2 production will remain a minor sink for NOx in the Southeast US in the future even as NOx emissions continue to decline.
Abstract: Formation of ozone and organic aerosol in continental atmospheres depends on whether isoprene emitted by vegetation is oxidized by the high-NOx pathway (where peroxy radicals react with NO) or by low-NOx pathways (where peroxy radicals react by alternate channels, mostly with HO2). We used mixed layer observations from the SEAC4RS aircraft campaign over the Southeast US to test the ability of the GEOS-Chem chemical transport model at different grid resolutions (0.25° × 0.3125°, 2° × 2.5°, 4° × 5°) to simulate this chemistry under high-isoprene, variable-NOx conditions. Observations of isoprene and NOx over the Southeast US show a negative correlation, reflecting the spatial segregation of emissions; this negative correlation is captured in the model at 0.25° × 0.3125° resolution but not at coarser resolutions. As a result, less isoprene oxidation takes place by the high-NOx pathway in the model at 0.25° × 0.3125° resolution (54%) than at coarser resolution (59%). The cumulative probability distribution functions (CDFs) of NOx, isoprene, and ozone concentrations show little difference across model resolutions and good agreement with observations, while formaldehyde is overestimated at coarse resolution because excessive isoprene oxidation takes place by the high-NOx pathway with high formaldehyde yield. The good agreement of simulated and observed concentration variances implies that smaller-scale non-linearities (urban and power plant plumes) are not important on the regional scale. Correlations of simulated vs. observed concentrations do not improve with grid resolution because finer modes of variability are intrinsically more difficult to capture. Higher model resolution leads to decreased conversion of NOx to organic nitrates and increased conversion to nitric acid, with total reactive nitrogen oxides (NOy) changing little across model resolutions. Model concentrations in the lower free troposphere are also insensitive to grid resolution. The overall low sensitivity of modeled concentrations to grid resolution implies that coarse resolution is adequate when modeling continental boundary layer chemistry for global applications.
Abstract: One of the objectives of the Deep Convective Clouds and Chemistry (DC3) field experiment was to determine the scavenging of soluble trace gases by thunderstorms. We present an analysis of scavenging of hydrogen peroxide (H2O2) and methyl hydrogen peroxide (CH3OOH) from six DC3 cases that occurred in Oklahoma and northeast Colorado. Estimates of H2O2 scavenging efficiencies are comparable to previous studies ranging from 79 to 97% with relative uncertainties of 5–25%. CH3OOH scavenging efficiencies ranged from 12 to 84% with relative uncertainties of 18–558%. The wide range of CH3OOH scavenging efficiencies is surprising, as previous studies suggested that CH3OOH scavenging efficiencies would be <10%. Cloud chemistry model simulations of one DC3 storm produced CH3OOH scavenging efficiencies of 26–61% depending on the ice retention factor of CH3OOH during cloud drop freezing, suggesting ice physics impacts CH3OOH scavenging. The highest CH3OOH scavenging efficiencies occurred in two severe thunderstorms, but there is no obvious correlation between the CH3OOH scavenging efficiency and the storm thermodynamic environment. We found a moderate correlation between the estimated entrainment rates and CH3OOH scavenging efficiencies. Changes in gas-phase chemistry due to lightning production of nitric oxide and aqueous-phase chemistry have little effect on CH3OOH scavenging efficiencies. To determine why CH3OOH can be substantially removed from storms, future studies should examine effects of entrainment rate, retention of CH3OOH in frozen cloud particles during drop freezing, and lightning-NOx production.
Abstract: Nitrogen oxides (NOx) have fallen steadily across the US over the last 15 years. At the same time, NOx concentrations decrease on weekends relative to weekdays, largely without co-occurring changes in other gas-phase emissions, due to patterns of diesel truck activities. These trends taken together provide two independent constraints on the role of NOx in the nonlinear chemistry of atmospheric oxidation. In this context, we interpret interannual trends in wintertime ammonium nitrate (NH4NO3) in the San Joaquin Valley of California, a location with the worst aerosol pollution in the US and where a large portion of aerosol mass is NH4NO3. Here, we show that NOx reductions have simultaneously decreased nighttime and increased daytime NH4NO3 production over the last decade. We find a substantial decrease in NH4NO3 since 2000 and conclude that this decrease is due to reduced nitrate radical-initiated production at night in residual layers that are decoupled from fresh emissions at the surface. Further reductions in NOx are imminent in California, and nationwide, and we make a quantitative prediction of the response of NH4NO3. We show that the combination of rapid chemical production and efficient NH4NO3 loss via deposition of gas-phase nitric acid implies that high aerosol days in cities in the San Joaquin Valley air basin are responsive to local changes in NOx within those individual cities. Our calculations indicate that large decreases in NOx in the future will not only lower wintertime NH4NO3 concentrations but also cause a transition in the dominant NH4NO3 source from nighttime to daytime chemistry.
Abstract: Speciated particle-phase organic nitrates (pONs) were quantified using online chemical ionization MS during June and July of 2013 in rural Alabama as part of the Southern Oxidant and Aerosol Study. A large fraction of pONs is highly functionalized, possessing between six and eight oxygen atoms within each carbon number group, and is not the common first generation alkyl nitrates previously reported. Using calibrations for isoprene hydroxynitrates and the measured molecular compositions, we estimate that pONs account for 3% and 8% of total submicrometer organic aerosol mass, on average, during the day and night, respectively. Each of the isoprene- and monoterpenes-derived groups exhibited a strong diel trend consistent with the emission patterns of likely biogenic hydrocarbon precursors. An observationally constrained diel box model can replicate the observed pON assuming that pONs (i) are produced in the gas phase and rapidly establish gas–particle equilibrium and (ii) have a short particle-phase lifetime (∼2–4 h). Such dynamic behavior has significant implications for the production and phase partitioning of pONs, organic aerosol mass, and reactive nitrogen speciation in a forested environment.
Abstract: Recent analyses of atmospheric aerosols from different regions have demonstrated the ubiquitous presence of strong surfactants and evidenced surface tension values, σ, below 40 mN m-1, suspected to enhance the cloud-forming potential of these aerosols. In this work, this approach was further improved and combined with absolute concentration measurements of aerosol surfactants by colorimetric titration. This analysis was applied to PM2.5 aerosols collected at the Baltic station of Askö, Sweden, from July to October 2010. Strong surfactants were found in all the sampled aerosols, with σ = (32–40) ± 1 mN m-1 and concentrations of at least 27 ± 6 mM or 104 ± 21 pmol m-3. The absolute surface tension curves and critical micelle concentrations (CMC) determined for these aerosol surfactants show that (1) surfactants are concentrated enough in atmospheric particles to strongly depress the surface tension until activation, and (2) the surface tension does not follow the Szyszkowski equation during activation but is nearly constant and minimal, which provides new insights on cloud droplet activation. In addition, both the CMCs determined and the correlation (R2 ∼ 0.7) between aerosol surfactant concentrations and chlorophyll-a seawater concentrations suggest a marine and biological origin for these compounds.
Abstract: Predictions of O3, CO, total NOy and individual NOy species (NO, NO2, HNO3, PAN, alkyl nitrates and aerosol nitrate) from a fine resolution regional air quality modeling system for the South Coast Air Basin (SoCAB) and San Joaquin Valley Air Basin (SJVAB) of California are presented and evaluated for the 2008 ARCTAS-CARB campaign. The measurements of the chemical compounds from the fire plumes during the field campaign allow for the evaluation of the model's ability to simulate fire-influenced air masses as well. In general, the model successfully simulated the broad spatial distribution of chemical compounds in both air basins as well as the variation within the basins. Using inventories that reflect 2008 emissions levels, the model performed well in simulating NOx (NO + NO2) in SoCAB. Therefore, the under prediction of O3 over these areas is more likely caused by uncertainties with the VOC emissions, chemistry, or discrepancies in the meteorology. The model did not capture the relatively high levels of O3, and some reactive nitrogen species that were measured off shore of the SoCAB, indicating potential missing sources or the transport from on shore to off shore was not successfully captured. In SJVAB, the model had good performance in simulating different chemical compounds in the Fresno and Arvin areas. However, enhanced concentrations of O3, NOx, HNO3 and PAN near dairy farms were significantly underestimated in the model. Negative biases also exist for O3 and HNO3 near oil fields, suggesting larger uncertainties associated with these emission sources. While the model simulated the total NOy mixing ratios reasonably well, the prediction for partitioning between individual compounds showed larger uncertainties in the model simulation. Although the fire emissions inventory was updated to include the latest emissions estimates and speciation profiles, our model shows limited improvement in simulating the enhancement of O3, CO, and PAN under fire impact as compared to a previous version of the modeling system. Further improvements in simulating fire emissions, especially the timing and the plume injection heights, are desired in order to better simulate the impact of fires.
Abstract: High wintertime ozone levels have been observed in the Uintah Basin, Utah, a sparsely populated rural region with intensive oil and gas operations. The reactive nitrogen budget plays an important role in tropospheric ozone formation. Measurements were taken during three field campaigns in the winters of 2012, 2013 and 2014, which experienced varying climatic conditions. Average concentrations of ozone and total reactive nitrogen were observed to be 2.5 times higher in 2013 than 2012, with 2014 an intermediate year in most respects. However, photochemically active NOx (NO+NO2) remained remarkably similar all three years. Nitric acid comprised roughly half of NOz ( ≡ NOy−NOx) in 2013, with nighttime nitric acid formation through heterogeneous uptake of N2O5 contributing approximately 6 times more than daytime formation. In 2012, N2O5 and ClNO2 were larger components of NOz relative to HNO3. The nighttime N2O5 lifetime between the high-ozone year 2013 and the low-ozone year 2012 is lower by a factor of 2.6, and much of this is due to higher aerosol surface area in the high-ozone year of 2013. A box-model simulation supports the importance of nighttime chemistry on the reactive nitrogen budget, showing a large sensitivity of NOx and ozone concentrations to nighttime processes.
Abstract: Ozone pollution affects human health, especially in urban areas on hot sunny days. Its basic photochemistry has been known for decades and yet it is still not possible to correctly predict the high ozone levels that are the greatest threat. The CalNex_SJV study in Bakersfield CA in May/June 2010 provided an opportunity to examine ozone photochemistry in an urban area surrounded by agriculture. The measurement suite included hydroxyl (OH), hydroperoxyl (HO2), and OH reactivity, which are compared with the output of a photochemical box model. While the agreement is generally within combined uncertainties, measured HO2 far exceeds modeled HO2 in NOx-rich plumes. OH production and loss do not balance as they should in the morning, and the ozone production calculated with measured HO2 is a decade greater than that calculated with modeled HO2 when NO levels are high. Calculated ozone production using measured HO2 is twice that using modeled HO2, but this difference in calculated ozone production has minimal impact on the assessment of NOx-sensitivity or VOC-sensitivity for midday ozone production. Evidence from this study indicates that this important discrepancy is not due to the HO2 measurement or to the sampling of transported plumes but instead to either emissions of unknown organic species that accompany the NO emissions or unknown photochemistry involving nitrogen oxides and hydrogen oxides, possibly the hypothesized reaction OH + NO + O2.
Abstract: NOx (NOx ≡ NO + NO2) regulates O3 and HOx (HOx ≡ OH + HO2) concentrations in the upper troposphere. In the laboratory, it is difficult to measure rates and branching ratios of the chemical reactions affecting NOx at the low temperatures and pressures characteristic of the upper troposphere, making direct measurements in the atmosphere especially useful. We report quasi-Lagrangian observations of the chemical evolution of an air parcel following a lightning event that results in high NOx concentrations. These quasi-Lagrangian measurements obtained during the Deep Convective Clouds and Chemistry experiment are used to characterize the daytime rates for conversion of NOx to different peroxy nitrates, the sum of alkyl and multifunctional nitrates, and HNO3. We infer the following production rate constants [in (cm3/molecule)/s] at 225 K and 230 hPa: 7.2(±5.7) × 10-12 (CH3O2NO2), 5.1(±3.1) × 10-13 (HO2NO2), 1.3(±0.8) × 10-11 (PAN), 7.3(±3.4) × 10-12 (PPN), and 6.2(±2.9) × 10-12 (HNO3). The HNO3 and HO2NO2 rates are ~30-50% lower than currently recommended whereas the other rates are consistent with current recommendations to within ±30%. The analysis indicates that HNO3 production from the HO2 and NO reaction (if any) must be accompanied by a slower rate for the reaction of OH with NO2, keeping the total combined rate for the two processes at the rate reported for HNO3 production above.
Abstract: Gas- and aerosol-phase measurements of oxidants, biogenic volatile organic compounds (BVOCs) and organic nitrates made during the Southern Oxidant and Aerosol Study (SOAS campaign, Summer 2013) in central Alabama show that a nitrate radical (NO3) reaction with monoterpenes leads to significant secondary aerosol formation. Cumulative losses of NO3 to terpenes are correlated with increase in gas- and aerosol-organic nitrate concentrations made during the campaign. Correlation of NO3 radical consumption to organic nitrate aerosol formation as measured by aerosol mass spectrometry and thermal dissociation laser-induced fluorescence suggests a molar yield of aerosol-phase monoterpene nitrates of 23-44%. Compounds observed via chemical ionization mass spectrometry (CIMS) are correlated to predicted nitrate loss to BVOCs and show C10H17NO5, likely a hydroperoxy nitrate, is a major nitrate-oxidized terpene product being incorporated into aerosols. The comparable isoprene product C5H9NO5 was observed to contribute less than 1 % of the total organic nitrate in the aerosol phase and correlations show that it is principally a gas-phase product from nitrate oxidation of isoprene. Organic nitrates comprise between 30 and 45 % of the NOy budget during SOAS. Inorganic nitrates were also monitored and showed that during incidents of increased coarse-mode mineral dust, HNO3 uptake produced nitrate aerosol mass loading at a rate comparable to that of organic nitrate produced via NO3 + BVOCs.
Abstract: Recent observations suggest a large and unknown daytime source of nitrous acid (HONO) to the atmosphere. Multiple mechanisms have been proposed, many of which involve chemistry that reduces nitrogen dioxide (NO2) on some time scale. To examine the NO2 dependence of the daytime HONO source, we compare weekday and weekend measurements of NO2 and HONO in two U.S. cities. We find that daytime HONO does not increase proportionally to increases in same-day NO2, i.e., the local NO2 concentration at that time and several hours earlier. We discuss various published HONO formation pathways in the context of this constraint.
Abstract: Real time, atmospheric NO2 column profiles over the Mid-Atlantic states, during the July 2011 National Aeronautics and Space Administration (NASA) Deriving Information on Surface Conditions from Column and Vertically Resolved Observations to Air Quality (DISCOVER AQ) flight campaign, demonstrated that a cavity ring down spectrometer with a light emitting diode light source (LED-CRD) is a suitable technique for detecting NO2 in the boundary layer (BL) and lower free troposphere (LFT). Results from a side-by-side flight between a NASA P3 aircraft and a University of Maryland (UMD) Cessna 402B aircraft show that NO2 concentrations in ambient air from 0.08 nmol /mol (or ppbv) to 1.3 nmol/mol were consistent with NO2 measurements obtained via laser induced fluorescence (LIF) and photolysis followed by NO chemiluminescence (P-CL). The current LED-CRD, commercially available by Los Gatos Research (LGR), includes the modifications added by Castellanos et al. (Rev. Sci. Instrum. 80:113107, 2009) to compensate for baseline drift and humidity through built in zeroing and drying. Because of laser instability in the initial instrument, the laser light source in the Castellanos et al. (Rev. Sci. Instrum. 80:113107, 2009) instrument has been replaced with a light emitting diode. Six independent calibrations demonstrated the instrument's linearity up through 150 nmol/mol NO2 and excellent stability in calibration coefficient of 1.26 (± 3.7 %). The instrument detection limit is 80 pmol/mol. Aircraft measurements over the Mid-Atlantic are included showing horizontal and vertical distributions of NO2 during air quality episodes. During 23 research flights, NO2 profiles were measured west and generally upwind of the Baltimore/Washington, D.C. area in the morning and east (generally downwind) of the metropolitan region in the afternoon. Column contents (surface to 2,500 m altitude) were remarkably similar (≈3×1015 molecules/cm2) indicating that NO2 is widely distributed over the eastern US contributing to the regional (spatial scales of approximately 1000 km) nature of smog events.
Abstract: Organic nitrates in both gas and condensed (aerosol) phases were measured during the Uintah Basin Winter Ozone Study from January to February in 2012. A high degree of correlation between total aerosol volume at diameters less than 500 nm and the particulate organic nitrate concentration indicates that organic nitrates are a consistent, if not dominant, fraction of fine aerosol mass. In contrast, a similar correlation with sub-2.5 µm aerosol volume is weaker. The C : N atomic ratio inferred from field measurements of PM2.5 and particulate organic nitrate is 34 : 1. Calculations constrained by the observations indicate that both condensation of gas-phase nitrates and heterogeneous reactions of NO3 / N2O5 are responsible for introducing organic nitrate functionality into the aerosol and that the source molecules are alkanes. Extrapolating the results to urban aerosol suggests organic nitrate production from alkanes may be a major secondary organic aerosol source.
Abstract: A model intercomparison activity was inspired by the large suite of observations of atmospheric composition made during the International Polar Year (2008) in the Arctic. Nine global and two regional chemical transport models participated in this intercomparison and performed simulations for 2008 using a common emissions inventory to assess the differences in model chemistry and transport schemes. This paper summarizes the models and compares their simulations of ozone and its precursors and presents an evaluation of the simulations using a variety of surface, balloon, aircraft and satellite observations. Each type of measurement has some limitations in spatial or temporal coverage or in composition, but together they assist in quantifying the limitations of the models in the Arctic and surrounding regions. Despite using the same emissions, large differences are seen among the models. The cloud fields and photolysis rates are shown to vary greatly among the models, indicating one source of the differences in the simulated chemical species. The largest differences among models, and between models and observations, are in NOy partitioning (PAN vs. HNO3) and in oxygenated volatile organic compounds (VOCs) such as acetaldehyde and acetone. Comparisons to surface site measurements of ethane and propane indicate that the emissions of these species are significantly underestimated. Satellite observations of NO2 from the OMI (Ozone Monitoring Instrument) have been used to evaluate the models over source regions, indicating anthropogenic emissions are underestimated in East Asia, but fire emissions are generally overestimated. The emission factors for wildfires in Canada are evaluated using the correlations of VOCs to CO in the model output in comparison to enhancement factors derived from aircraft observations, showing reasonable agreement for methanol and acetaldehyde but underestimate ethanol, propane and acetone, while overestimating ethane emission factors.
Abstract: Alkenes are oxidized rapidly in the atmosphere by addition of OH and subsequently O2 leading to the formation of β-hydroxy peroxy radicals. These peroxy radicals react with NO to form β-hydroxy nitrates with a branching ratio α. We quantify α for C2–C8 alkenes at 295 K ± 3 and 993 hPa. The branching ratio can be expressed as α = (0.045 ± 0.016) × N − (0.11 ± 0.05) where N is the number of heavy atoms (excluding the peroxy moiety), and listed errors are 2σ. These branching ratios are larger than previously reported and are similar to those for peroxy radicals formed from H abstraction from alkanes. We find the isomer distributions of β-hydroxy nitrates formed under NO-dominated peroxy radical chemistry to be different than the isomer distribution of hydroxy hydroperoxides produced under HO2-dominated peroxy radical chemistry. Assuming unity yield for the hydroperoxides implies that the branching ratio to form β-hydroxy nitrates increases with substitution of RO2. Deuterium substitution enhances the branching ratio to form hydroxy nitrates in both propene and isoprene by a factor of ~ 1.5. The role of alkene chemistry in the Houston region is re-evaluated using the RONO2 branching ratios reported here. Small alkenes are found to play a significant role in present-day oxidant formation more than a decade (2013) after the 2000 Texas Air Quality Study identified these compounds as major contributors to photochemical smog in Houston.
Abstract: Methyl peroxy nitrate (CH3O2NO2) is a non-acyl peroxy nitrate that is important for photochemistry at low temperatures characteristic of the upper troposphere. We report the first measurements of CH3O2NO2, which we achieved through a new aircraft inlet configuration, combined with thermal-dissociation laser-induced fluorescence (TD-LIF) detection of NO2, and describe the accuracy, specificity, and interferences to CH3O2NO2 measurements. CH3O2NO2 is predicted to be a ubiquitous interference to upper-tropospheric NO2 measurements. We describe an experimental strategy for obtaining NO2 observations free of the CH3O2NO2 interference. Using these new methods, we made observations during two recent aircraft campaigns: the Deep Convective Clouds and Chemistry (DC-3) and the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) experiments. The CH3O2NO2 measurements we report have a detection limit (S/N = 2) of 15 pptv at 1 min averaging on a background of 200 pptv NO2 and an accuracy of ±40%. Observations are used to constrain the interference of pernitric acid (HO2NO2) to the CH3O2NO2 measurements, as HO2NO2 partially decomposes (~11%) along with CH3O2NO2 in the heated CH3O2NO2 channel used to detect CH3O2NO2.
Abstract: Brown carbon aerosol consists of light-absorbing organic particulate matter with wavelength-dependent absorption. Aerosol optical extinction, absorption, size distributions, and chemical composition were measured in rural Alabama during summer 2013. The field site was well located to examine sources of brown carbon aerosol, with influence by high biogenic organic aerosol concentrations, pollution from two nearby cities, and biomass burning aerosol. We report the optical closure between measured dry aerosol extinction at 365 nm and calculated extinction from composition and size distribution, showing agreement within experiment uncertainties. We find that aerosol optical extinction is dominated by scattering, with single-scattering albedo values of 0.94 ± 0.02. Black carbon aerosol accounts for 91 ± 9% of the total carbonaceous aerosol absorption at 365 nm, while organic aerosol accounts for 9 ± 9%. The majority of brown carbon aerosol mass is associated with biomass burning, with smaller contributions from biogenically derived secondary organic aerosol.
Abstract: The Deep Convective Clouds and Chemistry (DC3) field experiment produced an exceptional dataset on thunderstorms, including their dynamical, physical, and electrical structures, and their impact on the chemical composition of the troposphere. The field experiment gathered detailed information on the chemical composition of the inflow and outflow regions of midlatitude thunderstorms in northeast Colorado, West Texas to central Oklahoma, and northern Alabama. A unique aspect of the DC3 strategy was to locate and sample the convective outflow a day after active convection in order to measure the chemical transformations within the upper tropospheric convective plume. These data are being analyzed to investigate transport and dynamics of the storms, scavenging of soluble trace gases and aerosols, production of nitrogen oxides by lightning, relationships between lightning flash rates and storm parameters, chemistry in the upper troposphere that is affected by the convection, and related source characterization of the three sampling regions. DC3 also documented biomass burning plumes and the interactions of these plumes with deep convection. Capsule: DC3 brought together simultaneous measurements of storm kinematics, structure, electrical activity, and chemistry to improve our knowledge of how thunderstorms affect the chemical composition of the troposphere.
Abstract: Nitrogen dioxide (NO2) and total alkyl nitrates (ΣANs) were measured using thermal dissociation laser-induced fluorescence during the 2012 Uintah Basin Winter Ozone Study (UBWOS) in Utah, USA. The observed NO2 concentration was highest before sunrise and lowest in the late afternoon, suggestive of a persistent local source of NO2 coupled with turbulent mixing out of the boundary layer. In contrast, ΣANs co-varied with solar radiation with a noontime maximum, indicating that local photochemical production combined with rapid mixing and/or deposition was the dominant factor in determining the ΣAN concentrations. We calculate that ΣANs were a large fraction (~60%) of the HOx free radical chain termination and show that the temperature dependence of the alkyl nitrate yields enhances the role of ΣANs in local chemistry during winter by comparison to what would occur in the warmer temperatures of summer.
Abstract: The secondary organic aerosol (SOA) mass yields from NO3 oxidation of a series of biogenic volatile organic compounds (BVOCs), consisting of five monoterpenes and one sesquiterpene (α-pinene, β-pinene, Δ-3-carene, limonene, sabinene, and β-caryophyllene), were investigated in a series of continuous flow experiments in a 10 m3 indoor Teflon chamber. By making in situ measurements of the nitrate radical and employing a kinetics box model, we generate time-dependent yield curves as a function of reacted BVOC. SOA yields varied dramatically among the different BVOCs, from zero for α-pinene to 38–65% for Δ-3-carene and 86% for β-caryophyllene at mass loading of 10 μg m–3, suggesting that model mechanisms that treat all NO3 + monoterpene reactions equally will lead to errors in predicted SOA depending on each location’s mix of BVOC emissions. In most cases, organonitrate is a dominant component of the aerosol produced, but in the case of α-pinene, little organonitrate and no aerosol is formed.
Abstract: Measurements of HONO(g) and particulate nitrite (NO2-(p)) were made with a modified Ambient Ion Monitor – Ion Chromatography (AIM-IC) instrument during CalNex 2010 in Bakersfield, CA (CalNex-SJV). Observations of gas and PM2.5 water soluble composition showed accumulation of both species at night, followed by loss the next day. Intercomparison with a Stripping Coil-UV/Vis Absorption Photometer (SC-AP) demonstrated excellent agreement with the AIM-IC HONO(g) measurement (slope = 0.957, R2 = 0.86) and the particulate nitrite observations were validated to be free of known interferences for wet chemical instrumentation. The accumulation of nitrite into particulate matter was found to be enhanced when gaseous mixing ratios of HONO(g) were highest. Reactive uptake of HONO(g) on to lofted dust and the ground surface, forming a reservoir, is a potential mechanism to explain these observations.
The AIM-IC HONO(g) measurements were parameterized in a chemical model to calculate the ground surface daytime HONO(g) source strength at 4.5 m above the surface, found to be on the order of 1.27 ppb hr-1, to determine the relative importance of a surface reservoir. If all deposited nighttime HONO(g) is reemitted the following day, up to 30 % of the daytime HONO(g) source at CalNex-SJV may be accounted for. The observations of HONO(g) and NO2-(p) in Bakersfield, during CalNex, suggest a surface sink and source of HONO(g). Extension of currently accepted unknown daytime HONO(g) source reactions to include a potential surface HONO(g) reservoir should therefore be sound, but quantitation of the relative contributions of each surface source toward daytime HONO(g) production remains to be resolved.
Abstract: Exchange of NOx (NO+NO2) between the atmosphere and biosphere is important for air quality, climate change, and ecosystem nutrient dynamics. There are few direct ecosystem-scale measurements of the direction and rate of atmosphere–biosphere exchange of NOx. As a result, a complete description of the processes affecting NOx following emission from soils and/or plants as they transit from within the plant/forest canopy to the free atmosphere remains poorly constrained and debated. Here, we describe measurements of NO and NO2 fluxes and vertical concentration gradients made during the Biosphere Effects on AeRosols and Photochemistry EXperiment 2009. In general, during daytime we observe upward fluxes of NO and NO2 with counter-gradient fluxes of NO. We find that NOx fluxes from the forest canopy are smaller than calculated using observed flux–gradient relationships for conserved tracers and also smaller than measured soil NO emissions. We interpret these differences as primarily due to chemistry converting NOx to higher nitrogen oxides within the forest canopy, which might be part of a mechanistic explanation for the "canopy reduction factor" applied to soil NOx emissions in large-scale models.
Abstract: The San Joaquin Valley (SJV) experiences some of the worst ozone air quality in the US, frequently exceeding the California 8 h standard of 70.4 ppb. To improve our understanding of trends in the number of ozone violations in the SJV, we analyze observed relationships between organic reactivity, nitrogen oxides (NOx), and daily maximum temperature in the southern SJV using measurements made as part of California at the Nexus of Air Quality and Climate Change in 2010 (CalNex-SJV). We find the daytime speciated organic reactivity with respect to OH during CalNex-SJV has a temperature-independent portion with molecules typically associated with motor vehicles being the major component. At high temperatures, characteristic of days with high ozone, the largest portion of the total organic reactivity increases exponentially with temperature and is dominated by small, oxygenated organics and molecules that are unidentified. We use this simple temperature classification to consider changes in organic emissions over the last and next decade. With the CalNex-SJV observations as constraints, we examine the sensitivity of ozone production (PO3) to future NOx and organic reactivity controls. We find that PO3 is NOx-limited at all temperatures on weekends and on weekdays when daily maximum temperatures are greater than 29 °C. As a consequence, NOx reductions are the most effective control option for reducing the frequency of future ozone violations in the southern SJV.
Abstract: Biomass burning represents both a significant and highly variable source of NOx to the atmosphere. This variability stems from both the episodic nature of fires, and from fire conditions such as the modified combustion efficiency of the fire, the nitrogen content of the fuel and possibly other factors that have not been identified or evaluated by comparison with observations. Satellite instruments offer an opportunity to observe emissions from wildfires, providing a large suite of measurements which allow us to study mean behavior and variability on the regional scale in a statistically rigorous manner. Here we use space-based measurements of fire radiative power from the Moderate Resolution Imaging Spectroradiometer in combination with NO2 tropospheric column densities from the Ozone Monitoring Instrument to measure mean emission coefficients (ECs in g NO MJ−1) from fires for global biomes, and across a wide range of smaller-scale ecoregions, defined as spatially-distinct clusters of fires with similar fuel type. Mean ECs for all biomes fall between 0.250-0.362 g NO MJ-1, a range that is smaller than found in previous studies of biome-scale emission factors. The majority of ecoregion ECs fall within or near this range, implying that under most conditions, mean fire emissions of NOx per unit energy are similar between different regions regardless of fuel type or spatial variability. In contrast to these similarities, we find that about 24% of individual ecoregion ECs deviate significantly (with 95% confidence) from the mean EC for the associated biome, and a similar number of ecoregion ECs falls outside the range of all mean biome ECs, implying that there are some regions where fuel type-specific global emission parameterizations fail to capture local fire NOx emissions.
Abstract: Eight distinct hydroxy nitrates are stable products of the first step in the atmospheric oxidation of isoprene by OH. The subsequent chemical fate of these molecules affects global and regional production of ozone and aerosol as well as the location of nitrogen deposition. We synthesized and purified 3 of the 8 isoprene hydroxy nitrate isomers: (E/Z)-2-methyl-4-nitrooxybut-2-ene-1-ol and 3-methyl-2-nitrooxybut-3-ene-1-ol. Oxidation of these molecules by OH and ozone was studied using both chemical ionization mass spectrometry and thermo-dissociation laser induced fluorescence. The OH reaction rate constants at 300 K measured relative to propene at 745 Torr are (1.1 ± 0.2) × 10–10 cm3 molecule–1 s–1 for both the E and Z isomers and (4.2 ± 0.7) × 10–11 cm3 molecule–1 s–1 for the third isomer. The ozone reaction rate constants for (E/Z)-2-methyl-4-nitrooxybut-2-ene-1-ol are (2.7 ± 0.5) × 10–17 and (2.9 ± 0.5) × 10–17 cm3 molecule–1 s–1, respectively. 3-Methyl-2-nitrooxybut-3-ene-1-ol reacts with ozone very slowly, within the range of (2.5–5) × 10–19 cm3 molecule–1 s–1. Reaction pathways, product yields, and implications for atmospheric chemistry are discussed. A condensed mechanism suitable for use in atmospheric chemistry models is presented.
Abstract: The formation of organic nitrates (RONO2) represents an important NOx (NOx = NO + NO2) sink in the remote and rural continental atmosphere, thus impacting ozone production and secondary organic aerosol (SOA) formation. In these remote and rural environments, the organic nitrates are primarily derived from biogenic volatile organic compounds (BVOCs) such as isoprene and monoterpenes. Although there are numerous studies investigating the formation of SOA from monoterpenes, there are few studies investigating monoterpene gas-phase chemistry. Using a regional chemical transport model with an extended representation of organic nitrate chemistry, we investigate the processes controlling the production and fate of monoterpene nitrates (MTNs) over the boreal forest of Canada. MTNs account for 5–12% of total oxidized nitrogen over the boreal forest, and production via NO3 chemistry is more important than production via OH when the NOx mixing ratio is greater than 75 pptv. The regional responses are investigated for two oxidation pathways of MTNs: one that returns NOx to the atmosphere and one that converts MTNs into a nitrate that behaves like HNO3. The likely situation is in between, and these two assumptions bracket the uncertainty about this chemistry. In the case where the MTNs return NOx after oxidation, their formation represents a net chemical NOx loss that exceeds the net loss to peroxy nitrate formation. When oxidation of MTNs produces a molecule that behaves like HNO3, HNO3 and MTNs are nearly equal chemical sinks for NOx. This uncertainty in the oxidative fate of MTNs results in changes in NOx of 8–14%, in O3 of up to 3%, and in OH of 3–6% between the two model simulations.
Abstract: We examine spatial variations in the weekday–weekend pattern of NO2 over the Los Angeles metropolitan area using the Ozone Monitoring Instrument (OMI) and then compare the observations to calculations using the WRF-Chem model. We find that the spatial pattern of the weekday–weekend variations of the NO2 column in the model is significantly different than observed. A sensitivity study shows that the contrasting spatial pattern of NO2 on weekdays and weekends is a useful diagnostic of emissions and chemistry. These improvements suggest that constraints from space-based observations of the processes affecting urban photochemistry (e.g., spatial patterns of emissions, ratios of VOC to NOx emissions, rate constants) are possible at a level of detail not previously described.
Abstract: We implement a new isoprene oxidation mechanism in a global 3-D chemical transport model (GEOS-Chem). Model results are evaluated with observations for ozone, isoprene oxidation products, and related species from the International Consortium for Atmospheric Research on Transport and Transformation aircraft campaign over the eastern United States in summer 2004. The model achieves an unbiased simulation of ozone in the boundary layer and the free troposphere, reflecting canceling effects from recent model updates for isoprene chemistry, bromine chemistry, and HO2 loss to aerosols. Simulation of the ozone-CO correlation is improved relative to previous versions of the model, and this is attributed to a lower and reversible yield of isoprene nitrates, increasing the ozone production efficiency per unit of nitrogen oxides (NOx ≡ NO + NO2). The model successfully reproduces the observed concentrations of organic nitrates (∑ANs) and their correlations with HCHO and ozone. ∑ANs in the model is principally composed of secondary isoprene nitrates, including a major contribution from nighttime isoprene oxidation. The correlations of ∑ANs with HCHO and ozone then provide sensitive tests of isoprene chemistry and argue in particular against a fast isomerization channel for isoprene peroxy radicals. ∑ANs can provide an important reservoir for exporting NOx from the U.S. boundary layer. We find that the dependence of surface ozone on isoprene emission is positive throughout the U.S., even if NOx emissions are reduced by a factor of 4. Previous models showed negative dependences that we attribute to erroneous titration of OH by isoprene.
Abstract: Atmospheric photooxidation of isoprene is an important source of secondary organic aerosol (SOA) and there is increasing evidence that anthropogenic oxidant emissions can enhance this SOA formation. In this work, we use ambient observations of organosulfates formed from isoprene epoxydiols (IEPOX) and methacrylic acid epoxide (MAE) and a broad suite of chemical measurements to investigate the relative importance of nitrogen oxide (NO/NO2) and hydroperoxyl (HO2) SOA formation pathways from isoprene at a forested site in California. In contrast to IEPOX, the calculated production rate of MAE was observed to be independent of temperature. This is the result of the very fast thermolysis of MPAN at high temperatures that affects the distribution of the MPAN reservoir (MPAN / MPA radical) reducing the fraction that can react with OH to form MAE and subsequently SOA (FMAE formation). The strong temperature dependence of FMAE formation helps to explain our observations of similar concentrations of IEPOX-derived organosulfates (IEPOX-OS; ~1 ng m–3) and MAE-derived organosulfates (MAE-OS; ~1 ng m–3) under cooler conditions (lower isoprene concentrations) and much higher IEPOX-OS (~20 ng m–3) relative to MAE-OS (<0.0005 ng m–3) at higher temperatures (higher isoprene concentrations). A kinetic model of IEPOX and MAE loss showed that MAE forms 10−100 times more ring-opening products than IEPOX and that both are strongly dependent on aerosol water content when aerosol pH is constant. However, the higher fraction of MAE ring opening products does not compensate for the lower MAE production under warmer conditions (higher isoprene concentrations) resulting in lower formation of MAE-derived products relative to IEPOX at the surface. In regions of high NOx, high isoprene emissions and strong vertical mixing the slower MPAN thermolysis rate aloft could increase the fraction of MPAN that forms MAE resulting in a vertically varying isoprene SOA source.
Abstract: At the Rocky Mountain Biogenic Aerosol Study (BEACHON-RoMBAS) field campaign in the Colorado front range, July–August 2011, measurements of gas- and aerosol-phase organic nitrates enabled a study of the role of NOx (NOx = NO + NO2) in oxidation of forest-emitted VOCs and subsequent aerosol formation. Substantial formation of peroxy- and alkyl-nitrates is observed every morning, with an apparent 2.9% yield of alkyl nitrates from daytime RO2 + NO reactions. Aerosol-phase organic nitrates, however, peak in concentration during the night, with concentrations up to 140 ppt as measured by both optical spectroscopic and mass spectrometric instruments. The diurnal cycle in aerosol fraction of organic nitrates shows an equilibrium-like response to the diurnal temperature cycle, suggesting some reversible absorptive partitioning, but the full dynamic range cannot be reproduced by thermodynamic repartitioning alone. Nighttime aerosol organic nitrate is observed to be positively correlated with [NO2] × [O3] but not with [O3]. These observations support the role of nighttime NO3-initiated oxidation of monoterpenes as a significant source of nighttime aerosol. Nighttime production of organic nitrates exceeds daytime photochemical production at this site, which we postulate to be representative of the Colorado front range forests.
Abstract: The CMAQ (Community Multiscale Air Quality) us model in combination with observations for INTEX-NA/ICARTT (Intercontinental Chemical Transport Experiment–North America/International Consortium for Atmospheric Research on Transport and Transformation) 2004 are used to evaluate recent advances in isoprene oxidation chemistry and provide constraints on isoprene nitrate yields, isoprene nitrate lifetimes, and NOx recycling rates. We incorporate recent advances in isoprene oxidation chemistry into the SAPRC-07 chemical mechanism within the US EPA (United States Environmental Protection Agency) CMAQ model. The results show improved model performance for a range of species compared against aircraft observations from the INTEX-NA/ICARTT 2004 field campaign. We further investigate the key processes in isoprene nitrate chemistry and evaluate the impact of uncertainties in the isoprene nitrate yield, NOx (NOx = NO + NO2) recycling efficiency, dry deposition velocity, and RO2 + HO2 reaction rates. We focus our examination on the southeastern United States, which is impacted by both abundant isoprene emissions and high levels of anthropogenic pollutants. We find that NOx concentrations increase by 4–9% as a result of reduced removal by isoprene nitrate chemistry. O3 increases by 2 ppbv as a result of changes in NOx. OH concentrations increase by 30%, which can be primarily attributed to greater HOx production. We find that the model can capture observed total alkyl and multifunctional nitrates (ΣANs) and their relationship with O3 by assuming either an isoprene nitrate yield of 6% and daytime lifetime of 6 hours or a yield of 12% and lifetime of 4 h. Uncertainties in the isoprene nitrates can impact ozone production by 10% and OH concentrations by 6%. The uncertainties in NOx recycling efficiency appear to have larger effects than uncertainties in isoprene nitrate yield and dry deposition velocity. Further progress depends on improved understanding of isoprene oxidation pathways, the rate of NOx recycling from isoprene nitrates, and the fate of the secondary, tertiary, and further oxidation products of isoprene.
Abstract: Real time, atmospheric NO2 column profiles over the Mid-Atlantic states, during the July 2011 National Aeronautics and Space Administration (NASA) Deriving Information on Surface Conditions from Column and Vertically Resolved Observations to Air Quality (DISCOVER AQ) flight campaign, demonstrated that a cavity ring down spectrometer with a light emitting diode light source (LED-CRD) is a suitable technique for detecting NO2 in the boundary layer (BL) and lower free troposphere (LFT). Results from a side-by-side flight between a NASA P3 aircraft and a University of Maryland (UMD) Cessna 402B aircraft show that NO2 concentrations in ambient air from 0.08 nmol/mol (or ppbv) to 1.3 nmol/mol were consistent with NO2 measurements obtained via laser induced fluorescence (LIF) and photolysis followed by NO chemiluminescence (P-CL). The current LED-CRD, commercially available by Los Gatos Research (LGR), includes the modifications added by Castellanos et al. (Rev. Sci. Instrum. 80:113107, 2009) to compensate for baseline drift and humidity through built in zeroing and drying. Because of laser instability in the initial instrument, the laser light source in the Castellanos et al. (Rev. Sci. Instrum. 80:113107, 2009) instrument has been replaced with a light emitting diode. Six independent calibrations demonstrated the instrument’s linearity up through 150 nmol/mol NO2 and excellent stability in calibration coefficient of 1.26 (± 3.7 %). The instrument detection limit is 80 pmol/mol. Aircraft measurements over the Mid-Atlantic are included showing horizontal and vertical distributions of NO2 during air quality episodes. During 23 research flights, NO2 profiles were measured west and generally upwind of the Baltimore/Washington, D.C. area in the morning and east (generally downwind) of the metropolitan region in the afternoon. Column contents (surface to 2,500 m altitude) were remarkably similar (≈3 x 1015 molecules/cm2) indicating that NO2 is widely distributed over the eastern US contributing to the regional (spatial scales of approximately1000 km) nature of smog events.
Abstract: The presence of organic surfactants in atmospheric aerosol may lead to a depression of cloud droplet growth and evaporation rates affecting the radiative properties and lifetime of clouds. Both the magnitude and mechanism of this effect, however, remain poorly constrained. We have used Raman thermometry measurements of freely evaporating micro-droplets to determine evaporation coefficients for several concentrations of acetic acid, which is ubiquitous in atmospheric aerosol and has been shown to adsorb strongly to the air–water interface. We find no suppression of the evaporation kinetics over the concentration range studied (1–5 M). The evaporation coefficient determined for 2 M acetic acid is 0.53 ± 0.12, indistinguishable from that of pure water (0.62 ± 0.09).
Abstract: The formation of like-charge guanidinium-guanidinium contact ion pairs in water is evidenced and characterized by X-ray absorption spectroscopy and first-principles spectral simulations based on molecular dynamics sampling. Observed concentration-induced nitrogen K-edge resonance shifts result from Π* state mixing and the release of water molecules from each first solvation sphere as two solvated guanidinium ions associate into a stacked pair configuration. Possible biological implications of this counterintuitive cation-cation pairing are discussed.
Abstract: The California Research at the Nexus of Air Quality and Climate Change (CalNex) field study was conducted throughout California in May, June, and July of 2010. The study was organized to address issues simultaneously relevant to atmospheric pollution and climate change, including (1) emission inventory assessment, (2) atmospheric transport and dispersion, (3) atmospheric chemical processing, and (4) cloud-aerosol interactions and aerosol radiative effects. Measurements from networks of ground sites, a research ship, tall towers, balloon-borne ozonesondes, multiple aircraft, and satellites provided in situ and remotely sensed data on trace pollutant and greenhouse gas concentrations, aerosol chemical composition and microphysical properties, cloud microphysics, and meteorological parameters. This overview report provides operational information for the variety of sites, platforms, and measurements, their joint deployment strategy, and summarizes findings that have resulted from the collaborative analyses of the CalNex field study. Climate-relevant findings from CalNex include that leakage from natural gas infrastructure may account for the excess of observed methane over emission estimates in Los Angeles. Air-quality relevant findings include the following: mobile fleet VOC significantly declines, and NOx emissions continue to have an impact on ozone in the Los Angeles basin; the relative contributions of diesel and gasoline emission to secondary organic aerosol are not fully understood; and nighttime NO3 chemistry contributes significantly to secondary organic aerosol mass in the San Joaquin Valley. Findings simultaneously relevant to climate and air quality include the following: marine vessel emissions changes due to fuel sulfur and speed controls result in a net warming effect but have substantial positive impacts on local air quality.
Abstract: Limitations in the chemical characterization of tropospheric organic aerosol (OA) continue to impede attempts to fully understand its chemical sources and sinks. To assess the role of organic nitrates in OA, we used a new thermal dissociation – laser induced fluorescence based (TD-LIF) technique to obtain a high time resolution record of total aerosol organic nitrates (hereafter ΣANsaer) at the Bakersfield, CA supersite during the 2010 CalNex campaign. The TD-LIF measurements compare well with FT-IR measurements from collocated filter samples. These measurements show that ΣANs are a ubiquitous component of the OA with the —ONO2 subunit comprising on average 4.8% of the OA mass. Scaling this fraction by an estimate of the organic backbone mass yields an estimate that 17 - 23% of OA molecules contain nitrate functional groups. Measurements of both total ΣAN (gas + aerosol) and ΣANaer show that on average 21% of ΣANs are in the condensed phase, suggesting atmospheric organic nitrates have similar volatilities to analogous non-nitrate oxidized organic compounds. The fraction of ΣAN that is in the condensed phase increases with total OA concentration, providing direct evidence from the atmosphere that absorptive partitioning into OA has some control over the ΣAN phase partitioning. The specific molecular identity of the ΣAN is incompletely understood. Both biogenic hydrocarbons and long chain alkanes are calculated to be significant sources of low volatility nitrates in Bakersfield, and UPLC/ESI-Q-TOFMS measurements confirm the existence of particulate nitrooxy organosulfates derived from gas phase oxidation of both isoprene and monoterpenes.
Abstract: The evolution of atmospheric composition downwind of a city depends strongly on the concentration of OH within the plume. We use space-based observations of NO2, a molecule that affects both the sources and sinks of OH, to examine the functional dependence of OH concentration on the speed of the wind over Riyadh, Saudi Arabia. These observations illustrate the nonlinear dependence of the OH concentration on NO2 and on the rate of atmospheric mixing. We derive a range of NOx lifetimes of 5.5–8.0 h, lifetimes that correspond to an effective plume-averaged OH concentration of 7.6 × 106 molecules cm–3 at fast (26 km h–1) and 5.2 × 106 molecules cm–3 at slow (4 km h–1) wind speeds.
Abstract: The partitioning of reactive nitrogen (NOy) was measured over the remote North Pacific during spring 2006. Aircraft observations of NO, NO2, total peroxy nitrates (ΣPNs), total alkyl and multi-functional nitrates (ΣANs) and nitric acid (HNO3), made between 25° and 55° N, confirm a controlling role for peroxyacyl nitrates in NOx production in aged Asian outflow. ΣPNs account for more than 60% of NOy above 5 km, while thermal dissociation limits their contribution to less than 10% in the lower troposphere. Using simultaneous observations of NOx, ΣPNs, ΣANs, HNO3 and average wind speed, we calculate the flux of reactive nitrogen through the meridional plane of 150° W (between 20° and 55° N) to be 0.007 ± 0.002 Tg N day−1, which provides an upper limit of 23 ± 6.5% on the transport efficiency of NOy from East Asia. Observations of NOx, and HOx are used to constrain a 0-D photochemical box model for the calculation of net photochemical ozone production or tendency (Δ O3) as a function of aircraft altitude and NOx concentrations. The model analysis indicates that the photochemical environment of the lower troposphere (altitude < 6 km) over the north Pacific is one of net O3 destruction, with an experimentally determined crossover point between net O3 destruction and net O3 production of 60 pptv NOx. Qualitative indicators of integrated net O3 production derived from simultaneous measurements of O3 and light alkanes (Parrish et al., 1992), also indicate that the north Pacific is, on average, a region of net O3 destruction.
Abstract: In contrast with the textbook view of remote chemistry where HNO3 formation is the primary sink of nitrogen oxides, recent theoretical analyses show that formation of RONO2 (ΣANs) from isoprene and other terpene precursors is the primary net chemical loss of nitrogen oxides over the remote continents where the concentration of nitrogen oxides is low. This then increases the prominence of questions concerning the chemical lifetime and ultimate fate of ΣANs. We present observations of nitrogen oxides and organic molecules collected over the Canadian boreal forest during the summer that show that ΣANs account for ~ 20% of total oxidized nitrogen and that their instantaneous production rate is larger than that of HNO3. This confirms the primary role of reactions producing ΣANs as a control over the lifetime of NOx (NOx = NO + NO2) in remote, continental environments. However, HNO3 is generally present in larger concentrations than ΣANs indicating that the atmospheric lifetime of ΣANs is shorter than the HNO3 lifetime. We investigate a range of proposed loss mechanisms that would explain the inferred lifetime of ΣANs finding that in combination with deposition, two processes are consistent with the observations: (1) rapid ozonolysis of isoprene nitrates where at least ~ 40% of the ozonolysis products release NOx from the carbon backbone and/or (2) hydrolysis of particulate organic nitrates with HNO3 as a product. Implications of these ideas for our understanding of NOx and NOy budget in remote and rural locations are discussed.
Abstract: Nitrogen oxide (NOx) emissions from wildfires account for ~15% of the global total, inducing large fluctuations in the chemical production and loss rates of O3 and CH4 and thereby affecting Earth's radiative balance. NOx emissions from fires depend on fuel nitrogen (N) content, the relative contributions of flaming and smoldering combustion, and total biomass burned. Current understanding of the variability in these factors across biomes is limited by sparse observations. Here we use satellite-based measurements to study emission coefficients (ECs), a value proportional to NOx emitted per unit of biomass burned, from fires in African savannas. We show that ECs for NOx exhibit a pronounced seasonal cycle in woody savannas, with early-season ECs 20–40% above and late-season ECs 30–40% below the mean, while no cycle exists in nonwoody savannas. We discuss several possible mechanisms of the observed cycle including seasonal differences in fuel N content and modified combustion efficiency.
Abstract: The reactive uptake coefficients γ, for nitrate radical, NO3, on ∼100 nm diameter squalane and squalene aerosol were measured (1 atm pressure of N2 and 293 K). For squalane, a branched alkane, γNO3 of 2.8 × 10−3 was estimated. For squalene which contains 6 double bonds, γNO3 was found to be a function of degree of oxidation with an initial value of 0.18 ± 0.03 on fresh particles increasing to 0.82 ± 0.11 on average of over 3 NO3 reactions per squalene molecule in the aerosol. Synchrotron VUV-ionization aerosol mass spectrometry was used to detect the particle phase oxidation products that include as many as 3 NO3 subunits added to the squalene backbone. The fraction of squalene remaining in the aerosol follows first order kinetics under oxidation, even at very high oxidation equivalents, which suggests that the matrix remains a liquid upon oxidation. Our calculation indicates a much shorter chemical lifetime for squalene-like particle with respect to NO3 than its atmospheric lifetime to deposition or wet removal.
Abstract: Observations of tropospheric NO2 vertical column densities over the United States (US) for 2005–2011 are evaluated using the OMI Berkeley High Resolution (BEHR) retrieval algorithm. We assess changes in NO2 on day-of-week and interannual timescales to assess the impact of changes in emissions from mobile and non-mobile sources on the observed trends. We observe consistent decreases in cities across the US, with an average total reduction of 32 ± 7% across the 7 yr. Changes for large power plants have been more variable (−26 ± 12%) due to regionally-specific regulation policies. An increasing trend of 10–20% in background NO2 columns in the northwestern US is observed. We examine the impact of the economic recession on emissions and find that decreases in NO2 column densities over cities were moderate prior to the recession (−6 ± 5% yr−1), larger during the recession (−8 ± 5% yr−1), and then smaller after the recession (−3 ± 4% yr−1). Differences in the trends observed on weekdays and weekends indicate that prior to the economic recession, NO2 reductions were dominated by technological improvements to the light-duty vehicle fleet but that a decrease in diesel truck activity has contributed to emission reductions since the recession. We use the satellite observations to estimate a 34% decrease in NO2 from mobile sources in cities for 2005–2011 and use that value to infer changes in non-mobile sources. We find that reductions in NO2 from non-mobile sources in cities have been both more modest and more variable than NO2 reductions from mobile sources (−10 ± 13%).
Abstract: We present an analysis of the NOx budget in conditions of low NOx (NOx = NO + NO2) and high biogenic volatile organic compound (BVOC) concentrations that are characteristic of most continental boundary layers. Using a steady-state model, we show that below 500 pptv of NOx, the NOx lifetime is extremely sensitive to organic nitrate (RONO2) formation rates. We find that even for RONO2 formation values that are an order of magnitude smaller than is typical for continental conditions significant reductions in NOx lifetime, and consequently ozone production efficiency, are caused by nitrate forming reactions. Comparison of the steady-state box model to a 3-D chemical transport model (CTM) confirms that the concepts illustrated by the simpler model are a useful approximation of predictions provided by the full CTM. This implies that the regional and global budgets of NOx, OH, and ozone will be sensitive to assumptions regarding organic nitrate chemistry. Changes in the budgets of these species affect the representation of processes important to air quality and climate. Consequently, CTMs must include an accurate representation of organic nitrate chemistry in order to provide accurate assessments of past, present, and future air quality and climate. These findings suggest the need for further experimental constraints on the formation and fate of biogenic RONO2.
Abstract: N2O5 detection in the atmosphere has been accomplished using techniques which have been developed during the last decade. Most techniques use a heated inlet to thermally decompose N2O5 to NO3, which can be detected by either cavity based absorption at 662 nm or by laser-induced fluorescence. In summer 2007, a large set of instruments, which were capable of measuring NO3 mixing ratios, were simultaneously deployed in the atmosphere simulation chamber SAPHIR in Jülich, Germany. Some of these instruments measured N2O5 mixing ratios either simultaneously or alternatively. Experiments focused on the investigation of potential interferences from, e.g., water vapour or aerosol and on the investigation of the oxidation of biogenic volatile organic compounds by NO3. The comparison of N2O5 mixing ratios shows an excellent agreement between measurements of instruments applying different techniques (3 cavity ring-down (CRDS) instruments, 2 laser-induced fluorescence (LIF) instruments). Datasets are highly correlated as indicated by the square of the linear correlation coefficients, R2, which values were larger than 0.96 for the entire datasets. N2O5 mixing ratios well agree within the combined accuracy of measurements. Slopes of the linear regression range between 0.87 and 1.26 and intercepts are negligible. The most critical aspect of N2O5 measurements by cavity ring-down instruments is the determination of the inlet and filter transmission efficiency. Measurements here show that the N2O5 inlet transmission efficiency can decrease in the presence of high aerosol loads, and that frequent filter/inlet changing is necessary to quantitatively sample N2O5 in some environments. The analysis of data also demonstrates that a general correction for degrading filter transmission is not applicable for all conditions encountered during this campaign. Besides the effect of a gradual degradation of the inlet transmission efficiency aerosol exposure, no other interference for N2O5 measurements is found.
Abstract: Peroxynitrates are responsible for global scale transport of reactive nitrogen. Recent laboratory observations suggest that they may also play an important role in delivery of nutrients to plant canopies. We measured eddy covariance fluxes of total peroxynitrates (ΣPNs) and three individual peroxynitrates (APNs ≡ PAN + PPN + MPAN) over a ponderosa pine forest during the Biosphere Effects on AeRosols and Photochemistry EXperiment 2009 (BEARPEX 2009). Concentrations of these species were also measured at multiple heights above and within the canopy. While the above-canopy daytime concentrations are nearly identical for ΣPNs and APNs, we observed the downward flux of ΣPNs to be 30–60% slower than the flux of APNs. The vertical concentration gradients of ΣPNs and APNs vary with time of day and exhibit different temperature dependencies. These differences can be explained by the production of peroxynitrates other than PAN, PPN, and MPAN within the canopy (presumably as a consequence of biogenic VOC emissions) and upward fluxes of these PN species. The impact of this implied peroxynitrate flux on the interpretation of NOx fluxes and ecosystem N exchange is discussed.
Abstract: We describe the effects of nitrogen oxide (NOx) and organic reactivity reductions on the frequency of high ozone days in California's San Joaquin Valley. We use sixteen years of observations of ozone, nitrogen oxides, and temperature at sites upwind, within, and downwind of three cities to assess the probability of exceeding the California 8-h average ozone standard of 70.4 ppb at each location. The comprehensive data records in the region and the steep decreases in emissions over the last decade are sufficient to constrain the relative import of NOx and organic reactivity reductions on the frequency of violations. We show that high ozone has a large component that is due to local production, as the probability of exceeding the state standard is lowest for each city at the upwind site, increases in the city center, is highest at downwind locations, and then decreases at the receptor city to the south. We see that reductions in organic reactivity have been very effective in the central and northern regions of the San Joaquin but less so in the southern portion of the Valley. We find evidence for two distinct categories of reactivity sources: one source that has decreased and dominates at moderate temperatures, and a second source that dominates at high temperatures, particularly in the southern San Joaquin, and has not changed over the last twelve years. We show that NOx reductions are already effective or are poised to become so in the southern and central Valley, where violations are most frequent, as conditions in these regions have or are transitioning to NOx-limited chemistry when temperatures are hottest and high ozone most probable.
Abstract: The understanding of oxidation in forest atmospheres is being challenged by measurements of unexpectedly large amounts of hydroxyl (OH). A significant number of these OH measurements were made by laser-induced fluorescence in low-pressure detection chambers (called Fluorescence Assay with Gas Expansion (FAGE)) using the Penn State Ground-based Tropospheric Hydrogen Oxides Sensor (GTHOS). We deployed a new chemical removal method to measure OH in parallel with the traditional FAGE method in a California forest. The new method gives on average only 40–60% of the OH from the traditional method and this discrepancy is temperature dependent. Evidence indicates that the new method measures atmospheric OH while the traditional method is affected by internally generated OH, possibly from oxidation of biogenic volatile organic compounds. The improved agreement between OH measured by this new technique and modeled OH suggests that oxidation chemistry in at least one forest atmosphere is better understood than previously thought.
Abstract: Laboratory studies have established a number of chemical pathways by which nitrogen oxides (NOx) affect atmospheric organic aerosol (OA) production. However, these effects have not been directly observed in ambient OA. We report measurements of particulate organic nitrates in Bakersfield, California, the nighttime formation of which increases with NOx and is suppressed by high concentrations of organic molecules that rapidly react with nitrate radical (NO3)—evidence that multigenerational chemistry is responsible for organic nitrate aerosol production. This class of molecules represents about a third of the nighttime increase in OA, suggesting that most nighttime secondary OA is due to the NO3 product of anthropogenic NOx emissions. Consequently, reductions in NOx emissions should reduce the concentration of organic aerosol in Bakersfield and the surrounding region.
Abstract: Soils have been identified as a major source (~15%) of global nitrogen oxide (NOx) emissions. Parameterizations of soil NOx emissions (SNOx) commonly used in the current generation of chemical transport models were designed to capture mean seasonal behaviour. These parameterizations do not, however, respond quantitatively to the meteorological triggers that are observed to result in pulsed SNOx. Here we present a new parameterization of SNOx implemented within a global chemical transport model (GEOS-Chem). The parameterization represents available nitrogen (N) in soils using biome specific emission factors, online wet- and dry-deposition of N, and fertilizer and manure N derived from a spatially explicit dataset, distributed using seasonality derived from data obtained by the Moderate Resolution Imaging Spectrometer. Moreover, it represents the functional form of emissions derived from point measurements and ecosystem scale experiments including pulsing following soil wetting by rain or irrigation, and emissions that are a smooth function of soil moisture as well as temperature between 0 and 30 °C. This parameterization yields global above-soil SNOx of 10.7 Tg N yr−1, including 1.8 Tg N yr−1 from fertilizer N input (1.5% of applied N) and 0.5 Tg N yr−1 from atmospheric N deposition. Over the United States (US) Great Plains region, SNOx are predicted to comprise 15–40% of the tropospheric NO2 column and increase column variability by a factor of 2–4 during the summer months due to chemical fertilizer application and warm temperatures. SNOx enhancements of 50–80% of the simulated NO2 column are predicted over the African Sahel during the monsoon onset (April–June). In this region the day-to-day variability of column NO2 is increased by a factor of 5 due to pulsed-N emissions. We evaluate the model by comparison with observations of NO2 column density from the Ozone Monitoring Instrument (OMI). We find that the model is able to reproduce the observed interannual variability of NO2 (induced by pulsed-N emissions) over the US Great Plains. We also show that the OMI mean (median) NO2 observed during the overpass following first rainfall over the Sahel is 49% (23%) higher than in the five days preceding. The measured NO2 on the day after rainfall is still 23% (5%) higher, providing a direct measure of the pulse's decay time of 1–2 days. This is consistent with the pulsing representation used in our parameterization and much shorter than 5–14 day pulse decay length used in current models.
Abstract: Alkyl and multifunctional organic nitrates, molecules of the chemical form RONO2, are products of chain terminating reactions in the tropospheric HOx and NOx catalytic cycles and thereby impact ozone formation locally. Many of the molecules in the class have lifetimes that are long enough that they can be transported over large distances. If the RONO2 then decompose to deliver NOx to remote regions they affect ozone production rates in locations distant from the original NOx source. While measurements of total RONO2 (ΣANs) and small straight chain alkyl nitrates are routine, measurements of the specific multifunctional RONO2 molecules that are believed to dominate the total have rarely been reported and never reported in coincidence with ambient ΣANs measurements. Here we describe observations obtained during the BEARPEX 2009 experiment including ΣANs and a suite of multifunctional nitrates including isoprene derived hydroxynitrates, oxidation products of those nitrates, 2-methyl-3-buten-2-ol (MBO) derived hydroxynitrates, and monoterpene nitrates. At the BEARPEX field site, the sum of the individual biogenically derived nitrates account for two-thirds of the ΣANs, confirming predictions of the importance of biogenic nitrates to the NOy budget. Isoprene derived nitrates, transported to the site, are a much larger fraction of the ΣANs at the site than the nitrates derived from the locally emitted MBO. Evidence for additional nitrates, possibly from nocturnal chemistry of isoprene and α-pinene, is presented.
Abstract: Tropospheric ozone is the third strongest greenhouse gas, and has the highest uncertainty in radiative forcing of the top five greenhouse gases. Throughout the troposphere, ozone is produced by radical oxidation of nitrogen oxides (NOx = NO + NO2). In the upper troposphere (8–10 km), current chemical transport models under-estimate nitrogen dioxide (NO2) observations. Improvements to simulated NOx production from lightning have increased NO2 predictions, but the predictions in the upper troposphere remain biased low. The upper troposphere has low temperatures (T < 250 K) that increase the uncertainty of many important chemical reaction rates. This study constrains uncertain reaction rates by combining model predictions with measurements from the Intercontinental Chemical Transport Experiment-North America observational campaign. The results show that the nitric acid formation rate, which is the dominant sink of NO2 and radicals, is currently over-estimated by 22% in the upper troposphere. The results from this study suggest that the temperature sensitivity of nitric acid formation is lower than currently recommended. Since the formation of nitric acid removes nitrogen dioxide and radicals that drive the production of ozone, the revised reaction rate will affect ozone concentrations in upper troposphere impacting climate and air quality in the lower troposphere.
Abstract: Inference of NOx emissions (NO+NO2) from satellite observations of tropospheric NO2 column requires knowledge of NOx lifetime, usually provided by chemical transport models (CTMs). However, it is known that species subject to non-linear sources or sinks, such as ozone, are susceptible to biases in coarse-resolution CTMs. Here we compute the resolution-dependent bias in predicted NO2 column, a quantity relevant to the interpretation of space-based observations. We use 1-D and 2-D models to illustrate the mechanisms responsible for these biases over a range of NO2 concentrations and model resolutions. We find that predicted biases are largest at coarsest model resolutions with negative biases predicted over large sources and positive biases predicted over small sources. As an example, we use WRF-CHEM to illustrate the resolution necessary to predict 10 AM and 1 PM NO2 column to 10 and 25% accuracy over three large sources, the Four Corners power plants in NW New Mexico, Los Angeles, and the San Joaquin Valley in California for a week-long simulation in July 2006. We find that resolution in the range of 4–12 km is sufficient to accurately model nonlinear effects in the NO2 loss rate.
Abstract: Detailed comparisons of airborne CH2O measurements acquired by tunable diode laser absorption spectroscopy with steady state box model calculations were carried out using data from the 2006 INTEX-B and MILARGO campaign in order to improve our understanding of hydrocarbon oxidation processing. This study includes comparisons over Mexico (including Mexico City), the Gulf of Mexico, parts of the continental United States near the Gulf coast, as well as the more remote Pacific Ocean, and focuses on comparisons in the boundary layer. Select previous comparisons in other campaigns have highlighted some locations in the boundary layer where steady state box models have tended to underpredict CH2O, suggesting that standard steady state modeling assumptions might be unsuitable under these conditions, and pointing to a possible role for unmeasured hydrocarbons and/or additional primary emission sources of CH2O. Employing an improved instrument, more detailed measurement-model comparisons with better temporal overlap, up-to-date measurement and model precision estimates, up-to-date rate constants, and additional modeling tools based on both Lagrangian and Master Chemical Mechanism (MCM) runs, we have explained much of the disagreement between observed and predicted CH2O as resulting from non-steady-state atmospheric conditions in the vicinity of large pollution sources, and have quantified the disagreement as a function of plume lifetime (processing time). We show that in the near field (within ~4 to 6 h of the source), steady-state models can either over-or-underestimate observations, depending on the predominant non-steady-state influence. In addition, we show that even far field processes (10–40 h) can be influenced by non-steady-state conditions which can be responsible for CH2O model underestimations by ~20%. At the longer processing times in the 10 to 40 h range during Mexico City outflow events, MCM model calculations, using assumptions about initial amounts of high-order NMHCs, further indicate the potential importance of CH2O produced from unmeasured and multi-generation hydrocarbon oxidation compounds, particularly methylglyoxal, 3-hydroxypropanal, and butan-3-one-al.
Abstract: A relaxed eddy accumulation (REA) system combined with a nitrous acid (HONO) analyzer was developed to measure atmospheric HONO vertical fluxes. The system consists of three major components: (1) a fast-response sonic anemometer measuring both vertical wind velocity and air temperature, (2) a fast-response controlling unit separating air motions into updraft and downdraft samplers by the sign of vertical wind velocity, and (3) a highly sensitive HONO analyzer based on aqueous long path absorption photometry that measures HONO concentrations in the updrafts and downdrafts. A dynamic velocity threshold (±0.5σw, where σw is a standard deviation of the vertical wind velocity) was used for valve switching determined by the running means and standard deviations of the vertical wind velocity. Using measured temperature as a tracer and the average values from two field deployments, the flux proportionality coefficient, β, was determined to be 0.42 ± 0.02, in good agreement with the theoretical estimation. The REA system was deployed in two ground-based field studies. In the California Research at the Nexus of Air Quality and Climate Change (CalNex) study in Bakersfield, California in summer 2010, measured HONO fluxes appeared to be upward during the day and were close to zero at night. The upward HONO flux was highly correlated to the product of NO2 and solar radiation. During the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX 2009) at Blodgett Forest, California in July 2009, the overall HONO fluxes were small in magnitude and were close to zero. Causes for the different HONO fluxes in the two different environments are briefly discussed.
Abstract: We present roughly one month of high time-resolution, direct, in situ measurements of gas-phase glyoxal acquired during the BEARPEX 2007 field campaign. The research site, located on a ponderosa pine plantation in the Sierra Nevada mountains, is strongly influenced by biogenic volatile organic compounds (BVOCs); thus this data adds to the few existing measurements of glyoxal in BVOC-dominated areas. The short lifetime of glyoxal of ~1 h, the fact that glyoxal mixing ratios are much higher during high temperature periods, and the results of a photochemical model demonstrate that glyoxal is strongly influenced by BVOC precursors during high temperature periods. A zero-dimensional box model using near-explicit chemistry from the Leeds Master Chemical Mechanism v3.1 was used to investigate the processes controlling glyoxal chemistry during BEARPEX 2007. The model showed that MBO is the most important glyoxal precursor (~67 %), followed by isoprene (~26 %) and methylchavicol (~6 %), a precursor previously not commonly considered for glyoxal production. The model calculated a noon lifetime for glyoxal of ~0.9 h, making glyoxal well suited as a local tracer of VOC oxidation in a forested rural environment; however, the modeled glyoxal mixing ratios over-predicted measured glyoxal by a factor 2 to 5. Loss of glyoxal to aerosol was not found to be significant, likely as a result of the very dry conditions, and could not explain the over-prediction. Although several parameters, such as an approximation for advection, were found to improve the model measurement discrepancy, reduction in OH was by far the most effective. Reducing model OH concentrations to half the measured values decreased the glyoxal over-prediction from a factor of 2.4 to 1.1, as well as the overprediction of HO2 from a factor of 1.64 to 1.14. Our analysis has shown that glyoxal is particularly sensitive to OH concentration compared to other BVOC oxidation products. This relationship arises from (i) the predominantly secondary- or higher-generation production of glyoxal from (mainly OH-driven, rather than O3-driven) BVOC oxidation at this site and (ii) the relative importance of photolysis in glyoxal loss as compared to reaction with OH. We propose that glyoxal is a useful tracer for OH-driven BVOC oxidation chemistry.
Abstract: We retrieve slant column NO2 from the super-zoom mode of the Ozone Monitoring Instrument (OMI) to explore its utility for understanding NOx emissions and variability. Slant column NO2 is operationally retrieved from OMI (Boersma et al., 2007; Bucsela et al., 2006) with a nadir footprint of 13 × 24 km2, the result of averaging eight detector elements on board the instrument. For 85 orbits in late 2004, OMI reported observations from individual "super-zoom" detector elements (spaced at 13 × 3 km2 at nadir). We assess the spatial response of these individual detector elements in-flight and determine an upper-bound on spatial resolution of 9 km, in good agreement with on-ground calibration (7 km FWHM). We determine the precision of the super-zoom mode to be 2.1 × 1015 molecules cm−2, approximately a factor of √8 lower than an identical retrieval at operational scale as expected if random noise dominates the uncertainty. We retrieve slant column NO2 over the Satpura power plant in India; Seoul, South Korea; Dubai, United Arab Emirates; and a set of large point sources on the Rihand Reservoir in India using differential optical absorption spectroscopy (DOAS). Over these sources, the super-zoom mode of OMI observes variation in slant column NO2 of up to 30 × the instrumental precision within one operational footprint.
Abstract: We present a new retrieval of tropospheric NO2 vertical column density from the Ozone Monitoring Instrument (OMI) based on high spatial and temporal resolution terrain and profile inputs. We compare our NO2 product, the Berkeley High-Resolution (BEHR) product, with operational retrievals and find that the operational retrievals are biased high (30 %) over remote areas and biased low (8 %) over urban regions. Additionally, we find non-negligible impacts on the retrieved NO2 column for terrain pressure (±20 %), albedo (±40 %), and NO2 vertical profile (−75 %–+10 %). We validate the operational and BEHR products using boundary layer aircraft observations from the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS-CA) field campaign which occurred in June 2008 in California. Results indicate that columns derived using our boundary layer extrapolation method show good agreement with satellite observations (R2 = 0.65–0.83; N = 68) and provide a more robust validation of satellite-observed NO2 column than those determined using full vertical spirals (R2 = 0.26; N = 5) as in previous work. Agreement between aircraft observations and the BEHR product (R2 = 0.83) is better than agreement with the operational products (R2 = 0.65–0.72). We also show that agreement between satellite and aircraft observations can be further improved (e.g. BEHR: R2 = 0.91) using cloud information from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument instead of the OMI cloud product. These results indicate that much of the variance in the operational products can be attributed to coarse resolution terrain pressure, albedo, and profile parameters implemented in the retrievals.
Abstract: Observations of NOx in the Sacramento, CA region show that mixing ratios decreased by 30 % between 2001 and 2008. Here we use an observation-based method to quantify net ozone (O3) production rates in the outflow from the Sacramento metropolitan region and examine the O3 decrease resulting from reductions in NOx emissions. This observational method does not rely on assumptions about detailed chemistry of ozone production, rather it is an independent means to verify and test these assumptions. We use an instantaneous steady-state model as well as a detailed 1-D plume model to aid in interpretation of the ozone production inferred from observations. In agreement with the models, the observations show that early in the plume, the NOx dependence for Ox (Ox = O3 + NO2) production is strongly coupled with temperature, suggesting that temperature-dependent biogenic VOC emissions and other temperature-related effects can drive Ox production between NOx-limited and NOx-suppressed regimes. As a result, NOx reductions were found to be most effective at higher temperatures over the 7 year period. We show that violations of the California 1-h O3 standard (90 ppb) in the region have been decreasing linearly with decreases in NOx (at a given temperature) and predict that reductions of NOx concentrations (and presumably emissions) by an additional 30% (relative to 2007 levels) will eliminate violations of the state 1 h standard in the region. If current trends continue, a 30% decrease in NOx is expected by 2012, and an end to violations of the 1 h standard in the Sacramento region appears to be imminent.
Abstract: We use observations of fire radiative power (FRP) from the Moderate Resolution Imaging Spectroradiometer~(MODIS) and tropospheric NO2 column measurements from the Ozone Monitoring Instrument (OMI) to derive NO2 wildfire emission coefficients (g MJ−1) for three land types over California and Nevada. Retrieved emission coefficients were 0.279±0.077, 0.342±0.053, and 0.696±0.088 g MJ−1 NO2 for forest, grass and shrub fuels, respectively. These emission coefficients reproduce ratios of emissions with fuel type reported previously using independent methods. However, the magnitude of these coefficients is lower than prior estimates. While it is possible that a negative bias in the OMI NO2 retrieval over regions of active fire emissions is partly responsible, comparison with several other studies of fire emissions using satellite platforms indicates that current emission factors may overestimate the contributions of flaming combustion and underestimate the contributions of smoldering combustion to total fire emissions. Our results indicate that satellite data can provide an extensive characterization of the variability in fire NOx emissions; 67 % of the variability in emissions in this region can be accounted for using an FRP-based parameterization.
Abstract: Validation is the essential part of satellite remote sensing, since the retrieved data must be fit-for-purpose and their significance quantified, whether they are for scientific research or environmental monitoring. Data are validated by comparing satellite data sets with those obtained from ground-based, balloon and airborne instrumentation, or from instruments on other satellites, or with the output of models; all can be fraught with sampling difficulties and comparability. Chapter 7 discusses these problems in some detail and indicates the quality assurance that is used in the field. The possibilities of optimising retrieval algorithms are dealt with, as well the problem of instrument degradation over time. The differing needs for data on trace gases and cloud and aerosol data are mentioned, as are the use of correlative methods. The chapter concludes with requirements for future measurements and possible validation strategies.
Abstract: Urban O3 is produced by photochemical chain reactions that amplify background O3 in mixtures of gaseous nitrogen oxides (NOx) and organic molecules. Current thinking treats NOx and organics as independent variables that limit O3 production depending on the NOx to organic ratio; in this paradigm, reducing organics either has no effect or reduces O3. We describe a theoretical counterexample where NOx and organics are strongly coupled and reducing organics increases O3 production, and illustrate the example with observations from Mexico City. This effect arises from chain termination in the HOx and NOx cycles via organic nitrate production. We show that reductions in VOC reactivity that inadvertently reduce organic nitrate production rates will be counterproductive without concurrent reductions in NOx or other organics.
Abstract: Using measurements from the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) experiment, we show that methyl peroxy nitrate (CH3O2NO2) is present in concentrations of ~5–15 pptv in the springtime arctic upper troposphere. We investigate the regional and global effects of CH3O2NO2 by including its chemistry in the GEOS-Chem 3-D global chemical transport model. We find that at temperatures below 240 K inclusion of CH3O2NO2 chemistry results in decreases of up to ~20 % in NOx, ~20 % in N2O5, ~5 % in HNO3, ~2 % in ozone, and increases in methyl hydrogen peroxide of up to ~14 %. Larger changes are observed in biomass burning plumes lofted to high altitude. Additionally, by sequestering NOx at low temperatures, CH3O2NO2 decreases the cycling of HO2 to OH, resulting in a larger upper tropospheric HO2 to OH ratio. These results may impact some estimates of lightning NOx sources as well as help explain differences between models and measurements of upper tropospheric composition.
Abstract: The formation of organic nitrates and secondary organic aerosol (SOA) were monitored during the NO3 + limonene reaction in the atmosphere simulation chamber SAPHIR at Research Center Jülich. The 24-h run began in a purged, dry, particle-free chamber and comprised two injections of limonene and oxidants, such that the first experiment measured SOA yield in the absence of seed aerosol, and the second experiment yields in the presence of 10 μg m−3 seed organic aerosol. After each injection, two separate increases in aerosol mass were observed, corresponding to sequential oxidation of the two limonene double bonds. Analysis of the measured NO3, limonene, product nitrate concentrations, and aerosol properties provides mechanistic insight and constrains rate constants, branching ratios and vapor pressures of the products. The organic nitrate yield from NO3 + limonene is ≈30%. The SOA mass yield was observed to be 25–40%. The first injection is reproduced by a kinetic model. PMF analysis of the aerosol composition suggests that much of the aerosol mass results from combined oxidation by both O3 and NO3, e.g., oxidation of NO3 + limonene products by O3. Further, later aerosol nitrate mass seems to derive from heterogeneous uptake of NO3 onto unreacted aerosol alkene.
Abstract: In a companion paper, we introduced the Chemistry of Atmosphere-Forest Exchange (CAFE) model, a vertically-resolved 1-D chemical transport model designed to probe the details of near-surface reactive gas exchange. Here, we apply CAFE to noontime observations from the 2007 Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX-2007). In this work we evaluate the CAFE modeling approach, demonstrate the significance of in-canopy chemistry for forest-atmosphere exchange and identify key shortcomings in the current understanding of intra-canopy processes. CAFE generally reproduces BEARPEX-2007 observations but requires an enhanced radical recycling mechanism to overcome a factor of 6 underestimate of hydroxyl (OH) concentrations observed during a warm (~29 °C) period. Modeled fluxes of acyl peroxy nitrates (APN) are quite sensitive to gradients in chemical production and loss, demonstrating that chemistry may perturb forest-atmosphere exchange even when the chemical timescale is long relative to the canopy mixing timescale. The model underestimates peroxy acetyl nitrate (PAN) fluxes by 50% and the exchange velocity by nearly a factor of three under warmer conditions, suggesting that near-surface APN sinks are underestimated relative to the sources. Nitric acid typically dominates gross dry N deposition at this site, though other reactive nitrogen (NOy) species can comprise up to 28% of the N deposition budget under cooler conditions. Upward NO2 fluxes cause the net above-canopy NOy flux to be ~30% lower than the gross depositional flux. CAFE under-predicts ozone fluxes and exchange velocities by ~20%. Large uncertainty in the parameterization of cuticular and ground deposition precludes conclusive attribution of non-stomatal fluxes to chemistry or surface uptake. Model-measurement comparisons of vertical concentration gradients for several emitted species suggests that the lower canopy airspace may be only weakly coupled with the upper canopy. Future efforts to model forest-atmosphere exchange will require a more mechanistic understanding of non-stomatal deposition and a more thorough characterization of in-canopy mixing processes.
Abstract: Regional and global chemical transport models underpredict NOx (NO + NO2) in the upper troposphere where it is a precursor to the greenhouse gas ozone. The NOx bias has been shown in model evaluations using aircraft data (Singh et al., 2007) and total column NO2 (molecules cm−2) from satellite observations (Napelenok et al., 2008). The causes of NOx underpredictions have yet to be fully understood due to the interconnected nature of simulated emission, transport, and chemistry processes. Recent observation-based studies, in the upper troposphere, identify chemical rate coefficients as a potential source of error (Olson et al., 2006; Ren et al., 2008). Since typical chemistry evaluation techniques are not available for upper tropospheric conditions, this study develops an evaluation platform from in situ observations, stochastic convection, and deterministic chemistry. We derive a stochastic convection model and optimize it using two simulated datasets of time since convection, one based on meteorology, and the other on chemistry. The chemistry surrogate for time since convection is calculated using seven different chemical mechanisms, all of which predict shorter time since convection than our meteorological analysis. We evaluate chemical simulations by inter-comparison and by pairing results with observations based on NOx:HNO3, a photochemical aging indicator. Inter-comparison reveals individual chemical mechanism biases and recommended updates. Evaluation against observations shows that all chemical mechanisms overpredict NOx removal relative to long-lived methanol and carbon monoxide. All chemical mechanisms underpredict observed NOx by at least 30%, and further evaluation is necessary to refine simulation sensitivities to initial conditions and chemical rate uncertainties.
Abstract: We analyze detailed atmospheric gas/aerosol composition data acquired during the 2008 NASA ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) airborne campaign performed at high northern latitudes in spring (ARCTAS-A) and summer (ARCTAS-B) and in California in summer (ARCTAS-CARB). Biomass burning influences were widespread throughout the ARCTAS campaign. MODIS data from 2000 to 2009 indicated that 2008 had the second largest fire counts over Siberia and a more normal Canadian boreal forest fire season. Near surface arctic air in spring contained strong anthropogenic signatures indicated by high sulfate. In both spring and summer most of the pollution plumes transported to the Arctic region were from Europe and Asia and were present in the mid to upper troposphere and contained a mix of forest fire and urban influences. The gas/aerosol composition of the high latitude troposphere was strongly perturbed at all altitudes in both spring and summer. The reactive nitrogen budget was balanced with PAN as the dominant component. Mean ozone concentrations in the high latitude troposphere were only minimally perturbed (<5 ppb), although many individual pollution plumes sampled in the mid to upper troposphere, and mixed with urban influences, contained elevated ozone (ΔO3/ΔCO = 0.11 ± 0.09 v/v). Emission and optical characteristics of boreal and California wild fires were quantified and found to be broadly comparable. Greenhouse gas emission estimates derived from ARCTAS-CARB data for the South Coast Air Basin of California show good agreement with state inventories for CO2 and N2O but indicate substantially larger emissions of CH4. Simulations by multiple models of transport and chemistry were found to be broadly consistent with observations with a tendency towards under prediction at high latitudes.
Abstract: We determine enhancement ratios for NOx, PAN, and other NOy species from boreal biomass burning using aircraft data obtained during the ARCTAS-B campaign and examine the impact of these emissions on tropospheric ozone in the Arctic. We find an initial emission factor for NOx of 1.06 g NO per kg dry matter (DM) burned, much lower than previous observations of boreal plumes, and also one third the value recommended for extratropical fires. Our analysis provides the first observational confirmation of rapid PAN formation in a boreal smoke plume, with 40% of the initial NOx emissions being converted to PAN in the first few hours after emission. We find little clear evidence for ozone formation in the boreal smoke plumes during ARCTAS-B in either aircraft or satellite observations, or in model simulations. Only a third of the smoke plumes observed by the NASA DC8 showed a correlation between ozone and CO, and ozone was depleted in the plumes as often as it was enhanced. Special observations from the Tropospheric Emission Spectrometer (TES) also show little evidence for enhanced ozone in boreal smoke plumes between 15 June and 15 July 2008. Of the 22 plumes observed by TES, only 4 showed ozone increasing within the smoke plumes, and even in those cases it was unclear that the increase was caused by fire emissions. Using the GEOS-Chem atmospheric chemistry model, we show that boreal fires during ARCTAS-B had little impact on the median ozone profile measured over Canada, and had little impact on ozone within the smoke plumes observed by TES.
Abstract: We examine the interannual variability in the NO2 column over North America measured by the Ozone Monitoring Instrument (OMI) in 2005–2008. By comparison to a model of soil NOx emissions driven by the North American Regional Reanalysis precipitation and 0–10 cm soil temperature fields, we show the source of this observed interannual variability over much of the central United States in June is fertilizer application. We find that dry, warm conditions followed by convective precipitation induces pulsed emissions of NOx over the agricultural Great Plains. In June 2006 we infer a 50% increase in soil NOx emission and a 30% increase in the tropospheric NO2 column relative to the June 2005–2008 mean. In a case-study of fertilized corn and soybean fields over SE South Dakota, we find an associated rain-induced pulsing event reaching 4.6×1015 molec cm−2, equivalent to a surface concentration of ~2 ppbv. We calculate that soil NOx emissions resulted in a mean daily maximum 8-h ozone enhancement over the agricultural Great Plains of 5 ppbv in June 2006 (with predicted events reaching 16 ppbv) compared with a mean enhancement of 3 ppbv for soil NOx in the years 2005–2008.
Abstract: We interpret observations from the Intercontinental Chemical Transport Experiment, Phase B (INTEX-B) in spring 2006 using a global chemical transport model (GEOS-Chem) to evaluate sensitivities of the free troposphere above the North Pacific Ocean and North America to Asian anthropogenic emissions. We develop a method to use satellite observations of tropospheric NO2 columns to provide timely estimates of trends in NOx emissions. NOx emissions increased by 33% for China and 29% for East Asia from 2003 to 2006. We examine measurements from three aircraft platforms from the INTEX-B campaign, including a Canadian Cessna taking vertical profiles of ozone near Whistler Peak. The contribution to the mean simulated ozone profiles over Whistler below 5.5 km is at least 7.2 ppbv for Asian anthropogenic emissions and at least 3.5 ppbv for global lightning NOx emissions. Tropospheric ozone columns from OMI exhibit a broad Asian outflow plume across the Pacific, which is reproduced by simulation. Mean modelled sensitivities of Pacific (30° N–60° N) tropospheric ozone columns are at least 4.6 DU for Asian anthropogenic emissions and at least 3.3 DU for lightning, as determined by simulations excluding either source. Enhancements of ozone over Canada from Asian anthropogenic emissions reflect a combination of trans-Pacific transport of ozone produced over Asia, and ozone produced in the eastern Pacific through decomposition of peroxyacetyl nitrates (PANs). A sensitivity study decoupling PANs globally from the model's chemical mechanism establishes that PANs increase ozone production by removing NOx from regions of low ozone production efficiency (OPE) and injecting it into regions with higher OPE, resulting in a global increase in ozone production by 2% in spring 2006. PANs contribute up to 4 ppbv to surface springtime ozone concentrations in western Canada. Ozone production due to PAN transport is greatest in the eastern Pacific; commonly occurring transport patterns advect this ozone northeastward into Canada. Transport events observed by the aircraft confirm that polluted airmasses were advected in this way.
Abstract: To better understand the processing of biogenic VOCs (BVOCs) in the pine forests of the US Sierra Nevada, we measured HCHO at Blodgett Research Station using Quantum Cascade Laser Spectroscopy (QCLS) during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) of late summer 2007. Four days of the experiment exhibited particularly copious HCHO, with midday peaks between 15–20 ppbv, while the other days developed delayed maxima between 8–14 ppbv in the early evening. From the expansive photochemical data set, we attempt to explain the observed HCHO concentrations by quantifying the various known photochemical production and loss terms in its chemical budget. Overall, known chemistry predicts a factor of 3–5 times less HCHO than observed. By examining diurnal patterns of the various budget terms we conclude that, during the high HCHO period, local, highly reactive oxidation chemistry produces an abundance of formaldehyde at the site. The results support the hypothesis of previous work at Blodgett Forest suggesting that large quantities of oxidation products, observed directly above the ponderosa pine canopy, are evidence of profuse emissions of very reactive volatile organic compounds (VR-VOCs) from the forest. However, on the majority of days, under generally cooler and more moist conditions, lower levels of HCHO develop primarily influenced by the influx of precursors transported into the region along with the Sacramento plume.
Abstract: Alkyl and multifunctional nitrates (RONO2, ΣANs) have been observed to be a significant fraction of NOy in a number of different chemical regimes. Their formation is an important free radical chain termination step ending production of ozone and possibly affecting formation of secondary organic aerosol. ΣANs also represent a potentially large, unmeasured contribution to OH reactivity and are a major pathway for the removal of nitrogen oxides from the atmosphere. Numerous studies have investigated the role of nitrate formation from biogenic compounds and in the remote atmosphere. Less attention has been paid to the role ΣANs may play in the complex mixtures of hydrocarbons typical of urban settings. Measurements of total alkyl and multifunctional nitrates, NO2, total peroxy nitrates (ΣPNs), HNO3 and a representative suite of hydrocarbons were obtained from the NASA DC-8 aircraft during spring of 2006 in and around Mexico City and the Gulf of Mexico. ΣANs were observed to be 10–20% of NOy in the Mexico City plume and to increase in importance with increased photochemical age. We describe three conclusions: (1) Correlations of ΣANs with odd-oxygen (Ox) indicate a stronger role for ΣANs in the photochemistry of Mexico City than is expected based on currently accepted photochemical mechanisms, (2) ΣAN formation suppresses peak ozone production rates by as much as 40% in the near-field of Mexico City and (3) ΣANs play a significant role in the export of NOy from Mexico City to the Gulf Region.
Abstract: We present case studies identifying lightning-generated upper tropospheric NOx (LNOx) observed during NASA's Tropical Composition, Cloud and Climate Coupling Experiment (TC4) in July and August 2007. In the campaign, DC-8 aircraft missions, flown from Costa Rica, recorded in situ NO2 profiles near active storms and in relatively quiet areas. We combine these TC4 DC-8 data with satellite data from the Ozone Monitoring Instrument (OMI) to estimate the lightning-generated NO2 (LNO2), above background levels, in the observed OMI NO2 fields. We employ improved off-line processing techniques to customize the OMI retrieval for LNO2. Information on lightning flashes (primarily cloud-to-ground) observed by the Costa Rica Lightning Detection Network operated by the Instituto Costarricense de Electricidad and the World Wide Lightning Location Network were examined over storms upwind of regions where OMI indicates enhanced LNO2. These flash data are compared with Tropical Rainfall Measuring Mission/Lightning Imaging Sensor satellite data to estimate total flashes. Finally, using [NOx]/[NO2] ratios from NASA's Global Modeling Initiative model, we estimate LNOx production per flash for four cases and obtain rates of ∼100-250 mol/flash. These are consistent with rates derived from previous studies of tropical and subtropical storms and below those from modeling of observed midlatitude storms. In our study, environments with stronger anvil-level winds were associated with higher production rates. LIS flash footprint data for one of the low-LNOx production cases with weak upper tropospheric winds suggest below-average flash lengths for this storm. LNOx enhancements over background determined from the OMI data were in less than, but roughly proportional to, aircraft estimates.
Abstract: A novel instrument is described that quantifies total particle-phase organic nitrates in real time with a detection limit of 0.11 μg m−3 min−1, 45 ppt min−1 (−ONO2). Aerosol nitrates are separated from gas-phase nitrates with a short residence time activated carbon denuder. Detection of organic molecules containing −ONO2 subunits is accomplished using thermal dissociation coupled to laser induced fluorescence detection of NO2. This instrument is capable of high time resolution (seconds) measurements of particle-phase organic nitrates, without interference from inorganic nitrate. Here we use it to quantify organic nitrates in secondary organic aerosol generated from high-NOx photooxidation of limonene, α-pinene, Δ-3-carene, and tridecane. In these experiments the organic nitrate moiety is observed to be 6−15% of the total SOA mass.
Abstract: Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH) in the lower troposphere. Understanding HONO chemistry, particularly its sources and contribution to HOx (=OH+HO2) production, is very important for understanding atmospheric oxidation processes. A highly sensitive instrument for detecting atmospheric HONO based on wet chemistry followed by liquid waveguide long path absorption photometry was deployed in the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) at Blodgett Forest, California in late summer 2007. The median diurnal variation shows minimum HONO levels of about 20–30 pptv during the day and maximum levels of about 60–70 pptv at night, a diurnal pattern quite different from the results at various other forested sites. Measured HONO/NO2 ratios for a 24-h period ranged from 0.05 to 0.13 with a mean ratio of 0.07. Speciation of reactive nitrogen compounds (NOy) indicates that HONO accounted for only ~3% of total NOy. However, due to the fast HONO loss through photolysis, a strong HONO source (1.59 ppbv day−1) existed in this environment in order to sustain the observed HONO levels, indicating the significant role of HONO in NOy cycling. The wet chemistry HONO measurements were compared to the HONO measurements made with a Chemical Ionization Mass Spectrometer (CIMS) over a three-day period. Good agreement was obtained between the measurements from the two different techniques. Using the expansive suite of photochemical and meteorological measurements, the contribution of HONO photolysis to HOx budget was calculated to be relatively small (6%) compared to results from other forested sites. The lower HONO mixing ratio and thus its smaller contribution to HOx production are attributed to the unique meteorological conditions and low acid precipitation at Blodgett Forest. Further studies of HONO in this kind of environment are needed to test this hypothesis and to improve our understanding of atmospheric oxidation and nitrogen budget.
Abstract: Current understanding of the vapor−liquid exchange kinetics of liquid water is incomplete, leading to uncertainties in modeling the climatic effects of clouds and aerosol. Initial studies of atmospherically relevant solutes (ammonium sulfate, sodium chloride) indicate that their effect on the evaporation kinetics of water is minimal, but all those constituent ions are also expected to be depleted in concentration at the air−water interface. We present measurements of the evaporation kinetics of water from 4 M sodium perchlorate solution, which is expected to have an enhanced concentration of perchlorate in the surface layer, using Raman thermometry of liquid microdroplets in a free evaporation regime. We determine the evaporation coefficient γe to be 0.47 ± 0.02, ca. 25% smaller than our measured value for pure water (0.62 ± 0.09). This change, while small, indicates that direct interactions between perchlorate ions and evaporating water molecules are affecting the evaporation mechanism and kinetics and suggests that other solutes with high surface affinities may also produce a similar influence in the atmosphere and elsewhere.
Abstract: We use observations from the April 2008 NASA ARCTAS aircraft campaign to the North American Arctic, interpreted with a global 3-D chemical transport model (GEOS-Chem), to better understand the sources and cycling of hydrogen oxide radicals (HOx≡H+OH+peroxy radicals) and their reservoirs (HOy≡HOx+peroxides) in the springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed HO2 and H2O2 concentrations. Computation of HOx and HOy gas-phase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hypothesize that this could reflect HO2 uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, through a mechanism that does not produce H2O2. We implemented such an uptake of HO2 by aerosol in the model using a standard reactive uptake coefficient parameterization with γ(HO2) values ranging from 0.02 at 275 K to 0.5 at 220 K. This successfully reproduces the concentrations and vertical distributions of the different HOx species and HOy reservoirs. HO2 uptake by aerosol is then a major HOx and HOy sink, decreasing mean OH and HO2 concentrations in the Arctic troposphere by 32% and 31% respectively. Better rate and product data for HO2 uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere.
Abstract: Peroxyacetyl nitrate (PAN) and its chemical analogues are increasingly being quantified in the ambient atmosphere by thermal dissociation (TD) followed by detection of either the peroxyacyl radical or the NO2 product. Here we present details of the technique developed at University of California, Berkeley which detects the sum of all peroxynitrates (ΣPNs) via laser-induced fluorescence (LIF) of the NO2 product. We review the various deployments and compare the Berkeley ΣPNs measurements with the sums of PAN and its homologue species detected individually by other instruments. The observed TD-LIF ΣPNs usually agree to within 10% with the summed individual species, thus arguing against the presence of significant concentrations of unmeasured PAN-type compounds in the atmosphere, as suggested by some photochemical mechanisms. Examples of poorer agreement are attributed to a sampling inlet design that is shown to be inappropriate for high NOx conditions. Interferences to the TD-LIF measurements are described along with strategies to minimize their effects.
Abstract: We describe ground and space-based measurements of spatial and temporal variation of NO2 in four California metropolitan regions. The measurements of weekly cycles and trends over the years 2005−2008 observed both from the surface and from space are nearly identical to each other. Observed decreases in Los Angeles and the surrounding cities are 46% on weekends and 9%/year from 2005−2008. Similar decreases are observed in the San Francisco Bay area and in Sacramento. In the San Joaquin Valley cities of Fresno and Bakersfield weekend decreases are much smaller, only 27%, and the decreasing trend is only 4%/year. We describe evidence that the satellite observations provide a uniquely complete view of changes in spatial patterns over time. For example, we observe variations in the spatial pattern of weekday−weekend concentrations in the Los Angeles basin with much steeper weekend decreases at the eastern edge of the basin. We also observe that the spatial extent of high NO2 in the San Joaquin Valley has not receded as much as it has for other regions in the state. Analysis of these measurements is used to describe observational constraints on temporal trends in emission sources in the different regions.
Abstract: We present a sensitivity analysis of the tropospheric NO2 retrieval from the Ozone Monitoring Instrument (OMI) using measurements from the Dutch Aerosol and Nitrogen Dioxide Experiments for Validation of OMI and SCIAMACHY (DANDELIONS) and Intercontinental Chemical Transport Experiment-B (INTEX-B) campaigns held in 2006. These unique campaigns covered a wide range of pollution conditions and provided detailed information on the vertical distribution of NO2. During the DANDELIONS campaign, tropospheric NO2 profiles were measured with a lidar in a highly polluted region of the Netherlands. During the INTEX-B campaign, NO2 profiles were measured using laser-induced fluorescence onboard an aircraft in a range of meteorological and polluted conditions over the Gulf of Mexico and the east Pacific. We present a comparison of measured profiles with a priori profiles used in the OMI tropospheric NO2 retrieval algorithm. We examine how improvements in surface albedo estimates improve the OMI NO2 retrieval. From these comparisons we find that the absolute average change in tropospheric columns retrieved with measured profiles and improved surface albedos is 23% with a standard deviation of 27% and no trend in the improved being larger or smaller than the original. We show that these changes occur in case studies related to pollution in the southeastern United States and pollution outflow in the Gulf of Mexico. We also examine the effects of using improved Mexico City terrain heights on the OMI NO2 product.
Abstract: The Sulfur Transport and dEposition Model (STEM) is applied to the analysis of observations obtained during the Intercontinental Chemical Transport Experiment-Phase B (INTEX-B), conducted over the eastern Pacific Ocean during spring 2006. Predicted trace gas and aerosol distributions over the Pacific are presented and discussed in terms of transport and source region contributions. Trace species distributions show a strong west (high) to east (low) gradient, with the bulk of the pollutant transport over the central Pacific occurring between ~20° N and 50° N in the 2–6 km altitude range. These distributions are evaluated in the eastern Pacific by comparison with the NASA DC-8 and NSF/NCAR C-130 airborne measurements along with observations from the Mt. Bachelor (MBO) surface site. Thirty different meteorological, trace gas and aerosol parameters are compared. In general the meteorological fields are better predicted than gas phase species, which in turn are better predicted than aerosol quantities. PAN is found to be significantly overpredicted over the eastern Pacific, which is attributed to uncertainties in the chemical reaction mechanisms used in current atmospheric chemistry models in general and to the specifically high PAN production in the SAPRC-99 mechanism used in the regional model. A systematic underprediction of the elevated sulfate layer in the eastern Pacific observed by the C-130 is another issue that is identified and discussed. Results from source region tagged CO simulations are used to estimate how the different source regions around the Pacific contribute to the trace gas species distributions. During this period the largest contributions were from China and from fires in South/Southeast and North Asia. For the C-130 flights, which operated off the coast of the Northwest US, the regional CO contributions range as follows: China (35%), South/Southeast Asia fires (35%), North America anthropogenic (20%), and North Asia fires (10%). The transport of pollution into the western US is studied at MBO and a variety of events with elevated Asian dust, and periods with contributions from China and fires from both Asia and North America are discussed. The role of heterogeneous chemistry on the composition over the eastern Pacific is also studied. The impacts of heterogeneous reactions at specific times can be significant, increasing sulfate and nitrate aerosol production and reducing gas phase nitric acid levels appreciably (~50%).
Abstract: Four hydroxynitrates (R(OH)R'ONO2) representative of atmospheric volatile organic compound (VOC) oxidation products were synthesized, nebulized and sampled into an Aerodyne High Resolution Time of Flight Aerosol Mass Spectrometer (HR-ToF-AMS). The resulting mass spectrum was used to evaluate calibration factors for elemental analysis of organic nitrates by AMS, and to determine the distribution of nitrogen in the detected fragments in a search for an AMS signature of organic nitrates. We found that 30% of the detected nitrogen mass is in the NO+ and NO2+ fragments, 12% at NHx+ fragments, 5% at CxHyOzN+ fragments, and 53% at various CxHyN+ fragments. Elemental analysis indicated that nitrogen was detected with higher efficiency than carbon and hydrogen, but oxygen was detected with reduced efficiency compared to previously reported results for a suite of organics which did not include organic nitrates. The results are used to suggest the maximum corrections to ambient O:C and N:C ratios based on AMS measurements.
Abstract: NO2 concentrations were measured by various instruments during the NO3Comp campaign at the atmosphere simulation chamber SAPHIR at Forschungszentrum Jülich, Germany, in June 2007. Analytical methods included photolytic conversion with chemiluminescence (PC-CLD), broadband cavity ring-down spectroscopy (BBCRDS), pulsed cavity ring-down spectroscopy (CRDS), incoherent broadband cavity-enhanced absorption spectroscopy (IBB-CEAS), and laser-induced fluorescence (LIF). All broadband absorption spectrometers were optimized for the detection of the main target species of the campaign, NO3, but were also capable of detecting NO2 simultaneously with reduced sensitivity. NO2 mixing ratios in the chamber were within a range characteristic of polluted, urban conditions, with a maximum mixing ratio of approximately 75 ppbv. The overall agreement between measurements of all instruments was excellent. Linear fits of the combined data sets resulted in slopes that differ from unity only within the stated uncertainty of each instrument. Possible interferences from species such as water vapor and ozone were negligible under the experimental conditions.
Abstract: Air quality in the outflow from urban centers affects millions of people, as well as, natural and managed ecosystems downwind. In locations where there are large sources of biogenic VOCs downwind of urban centers, the outflow is characterized by a high VOC reactivity due to biogenic emissions and low NOx. However most field and chamber studies have focused on limiting cases of high NOx or of near zero NOx. Recent measurements of a wide suite of VOCs, O3 and meteorological parameters at several locations within the Sacramento urban plume have provided a detailed benchmark for testing our understanding of chemistry in a plume transitioning from high NOx to low NOx and high VOC reactivity. As an additional simplification, the strong mountain valley circulation in the region makes this urban plume a physical realization of a nearly idealized Lagrangian plume. Here, we describe a model of this plume. We use a Lagrangian model representing chemistry based on the Master Chemical Mechanism (MCM) v3.1 along with mixing and deposition. We discuss the effects of entrainment of background air, the branching ratio for the production of isoprene nitrates and the effects of soil NOx emissions on the composition of the evolving plume. The model predicts that after 2–3 h of chemical processing only 45% of the peroxynitrates (ΣPNs) are PAN and that most (69%) RONO2 are secondary alkyl nitrate products of the reaction of OH with RONO2. We find the model is more consistent with the observations if: a) the yield of ΣPNs from large and multi-functional aldehydes is close to zero; and b) the reaction between OH and RONO2 produces multifunctional nitrates as opposed to either HNO3 or NO2 as is typical in most currently adopted reaction mechanisms. Model results also show that adding NOx emissions throughout the transect increases the available NOx in the downwind regions, but modeled ozone concentrations were little affected by the increased NOx.
Abstract: This paper provides a synthesis of results that have emerged from recent modeling studies of the potential sensitivity of U.S. regional ozone (O3) concentrations to global climate change (ca. 2050). This research has been carried out under the auspices of an ongoing U.S. Environmental Protection Agency (EPA) assessment effort to increase scientific understanding of the multiple complex interactions among climate, emissions, atmospheric chemistry, and air quality. The ultimate goal is to enhance the ability of air quality managers to consider global change in their decisions through improved characterization of the potential effects of global change on air quality, including O3. The results discussed here are interim, representing the first phase of the EPA assessment. The aim in this first phase was to consider the effects of climate change alone on air quality, without accompanying changes in anthropogenic emissions of precursor pollutants. Across all of the modeling experiments carried out by the different groups, simulated global climate change causes increases of a few to several parts per billion (ppb) in summertime mean maximum daily 8-h average O3 concentrations over substantial regions of the country. The different modeling experiments in general do not, however, simulate the same regional patterns of change. These differences seem to result largely from variations in the simulated patterns of changes in key meteorological drivers, such as temperature and surface insolation. How isoprene nitrate chemistry is represented in the different modeling systems is an additional critical factor in the simulated O3 response to climate change.
Abstract: Aqueous evaporation and condensation kinetics are poorly understood, and uncertainties in their rates affect predictions of cloud behavior and therefore climate. We measured the cooling rate of 3M ammonium sulfate droplets undergoing free evaporation via Raman thermometry. Analysis of the measurements yields a value of 0.58 ± 0.05 for the evaporation coefficient, identical to that previously determined for pure water. These results imply that subsaturated aqueous ammonium sulfate, which is the most abundant inorganic component of atmospheric aerosol, does not affect the vapor–liquid exchange mechanism for cloud droplets, despite reducing the saturation vapor pressure of water significantly.
Abstract: In-situ airborne measurements of trace gases, aerosol size distributions, chemistry and optical properties were conducted over Mexico and the Eastern North Pacific during MILAGRO and INTEX-B. Heterogeneous reactions between secondary aerosol precursor gases and mineral dust lead to sequestration of sulfur, nitrogen and chlorine in the supermicrometer particulate size range. Simultaneous measurements of aerosol size distributions and weak-acid soluble calcium result in an estimate of 11 wt% of CaCO3 for Asian dust. During transport across the North Pacific, ~5–30% of the CaCO3 is converted to CaSO4 or Ca(NO3)2 with an additional ~4% consumed through reactions with HCl. The 1996 to 2008 record from the Mauna Loa Observatory confirm these findings, indicating that, on average, 19% of the CaCO3 has reacted to form CaSO4 and 7% has reacted to form Ca(NO3)2 and ~2% has reacted with HCl. In the nitrogen-oxide rich boundary layer near Mexico City up to 30% of the CaCO3 has reacted to form Ca(NO3)2 while an additional 8% has reacted with HCl. These heterogeneous reactions can result in a ~3% increase in dust solubility which has an insignificant effect on their optical properties compared to their variability in-situ. However, competition between supermicrometer dust and submicrometer primary aerosol for condensing secondary aerosol species led to a 25% smaller number median diameter for the accumulation mode aerosol. A 10–25% reduction of accumulation mode number median diameter results in a 30–70% reduction in submicrometer light scattering at relative humidities in the 80–95% range. At 80% RH submicrometer light scattering is only reduced ~3% due to a higher mass fraction of hydrophobic refractory components in the dust-affected accumulation mode aerosol. Thus reducing the geometric mean diameter of the submicrometer aerosol has a much larger effect on aerosol optical properties than changes to the hygroscopic:hydrophobic mass fractions of the accumulation mode aerosol. In the presence of dust, nitric acid concentrations are reduced to <50% of total nitrate (nitric acid plus particulate nitrate). NOy as a fraction of total nitrogen (NOy plus particulate nitrate), is reduced from >85% to 60–80% in the presence of dust. These observations support previous model studies which predict irreversible sequestration of reactive nitrogen species through heterogeneous reactions with mineral dust during long-range transport.
Abstract: Acyl peroxy nitrates (APNs, also known as PANs) are formed from the oxidation of aldehydes and other oxygenated VOC (oVOC) in the presence of NO2. There are both anthropogenic and biogenic oVOC precursors to APNs, but a detailed evaluation of this chemistry against observations has proven elusive. Here we describe measurements of PAN, PPN, and MPAN along with the majority of chemicals that participate in their production and loss, including OH, HO2, numerous oVOC, and NO2. Observations were made during the Biosphere Effects on AeRosols and Photochemistry Experiment (BEARPEX 2007) in the outflow of the Sacramento urban plume. These observations are used to evaluate a detailed chemical model of APN ratios and concentrations. We find that the ratios of APNs are nearly independent of the loss mechanisms and thus an especially good test of our understanding of their sources. We show that oxidation of methylvinyl ketone, methacrolein, methyl glyoxal, biacetyl and acetaldehyde are all significant sources of the PAN+peroxy acetyl (PA) radical reservoir, accounting for 26%, 2%, 7%, 20%, and 45%, of the production rate on average during the campaign, respectively. At high temperatures, when upwind isoprene emissions are highest, oxidation of non-acetaldehyde PA radical sources contributes over 60% to the total PA production rate, with methylvinyl ketone being the most important of the isoprene-derived sources. An analysis of absolute APN concentrations reveals a missing APN sink that can be resolved by increasing the PA+∑RO2 rate constant by a factor of 3.
Abstract: Alkyl nitrates and secondary organic aerosol (SOA) produced during the oxidation of isoprene by nitrate radicals has been observed in the SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) chamber. A 16 h dark experiment was conducted with temperatures at 289–301 K, and maximum concentrations of 11 ppb isoprene, 62.4 ppb O3 and 31.1 ppb NOx. We find the yield of nitrates is 70±8% from the isoprene + NO3 reaction, and the yield for secondary dinitrates produced in the reaction of primary isoprene nitrates with NO3 is 40±20%. We find an effective rate constant for reaction of NO3 with the group of first generation oxidation products to be 7×10−14 molecule−1 cm3 s−1. At the low total organic aerosol concentration in the chamber (max=0.52 μg m−3) we observed a mass yield (ΔSOA mass/Δisoprene mass) of 2% for the entire 16 h experiment. However a comparison of the timing of the observed SOA production to a box model simulation of first and second generation oxidation products shows that the yield from the first generation products was <0.7% while the further oxidation of the initial products leads to a yield of 14% (defined as ΔSOA/Δisoprene2x where Δisoprene2x is the mass of isoprene which reacted twice with NO3). The SOA yield of 14% is consistent with equilibrium partitioning of highly functionalized C5 products of isoprene oxidation.
Abstract: Observations of NO, NO2, total peroxy nitrates (ΣPNs), total alkyl nitrates (ΣANs), HNO3, CO, O3, and meteorological parameters were obtained from October 2000 through February 2002 at 1315 m a.s.l., 38.9° N, 120.6° W on Sierra Pacific Industries land, adjacent to the University of California Blodgett Forest Research Station (UC-BFRS). We describe the data set with emphasis on the diurnal cycles during summertime 2001. We show that transport of the Sacramento urban plume is a primary factor responsible for diurnal variation in total reactive nitrogen mixing ratios as well as in NOx, ΣPNs and ΣANs, all of which exhibit a late afternoon/early evening peak. In contrast, HNO3 has a peak just after local noon indicating that HNO3 is in near steady state during the day with production due to photochemistry and removal by deposition and mixing with the background free troposphere. Boundary layer dynamics influence mixing ratios of all species in the early morning. Analysis of the morning feature suggests that higher mixing ratios of NOx and HNO3 persist in the residual layer than in the nocturnal boundary layer indicating the presence of nocturnal sinks of both species. Nighttime observations also indicate large HNO3 and ΣANs production through oxidation of alkenes by NO3.
Abstract: Oxidation of isoprene through reaction with NO3 radicals is a significant sink for isoprene that persists after dark. The main products of the reaction are multifunctional nitrates. These nitrates constitute a significant NOx sink in the nocturnal boundary layer and they likely play an important role in formation of secondary organic aerosol. Products of the isoprene+NO3 reaction will, in many locations, be abundant enough to affect nighttime radical chemistry and to persist into daytime where they may represent a source of NOx. Product formation in the isoprene + NO3 reaction was studied in a smog chamber at Purdue University. Isoprene nitrates and other hydrocarbon products were observed using Proton Transfer Reaction-Mass Spectrometry (PTR-MS) and reactive nitrogen products were observed using Thermal Dissociation–Laser Induced Fluorescence (TD-LIF). The organic nitrate yield is found to be 65±12% of which the majority was nitrooxy carbonyls and the combined yield of methacrolein and methyl vinyl ketone (MACR+MVK) is found to be ∼10%. PTR-MS measurements of nitrooxy carbonyls and TD-LIF measurements of total organic nitrates agreed well. The PTR-MS also observed a series of minor oxidation products which were tentatively identified and their yields quantified These other oxidation products are used as additional constraints on the reaction mechanism.
Abstract: Fast measurements of aerosol and gas-phase constituents coupled with the ISORROPIA-II thermodynamic equilibrium model are used to study the partitioning of semivolatile inorganic species and phase state of Mexico City aerosol sampled at the T1 site during the MILAGRO 2006 campaign. Overall, predicted semivolatile partitioning agrees well with measurements. PM2.5 is insensitive to changes in ammonia but is to acidic semivolatile species. For particle sizes up to 1μm diameter, semi-volatile partitioning requires 15–30 min to equilibrate; longer time is typically required during the night and early morning hours. Aerosol and gas-phase speciation always exhibits substantial temporal variability, so that aerosol composition measurements (bulk or size-resolved) obtained over large integration periods are not reflective of its true state. When the aerosol sulfate-to-nitrate molar ratio is less than unity, predictions improve substantially if the aerosol is assumed to follow the deliquescent phase diagram. Treating crustal species as "equivalent sodium" (rather than explicitly) in the thermodynamic equilibrium calculations introduces important biases in predicted aerosol water uptake, nitrate and ammonium; neglecting crustals further increases errors dramatically. This suggests that explicitly considering crustals in the thermodynamic calculations is required to accurately predict the partitioning and phase state of aerosols.
Abstract: The yields of organic nitrates and of secondary organic aerosol (SOA) particle formation were measured for the reaction NO3+β-pinene under dry and humid conditions in the atmosphere simulation chamber SAPHIR at Research Center Jülich. These experiments were conducted at low concentrations of NO3 (NO3+N2O5<10 ppb) and β-pinene (peak~15 ppb), with no seed aerosol. SOA formation was observed to be prompt and substantial (~50% mass yield under both dry conditions and at 60% RH), and highly correlated with organic nitrate formation. The observed gas/aerosol partitioning of organic nitrates can be simulated using an absorptive partitioning model to derive an estimated vapor pressure of the condensing nitrate species of pvap~5×10−6 Torr (6.67×10−4 Pa), which constrains speculation about the oxidation mechanism and chemical identity of the organic nitrate. Once formed the SOA in this system continues to evolve, resulting in measurable aerosol volume decrease with time. The observations of high aerosol yield from NOx-dependent oxidation of monoterpenes provide an example of a significant anthropogenic source of SOA from biogenic hydrocarbon precursors. Estimates of the NO3+β-pinene SOA source strength for California and the globe indicate that NO3 reactions with monoterpenes are likely an important source (0.5–8% of the global total) of organic aerosol on regional and global scales.
Abstract: Formation of isoprene nitrates (INs) is an important free radical chain termination step ending production of ozone and possibly affecting formation of secondary organic aerosol. Isoprene nitrates also represent a potentially large, unmeasured contribution to OH reactivity and are a major pathway for the removal of nitrogen oxides from the atmosphere. Current assessments indicate that formation rates of isoprene nitrates are uncertain to a factor of 2–3 and the subsequent fate of isoprene nitrates remains largely unconstrained by laboratory, field or modeling studies. Measurements of total alkyl and multifunctional nitrates (ΣANs), NO2, total peroxy nitrates (ΣPNs), HNO3, CH2O, isoprene and other VOC were obtained from the NASA DC-8 aircraft during summer 2004 over the continental US during the INTEX-NA campaign. These observations represent the first characterization of ΣANs over a wide range of land surface types and in the lower free troposphere. ΣANs were a significant, 12–20%, fraction of NOy throughout the experimental domain and ΣANs were more abundant when isoprene was high. We use the observed hydrocarbon species to calculate the relative contributions of ΣAN precursors to their production. These calculations indicate that isoprene represents at least three quarters of the ΣAN source in the summertime continental boundary layer of the US. An observed correlation between ΣANs and CH2O is used to place constraints on nitrate yields from isoprene oxidation, atmospheric lifetimes of the resulting nitrates and recycling efficiencies of nitrates during subsequent oxidation. We find reasonable fits to the data using sets of production rates, lifetimes and recycling efficiencies of INs as follows (4.4%, 16 h, 97%), (8%, 2.5 h, 79%) and (12%, 95 min, 67%). The analysis indicates that the lifetime of ΣANs as a pool of compounds is considerably longer than the lifetime of the individual isoprene nitrates to reaction with OH, implying that the organic nitrate functionality is at least partially maintained through a second oxidation cycle.
Abstract: This study explores the upper tropospheric anticyclone above eastern North America and its influence on the summertime buildup and decay of lightning NOx (LNOx) and thunderstorm outflow. LNOx transport is simulated with a particle dispersion model that releases a LNOx tracer from the locations of millions of cloud-to-ground lightning flashes during May–September 2004 and 2006. On average, upper tropospheric zonal flow in May transitions to a closed anticyclone above northern Mexico and the southern United States in July that strengthens in August and rapidly decays in September. Concentrations of the LNOx tracer reach a maximum above the southern United States and Gulf of Mexico in July and August. Fourteen study sites across North America exhibit high day-to-day variability of the LNOx tracer in the upper troposphere during summer, with the sites most heavily influenced by the North American summer monsoon having the greatest background concentrations. During late spring and September the western sites have low concentrations with little variability. In general, the west coast sites plus Barbados have the most aged thunderstorm outflow, while the east coast sites have the least aged outflow. More than 80% of summertime upper tropospheric NOx above the eastern United States is produced by lightning. To produce the best available observation-based view of upper troposphere NOx above North America, measurements from six aircraft campaigns are combined in a single composite plot. The modeled upper tropospheric NOx matches the general continental-scale distribution of NOx in the composite plot, supporting the dominant role of LNOx in the simulations.
Abstract: During the Biosphere Effects on AeRosols and Photochemistry EXperiment 2007 (BEARPEX-2007), we observed eddy covariance (EC) fluxes of speciated acyl peroxy nitrates (APNs), including peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN) and peroxymethacryloyl nitrate (MPAN), above a Ponderosa pine forest in the western Sierra Nevada. All APN fluxes are net downward during the day, with a median midday PAN exchange velocity of −0.3 cm s−1; nighttime storage-corrected APN EC fluxes are smaller than daytime fluxes but still downward. Analysis with a standard resistance model shows that loss of PAN to the canopy is not controlled by turbulent or molecular diffusion. Stomatal uptake can account for 25 to 50% of the observed downward PAN flux. Vertical gradients in the PAN thermal decomposition (TD) rate explain a similar fraction of the flux, suggesting that a significant portion of the PAN flux into the forest results from chemical processes in the canopy. The remaining "unidentified" portion of the net PAN flux (~15%) is ascribed to deposition or reactive uptake on non-stomatal surfaces (e.g. leaf cuticles or soil). Shifts in temperature, moisture and ecosystem activity during the summer – fall transition alter the relative contribution of stomatal uptake, non-stomatal uptake and thermochemical gradients to the net PAN flux. Daytime PAN and MPAN exchange velocities are a factor of 3 smaller than those of PPN during the first two weeks of the measurement period, consistent with strong intra-canopy chemical production of PAN and MPAN during this period. Depositional loss of APNs can be 3–21% of the gross gas-phase TD loss depending on temperature. As a source of nitrogen to the biosphere, PAN deposition represents approximately 4–19% of that due to dry deposition of nitric acid at this site.
Abstract: The measurement of OH reactivity, the inverse of the OH lifetime, provides a powerful tool to investigate atmospheric photochemistry. A new airborne OH reactivity instrument was designed and deployed for the first time on the NASA DC-8 aircraft during the second phase of Intercontinental Chemical Transport Experiment-B (INTEX-B) campaign, which was focused on the Asian pollution outflow over Pacific Ocean and was based in Hawaii and Alaska. The OH reactivity was measured by adding OH, generated by photolyzing water vapor with 185 nm UV light in a moveable wand, to the flow of ambient air in a flow tube and measuring the OH signal with laser induced fluorescence. As the wand was pulled back away from the OH detector, the OH signal decay was recorded; the slope of −Δln(signal)/Δtime was the OH reactivity. The overall absolute uncertainty at the 2σ confidence levels is about 1 s−1 at low altitudes (for decay about 6 s−1), and 0.7 s−1 at high altitudes (for decay about 2 s−1). From the median vertical profile obtained in the second phase of INTEX-B, the measured OH reactivity (4.0±1.0 s−1) is higher than the OH reactivity calculated from assuming that OH was in steady state (3.3±0.8 s−1), and even higher than the OH reactivity that was calculated from the total measurements of all OH reactants (1.6±0.4 s−1). Model calculations show that the missing OH reactivity is consistent with the over-predicted OH and under-predicted HCHO in the boundary layer and lower troposphere. The over-predicted OH and under-predicted HCHO suggest that the missing OH sinks are most likely related to some highly reactive VOCs that have HCHO as an oxidation product.
Abstract: The evaporation rate of D2O has been determined by Raman thermometry of a droplet train (12–15 μm diameter) injected into vacuum (~10-5 torr). The cooling rate measured as a function of time in vacuum was fit to a model that accounts for temperature gradients between the surface and the core of the droplets, yielding an evaporation coefficient (γe) of 0.57±0.06. This is nearly identical to that found for H2O (0.62±0.09) using the same experimental method and model, and indicates the existence of a kinetic barrier to evaporation. The application of a recently developed transition-state theory (TST) model suggests that the kinetic barrier is due to librational and hindered translational motions at the liquid surface, and that the lack of an isotope effect is due to competing energetic and entropic factors. The implications of these results for cloud and aerosol particles in the atmosphere are discussed.
Abstract: We use an ensemble of aircraft, satellite, sonde, and surface observations for April–May 2006 (NASA/INTEX-B aircraft campaign) to better understand the mechanisms for transpacific ozone pollution and its implications for North American air quality. The observations are interpreted with a global 3-D chemical transport model (GEOS-Chem). OMI NO2 satellite observations constrain Asian anthropogenic NOx emissions and indicate a factor of 2 increase from 2000 to 2006 in China. Satellite observations of CO from AIRS and TES indicate two major events of Asian transpacific pollution during INTEX-B. Correlation between TES CO and ozone observations shows evidence for transpacific ozone pollution. The semi-permanent Pacific High and Aleutian Low cause splitting of transpacific pollution plumes over the Northeast Pacific. The northern branch circulates around the Aleutian Low and has little impact on North America. The southern branch circulates around the Pacific High and some of that air impacts western North America. Both aircraft measurements and model results show sustained ozone production driven by peroxyacetylnitrate (PAN) decomposition in the southern branch, roughly doubling the transpacific influence from ozone produced in the Asian boundary layer. Model simulation of ozone observations at Mt. Bachelor Observatory in Oregon (2.7 km altitude) indicates a mean Asian ozone pollution contribution of 9±3 ppbv to the mean observed concentration of 54 ppbv, reflecting mostly an enhancement in background ozone rather than episodic Asian plumes. Asian pollution enhanced surface ozone concentrations by 5–7 ppbv over western North America in spring 2006. The 2000–2006 rise in Asian anthropogenic emissions increased this influence by 1–2 ppbv.
Abstract: Measurements of formaldehyde (CH2O) from a tunable diode laser absorption spectrometer (TDLAS) were acquired onboard the NASA DC-8 aircraft during the summer 2004 INTEX-NA campaign to test our understanding of convection and CH2O production mechanisms in the upper troposphere (UT, 6–12 km) over continental North America and the North Atlantic Ocean. The present study utilizes these TDLAS measurements and results from a box model to (1) establish sets of conditions by which to distinguish “background” UT CH2O levels from those perturbed by convection and other causes; (2) quantify the CH2O precursor budgets for both air mass types; (3) quantify the fraction of time that the UT CH2O measurements over North America and North Atlantic are perturbed during the summer of 2004; (4) provide estimates for the fraction of time that such perturbed CH2O levels are caused by direct convection of boundary layer CH2O and/or convection of CH2O precursors; (5) assess the ability of box models to reproduce the CH2O measurements; and (6) examine CH2O and HO2 relationships in the presence of enhanced NO. Multiple tracers were used to arrive at a set of UT CH2O background and perturbed air mass periods, and 46% of the TDLAS measurements fell within the latter category. In general, production of CH2O from CH4 was found to be the dominant source term, even in perturbed air masses. This was followed by production from methyl hydroperoxide, methanol, PAN-type compounds, and ketones, in descending order of their contribution. At least 70% to 73% of the elevated UT observations were caused by enhanced production from CH2O precursors rather than direct transport of CH2O from the boundary layer. In the presence of elevated NO, there was a definite trend in the CH2O measurement–model discrepancy, and this was highly correlated with HO2 measurement–model discrepancies in the UT.
Abstract: Measurements of exchange of reactive nitrogen oxides between the atmosphere and a ponderosa pine forest in the Sierra Nevada Mountains are reported. During winter, we observe upward fluxes of NO2, and downward fluxes of total peroxy and peroxy acyl nitrates (ΣPNs), total gas and particle phase alkyl and multifunctional alkyl nitrates (ΣANs(g+p)), and the sum of gaseous HNO3 and semi-volatile NO3− particles (HNO3(g+p)). We use calculations of the vertical profile and flux of NO, partially constrained by observations, to show that net midday ΣNOyi fluxes in winter are –4.9 ppt m s−1. The signs and magnitudes of these wintertime individual and ΣNOyi fluxes are in the range of prior measurements. In contrast, during summer, we observe downward fluxes only of ΣANs(g+p), and upward fluxes of HNO3(g+p), ΣPNs and NO2 with signs and magnitudes that are unlike most, if not all, previous observations and analyses of fluxes of individual nitrogen oxides. The results imply that the mechanisms contributing to NOy fluxes, at least at this site, are much more complex than previously recognized. We show that the observations of upward fluxes of HNO3(g+p) and ΣPNs during summer are consistent with oxidation of NO2 and acetaldehyde by an OH x residence time of 1.1×1010 molec OH cm−3 s, corresponding to 3 to 16×107 molecules cm−3 OH within the forest canopy for a 420 to 70 s canopy residence time. We show that ΣAN(g+p) fluxes are consistent with this range in OH if the reaction of OH with ΣANs produces either HNO3 or NO2 with a 6–30% yield. Calculations of NO fluxes constrained by the NO2 observations and the inferred OH indicate that NOx fluxes are downward into the canopy because of the substantial conversion of NOx to HNO3 and ΣPNs in the canopy. Even so, we derive that NOx emission fluxes of ~15 ng(N) m−2 s−1 at midday during summer are required to balance the NOx and NOy flux budgets. These fluxes are partly explained by estimates of soil emissions (estimated to be between 3 and 6 ng(N) m−2 s-1). One possibility for the remainder of the NOx source is large HONO emissions. Alternatively, the 15 ng(N) m−2 s−1 emission estimate may be too large, and the budget balanced if the deposition of HNO3 and ΣPNs is slower than we estimate, if there are large errors in either our understanding of peroxy radical chemistry, or our assumptions that the budget is required to balance because the fluxes do not obey similarity theory.
Abstract: We compare tropospheric NO2 column measurements from the Ozone Monitoring Instrument (OMI) aboard the EOS Aura satellite with coincident in situ aircraft measurements on vertical spirals over the southern United States, Mexico, and the Gulf of Mexico during the INTEX-B campaign in March 2006. Good correlation with no significant bias (r2=0.67, slope=0.99±0.17, n=12) is found for the ensemble of comparisons when the aircraft could spiral sufficiently low to sample most of the NO2 column. Urban spirals where large extrapolations were needed below the aircraft floor (1000 ft) showed poorer agreement. We use the OMI observations together with a global chemical transport model (GEOS-Chem) to estimate emissions of nitrogen oxides over the eastern United States and Mexico in March 2006. Comparison to EPA's National Emissions Inventory 1999 (NEI99) calls for a decrease in power plant emissions and an increase in on-road vehicle emissions relative to that inventory. The rise in vehicular emissions is offsetting the reduction in power plant and industry emissions. These findings are consistent with independent assessments. Our OMI-derived emission estimates for Mexico are higher by a factor of 2.0±0.5 than bottom-up emissions, similar to a comparison between the recently released Mexican NEI99 inventory and the bottom-up showing that the Mexican NEI99 inventory is 1.6–1.8× higher.
Abstract: We present an analysis of in situ NO2 measurements from aircraft experiments between summer 2004 and spring 2006. The data are from the INTEX-A, PAVE, and INTEX-B campaigns and constitute the most comprehensive set of tropospheric NO2 profiles to date. Profile shapes from INTEX-A and PAVE are found to be qualitatively similar to annual mean profiles from the GEOS-Chem model. Using profiles from the INTEX-B campaign, we perform error-weighted linear regressions to compare the Ozone Monitoring Instrument (OMI) tropospheric NO2 columns from the near-real-time product (NRT) and standard product (SP) with the integrated in situ columns. Results indicate that the OMI SP algorithm yields NO2 amounts lower than the in situ columns by a factor of 0.86 (±0.2) and that NO2 amounts from the NRT algorithm are higher than the in situ data by a factor of 1.68 (±0.6). The correlation between the satellite and in situ data is good (r = 0.83) for both algorithms. Using averaging kernels, the influence of the algorithm's a priori profiles on the satellite retrieval is explored. Results imply that air mass factors from the a priori profiles are on average slightly larger (∼10%) than those from the measured profiles, but the differences are not significant.
Abstract: We describe an approach for characterizing selective binding between oppositely charged ionic functional groups under biologically relevant conditions. Relative shifts in K-shell x-ray absorption spectra of aqueous cations and carboxylate anions indicate the corresponding binding strengths via perturbations of carbonyl antibonding orbitals. XAS spectra measured for aqueous formate and acetate solutions containing lithium, sodium, and potassium cations reveal monotonically stronger binding of the lighter metals, supporting recent results from simulations and other experiments. The carbon K-edge spectra of the acetate carbonyl feature centered near 290 eV clearly indicate a preferential interaction of sodium versus potassium, which was less apparent with formate. These results are in accord with the Law of Matching Water Affinities, relating relative hydration strengths of ions to their respective tendencies to form contact ion pairs. Density functional theory calculations of K-shell spectra support the experimental findings.
Abstract: We describe observations of atmospheric reactive nitrogen compounds including NO, NO2, total peroxy nitrates, total alkyl nitrates, and HNO3 and their correlation with temperature. The measurements were made at a rural location 1315 m a.s.l. on the western slope of the Sierra Nevada Mountains in California during summer of 2001. The ratio of HNO3 to its source molecule, NO2, and the ratio of HNO3 to all other higher oxides of nitrogen (NOz) both increase with increasing temperature. Analysis of these increases suggests they are due to a steep increase in OH of between a factor of 2 and 3 over the range 18–32°C. Total peroxy nitrates decrease and total alkyl nitrates increase over the same temperature range. The decrease in the total peroxy nitrates is shown to be much less than expected if the rate of thermal decomposition were the sole important factor. This observation is consistent with the increase in OH inferred from the temperature trends in the HNO3/NO2 ratio.
Abstract: OH and HO2 were measured with the Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) as part of a large measurement suite from the NASA DC-8 aircraft during the Intercontinental Chemical Transport Experiment-A (INTEX-A). This mission, which was conducted mainly over North America and the western Atlantic Ocean in summer 2004, was an excellent test of atmospheric oxidation chemistry. The HOx results from INTEX-A are compared to those from previous campaigns and to results for other related measurements from INTEX-A. Throughout the troposphere, observed OH was generally 0.95 of modeled OH; below 8 km, observed HO2 was generally 1.20 of modeled HO2. This observed-to-modeled comparison is similar to that for TRACE-P, another midlatitude study for which the median observed-to-modeled ratio was 1.08 for OH and 1.34 for HO2, and to that for PEM-TB, a tropical study for which the median observed-to-modeled ratio was 1.17 for OH and 0.97 for HO2. HO2 behavior above 8 km was markedly different. The observed-to-modeled HO2 ratio increased from ∼1.2 at 8 km to ∼3 at 11 km with the observed-to-modeled ratio correlating with NO. Above 8 km, the observed-to-modeled HO2 and observed NO were both considerably greater than observations from previous campaigns. In addition, the observed-to-modeled HO2/OH, which is sensitive to cycling reactions between OH and HO2, increased from ∼1.5 at 8 km to almost 3.5 at 11 km. These discrepancies suggest a large unknown HOx source and additional reactants that cycle HOx from OH to HO2. In the continental planetary boundary layer, the observed-to-modeled OH ratio increased from 1 when isoprene was less than 0.1 ppbv to over 4 when isoprene was greater than 2 ppbv, suggesting that forests throughout the United States are emitting unknown HOx sources. Progress in resolving these discrepancies requires a focused research activity devoted to further examination of possible unknown OH sinks and HOx sources
Abstract: Volatile organic compound (VOC) reactivity in central California is examined using a photochemical air quality model (the Community Multiscale Air Quality model; CMAQ) and ground-based measurements to evaluate the contribution of VOC to photochemical activity. We classify VOC into four categories: anthropogenic, biogenic, aldehyde, and other oxygenated VOC. Anthropogenic and biogenic VOC consist of primary emissions, while aldehydes and other oxygenated VOC include both primary anthropogenic emissions and secondary products from primary VOC oxidation. To evaluate the model treatment of VOC chemistry, we compare calculated and modeled OH and VOC reactivities using the following metrics: 1) cumulative distribution functions of NOx concentration and VOC reactivity (ROH,VOC), 2) the relationship between ROH,VOC and NOx, 3) total OH reactivity (ROH,total) and speciated contributions, and 4) the relationship between speciated ROH,VOC and NOx. We find that the model predicts ROH,total to within 25–40% at three sites representing urban (Sacramento), suburban (Granite Bay) and rural (Blodgett Forest) chemistry. However in the urban area of Fresno, the model under predicts NOx and VOC emissions by a factor of 2–3. At all locations the model is consistent with observations of the relative contributions of total VOC. In urban areas, anthropogenic and biogenic ROH,VOC are predicted fairly well over a range of NOx conditions. In suburban and rural locations, anthropogenic and other oxygenated ROH,VOC relationships are reproduced, but calculated biogenic and aldehyde ROH,VOC are often poorly characterized by measurements, making evaluation of the model with available data unreliable. In central California, 30–50% of the modeled urban VOC reactivity is due to aldehydes and other oxygenated species, and the total oxygenated ROH,VOC is nearly equivalent to anthropogenic VOC reactivity. In rural vegetated regions, biogenic and aldehyde reactivity dominates. This indicates that more attention needs to be paid to the accuracy of models and measurements of both primary emissions of oxygenated VOC and secondary production of oxygenates, especially formaldehyde and other aldehydes, and that a more comprehensive set of oxygenated VOC measurements is required to include all of the important contributions to atmospheric reactivity.
Abstract: Day-of-week patterns in human activities can be used to examine the ways in which differences in primary emissions result in changes in the rates of photochemical reactions, and the production of secondary pollutants. Data from twelve California Air Resources Board monitoring sites in Sacramento, CA, and the downwind Mountain Counties air basin are analyzed to reveal day of week patterns in ozone and its precursors in the summers of 1998–2002. Measurements of non-methane hydrocarbons are available for the summers of 2001–2003 at three of these sites and NOx at six of these sites for the full time period. This routine monitoring data is complemented by data sets of ozone and nitrogen oxide concentrations obtained in the summers of 2001 and 2003 at three sites in the region and comprehensive measurements of VOC reactivity at two sites in 2001. Daytime concentrations of nitrogen oxides (NOx≡NO+NO2) are approximately 35% lower on weekends at all the sites, whereas the VOC reactivity changes by less than 10%. All six sites in the Sacramento Valley have higher 8-h maximum average ozone on the weekend and are more likely to exceed the national standard of 85 ppb on the weekend. In contrast, all the sites in the Mountain Counties are less likely to exceed the federal ozone standard on the weekend. Analysis of the day-of-week trends in odd oxygen show that the weekend effect of ozone within Sacramento is strongly influenced by NO sources close to the monitoring sites. This suggests that ozone measurements from monitoring sites close to highways, including two rural locations, may not be representative of the regional abundance, and lead to underestimates of long term exposure for humans and ecosystems.
Abstract: We present the first regional-scale chemistry simulation investigating the effects of biogenic 2-methyl-3-buten-2-ol (MBO) emissions on air quality. In a central California model domain, MBO emissions have a distinctly different regional pattern than isoprene but have similar daily maxima of about 5 mg m−2 hr−1. MBO oxidation causes an increase in ozone, formaldehyde, acetone and consequently hydrogen radical production (PHOx). The addition of MBO increases the daily maximum ozone as much as 3 ppb near source regions (2–5% in rural areas) and as much as 1 ppb in the Central Valley. Formaldehyde concentrations increase by as much as 1 ppb (40%) over the Sierra Nevada Mountains, increasing the production of HOx by 10–20% and accelerating local chemistry. This indicates that inclusion of MBO and other biogenic oxygenated emissions in regional simulations in the western and southeastern United States is essential for accurate representation of ozone and HOx.
Abstract: X-ray absorption spectra of aqueous 4 and 6 M potassium hydroxide solutions have been measured near the oxygen K edge. Upon addition of KOH to water, a new spectral feature (532.5 eV) emerges at energies well below the liquid water pre-edge feature (535 eV) and is attributed to OH- ions. In addition to spectral changes explicitly due to absorption by solvated OH- ions, calculated XA spectra indicate that first-solvation-shell water molecules exhibit an absorption spectrum that is unique from that of bulk liquid water. It is suggested that this spectral change results primarily from direct electronic perturbation of the unoccupied molecular orbitals of first-shell water molecules and only secondarily from geometric distortion of the local hydrogen bond network within the first hydration shell. Both the experimental and the calculated XA spectra indicate that the nature of the interaction between the OH- ion and the solvating water molecules is fundamentally different than the corresponding interactions of aqueous halide anions with respect to this direct orbital distortion. Analysis of the Mulliken charge populations suggests that the origin of this difference is a disparity in the charge asymmetry between the hydrogen atoms of the solvating water molecules. The charge asymmetry is induced both by electric field effects due to the presence of the anion and by charge transfer from the respective ions. The computational results also indicate that the OH- ion exists with a predominately “hyper-coordinated” solvation shell and that the OH- ion does not readily donate hydrogen bonds to the surrounding water molecules.
Abstract: We describe a new laboratory-based method for in situ detection of nitrous acid (HONO) using a combination of thermal dissociation (TD) and chemiluminescent (CL) detection of nitric oxide. A prototype was built using a commercial NO sensor. Laboratory tests for possible chemical interferences show that measurements are affected in predictable ways by NO2, peroxy nitrates, alkyl nitrates, HNO3, O3 and H2O.
Abstract: Global ozone analyses, based on assimilation of stratospheric profile and ozone column measurements, and NOy predictions from the Real-time Air Quality Modeling System (RAQMS) are used to estimate the ozone and NOy budget over the continental United States during the July–August 2004 Intercontinental Chemical Transport Experiment–North America (INTEX-A). Comparison with aircraft, satellite, surface, and ozonesonde measurements collected during INTEX-A show that RAQMS captures the main features of the global and continental U.S. distribution of tropospheric ozone, carbon monoxide, and NOy with reasonable fidelity. Assimilation of stratospheric profile and column ozone measurements is shown to have a positive impact on the RAQMS upper tropospheric/lower stratosphere ozone analyses, particularly during the period when SAGE III limb scattering measurements were available. Eulerian ozone and NOy budgets during INTEX-A show that the majority of the continental U.S. export occurs in the upper troposphere/lower stratosphere poleward of the tropopause break, a consequence of convergence of tropospheric and stratospheric air in this region. Continental U.S. photochemically produced ozone was found to be a minor component of the total ozone export, which was dominated by stratospheric ozone during INTEX-A. The unusually low photochemical ozone export is attributed to anomalously cold surface temperatures during the latter half of the INTEX-A mission, which resulted in net ozone loss during the first 2 weeks of August. Eulerian NOy budgets are shown to be very consistent with previously published estimates. The NOy export efficiency was estimated to be 24%, with NOx + PAN accounting for 54% of the total NOy export during INTEX-A.
Abstract: We analyze aircraft observations obtained during INTEX-A (1 July to 14 August 2004) to examine the summertime influence of Asian pollution in the free troposphere over North America. By applying correlation analysis and principal component analysis (PCA) to the observations between 6 and 12 km, we find dominant influences from recent convection and lightning (13% of observations), Asia (7%), the lower stratosphere (7%), and boreal forest fires (2%), with the remaining 71% assigned to background. Asian air masses are marked by high levels of CO, O3, HCN, PAN, C2H2, C6H6, methanol, and SO42–. The partitioning of NOy species in the Asian plumes is dominated by PAN (∼600 pptv), with varying NOx/HNO3 ratios in individual plumes, consistent with individual transit times of 3–9 days. Export of Asian pollution occurred in warm conveyor belts of midlatitude cyclones, deep convection, and in typhoons. Compared to Asian outflow measurements during spring, INTEX-A observations display lower levels of anthropogenic pollutants (CO, C3H8, C2H6, C6H6) due to shorter summer lifetimes; higher levels of biogenic tracers (methanol and acetone) because of a more active biosphere; and higher levels of PAN, NOx, HNO3, and O3 reflecting active photochemistry, possibly enhanced by efficient NOy export and lightning. The high ΔO3/ΔCO ratio (0.76 mol/mol) in Asian plumes during INTEX-A is due to strong photochemical production and, in some cases, mixing with stratospheric air along isentropic surfaces. The GEOS-Chem global model captures the timing and location of the Asian plumes. However, it significantly underestimates the magnitude of observed enhancements in CO, O3, PAN and NOx.
Abstract: The formation of organic nitrates during the oxidation of the biogenic hydrocarbon isoprene can strongly affect boundary layer concentrations of ozone and nitrogen oxides (NOx = NO + NO2). We constrain uncertainties in the chemistry of these isoprene nitrates using chemical transport model simulations in conjunction with observations over the eastern United States from the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign during summer 2004. The model best captures the observed boundary layer concentrations of organic nitrates and their correlation with ozone using a 4% yield of isoprene nitrate production from the reaction of isoprene hydroxyperoxy radicals with NO, a recycling of 40% NOx when isoprene nitrates react with OH and ozone, and a fast dry deposition rate of isoprene nitrates. Simulated boundary layer concentrations are only weakly sensitive to the rate of photochemical loss of the isoprene nitrates. An 8% yield of isoprene nitrates degrades agreement with the observations somewhat, but concentrations are still within 50% of observations and thus cannot be ruled out by this study. Our results indicate that complete recycling of NOx from the reactions of isoprene nitrates and slow rates of isoprene nitrate deposition are incompatible with the observations. We find that ∼50% of the isoprene nitrate production in the model occurs via reactions of isoprene (or its oxidation products) with the NO3 radical, but note that the isoprene nitrate yield from this pathway is highly uncertain. Using recent estimates of rapid reaction rates with ozone, 20–24% of isoprene nitrates are lost via this pathway, implying that ozonolysis is an important loss process for isoprene nitrates. Isoprene nitrates are shown to have a major impact on the nitrogen oxide (NOx = NO + NO2) budget in the summertime U.S. continental boundary layer, consuming 15–19% of the emitted NOx, of which 4–6% is recycled back to NOx and the remainder is exported as isoprene nitrates (2–3%) or deposited (8–10%). Our constraints on reaction rates, branching ratios, and deposition rates need to be confirmed through further laboratory and field measurements. The model systematically underestimates free tropospheric concentrations of organic nitrates, indicating a need for future investigation of the processes controlling the observed distribution.
Abstract: A theoretical model of liquid water evaporation has been developed to interpret results from a recent experimental investigation of isotope fractionation during free evaporation [Cappa et al. J. Phys. Chem. B 2005, 109 (51), 24391]. It is established that the free evaporation isotope fractionation factors (αevap) are primarily influenced by the nature of the intermolecular interactions between water molecules, namely, the condensed phase hindered translational and librational frequencies at the surface. The dependence of αevap on the isotopic composition of the liquid can be understood in terms of small variations in these frequencies with isotopic composition. This result suggests that the explicit nature of the solvation environment directly influences evaporation rates from liquids. The sensitivity of the calculated evaporation coefficient for liquid water to both temperature and isotope composition is also explored.
Abstract: Satellite observations have the potential to provide an accurate picture of atmospheric chemistry and air quality on a variety of spatial and temporal scales. A key consideration in the design of new instruments is the spatial resolution required to effectively monitor air quality from space. In this paper, variograms have been used to address this issue by calculating the horizontal length scales of ozone within the boundary layer and free troposphere using both in situ aircraft data from five different NASA aircraft campaigns and simulations with an air-quality model. For both the observations and the model, the smallest scale features were found in the boundary layer, with a characteristic scale of about 50 km which increased to greater than 150 km above the boundary layer. The length scale changes with altitude. It is shown that similar length scales are derived based on a totally independent approach using constituent lifetimes and typical wind speeds. To date, the spaceborne observations of tropospheric constituents have been from several instruments including TOMS, GOME, MOPITT, TES, and OMI which, in general, have different weighting functions that need to be considered, and none really measures at the surface. A further complication is that most satellite measurements (such as those of OMI and GOME) are of the vertically integrated column. In this paper, the length scales in the column measurements were also of the order of 50 km. To adequately resolve the 50-km features, a horizontal resolution of at least 10 km would be desirable.
Abstract: We use observations from two aircraft during the ICARTT campaign over the eastern United States and North Atlantic during summer 2004, interpreted with a global 3-D model of tropospheric chemistry (GEOS-Chem) to test current understanding of regional sources, chemical evolution, and export of NOx. The boundary layer NOx data provide top-down verification of a 50% decrease in power plant and industry NOx emissions over the eastern United States between 1999 and 2004. Observed NOx concentrations at 8–12 km altitude were 0.55 ± 0.36 ppbv, much larger than in previous U.S. aircraft campaigns (ELCHEM, SUCCESS, SONEX) though consistent with data from the NOXAR program aboard commercial aircraft. We show that regional lightning is the dominant source of this upper tropospheric NOx and increases upper tropospheric ozone by 10 ppbv. Simulating ICARTT upper tropospheric NOx observations with GEOS-Chem requires a factor of 4 increase in modeled NOx yield per flash (to 500 mol/flash). Observed OH concentrations were a factor of 2 lower than can be explained from current photochemical models, for reasons that are unclear. A NOy-CO correlation analysis of the fraction f of North American NOx emissions vented to the free troposphere as NOy (sum of NOx and its oxidation products) shows observed f = 16 ± 10% and modeled f = 14 ± 9%, consistent with previous studies. Export to the lower free troposphere is mostly HNO3 but at higher altitudes is mostly PAN. The model successfully simulates NOy export efficiency and speciation, supporting previous model estimates of a large U.S. anthropogenic contribution to global tropospheric ozone through PAN export.
Abstract: A comprehensive group of reactive nitrogen species (NO, NO2, HNO3, HO2NO2, PANs, alkyl nitrates, and aerosol-NO3−) were measured over North America during July/August 2004 from the NASA DC-8 platform (0.1–12 km). Nitrogen containing tracers of biomass combustion (HCN and CH3CN) were also measured along with a host of other gaseous (CO, VOC, OVOC, halocarbon) and aerosol tracers. Clean background air as well as air with influences from biogenic emissions, anthropogenic pollution, biomass combustion, convection, lightning, and the stratosphere was sampled over the continental United States, the Atlantic, and the Pacific. The North American upper troposphere (UT) was found to be greatly influenced by both lightning NOx and surface pollution lofted via convection and contained elevated concentrations of PAN, ozone, hydrocarbons, and NOx. Observational data suggest that lightning was a far greater contributor to NOx in the UT than previously believed. PAN provided a dominant reservoir of reactive nitrogen in the UT while nitric acid dominated in the lower troposphere (LT). Peroxynitric acid (HO2NO2) was present in sizable concentrations peaking at around 8 km. Aerosol nitrate appeared to be mostly contained in large soil based particles in the LT. Plumes from Alaskan fires contained large amounts of PAN and aerosol nitrate but little enhancement in ozone. A comparison of observed data with simulations from four 3-D models shows significant differences between observations and models as well as among models. We investigate the partitioning and interplay of the reactive nitrogen species within characteristic air masses and further examine their role in ozone formation.
Abstract: We describe measurements of total peroxy nitrates (ΣPNs), NO2, O3 and several aldehydes at Granite Bay, California, during the Chemistry and Transport of the Sacramento Urban Plume-2001 (CATSUP 2001) campaign, from 19 July–16 September 2001. We observed a strong photochemically driven variation of ΣPNs during the day with the median of 1.2 ppb at noon. Acetaldehyde, pentanal, hexanal and methacrolein had median abundances in the daytime of 1.2 ppb, 0.093 ppb, 0.14 ppb, and 0.27 ppb, respectively. We compare steady state and time dependent calculations of the dependence of ΣPNs on aldehydes, OH, NO and NO2 showing that the steady state calculations are accurate to ±30% between 10:00 and 18:00 h. We use the steady state calculation to investigate the composition of ΣPNs and the concentration of OH at Granite Bay. We find that PN molecules that have never been observed before make up an unreasonably large fraction of the ΣPNs unless we assume that there exists a PAN source that is much larger than the acetaldehyde source. We calculate that OH at the site varied between 2 and 7×106 molecule cm−3 at noon during the 8 weeks of the experiment.
Abstract: The first direct in situ measurements of HO2NO2 in the upper troposphere were performed from the NASA DC-8 during the Intercontinental Chemical Transport Experiment–North America 2004 with a chemical ionization mass spectrometer (CIMS). These measurements provide an independent diagnostic of HOx chemistry in the free troposphere and complement direct observations of HOx, because of the dual dependency of HO2NO2 on HOx and NOx. On average, the highest HO2NO2 mixing ratio of 76 pptv (median = 77 pptv, σ = 39 pptv) was observed at altitudes of 8–9 km. Simple steady state calculations of HO2NO2, constrained by measurements of HOx, NOx, and J values, are in good agreement (slope = 0.90, R2 = 0.60, and z = 5.5–7.5 km) with measurements in the midtroposphere where thermal decomposition is the major loss process. Above 8 km the calculated steady state HO2NO2 is in poor agreement with observed values (R2 = 0.20) and is typically larger by a factor of 2.4. Conversely, steady state calculations using model-derived HOx show reasonable agreement with the observed HO2NO2 in both the midtroposphere (slope = 0.96, intercept = 7.0, and R2 = 0.63) and upper troposphere (slope = 0.80, intercept = 32.2, and R2 = 0.58). These results indicate that observed HO2 and HO2NO2 are in poor agreement in the upper troposphere but that HO2NO2 levels are consistent with current photochemical theory.
Abstract: We present a statistical representation of the aggregate effects of deep convection on the chemistry and dynamics of the upper troposphere (UT) based on direct aircraft observations of the chemical composition of the UT over the eastern United States and Canada during summer. These measurements provide unique observational constraints on the chemistry occurring downwind of convection and the rate at which air in the UT is recycled. These results provide quantitative measures that can be used to evaluate global climate and chemistry models.
Abstract: The most extensive set of free tropospheric ozone measurements ever compiled across midlatitude North America was measured with daily ozonesondes, commercial aircraft and a lidar at 14 sites during July-August 2004. The model estimated stratospheric ozone was subtracted from all profiles, leaving a tropospheric residual ozone. On average the upper troposphere above midlatitude eastern North America contained 15 ppbv more tropospheric residual ozone than the more polluted layer between the surface and 2 km above sea level. Lowest ozone values in the upper troposphere were found above the two upwind sites in California. The upper troposphere above midlatitude eastern North America contained 16 ppbv more tropospheric residual ozone than the upper troposphere above three upwind sites, with the greatest enhancement above Houston, Texas, at 24 ppbv. Upper tropospheric CO measurements above east Texas show no statistically significant enhancement compared to west coast measurements, arguing against a strong influence from fresh surface anthropogenic emissions to the upper troposphere above Texas where the ozone enhancement is greatest. Vertical mixing of ozone from the boundary layer to the upper troposphere can only account for 2 ppbv of the 16 ppbv ozone enhancement above eastern North America; therefore the remaining 14 ppbv must be the result of in situ ozone production. The transport of NOx tracers from North American anthropogenic, biogenic, biomass burning, and lightning emissions was simulated for the upper troposphere of North America with a particle dispersion model. Additional box model calculations suggest the 24 ppbv ozone enhancement above Houston can be produced over a 10 day period from oxidation reactions of lightning NOx and background mixing ratios of CO and CH4. Overall, we estimate that 69–84% (11–13 ppbv) of the 16 ppbv ozone enhancement above eastern North America is due to in situ ozone production from lightning NOx with the remainder due to transport of ozone from the surface or in situ ozone production from other sources of NOx.
Abstract: Observations of day-of-week patterns and diurnal profiles of ozone, volatile organic compounds and nitrogen oxides are examined to assess the chemical and dynamical factors governing the daytime ozone accumulation and the distribution of chemically related species in Central California. Isoprene observations show that urban OH concentrations are higher on the weekend whereas rural OH concentrations are lower on the weekend, confirming that NOx concentrations have a direct effect on the rate of photochemical ozone production and that the transition from NOx-saturated (VOC-limited) to NOx-limited chemistry occurs between the city and the downwind rural counties. We quantify the extent to which mixing of ozone and its precursors from aloft contributes to the daytime accumulation of ozone at the surface in Sacramento. Ozone production in the rural Mountain Counties is currently NOx-limited and will decrease in response to NOx emission reductions in the Sacramento Valley. However, NOx emissions reductions of at least 50% (from weekday levels) are necessary to bring about a significant decrease in accumulation of ozone at the surface in the Sacramento Valley. The impact of NOx emission reductions on the frequency of exceeding the federal 8-hour ozone standard at an individual site will depend on the balance between reduced titration and the sign and magnitude of production changes. We further show that HNO3 production, which depends on the product of OH and NO2 mixing ratios, is a constant at high NOx, suggesting that NOx must be reduced below a threshold before nitrate aerosol can be expected to decrease.
Abstract: Observations of speciated nitrogen oxides, namely NO2, total peroxy nitrates (ΣPNs), total alkyl nitrates (ΣANs), and HNO3 by thermal dissociation laser induced fluorescence (TD-LIF), and supporting chemical and meteorological measurements at Big Hill (1860 m), a high elevation site in California's Sierra Nevada Mountains, are described. From May through October, terrain-driven winds in the region routinely bring air from Sacramento, 100 km southwest of the site, upslope over oak and pine forests to Big Hill during the day, while at night, the site often samples clean, dry air characteristic of the free troposphere. Winter differs mainly in that the meteorology does not favour the buildup of Sacramento's pollution over the Sierra Nevada range, and the urban-influenced air that is seen has been less affected by biogenic VOC emissions, resulting in longer lifetime for NO2 and a predominance of the inorganic forms of nitrogen oxides. Summertime observations at Big Hill can be compared with those from Granite Bay, a Sacramento suburb, and from the University of California's Blodgett Forest Research Station to examine the evolution of nitrogen oxides and ozone within the urban plume. Nitrogen oxide radicals (NO and NO2), which dominate total nitrogen oxides (NOy) at Granite Bay, are rapidly converted into HNO3, ΣPNs, and ΣANs, such that these compounds contribute 29, 30, and 21% respectively to the NOy budget in the plume at Big Hill. Nevertheless, the decreasing concentrations of NO2 as the plume is advected to Big Hill lead to decreases in the production rate of HNO3 and ozone. The data also demonstrate the role that temperature plays in sequestering NO2 into peroxy nitrates, effectively decreasing the rate of ozone production. The important contribution of ΣANs to NOy in the region suggests that they should be considered with regards to export of NOy from the boundary layer. Nocturnal observations of airmasses characteristic of the free troposphere showed lower NOy concentrations, which were dominated by HNO3 with a relatively small contribution from the organic nitrates.
Abstract: Recent theoretical and experimental studies of evaporation have suggested that on average, molecules in the higher-energy tail of the Boltzmann distribution are more readily transferred into the vapor during evaporation. To test these conclusions, the evaporative cooling rates of a droplet train of liquid water injected into vacuum have been studied via Raman thermometry. The resulting cooling rates are fit to an evaporative cooling model based on Knudsen's maximum rate of evaporation, in which we explicitly account for surface cooling. We have determined that the value of the evaporation coefficient (γe) of liquid water is 0.62 ± 0.09, confirming that a rate-limiting barrier impedes the evaporation rate. Such insight will facilitate the formulation of a microscopic mechanism for the evaporation of liquid water.
Abstract: It was recently suggested that liquid water primarily comprises hydrogen-bonded rings and chains, as opposed to the traditionally accepted locally tetrahedral structure (Wernet et al. Science 2004, 304, 995). This controversial conclusion was primarily based on comparison between experimental and calculated X-ray absorption spectra (XAS) using computer-generated ice-like 11-molecule clusters. Here we present calculations which conclusively show that when hydrogen-bonding configurations are chosen randomly, the calculated XAS does not reproduce the experimental XAS regardless of the bonding model employed (i.e., rings and chains vs tetrahedral). Furthermore, we also present an analysis of a recently introduced asymmetric water potential (Soper, A. K. J. Phys.: Condens. Matter 2005, 17, S3273), which is representative of the rings and chains structure, and make comparisons with the standard SPC/E potential, which represents the locally tetrahedral structure. We find that the calculated XAS from both potentials is inconsistent with the experimental XAS. However, we also show the calculated electric field distribution from the rings and chains structure is strongly bimodal and highly inconsistent with the experimental Raman spectrum, thus casting serious doubt on the validity of the rings and chains model for liquid water.
Abstract: Using a chemical transport model simulating ozone concentrations in central California, we evaluate the effects of variables associated with future changes in climate and ozone precursor emissions, including (1) increasing temperature; (2) increasing atmospheric water vapor; (3) increasing biogenic VOC emissions due to temperature; (4) projected decreases in anthropogenic NOx, VOC, and CO emissions in California for 2050; and (5) the influence of changing ozone, CO, and methane at the western boundary. Climatic changes expected for temperature, atmospheric water vapor, and biogenic VOC emissions each individually cause a 1–5% increase in the daily peak ozone. Projected reductions in anthropogenic emissions of 10–50% in NOx and 50–70% in VOCs and CO have the greatest single effect, reducing ozone by 8–15% in urban areas. Changes to the chemical boundary conditions lead to ozone increases of 6% in the San Francisco Bay area and along the west coast but only 1–2% inland. Simulations combining climate effects predict that ozone will increase 3–10% in various regions of California. This increase is partly offset by projected future emissions reductions, and a combined climate and emissions simulation yields ozone reductions of 3–9% in the Central Valley and almost no net change in the San Francisco Bay area. We find that different portions of the model domain have widely varying sensitivity to climate parameters. In particular, the San Francisco Bay region is more strongly influenced by temperature changes than inland regions, indicating that air quality in this region may worsen under future climate regimes.
Abstract: We retrieve tropospheric nitrogen dioxide (NO2) columns for May 2004 to April 2005 from the SCIAMACHY satellite instrument to derive top-down emissions of nitrogen oxides (NOx = NO + NO2) via inverse modeling with a global chemical transport model (GEOS-Chem). Simulated NO2 vertical profiles used in the retrieval are evaluated with airborne measurements over and downwind of North America (ICARTT); a northern midlatitude lightning source of 1.6 Tg N yr−1 minimizes bias in the retrieval. Retrieved NO2 columns are validated (r2 = 0.60, slope = 0.82) with coincident airborne in situ measurements. The top-down emissions are combined with a priori information from a bottom-up emission inventory with error weighting to achieve an improved a posteriori estimate of the global distribution of surface NOx emissions. Our a posteriori NOx emission inventory for land surface NOx emissions (46.1 Tg N yr−1) is 22% larger than the GEIA-based a priori bottom-up inventory for 1998, a difference that reflects rising anthropogenic emissions, especially from East Asia. A posteriori NOx emissions for East Asia (9.8 Tg N yr−1) exceed those from other continents. The a posteriori inventory improves the GEOS-Chem simulation of NOx, peroxyacetylnitrate, and nitric acid with respect to airborne in situ measurements over and downwind of New York City. The a posteriori is 7% larger than the EDGAR 3.2FT2000 global inventory, 3% larger than the NEI99 inventory for the United States, and 68% larger than a regional inventory for 2000 for eastern Asia. SCIAMACHY NO2 columns over the North Atlantic show a weak plume from lightning NOx.
Abstract: Nitrogen exchange between the atmosphere and biosphere directly influences atmospheric composition. While much is known about mechanisms of NO and N2O emissions, instrumentation for the study of mechanisms contributing to exchange of other major nitrogen species is quite limited. Here we describe the application of a new technique, thermal dissociation-laser induced fluorescence (TD-LIF), to eddy covariance measurements of the fluxes of NO2, total peroxy acyl and peroxy nitrates, total alkyl and multifunctional alkyl nitrates, and nitric acid. The technique offers the potential for investigating mechanisms of exchange of these species at the canopy scale over timescales from days to years. Examples of flux measurements at a ponderosa pine plantation in the mid-elevation Sierra Nevada Mountains in California are reported and used to evaluate instrument performance.
Abstract: This chapter contains sections titled:
Abstract: The release of NO and NO2 from frozen aqueous NaNO3 irradiated at 313 nm was studied using time-resolved spectroscopic techniques. The kinetic behavior of NO and NO2 signals during on-and-off illumination cycles confirms that NO2 is a primary photoproduct evolving from the outermost ice layers and reveals that NO is a secondary species generated deeper in the ice, whence it eventually emerges due to its inertness and larger diffusivity. NO is shown to be more weakly held than NO2 by ice in thermal desorption experiments on preirradiated samples. The partial control of gaseous emissions by mass transfer, and hence by the morphology and metamorphisms of polycrystalline ice, is established by (1) the nonmonotonic temperature dependence of NO and NO2 signals upon stepwise warming under continuous illumination, (2) the fact that the NO, NO2 or NOx (NOx ≡ NO + NO2) amounts released in bright thermograms performed under various heating ramps fail to scale with photon dose, due to irreversible losses in the adsorbed state. Because present NO/NO2 ratios are up to 10-fold smaller than those determined over sunlit snowpacks, we infer that the immediate precursors to NO mostly absorb at λ > λmax (NO3-) ~ 302 nm.
Abstract: Oxygen K-edge X-ray absorption spectra (XAS) of aqueous chloride solutions have been measured for Li+, Na+, K+, NH4+, C(NH2)3+, Mg2+, and Ca2+ at 2 and 4 M cation concentrations. Marked changes in the liquid water XAS are observed upon addition of the various monovalent cation chlorides that are nearly independent of the identity of the cation. This indicates that interactions with the dissolved monovalent cations do not significantly perturb the unoccupied molecular orbitals of water molecules in the vicinity of the cations and that water−chloride interactions are primarily responsible for the observed spectral changes. In contrast, the addition of the divalent cations engenders changes unique from the case of the monovalent cations, as well as from each other. Density functional theory calculations suggest that the ion-specific spectral variations arise primarily from direct electronic perturbation of the unoccupied orbitals due to the presence of the ions, probably as a result of differences in charge transfer from the water molecules onto the divalent cations.
Abstract: The oxygen K edge X-ray absorption spectra of aqueous HCl and NaCl solutions reveal distinct perturbations of the local water molecules by the respective solutes. While the addition of NaCl leads to large spectral changes, the effect of HCl on the observed X-ray absorption spectrum is surprisingly small. Density functional theory calculations suggest that this difference primarily reflects a strong blue shift of the hydrated proton (in either the Eigen (H9O4+) or Zundel (H2O5+) forms) spectrum relative to that of H2O, indicating the tighter binding of electrons in H3O+. This spectral shift counteracts the spectral changes that arise from direct electrostatic perturbation of water molecules in the first solvation shell of Cl-. Consequently, the observed spectral changes effected by HCl addition are minimal compared to those engendered by NaCl. Additionally, these results indicate that the effect of monovalent cations on the nature of the unoccupied orbitals of water molecules in the first solvation shell is negligible, in contrast to the large effects of monovalent anions.
Abstract: Soil NOx emission from agricultural regions in the western United States has been investigated using satellite observations of NO2 from the SCIAMACHY instrument. We show that the SCIAMACHY observations over a 2 million hectare agricultural region in Montana capture the short intense NOx pulses following fertilizer application and subsequent precipitation and we demonstrate that these variations can be reproduced by tuning the mechanistic parameters in an existing model of soil NOx emissions.
Abstract: The microscopic events engendering liquid water evaporation have received much attention over the last century, but remain incompletely understood. We present measurements of isotope fractionation occurring during free molecular evaporation from liquid microjets and show that the isotope ratios of evaporating molecules exhibit dramatic differences from equilibrium vapor values, strong variations with the solution deuterium mole fraction, and a clear temperature dependence. These results indicate the existence of an energetic barrier to evaporation and that the evaporation coefficient of water is less than unity. These new insights into water evaporation promise to advance our understanding of the processes that control the formation and lifetime of clouds in the atmosphere.
Abstract: The nitrogen K-edge spectra of aqueous proline and diglycine solutions have been measured by total electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at neutral and high pH. All observed spectral features have been assigned by comparison to the recently reported spectrum of aqueous glycine and calculated spectra of isolated amino acids and hydrated amino acid clusters. The sharp preedge resonances at 401.3 and 402.6 eV observed in the spectrum of anionic glycine indicate that the nitrogen terminus is in an “acceptor-only” configuration, wherein neither amine proton is involved in hydrogen bonding to the solvent, at high pH. The analogous 1s → σ*NH preedge transitions are absent in the NEXAFS spectrum of anionic proline, implying that the acceptor-only conformation observed in anionic glycine arises from steric shielding induced by free rotation of the amine terminus about the glycine CN bond. Anionic diglycine solutions exhibit a broadened 1s → π*CN resonance at 401.2 eV and a broad shoulder resonance at 403 eV, also suggesting the presence of an acceptor-only species. Although this assignment is not as unambiguous as for glycine, it implies that the nitrogen terminus of most proteins is capable of existing in an acceptor-only conformation at high pH. The NEXAFS spectrum of zwitterionic lysine solution was also measured, exhibiting features similar to those of both anionic and zwitterionic glycine, and leading us to conclude that the α amine group is present in an acceptor-only configuration, while the end of the butylammonium side chain is fully solvated.
Abstract: The unique chemical and physical properties of liquid water are a direct result of its highly directional hydrogen-bond (HB) network structure and associated dynamics. However, despite intense experimental and theoretical scrutiny spanning more than four decades, a coherent description of this HB network remains elusive. The essential question of whether continuum or multicomponent (“intact,” “broken bond,” etc.) models best describe the HB interactions in liquid water has engendered particularly intense discussion. Most notably, the temperature dependence of water's Raman spectrum has long been considered to be among the strongest evidence for a multicomponent distribution. Using a combined experimental and theoretical approach, we show here that many of the features of the Raman spectrum that are considered to be hallmarks of a multistate system, including the asymmetric band profile, the isosbestic (temperature invariant) point, and van't Hoff behavior, actually result from a continuous distribution. Furthermore, the excellent agreement between our newly remeasured Raman spectra and our model system further supports the locally tetrahedral description of liquid water, which has recently been called into question [Wernet, P., et al. (2004) Science 304, 995-999].
Abstract: The absolute accuracy and long-term precision of atmospheric measurements hinge on the quality of the instrumentation and calibration standards. To assess the consistency of the ozone (O3) and nitrogen oxides (NOx) standards maintained at the National Institute of Standards and Technology (NIST), these standards were compared through the gas-phase titration of O3 with nitric oxide (NO). NO and O3 were monitored using chemiluminescence and UV absorption, respectively. Nitrogen dioxide (NO2) was monitored directly by laser-induced fluorescence and indirectly by catalytic conversion to NO, followed by chemiluminescence. The observed equivalent loss of both NO and O3 and the formation of NO2 in these experiments was within 1% on average over the range of 40–200 nmol mol−1 of NO in excess O3, indicating that these instruments, when calibrated with the NIST O3 and NO standards and the NO2 permeation calibration system, are consistent to within 1% at tropospherically relevant mixing ratios of O3. Experiments conducted at higher initial NO mixing ratios or in excess NO are not in as good agreement. The largest discrepancies are associated with the chemiluminescence measurements. These results indicate the presence of systematic biases under these specific conditions. Prospects for improving these experiments are discussed.
Abstract: Temperature-programmed NO2 emissions from frozen aqueous NaNO3 solutions irradiated at 313 nm were monitored as function of nitrate concentration and heating rate, H, above −30 ºC. Emissions increase nonmonotonically with temperature, displaying transitions suggestive of underlying metamorphic transformations. Thus, NO2 emissions surge at ca. −8 ºC in frozen [NO3-] > 200 μM samples warmed at H = 0.70 °C min-1 under continuous irradiation, and also in the dark from samples that had been photolyzed at −30 ºC. The amounts of NO2 released in individual thermograms, ΣN, increase less than linearly with [NO3-] for the duration of experiments, revealing the significant loss of photogenerated NO2. The actual ΣN ∝ [NO3-]1/2 dependence (at constant H) is consistent with NO2 hydrolysis: 2NO2 + H2O → NO3- + NO2- + 2H+, overtaking NO2 desorption, even below the eutectic point (−18 ºC for aqueous NaNO3). The increasingly larger NO2 losses detected in longer experiments (at constant [NO3-]) are ascribed to secondary photolysis of trapped NO2. The relevance of present results to the interpretation of polar NO2 measurements is briefly analyzed.
Abstract: During the summer of 2001, NO2, total peroxy nitrates (ΣPNs), total alkyl nitrates (ΣANs), HNO3, volatile organic compounds (VOC), CO2, O3, and meteorological variables were measured at Granite Bay, CA. The diurnal variation in ΣPNs, ΣANs and HNO3 were all strongly correlated with sunlight, indicating both that they are photochemically produced and that they have a lifetime of a few hours at this site. The mixing ratios of ΣANs ranged as high as 2 ppbv. Mixing ratios at night averaged 0.4 ppbv. Odd-oxygen (Ox=O3+NO2) and ΣANs were strongly correlated reflecting both the common chemical source terms and the similar lifetimes of both species. Several approaches to interpreting the simultaneous variations of Ox and ΣANs are described, and used to derive a best estimate of the ΣAN yield from the VOC mixture at this site of 4.2% and an estimate of the range that is consistent with the observations of 3.9–5.8%. A yield of 4.2% implies termination of the HOx catalytic cycle by ΣAN formation once every 24 cycles. Analysis of the HNO3 observations in combination with the ΣAN and O3 measurements suggests that NOx terminations limit the HOx chain length to between 4.7 and 6.3.
Abstract: Measurements of the oxygen K-edge X-ray absorption spectrum (XAS) of aqueous sodium halide solutions demonstrate that ions significantly perturb the electronic structure of adjacent water molecules. The addition of halide salts to water engenders an increase in the preedge intensity and a decrease in the postedge intensity of the XAS, analogous to those observed when increasing the temperature of pure water. The main-edge feature exhibits unique behavior and becomes more intense when salt is added. Density functional theory calculations of the XAS indicate that the observed red shift of the water transitions as a function of salt concentration arises from a strong, direct perturbation of the unoccupied molecular orbitals on water by anions, and does not require significant distortion of the hydrogen bond network beyond the first solvation shell. This contrasts the temperature-dependent spectral variations, which result primarily from intensity changes of specific transitions due to geometric rearrangement of the hydrogen bond network.
Abstract: The carbon, nitrogen, and oxygen K-edge spectra were measured for aqueous solutions of glycine by total electron yield near-edge X-ray absorption fine structure (TEY NEXAFS) spectroscopy. The bulk solution pH was systematically varied while maintaining a constant amino acid concentration. Spectra were assigned through comparisons with both previous studies and ab initio computed spectra of isolated glycine molecules and hydrated glycine clusters. Nitrogen K-edge solution spectra recorded at low and moderate pH are nearly identical to those of solid glycine, whereas basic solution spectra strongly resemble those of the gas phase. The carbon 1s → π*C==O transition exhibits a 0.2 eV red shift at high pH due to the deprotonation of the amine terminus. This deprotonation also effects a 1.4 eV red shift in the nitrogen K-edge at high pH. Two sharp preedge features at 401.3 and 402.5 eV are also observed at high pH. These resonances, previously observed in the vapor-phase ISEELS spectrum of glycine, have been reassigned as transitions to σ* bound states. The observation of these peaks indicates that the amine moiety is in an acceptor-only hydrogen bond configuration at high pH. At low pH, the oxygen 1s → π*C==O transition exhibits a 0.25-eV red shift due to the protonation of the carboxylic acid terminus. These spectral differences indicate that the variations in electronic structure observed in the NEXAFS spectra are determined by the internal charge state and hydration environment of the molecule in solution.
Abstract: We report in situ measurements of N2O5, NO2, and O3 in Contra Costa County, California during January 2004. Mixing ratios of N2O5, an intermediate compound in the atmosphere's nocturnal cleansing process, ranged up to 200pmol/mol at night. The highest N2O5 concentrations were correlated with low O3 and high NO2 concentrations. The calculated steady state lifetime for N2O5 ranged from 5 to 30min. The total HNO3 produced by N2O5 hydrolysis over a 14h night was comparable to ambient NO2 concentrations, and is estimated to be a factor of nine bigger than the HNO3 produced during the day.
Abstract: Observations of NO2, total peroxy nitrates, total alkyl nitrates, and HNO3 along with a variety of correlative measurements over the full annual cycle (March 2003- February 2004) were collected at Big Hill CA (38.842 °N, 120.408 °W, 1860 m). The elevation and location of the Big Hill site, approximately 30 km west of Lake Tahoe, were chosen so that observations made there could be analyzed to quantify the influence of urban areas, such as Sacramento and San Francisco, which are upwind of Lake Tahoe during westerly flow. Examining the seasonal and diurnal behaviour of reactive nitrogen at the site shows that in the winter:
· total reactive nitrogen is lower, net flow at the surface is downhill and the urban plume rarely reaches the western rim of the Basin.
· individual episodes of high NO2 and inorganic nitrates associated with small-scale burning events along the western slope may generate HNO3 that can reach Tahoe
Combining our data with corresponding measurements at UC Blodgett Forest, we have developed a highly constrained model of the processes that govern reactive nitrogen distribution during the summer months in the region. Based on our analyses of the observations made, we can draw the following conclusions:
· During summer months, the Sacramento plume is the dominant source of reactive nitrogen in the region of the western slope of the Sierra Nevada, but this plume rarely reaches as far east as the Tahoe Basin
· HNO3 deposition is sufficiently fast that very little remains in the plume by the time it reaches high elevation sites near the western rim of the Lake Tahoe Basin
· At Big Hill, similar concentrations of HNO3 are found in airmasses coming from Sacramento (west) and Tahoe (east), demonstrating that urban areas to the west of Lake Tahoe cannot be identified as a net source of nitric acid to the Tahoe Basin
· Organic nitrates are significantly elevated in the plume compared to background conditions but their contribution to nitrogen deposition remains poorly understood
Yearlong measurements from Big Hill, just west of the Lake Tahoe Basin, show that the chemical processing, deposition, and dilution of urban emissions result in a negligible direct contribution of this upwind source to dry deposition of nitrogen oxides to Lake Tahoe.
Abstract: A strong temperature dependence of oxygen K–edge x–ray absorption fine structure features was observed for supercooled and normal liquid water droplets prepared from the breakup of a liquid microjet. Analysis of the data over the temperature range 251 to 288 kelvin (–22° to +15°C) yields a value of 1.5 ± 0.5 kilocalories per mole for the average thermal energy required to effect an observable rearrangement between the fully coordinated (“ice–like”) and distorted (“broken–donor”) local hydrogen-bonding configurations responsible for the pre-edge and post-edge features, respectively. This energy equals the latent heat of melting of ice with hexagonal symmetry (ice Ih) and is consistent with the distribution of hydrogen bond strengths obtained for the “overstructured” ST2 model of water.
Abstract: The influence of narrow optical resonances, which result from trapping of light rays via total internal reflection in water droplets, on the absorption of shortwave (SW) solar radiation has been estimated through high resolution Mie scattering calculations. Our results indicate that these resonances engender an increase in absorption of solar radiation by cloud droplets that is several W/m2 above the linear direct absorption process. Mie scattering calculations performed at the Δx = 0.1 (x = 2πr/λ) resolution typically implemented in cloudy sky radiative transfer models are shown to be insufficient for accurate determination of the attenuation of SW radiation when considered over relatively narrow wavelength ranges, consistent with the recent finding of Nussenzveig . However, for broadband calculations we find positive and negative errors in Mie calculations at Δx = 0.1 nearly cancel resulting in reasonable estimates of SW attenuation.
Abstract: Observations of total alkyl nitrates (ΣANs) were obtained using thermal dissociation-laser-induced fluorescence at La Porte, Texas, from 15 August to 15 September 2000, along with an extensive suite of other nitrogen oxides, hydrocarbons, and O3. The ΣAN mixing ratios ranged as high as 5.2 ppbv. The median midday mixing ratio was 1.2 ppbv, and the median nighttime mixing ratio was 0.26 ppbv. These are higher mixing ratios than the sum of individual nitrates in virtually every prior study. The diurnal variation of ΣANs was similar to that of HNO3 and of total peroxy nitrates, with a peak near 1300 local time (LT) indicating a photochemical source. Mixing ratios decreased rapidly in the afternoon, suggesting that ΣAN deposition is nearly as fast as HNO3 deposition. The observed correlation between O3 and ΣANs has a slope that increases from 29 (R2 = 0.73) ΔOx/ΔΣANs at 0900–1200 LT to 41 (R2 = 0.74) ΔOx/ΔΣANs at 1400–1800 LT. We present calculations constrained by the observed hydrocarbons showing that both the mixing ratio of ΣANs and the correlation of ΣANs with O3 are to be expected on the basis of the branching ratios for alkyl nitrate formation in the RO2 + NO reaction.
Abstract: The U.S. Weather Research Program convenes expert working groups on a one-time basis to identify critical research needs in various problem areas. The most recent expert working group was charged to “identify and delineate critical meteorological research issues related to the prediction of air quality.” In this context, “prediction” is denoted as “forecasting” and includes the depiction and communication of the present chemical state of the atmosphere, extrapolation or nowcasting, and numerical prediction and chemical evolution on time scales up to several days. Emphasis is on the meteorological aspects of air quality.
The problem of air quality forecasting is different in many ways from the problem of weather forecasting. The latter typically is focused on prediction of severe, adverse weather conditions, while the meteorology of adverse air quality conditions frequently is associated with benign weather. Boundary layer structure and wind direction are perhaps the two most poorly determined meteorological variables for regional air quality prediction. Meteorological observations are critical to effective air quality prediction, yet meteorological observing systems are designed to support prediction of severe weather, not the subtleties of adverse air quality. Three-dimensional meteorological and chemical observations and advanced data assimilation schemes are essential. In the same way, it is important to develop high-resolution and self-consistent databases for air quality modeling; these databases should include land use, vegetation, terrain elevation, and building morphology information, among others. New work in the area of chemically adaptive grids offers significant promise and should be pursued. The quantification and effective communication of forecast uncertainty are still in their early stages and are very important for decision makers; this also includes the visualization of air quality and meteorological observations and forecasts. Research is also needed to develop effective metrics for the evaluation and verification of air quality forecasts so that users can understand the strengths and weaknesses of various modeling schemes. Last, but not of least importance, is the need to consider the societal impacts of air quality forecasts and the needs that they impose on researchers to develop effective and useful products.
Abstract: The chemistry of peroxynitric acid (HO2NO2) and methyl peroxynitrate (CH3O2NO2)is predicted to be particularly important in the upper troposphere where temperatures are frequently low enough that these compounds do not rapidly decompose. At temperatures below 240K, we calculate that about 20% of NOy in the mid- and high-latitude upper troposphere is HO2NO2. Under these conditions, the reaction of OH with HO2NO2 is estimated to account for as much as one third of the permanent loss of hydrogen radicals. During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign, we used thermal dissociation laser-induced fluorescence (TD-LIF) to measure the sum of peroxynitrates (ΣPNs≡ HO2NO2+CH3O2NO2+PAN+PPN+...) aboard the NCAR C-130 research aircraft. We infer the sum of HO2NO2 and CH3O2NO2 as the difference between ΣPN measurements and gas chromatographic measurements of the two major peroxy acyl nitrates, peroxy acetyl nitrate (PAN) and peroxy propionyl nitrate (PPN). Comparison with NOy and other nitrogen oxide measurements confirms the importance of HO2NO2 and CH3O2NO2 to the reactive nitrogen budget and shows that current thinking about the chemistry of these species is approximately correct. During the spring high latitude conditions sampled during the TOPSE experiment, the model predictions of the contribution of (HO2NO2+CH3O2NO2) to NOy are highly temperature dependent: on average 30% of NOy at 230K, 15% of NOy at 240K, and <5% of NOy above 250K. The temperature dependence of the inferred concentrations corroborates the contribution of overtone photolysis to the photochemistry of peroxynitric acid. A model that includes IR photolysis (J=1x10-5s-1) agreed with the observed sum of HO2NO2+CH3O2NO2 to better than 35% below 240K where the concentration of these species is largest.
Abstract: We describe a prototype designed for in situ detection of the nitrate radical (NO3) by laser-induced fluorescence (LIF) and of N2O5 by thermal dissociation followed by LIF detection of NO3. An inexpensive 36 mW continuous wave multi-mode diode laser at 662 nm is used to excite NO3 in the B2E‘(0000) ← X2A'2(0000) band. Fluorescence is collected from 700 to 750 nm. The prototype has a sensitivity to NO3 of 76 ppt for a 60 s integration with an accuracy of 8%. Although this sensitivity is adequate for studies of N2O5 in many environments, it is much less sensitive (about 300 times) than expected based on a comparison of previously measured photophysical properties of NO2 and NO3. This implies much stronger nonradiative coupling of electronic states in NO3 than in NO2.
Abstract: We describe measurements of NO2, total peroxy nitrates (ΣPNs), total alkyl nitrates (ΣANs), and HNO3 using thermal dissociation followed by laser-induced fluorescence detection of NO2 at three continental locations. The ΣAN observations are unique and provide novel constraints on atmospheric photochemistry. At a rural site in California, measurements over a full annual cycle show that ΣANs are routinely 10–20% of NOy. At this rural site, at a suburban site in California and an urban site in Houston, Texas, both the absolute concentration of ΣANs and the fraction of the higher oxides of nitrogen (NOz) represented by ΣANs are greater than or equal to values reported in any prior observations. Although the contrast with prior observations is striking, we show that large abundances of ΣANs are consistent with simple chemical models of tropospheric ozone production and with the few prior comprehensive model studies. We also show that ΣANs are a large part, if not all, of the “missing NOy” reported in many prior experiments and emphasize that the ratios of ΣANs/NOz and of O3/ΣANs are especially useful for evaluating chemical models and comparing observations at different sites.
Abstract: Variations in the isotopic content (18O/16O and D/H ratios) of water in the natural environment provide a valuable tracer of the present-day global hydrologic cycle and a record of the climate over at least 400,000 years that is preserved in glacial ice. The interpretation of observed isotopic ratios in water vapor, rain, snow, and ice depends on our understanding of the processes (mainly phase changes) that produce isotopic fractionation. Whereas equilibrium isotopic fractionation is well understood, kinetic effects, or diffusion-controlled fractionation, has a limited experimental foundation. Kinetic effects are significant during evaporation into unsaturated air and during condensation to form ice from vapor. Kinetic effects are also thought to control the deuterium excess (d = δD − 8δ18O) of precipitation. We describe experiments to observe kinetic effects associated with evaporation. Analysis of our own and previous experiments shows that surface cooling of the liquid is a crucial variable affecting fractionation from evaporating water that has not been properly considered before. Including the effects of evaporative surface cooling reconciles observed D/H fractionation with kinetic theory and removes the need to invoke an unusual size for the HDO molecule. Thus the isotopic molecular diffusivity ratios are D(H218O)/D(H216O) = 0.9691 and D(HD16O)/D(H216O) = 0.9839. Implications of this work for representation of kinetic fractionation in global circulation models and cloud physics models are briefly discussed.
Abstract: Measurements of NO2 were taken at Cornelia Fort Airpark in Nashville, TN, during the 1999 Southern Oxidant Study using three different techniques: photolysis to NO followed by chemiluminescence (PCL), laser-induced fluorescence (LIF), and differential optical absorption spectroscopy (DOAS). This was an informal comparison of these techniques conducted during the 1999 Southern Oxidant Study. The PCL and LIF instruments were connected to a common manifold that sampled at the top of a 10-m-walkup tower. The DOAS instrument sampled over a 1.37-km-long light path with end points at 2 and 35 m above ground. The range of NO2 mixing ratios measured was 0.75 ppbv to over 60 ppbv and the median value was nearly 3 ppbv. While preliminary data analysis showed overall agreement between the LIF and PCL instruments to within 1% (least squares slope = 0.99; r2 = 0.98), subsequent analysis revealed a discontinuous shift of about 12% in the PCL data, which was confirmed by comparison to the DOAS data. A leak in the PCL inlet system was the likely cause. After adjustment of the affected PCL data, a comparison of all the coincident measurements showed high correlation (r2 > 0.99) and overall agreement to within 5%. Analysis of the ratios of PCL NO2 to LIF NO2 showed that greater than 90% of individual data points agree to within the total combined instrumental uncertainties. However, the comparison over short time periods is more precise than the average over the campaign. We attribute this to the need for improved PCL instrument data reduction procedures. The two in situ instruments were also operated side by side a year later in Houston, TX, with similar results.
Abstract: During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) aircraft program, ozone depletion events (ODEs) in the high latitude surface layer were investigated using lidar and in situ instruments. Flight legs of 100 km or longer distance were flown 32 times at 30 m altitude over a variety of regions north of 58° between early February and late May 2000. ODEs were found on each flight over the Arctic Ocean but their occurrence was rare at more southern latitudes. However, large area events with depletion to over 2 km altitude in one case were found as far south as Baffin Bay and Hudson Bay and as late as 22 May. There is good evidence that these more southern events did not form in situ but were the result of export of ozone-depleted air from the surface layer of the Arctic Ocean. Surprisingly, relatively intact transport of ODEs occurred over distances of 900–2000 km and in some cases over rough terrain. Accumulation of constituents in the frozen surface over the dark winter period cannot be a strong prerequisite of ozone depletion since latitudes south of the Arctic Ocean would also experience a long dark period. Some process unique to the Arctic Ocean surface or its coastal regions remains unidentified for the release of ozone-depleting halogens. There was no correspondence between coarse surface features such as solid ice/snow, open leads, or polynyas with the occurrence of or intensity of ozone depletion over the Arctic or subarctic regions. Depletion events also occurred in the absence of long-range transport of relatively fresh “pollution” within the high latitude surface layer, at least in spring 2000. Direct measurements of halogen radicals were not made. However, the flights do provide detailed information on the vertical structure of the surface layer and, during the constant 30 m altitude legs, measurements of a variety of constituents including hydroxyl and peroxy radicals. A summary of the behavior of these constituents is made. The measurements were consistent with a source of formaldehyde from the snow/ice surface. Median NOx in the surface layer was 15 pptv or less, suggesting that surface emissions were substantially converted to reservoir constituents by 30 m altitude and that ozone production rates were small (0.15–1.5 ppbv/d) at this altitude. Peroxyacetylnitrate (PAN) was by far the major constituent of NOy in the surface layer independent of the ozone mixing ratio.
Abstract: Routine observations of atmospheric NO2 at concentrations ranging from 0.1 to 100 parts per billion are needed for air quality monitoring and for the evaluation of photochemical models. We have designed, constructed, and field tested a relatively inexpensive and specific NO2 sensor using laser-induced fluorescence. The instrument combines a commercial cw external-cavity tunable diode laser (640 nm) and a continuous supersonic expansion. The total package is completely automated, has a modest size of 0.5 m3 and 118 kg, and could be manufactured at competitive prices with the current generation of instruments. The sensitivity of the instrument is 145 parts per trillion by volume min−1 (signal-to-noise ratio of 2), which is more than adequate for monitoring purposes.
Abstract: Tropospheric O3 concentrations are functions of the chain lengths of NOx (NOx ≡ NO + NO2) and HOx (HOx ≡ OH + HO2 + RO2) radical catalytic cycles. For a fixed HOx source at low NOx concentrations, kinetic models indicate the rate of O3 production increases linearly with increases in NOx concentrations (NOx limited). At higher NOx concentrations, kinetic models predict ozone production rates decrease with increasing NOx (NOx saturated). We present observations of NO, NO2, O3, OH, HO2, H2CO, actinic flux, and temperature obtained during the 1999 Southern Oxidant Study from June 15 to July 15, 1999, at Cornelia Fort Airpark, Nashville, Tennessee. The observations are used to evaluate the instantaneous ozone production rate (PO3) as a function of NO abundances and the primary HOx production rate (PHOx). These observations provide quantitative evidence for the response of PO3 to NOx. For high PHOx (0.5 < PHOx < 0.7 ppt/s), O3 production at this site increases linearly with NO to ∼500 ppt. PO3 levels out in the range 500–1000 ppt NO and decreases for NO above 1000 ppt. An analysis along chemical coordinates indicates that models of chemistry controlling peroxy radical abundances, and consequently PO3, have a large error in the rate or product yield of the RO2 + HO2 reaction for the classes of RO2 that predominate in Nashville. Photochemical models and our measurements can be forced into agreement if the product of the branching ratio and rate constant for organic peroxide formation, via RO2 + HO2 → ROOH + O2, is reduced by a factor of 3–12. Alternatively, these peroxides could be rapidly photolyzed under atmospheric conditions making them at best a temporary HOx reservoir. This result implies that O3 production in or near urban areas with similar hydrocarbon reactivity and HOx production rates may be NOx saturated more often than current models suggest.
Abstract: The capping of stationary source emissions of NOx in 22 states and the District of Columbia is federally mandated by the NOx SIP Call legislation with the intended purpose of reducing downwind O3 concentrations. Monitors for NO, NO2, and the reactive oxides of nitrogen into which these two compounds are converted will record data to evaluate air quality model (AQM) predictions. Guidelines for testing these models indicate the need for semicontinuous measurements as close to real time as possible but no less frequently than once per hour. The measurement uncertainty required for AQM testing must be less than ±20% (±10% for NO2) at mixing ratios of 1 ppbv and higher for NO, individual NOz component compounds, and NOy. This article is a review and discussion of different monitoring methods, some currently used in research and others used for routine monitoring. The performance of these methods is compared with the monitoring guidelines. Recommendations for advancing speciated and total NOy monitoring technology and a listing of demonstrated monitoring approaches are also presented.
Abstract: Measurements of anthropogenic hydrocarbons, ozone, and meteorological variables were obtained during the summer of 1997 near the University of California Blodgett Forest Research Station on the western slope of the Sierra Nevada Mountains. These measurements and related observations obtained upwind in Sacramento and Folsom, California, by the California Air Resources Board and the National Weather Service demonstrate that the transport of the Sacramento plume is controlled by extremely consistent, terrain-driven winds that draw polluted air into the Sierra Nevada by day and flush the mountains at night with clean, continental background air. In effect the plume serves as a mesoscale (100 km) daytime flow reactor where the daily evolution of the Sacramento urban plume can be characterized as a Lagrangian air parcel transported from the urban core into the sparsely populated Sierra Nevada mountains. Using observations of slowly reacting anthropogenic hydrocarbons, we demonstrate that at the peak impact of the Sacramento plume the air at Blodgett Forest can be characterized as a mixture of 32% air from the urban core and 68% from the relatively clean background. From measurements of more reactive hydrocarbons we infer an average daytime OH concentration of 1.1 × 107 molecules cm−3 during the transit of the urban plume.
Abstract: This paper describes a new instrument that uses a combination of thermal dissociation and laser-induced fluorescence detection of NO2 for in situ detection of the sum total peroxy nitrates, the sum total of alkyl nitrates and hydroxyalkyl nitrates, and HNO3. The instrument is capable of routine, continuous in situ measurements of these three classes of compounds that are accurate (15%) with a low detection limit (90 parts per trillion (ppt) 10 s−1, S/N ratio = 2 on a background of 1 ppb NO2 and 30 ppt 10 s−1 on a background of 100 ppt NO2). Theoretical analysis of potential interferences combined with laboratory experiments that test for interferences show that rapidly cooling the gas and dropping the pressure after the thermal dissociation reduces interferences to the order of 1–5%. Observations in ambient air at the University of California Blodgett Forest Research Station demonstrate the capabilities of this instrument under field conditions. These field observations are compared with independent total NOy observations.
Abstract: During the 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission, simultaneous in situ observations of NOx and HOx radicals, their precursors, and the radiation field were obtained in the lower stratosphere. We use these observations to evaluate the primary mechanisms that control NOx−HNO3 exchange and to understand their control over the partitioning between NO2 and HNO3 in regions of continuous sunlight. We calculate NOx production (PNOx) and loss (LNOx) in a manner directly constrained by the in situ measurements and current rate constant recommendations, using approaches for representing albedo, overhead O3 and [OH] that reduce model uncertainty. We find a consistent discrepancy of 18% between modeled rates of NOx production and loss (LNOx = 1.18PNOx) which is within the measurement uncertainty of ±27%. The partitioning between NOx production processes is [HNO3 + OH (41 ± 2)%; HNO3 + hν (59 ± 2)%] and between NOx loss processes is [NO2 + OH, 90% to >97%; BrONO2 + H2O, 10% to <3%]. The steady-state description of NOx−HNO3 exchange reveals the significant influence of the tight correlation between the photolysis rate of HNO3 and [OH] established by in situ measurements throughout the lower stratosphere. Parametrizing this relationship, we find (1) the steady-state value of [NO2]24h-avg/[HNO3] in the continuously sunlit, lower stratosphere is a function only of temperature and number density, and (2) the partitioning of NOx production between HNO3 + OH and HNO3 + hν is nearly constant throughout most of the lower stratosphere. We describe a methodology (functions of latitude, day, temperature, and pressure) for accurately predicting the steady-state value of [NO2]24h-avg/[HNO3] and the partitioning of NOx production within these regions. The results establish a metric to compare observations of [NO2]24h-avg/[HNO3] within the continuously sunlit region and provide a simple diagnostic for evaluating the accuracy of models that attempt to describe the coupled NOx−HOx photochemistry in the lower stratosphere.
Abstract: Extensive measurement campaigns by the NASA ER-2 research aircraft have obtained a nearly pole-to-pole database of the species that control HOx (OH + HO2) chemistry. The wide dynamic range of these in situ measurements provides an opportunity to demonstrate empirically the mechanisms that control the HOx system. Measurements in the lower stratosphere show a remarkably tight correlation of OH concentration with the solar zenith angle (SZA). This correlation is nearly invariant over latitudes ranging from 70° S to 90° N and all seasons. An analysis of the production and loss of HOx in terms of the rate determining steps of reaction sequences developed by Johnston and Podolske and Johnston and Kinnison is used to clarify the behavior of the system and to directly test our understanding of the system with observations. Calculations using in situ measurements show that the production rate of HOx is proportional to O3 and ultraviolet radiation flux. The loss rate is proportional to the concentration and the partitioning of NOy (reactive nitrogen) and the concentration of HO2. In the absence of heterogeneous reactions, the partitioning of NOy is controlled by O3 and HOx and the concentration of HO2 is controlled by NOy and O3, so that the removal rate of OH is buffered against changes in the correlation of O3 and NOy. The heterogeneous conversion of NO2 to HNO3 is not an important net source of HOx because production and removal sequences are nearly balanced. Changes in NOy partitioning resulting from heterogeneous chemistry have a large effect on the loss rates of HOx, but little or no impact on the measured abundance of OH. The enhanced loss rates at high NO2/HNO3 are offset in the data set examined here by enhanced production rates resulting from increased photolysis rates resulting from the decreased O3 column above the ER-2.
Abstract: In situ observations of OH and HO2 from the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA), Stratospheric TRacers of Atmospheric Transport (STRAT), and Polar Ozone Loss in the Arctic Region in Summer (POLARIS) NASA ER-2 field campaigns are used to examine the partitioning of HOx in the lower stratosphere (tropopause to ∼21 km) and upper troposphere (∼10 km to tropopause). These measurements span a latitude range from 70°S to 90°N and a variety of atmospheric conditions as a result of seasonal changes and altitude. The response of the observed [HO2]/[OH] to changes in temperature, [O3], [CO], [NO], [ClO], and [BrO] is investigated. The measured ratio is accurately described (∼±10%) by a steady-state model constrained by the measured mixing ratios of O3, CO, NO, ClO, and BrO, where the model is valid for conditions of HOx cycling much faster than HOx production and loss. The concentration of HO2 depends on [OH], which, to first order, has been observed to be a simple function of the solar zenith angle in the lower stratosphere.
Abstract: Reactions that inter-convert OH and HO2 are directly involved in the catalytic removal of O3 in the lower stratosphere and in the catalytic production of O3 in the upper troposphere. The agreement between the measured and modeled [HO2]/[OH] tests our current understanding of this important chemistry. Recent changes to the recommended rate constants for OH+O3 and HO2+O3 call into question how accurately the chemistry of the stratosphere is understood. [HO2]/[OH] calculated with the new recommendations is 48% higher than the observations throughout the lower stratosphere, exceeding the uncertainty limits of the observations (20%). The extensive atmospheric data set allows tests of the rates of the individual processes that couple these free radicals. This work shows that the discrepancy is largest when the ratio is controlled by the reactions of OH and HO2 with ozone.
Abstract: We examine inorganic chlorine (Cly,) partitioning in the summer lower stratosphere using in situ ER-2 aircraft observations made during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign. New steady state and numerical models estimate [ClONO2]/[HCl] using currently accepted photochemistry. These models are tightly constrained by observations with OH (parameterized as a function of solar zenith angle) substituting for modeled HO2 chemistry. We find that inorganic chlorine photochemistry alone overestimates observed [ClONO2]/[HCl] by approximately 55–60% at mid and high latitudes. On the basis of POLARIS studies of the inorganic chlorine budget, [ClO]/[ClONO2], and an intercomparison with balloon observations, the most direct explanation for the model-measurement discrepancy in Cly, partitioning is an error in the reactions, rate constants, and measured species concentrations linking HCl and ClO (simulated [ClO]/[HCl] too high) in combination with a possible systematic error in the ER-2 ClONO2 measurement (too low). The high precision of our simulation (±15% 1σ for [ClONO2]/[HCl], which is compared with observations) increases confidence in the observations, photolysis calculations, and laboratory rate constants. These results, along with other findings, should lead to improvements in both the accuracy and precision of stratospheric photochemical models.
Abstract: In situ observations of NO2, NO, NOy, ClONO2, OH, O3, aerosol surface area, spectrally resolved solar radiation, pressure and temperature obtained from the ER-2 aircraft during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) experiments are used to examine the factors controlling the fast photochemistry connecting NO and NO2 and the slower chemistry connecting NOx and HNO3. Our analysis uses “chemical coordinates” to examine gradients of the difference between a model and precisely calibrated measurements to provide a quantitative assessment of the accuracy of current photochemical models. The NO/NO2 analysis suggests that reducing the activation energy for the NO+O3 reaction by 1.7 kJ/mol will improve model representation of the temperature dependence of the NO/NO2 ratio in the range 215–235 K. The NOx/HNO3 analysis shows that systematic errors in the relative rate coefficients used to describe NOx loss by the reaction OH + NO2
Abstract: A one-dimensional model of meridional water vapor transport is used to evaluate the factors that control the spatial and temporal variations of oxygen (δ18O) and hydrogen (δD) isotopic ratios in global precipitation. The model extends Rayleigh descriptions of isotopes in precipitation by including (1) effects of recharge to air masses by evaporation and (2) horizontal transport by both eddy fluxes and advection. Globally, spatial variations in precipitation δ18O and δD depend on the ratio of evaporation to the product of horizontal moisture flux and horizontal temperature gradient. At low latitudes, where this ratio is large, precipitation δ18O and δD are closely tied to the isotopic ratios of oceanic evaporation. At high latitudes the ratio is small, and δ18O and δD are controlled by the ratio of advective transport to eddy transport. Transport by eddy fluxes induces less fractionation than transport by advection, resulting in a smaller gradient of isotopic ratios with temperature. The model-predicted temporal relationships between δ18O (or δD) of Antarctic precipitation and temperature do not necessarily coincide with the modern spatial relationship and depend strongly on the proximity of the precipitation site to the ocean evaporation source. Sensitivity of δ18O to temporal changes in local surface temperature is low at coastal sites and increases with distance inland. These results suggest a possible explanation of the apparent discrepancy between borehole temperature inversion estimates of glacial temperatures and temperatures inferred from the modern spatial δ18O—surface temperature relationship.
Abstract: In situ measurements of radical and long-lived species were made in the lower Arctic stratosphere (18 to 20 km) between spring and early autumn in 1997. The measurements include O3, ClO, OH, HO2, NO, NO2, N2O, CO, and overhead O3. A photochemical box model constrained by these and other observations is used to compute the diurnally averaged destruction and production rates of O3 in this region. The rates show a strong dependence on solar exposure and ambient O3. Total destruction rates, which reach 19%/month in summer, reveal the predominant role of NOx and HOx catalytic cycles throughout the period. Production of O3 is significant only in midsummer air parcels. A comparison of observed O3 changes with destruction rates and transport effects indicates the predominant role of destruction in spring and an increased role of transport by early autumn.
Abstract: We describe a time-gated laser-induced fluorescence instrument designed for accurate (±5%, 1σ), continuous, autonomous, in situ observations of NO2 with the sensitivity (15 ppt/10 s at S/N = 2) and portability necessary to study NO2 anywhere in the troposphere. The technique is advantageous because it is spectroscopically specific and direct in that it does not require conversion of NO2 into another species (e.g., NO) prior to detection, eliminating a class of potential interferences. Performance of the instrument is illustrated with 15 weeks (July−Oct 1998) of observations at the University of California, Blodgett Forest field station located in the foothills of the Sierra Nevada and 4 weeks (June 15 − July 15, 1999) in Nashville, TN during the Southern Oxidants Study. Ambient concentrations of NO2 at Blodgett Forest varied from below 50 ppt to 4000 ppt and NO2 ranged from 5 to 50% of the total reactive nitrogen; while in Nashville, TN, concentrations ranged from 1 to 75 ppb.
Abstract: Abstract: We use the first simultaneous in situ measurements of ClONO2, ClO, and HCl acquired using the NASA ER-2 aircraft during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission to test whether these three compounds quantitatively account for total inorganic chlorine (Cly) in the lower stratosphere in 1997. We find (ClO + ClONO2 + HCl)/Cly = 0.92±0.10, where Cly is inferred from in situ measurements of organic chlorine source gases. These observations are consistent with our current understanding of the budget and partitioning of Cly in the lower stratosphere. We find no evidence in support of missing inorganic chlorine species that compose a significant fraction of Cly. We apply the analysis to earlier ER-2 observations dating from 1991 to investigate possible causes of previously observed discrepancies in the inorganic chlorine budget. Using space shuttle, satellite, balloon, and aircraft measurements in combination with ER-2 data, we find that the discrepancy is unlikely to have been caused by missing chlorine species or an error in the photolysis rate of chlorine nitrate. We also find that HCl/Cly is not significantly controlled by aerosol surface area density in the lower stratosphere.
Abstract: The first in situ measurements of ClONO2 in the lower stratosphere, acquired using the NASA ER-2 aircraft during the Polar Ozone Loss in the Arctic Region in Summer (POLARIS) mission, are combined with simultaneous measurements of ClO, NO2, temperature, pressure, and the calculated photolysis rate coefficient (JClONO2) to examine the balance between production and loss of ClONO2. The observations demonstrate that the ClONO2 photochemical steady state approximation, [ClONO2]PSS = k × [ClO] × [NO2] / JClONO2, is in good agreement with the direct measurement, [ClONO2]MEAS. For the bulk of the data (80%), where T>220 K and latitudes >45°N, [ClONO2]PSS = 1.15±0.36 (1σ) × [ClONO2]MEAS, while for T<220 K and latitudes <45°N the result is somewhat less at 1.01±0.30. The cause of the temperature and/or latitude trend is unidentified. These results are independent of solar zenith angle and air density, thus there is no evidence in support of a pressure-dependent quantum yield for photodissociation of ClONO2 at wavelengths >300 nm. These measurements confirm the mechanism by which active nitrogen (NOx = NO + NO2) controls the abundance of active chlorine (Clx = ClO + Cl) in the stratosphere.
Abstract: Volume mixing ratio profiles of HCl, HOCl, ClNO3, CH3Cl, CFC-12, CFC-11, CCl4, HCFC-22, and CFC-113 were measured simultaneously from 9 to 38 km by the Jet Propulsion Laboratory MkIV Fourier Transform Infrared solar absorption spectrometer during two balloon flights from Fairbanks, Alaska (64.8°N), on May 8 and July 8, 1997. The altitude variation of total organic chlorine (CCly), total inorganic chlorine (Cly), and the nearly constant value (3.7±0.2 ppbv) of their sum (ClTOT) demonstrates that the stratospheric chlorine species available to react with O3 are supplied by the decomposition of organic chlorinated compounds whose abundances are well quantified. Measured profiles of HCl and ClNO3 agree well with profiles found by photochemical model (differences <10% for altitudes below 35 km) constrained by various other constituents measured by MkIV. The production of HCl by ClO + OH plays a relatively small role in the partitioning of HCl and ClNO3 for the sampled air masses. However, better agreement with the measured profiles of HCl and ClNO3 is obtained when this source of HCl is included in the model. Both the measured and calculated [ClNO3]/[HCl] ratios exhibit the expected near linear variation with [O3]2/[CH4] over a broad range of altitudes. MkIV measurements of HCl, ClNO3, and CCly agree well with ER-2 in situ observations of these quantities for directly comparable air masses. These results demonstrate good understanding of the budget of stratospheric chlorine and that the partitioning of inorganic chlorine is accurately described by photochemical models that employ JPL97 reaction rates and production of HCl from ClO + OH for the environmental conditions encountered: relatively warm temperatures, long periods of solar illumination, and relatively low aerosol surface areas.
Abstract: Observations of high NOx/NOy ratios (overall 40% larger than modelled values) during the Polar Ozone Loss in the Arctic Region in Summer campaign have led us to re-examine the heterogeneous chemistry of stratospheric aerosol particles during the polar summer period, using the Integrated MicroPhysics and Aerosol Chemistry on Trajectories model. The warm summer temperatures (up to 235 K) imply very concentrated sulphuric acid solutions (80 wt %). On the one hand, these solutions are more likely to freeze, into sulphuric acid monohydrate (SAM), reducing the efficiency of the N2O5 hydrolysis reaction. Including this freezing process increases NOx/NOy ratios but does not improve model/measurement agreement: in polar spring, SAM formation causes the NOx/NOy ratio to be overpredicted whereas freezing has a much smaller effect on nitrogen chemistry during the continuous solar exposure of polar summer. On the other hand, if sulphate aerosols remain liquid, the high acidity may promote acid-catalysed reactions. The most important reaction is CH2O + HNO3, which effectively increases NOx/NOy ratios across a wide range of conditions, improving agreement with measurements. Furthermore, the production of HONO can either enhance gas-phase OH concentrations or promote secondary liquid reactions, including HONO + HNO3 and HONO + HCl. Primary uncertainties include the uptake coefficient of CH2O relevant to reaction with HNO3, the amount of HONO available for secondary reaction, and the relative rates of HONO reaction with HNO3 and HCl. The fate of the formic acid product, whose presence in the stratosphere may be an indicator for the CH2O reaction, and the impact on the stratospheric hydrogen budget are also discussed.
Abstract: Stratospheric measurements of NO, NO2, O3, ClO, and HO2 were made during spring, early summer, and late summer in the Arctic region during 1997 as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) field campaign. In the sunlit atmosphere, NO2 and NO are in steady state through NO2 photolysis and reactions involving O3, ClO, BrO, and HO2. By combining observations of O3, ClO, and HO2, observed and modeled values of the NO2 photolysis rate coefficient (JNO2), and model estimates of BrO, several comparisons are made between steady state and measured values of both NO2 and JNO2. An apparent seasonal dependence in discrepancies between calculated and measured values was found; however, a source for this dependence could not be identified. Overall, the mean linear fits in the various comparisons show agreement within 19%, well within the combined uncertainties (±50 to 70%). These results suggest that photochemistry controlling the NO2/NO abundance ratio is well represented throughout much of the sunlit lower stratosphere. A reduction in the uncertainty of laboratory determinations of the rate coefficient of NO + O3 → NO2 + O2 would aid future analyses of these or similar atmospheric observations.
Abstract: On May 8, 1997, vertical profiles of over 30 different gases were measured remotely in solar occultation by the Jet Propulsion Laboratory MkIV Interferometer during a balloon flight launched from Fairbanks, Alaska. These gases included H2O, N2O, CH4, CO, NOx, NOy, HCl, ClNO3, CCl2F2, CCl3F, CCl4, CHClF2, CClF2CCl2F, SF6, CH3Cl, and C2H6, all of which were also measured in situ by instruments on board the NASA ER-2 aircraft, which was making flights from Fairbanks during this same early May time period as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) experiment. A comparison of the gas volume mixing ratios in the upper troposphere and lower stratosphere reveals agreement better than 5% for most gases. The three significant exceptions to this are SF6 and CCl4 for which the remote measurements exceed the in situ observations by 15–20% at all altitudes, and H2O for which the remote measurements are up to 30% smaller than the in situ observations near the hygropause.
Abstract: This paper describes recent measurements of carbonaceous aerosol made by wire impactors during the Photochemistry Ozone Loss in the Arctic Region in Summer (POLARIS) campaign and assesses their role in stratospheric photochemistry. Ninety-five percent of the carbonaceous aerosol collected during this campaign was in the form of black carbon aerosol (BCA), or soot. A new method of analyzing impactor samples is described that accounts for particle bounce and models the BCA as fractal aggregates to modify the aerodynamic collection efficiency and determine particle surface area. Results are compared to previously used methods. The new method results in an increase in the measured BCA number density of 4 times, surface area density of ∼15 times, and an increase in mass loading of 6.15 times over one previously used approach. Average values of number, surface area, and mass densities are 0.06 no./cm3,0.03 μm2/ cm3, and 0.64 ng/m3, respectively. BCA number densities are ∼1% of total aerosol number density, and BCA surface area density is ∼10% of the measured sulfuric acid aerosol surface area. Including heterogeneous reactions on BCA in a photochemical model can affect photochemistry leading to renoxification and increased ozone depletion. However, these predicted effects are not supported by the POLARIS observations, in particular, the NOx/NOy ratios. The laboratory data is not conclusive enough to determine to what extent the heterogeneous reaction is catalytic or carbon consuming. Including catalytic reactions on BCA does not statistically improve the agreement between model and measurement in any of the several scenarios considered. Furthermore, if the reactions cause even partial carbon oxidation, the BCA would be consumed at a rate inconsistent with POLARIS observations. These inconsistencies lead us to conclude that the presence of BCA in the stratosphere did not affect stratospheric photochemistry during POLARIS.
Abstract: This report assesses the potential atmospheric impacts of a proposed fleet of high-speed civil transport (HSCT) aircraft. The purpose of the report is to assess the effects of HSCT's on atmospheric composition and climate in order to provide a scientific basis for making technical, commercial, and environmental policy decisions regarding the HSCT fleet. The work summarized here was carried out as part of NASA's Atmospheric Effects of Aviation Project (a component of the High-Speed Research Program) as well as other NASA, U.S., and international research programs. The principal focus is on change in stratospheric ozone concentrations. The impact on climate change is also a concern. The report describes progress in understanding atmospheric processes, the current state of understanding of HSCT emissions, numerical model predictions of HSCT impacts, the principal uncertainties in atmospheric predictions, and the associated sensitivities in predicted effects of HSCT's.
Abstract: Measurements of the concentrations of OH and HO2 (HOx) in the high-latitude lower stratosphere imply the existence of unknown photolytic sources of HOx. The strength of the additional HOx source required to match the observations depends only weakly on solar zenith angle (SZA) for 80° < SZA < 93°. The wavelengths responsible for producing this HOx must be longer than 650 nm because the flux at shorter wavelengths is significantly attenuated at high SZA by scattering and absorption. Provided that the sources involve only a single photon, the strength of the bonds being broken must be < 45 kcal mole−1. We speculate that peroxynitric acid (HNO4) dissociates after excitation to an unknown excited state with an integrated band cross section of 2-3 × 10−20 cm² molecule−1 nm (650 < λ < 1250 nm).
Abstract: Extensive airborne measurements of the reactive nitrogen reservoir (NOy) and its component nitric oxide (NO) have been made in the lower stratosphere. Box model simulations that are constrained by observations of radical and long-lived species and which include heterogeneous chemistry systematically underpredict the NOx (= NO + NO2) to NOy ratio. The model agreement is substantially improved if newly measured rate coefficients for the OH + NO2 and OH + HNO3 reactions are used. When included in 2-D models, the new rate coefficients significantly increase the calculated ozone loss due to NOx and modestly change the calculated ozone abundances in the lower stratosphere. Ozone changes associated with the emissions of a fleet of supersonic aircraft are also altered.
Abstract: Simultaneous in situ measurements of HCl and ClO have been made for the first time in the southern hemisphere, allowing a systematic study of the processes governing chlorine activation between 15 and 20 km in the 1994 Antarctic winter. Data for several other gases (O3, NO, NOy, OH, HO2, N2O, CH4, CO, H2O, CFCs), particulates, and meteorological parameters were collected from the ER-2 aircraft out of New Zealand as part of the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements of Atmospheric Effects of Stratospheric Aircraft (ASHOE/MAESA) campaign. Observations from the ER-2 in the fall (April–May), prior to polar night, show that chlorine activation begins with 60–75% of inorganic chlorine as HCl. By midwinter (July–August), near-total removal of HCl is observed. The wintertime loss of HCl in air recently exposed to extreme temperatures is found to be correlated with high levels of reactive chlorine (ClO and its dimer, Cl2O2) in the linear fashion expected from the stoichiometry of the heterogeneous reaction of hydrochloric acid with chlorine nitrate on polar stratospheric clouds (PSCs): HCl + ClONO2 → Cl2 + HNO3. To constrain the role of different heterogeneous reactions and PSC types, we have used a photochemical trajectory model which includes heterogeneous sulfate and PSC chemistry. Model calculations of the evolution of reactive gases are compared with the in situ observations. In addition, simultaneous measurements of OH and HO2 are used as a diagnostic for the occurrence of the heterogeneous reaction HOCl + HCl → Cl2 + H2O, which contributes to suppressed levels of HOx inside the vortex. It is shown that the amount of chlorine activation is not strongly dependent on the composition of PSCs. However, HOx levels exhibit different signatures depending on the type of heterogeneous surfaces that affected chlorine activation. Furthermore, this analysis implies that in the edge region of the Antarctic vortex, the observed near-total removal of HCl can result from latitudinal excursions of air parcels in and out of sunlight during the winter, which photochemically resupply HOCl and ClONO2 as oxidation partners for HCl.
Abstract: The generation of sulfuric acid aerosols in aircraft exhaust has emerged as a critical issue in determining the impact of supersonic aircraft on stratospheric ozone. It has long been held that the first step in the mechanism of aerosol formation is the oxidation of SO2 emitted from the engine by OH in the exhaust plume. We report in situ measurements of OH and HO2 in the exhaust plumes of a supersonic (Air France Concorde) and a subsonic (NASA ER-2) aircraft in the lower stratosphere. These measurements imply that reactions with OH are responsible for oxidizing only a small fraction of SO2 (2%), and thus cannot explain the large number of particles observed in the exhaust wake of the Concorde.
Abstract: During the 1993 NASA Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE), anomalously low nitric oxide (NO) was found in a distinct sunlit layer located above the mid-latitude tropopause. The presence of a significant amount of reactive nitrogen (NOy) in the layer implies the systematic removal of NO, which is without precedent in stratospheric in situ observations. Large increases in measured chlorine monoxide (ClO) and the hydroperoxyl radical (HO2) also were observed in the layer. Heterogeneous reaction rate constants of chlorine nitrate (ClONO2) with hydrogen chloride (HCl) and H2O to form nitric acid (HNO3) on sulfate aerosol are enhanced in the NO removal layer by local increases in H2O and aerosol surface area. The associated conversion of NOx (= NO + NO2) to HNO3 is the most likely cause of the observed low NO and NOx/NOy values and high ClO values.
Abstract: Recent aircraft and balloon borne measurements of OH and H02 are reviewed. The authors demonstrate the ability of the laser-induced fluorescence technique to provide accurate, high signal to noise ratio measurements of OH throughout the upper troposphere and stratosphere. HO2 is measured as OH after gas phase chemical titration with nitric oxide. The addition of the HOx measurement capability to the suite of instruments aboard the NASA ER-2 aircraft has provided a wealth of new information about the processes that determine the concentration of ozone in the lower stratosphere. These simultaneous, in situ measurements provide a unique test of our understanding of the mechanisms that control the odd-hydrogen chemistry of the lower atmosphere.
Abstract: The response of ClO concentrations to changes in NO2 concentrations has been inferred from simultaneous observations of [ClO], [NO], [NO2] and [O3] in the mid-latitude lower stratosphere. This analysis demonstrates that [ClO] is inversely correlated with [NO2], consistent with formation and photolysis of [ClONO2]. A factor of ten range in the concentration of NO2 was sampled (0.1 to 1×109 mol/cm³), with a comparable range in the ratio of [ClO] to total available inorganic chlorine (1% ≤ [ClO]/[Cly] ≤ 5%). This analysis leads to an estimate of [ClONO2]/[Cly] = 0.12 (×/÷2), in the mid-latitude, lower-stratospheric air masses sampled.
Abstract: In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained through sunrise and sunset in the lower stratosphere during SPADE are compared to results from a photochemical model constrained by observed concentrations of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N2O5 on sulfate aerosols agree with measured concentrations of NO, NO2, and ClO throughout the day, but fail to account for high concentrations of OH and HO2 observed near sunrise and sunset. The morning burst of [OH] and [HO2] coincides with the rise of [NO] from photolysis of NO2, suggesting a new source of HOx that photolyzes in the near UV (350 to 400 nm) spectral region. A model that allows for the heterogeneous production of HNO2 results in an excellent simulation of the diurnal variations of [OH] and [HO2].
Abstract: In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained in the lower stratosphere during SPADE are compared to results from a photochemical model that assimilates measurements of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N2O5 agree well with measured concentrations of NO and ClO, but concentrations of HO2 and OH are underestimated by 10 to 25%, concentrations of NO2 are overestimated by 10 to 30%, and concentrations of HCl are overestimated by a factor of 2. Discrepancies for [OH] and [HO2] are reduced if we allow for higher yields of O(1D) from O3 photolysis and for heterogeneous production of HNO2. The data suggest more efficient catalytic removal of O3 by hydrogen and halogen radicals relative to nitrogen oxide radicals than predicted by models using recommended rates and cross sections. Increases in [O3] in the lower stratosphere may be larger in response to inputs of NOy from supersonic aircraft than estimated by current assessment models.
Abstract: The Stratospheric Photochemistry, Aerosols, and Dynamics Expedition (SPADE) made in situ observations of the composition of the lower stratosphere from the NASA ER-2 aircraft at latitudes from 15°N to 60°N, during November 1992 and April, May and October 1993. SPADE followed the Airborne Arctic Stratospheric Expedition II (AASE-II, September 1991 to March 1992) by 8 months. Together the two missions provide a record of stratospheric trace species and aerosols at middle and high latitudes spanning the input and decay of debris from the eruption of Mt. Pinatubo. New instruments deployed for SPADE include sensors to measure OH, HO2, H2O, CO2, NO2 and the UV/visible radiation field, complementing sensors previously deployed on the ER-2 (ClO, BrO, NO, NOy, N2O, O3, H2O, HCl, CH4, CFC-11, CFC-113, and aerosol number and size distribution). The data provide the first simultaneous in situ measurements of radicals and reservoir species to include representatives from all the important families of stratospheric reactants, observed as functions of time of day (at ∼18 km), latitude and altitude (15–20 km). The results place strong new constraints on models of stratospheric photochemistry. For example, measurements of the radicals HO2, NO2, ClO and BrO allow for a nearly completely empirical evaluation of local rates for photochemical removal of ozone (by known catalytic cycles).
Highly precise observations of the seasonal cycle and interannual changes in CO2, combined with data for N2O and other tracer species, provide new insights into rates for transport in the lower stratosphere. High resolution data obtained during SPADE by the ER-2 (in its own wake) and in AASE-II by the DC-8 (in wakes from commercial aircraft) provide support for engineering models of NOx emissions from subsonic jet aircraft at cruise conditions. Observations from the DC-8 during AASE-II, defining global distributions of NOx and NOy near the tropopause, and of HF, HCl, ClNO3, and HNO3 column abundances, provide new information on the processes influencing polar ozone loss.
Abstract: Measurements of the ratio OH/HO2, NO, O3, ClO, and BrO were obtained at altitudes from 15–20 km and latitudes from 15–60°N. A method is presented for interpreting these simultaneous in situ observations that constrains the rates of chemical transformations that 1) are responsible for over half the ozone removal rate in the lower stratosphere via reactions of HO2 and 2) control the abundance of HO2 through coupling to nitrogen and halogen radicals. The results show our understanding of the chemical reactions controlling the partitioning of OH and HO2 is complete and accurate and that the potential effects of “missing chemistry” are strictly constrained in the region of the atmosphere encompassed by the observations. The analysis demonstrates that the sensitivity of the ratio OH/HO2 to changes in NO is described to within 12% by current models. This reduces by more than a factor of 2 the effect of uncertainty in the coupling of hydrogen and nitrogen radicals on the analysis of the potential effects of perturbations to odd nitrogen in the lower stratosphere.
Abstract: The potential energy surface for the He–CO van der Waals interaction is calculated by the supermolecular approach using fourth–order Mo/ller–Plesset perturbation theory (MP4) with a large basis set containing bond functions. The rovibrational energies of He–CO are then calculated by the collocation method. Our ab initiosurface has a single near T–shaped minimum (R m =3.49 Å, θ=120°, V m =−20.32 cm−1), in agreement with a recent experimental potential (R=3.394 Å, θ=121.3°, V m =−22.91 cm−1), determined from high–resolution spectroscopic measurements, but significantly different from a previously published ab initiosurface for this system. The calculated rovibrational energies are in good agreement with experiment. The explicit dependence of the intermolecular potential on the CO bond distance is also studied by MP4 calculations, and the results confirm the experimental observation that the intermolecular coordinates are approximately uncoupled from the CO bond distance.
Abstract: Simultaneous in situ measurements of the concentrations of OH, HO2, ClO, BrO, NO, and NO2 demonstrate the predominance of odd-hydrogen and halogen free-radical catalysis in determining the rate of removal of ozone in the lower stratosphere during May 1993. A single catalytic cycle, in which the rate-limiting step is the reaction of HO2 with ozone, accounted for nearly one-half of the total O3 removal in this region of the atmosphere. Halogen-radical chemistry was responsible for approximately one-third of the photochemical removal of O3; reactions involving BrO account for one-half of this loss. Catalytic destruction by NO2, which for two decades was considered to be the predominant loss process, accounted for less than 20 percent of the O3 removal. The measurements demonstrate quantitatively the coupling that exists between the radical families. The concentrations of HO2 and ClO are inversely correlated with those of NO and NO2. The direct determination of the relative importance of the catalytic loss processes, combined with a demonstration of the reactions linking the hydrogen, halogen, and nitrogen radical concentrations, shows that in the air sampled the rate of O3 removal was inversely correlated with total NOx loading.
Abstract: The three–dimensional intermolecular potential energy surface (IPS) for Ar–NH3 has been determined from a least–squares fit to 61 far infrared and microwave vibration–rotation–tunneling (VRT) measurements and to temperature–dependent second virial coefficients. The three intermolecular coordinates (R,θ,φ) are treated without invoking any approximations regarding their separability, and the NH3 inversion–tunneling motion is included adiabatically. A surface with 13 variable parameters has been optimized to accurately reproduce the spectroscopic observables, using the collocation method to treat the coupled multidimensional dynamics within a scattering formalism. Anisotropy in the IPS is found to significantly mix the free rotor basis functions. The 149.6 cm−1 global minimum on this surface occurs with the NH3 symmetry axis nearly perpendicular to the van der Waals bond axis (θ=96.6°), at a center–of–mass separation of 3.57 Å, and with the Ar atom midway between two of the NH3 hydrogen atoms (φ=60°). The position of the global minimum is very different from the center–of–mass distance extracted from microwave spectroscopic studies. Long–range (R≥3.8 Å) attractive interactions are greatest when either a N–H bond or the NH3 lone pair is directed toward the argon. Comparisons with ab initiosurfaces for this molecule as well as the experimentally determined IPS for Ar–H2O are presented.
Abstract: The odd–hydrogen radicals OH and HO2 are central to most of the gas–phase chemical transformations that occur in the atmosphere. Of particular interest is the role that these species play in controlling the concentration of stratosphericozone. This paper describes an instrument that measures both of these species at volume mixing ratios below one part in 1014 in the upper troposphere and lower stratosphere. The hydroxyl radical (OH) is measured by laser induced fluorescence at 309 nm. Tunable UV light is used to pump OH to the first electronic state (A 2Σ+(v’=1) ← X2Π3/2 (v‘=0)) near 282 nm. The laser light is produced by a high–repetition rate pulsed dye–laser powered with all solid–state pump lasers. HO2 is measured as OH after gas–phase titration with nitric oxide. Measurements aboard a NASA ER–2 aircraft demonstrate the capability of this instrument to perform reliably with very high signal–to–noise ratios (≥30) achieved in short integration times (< 20 sec).
Abstract: Thirteen vibration–rotation–tunneling (VRT) bands of the CH4–H2O complex have been measured in the range from 18 to 35.5 cm−1 using tunable far infrared laserspectroscopy. The ground state has an average center of mass separation of 3.70 Å and a stretching force constant of 1.52 N/m, indicating that this complex is more strongly bound than Ar–H2O. The eigenvaluespectrum has been calculated with a variational procedure using a spherical expansion of a site–site ab initio intermolecular potential energy surface [J. Chem. Phys. 93, 7808 (1991)]. The computed eigenvalues exhibit a similar pattern to the observed spectra but are not in quantitative agreement. These observations suggest that both monomers undergo nearly free internal rotation within the complex.
Abstract: A new highly accurate and detailed intermolecular potentialsurface for Ar–H2O is derived by a direct nonlinear least squares fit to 37 far infrared, infrared, and microwave spectroscopicmeasurements. The new potential (denoted AW2) gives a much better description of the strong radial dependence of the anisotropic forces and of the binding energy than its predecessor, the AW1 surface [Cohen and Saykally, J. Phys. Chem. 94, 7991 (1990)]. The global minimum on the AW2 potential (D e =142.98 cm−1) occurs at the position R=3.636 Å, θ=74.3°, and φ=0°. At these coordinates the argon is located in the monomer plane between the perpendicular to the C2 axis (θ=90°) and the hydrogen bonded geometry (θ=55°). This orientation of the minimum is opposite of that found in recent ab initio calculations of Bulski et al. [J. Chem. Phys. 94, 8097 (1991)] and Chalisinski et al. [J. Chem. Phys. 94, 2807 (1991)]. Both sets of authors find a minimum at an antihydrogen bonded geometry corresponding to an orientation Ar–OH (θ=125°).
Abstract: We have measured and assigned more than 800 new far–infrared absorption lines and 12 new microwave absorption lines of the ammonia dimer. Our data are analyzed in combination with all previously measured far–infrared and microwave spectra for this cluster. The vibration–rotation–tunneling (VRT) states of the ammonia dimer connected by electric–dipole–allowed transitions are separated into three groups that correspond to different combinations of monomer rotational states: A+A states (states formed from the combination of two ammonia monomers in A states), A+E states, and E+E states. We present complete experimentally determined energy–level diagrams for the K a =0 and K a =1 levels of each group in the ground vibrational state of this complex. From these, we deduce that the appropriate molecular symmetry group for the ammonia dimer is G 144. This, in turn, implies that three kinds of tunneling motions are feasible for the ammonia dimer: interchange of the ‘‘donor’’ and ‘‘acceptor’’ roles of the monomers, internal rotation of the monomers about their C 3 symmetry axes, and quite unexpectedly, ‘‘umbrella’’ inversion tunneling.
In the K a =0 A+E and E+E states, the measured umbrella inversion tunneling splittings range from 1.1 to 3.3 GHz. In K a =1, these inversion splittings between two sets of E+E states are 48 and 9 MHz, while all others are completely quenched. Another surprise, in light of previous analyses of tunneling in the ammonia dimer, is our discovery that the interchange tunneling splittings are large. In the A+A and E+E states, they are 16.1 and 19.3 cm−1, respectively. In the A+E states, the measured 20.5 cm−1 splitting can result from a difference in ‘‘donor’’ and ‘‘acceptor’’ internal rotation frequencies that is increased by interchange tunneling. We rule out the possibility that the upper state of the observed far–infrared subbands is the very–low–frequency out–of–plane intermolecular vibration predicted in several theoretical studies [C. E. Dykstra and L. Andrews, J. Chem. Phys. 92, 6043 (1990); M. J. Frisch, J. E. Del Bene, J. S. Binkley, and H. F. Schaefer III, ibid. 84, 2279 (1986)]. In their structure determination, Nelson et al. assumed that monomer umbrella inversion tunneling was completely quenched and that ‘‘donor–acceptor’’ interchange tunneling was nearly quenched in the ammonia dimer [D. D. Nelson, G. T. Fraser, and W. Klemperer, J. Chem. Phys. 83, 6201 (1985); D. D. Nelson, W. Klemperer, G. T. Fraser, F. J. Lovas, and R. D. Suenram, ibid. 87, 6364 (1987)]. Our experimental results, considered together with the results of six–dimensional calculations of the VRT dynamics presented by van Bladel et al. in the accompanying paper [J. Chem. Phys. 97, 4750 (1992)], make it unlikely that the structure proposed by Nelson et al. for the ammonia dimer is the equilibrium structure.
Abstract: Measurements of the low-frequency van der Waals vibrations in weakly bound complexes by high-resolution laser spectroscopy provide a means to probe intermolecular forces at unprecedented levels of detail and precision. Several new methods are presently being used to record vibration-rotation-tunneling (VRT) transitions associated with the motions of the weak bonds in van der Waals clusters. The most direct measurements are those probing only the van der Waals modes themselves, which occur at far infrared wavelengths. This article presents a review of the information on both intermolecular forces and intramolecular dynamics that has been obtained from far-infrared VRT spectra of 18 complexes during the part several years. Some rotationally resolved measurements of van der Waals modes observed in combination with electronic or vibrational excitation are also discussed.
Abstract: Five new vibration–rotation tunneling states of Ar–H2O [the Σ and Π(111) and the Σ and Π(212) internal rotor states and the n=1, Π(101) stretching–internal rotor combination level] have been accessed by tunable far–infrared laser spectroscopy. The measured vibrational band origins of transitions to these states are within 2% of predictions made from an anisotropic three–dimensional intermolecular potentialsurface (denoted AW1) derived from a nonlinear least–squares fit to previous far–infrared spectral data [J. Phys. Chem. 9 4, 7991(1990)]. This provides strong evidence that the AW1 intermolecular potentialsurface incorporates much of the essential physics of the intermolecular forces which bind the cluster. However, larger deviations from the predictions are found in the observed rotational term values. A detailed analysis of these deviations clearly demonstrates the need for even stronger angular–vradial coupling in the Ar–H2O intermolecular potential than the already substantial coupling present in the AW1 surface. Specifically, the presently observed Σ(111) state and the n=1, Σ(000) state are found to be approximately 65:35 mixtures of the basis states which represent pure stretching and internal rotation. The Σ(212) level is found to be mixed just as strongly with n=2, Σ(101). The formalism for accurately deperturbing vibration–rotation–tunneling states coupled by Coriolis interactions used in the above analysis is presented.
Abstract: The state of the art in far infrared (FIR) spectroscopy is reviewed. The development of tunable, coherent FIR radiation sources is discussed. Applications of tunable FIR laserspectrometers for measurement of rotational spectra and dipole moments of molecular ions and free radicals, vibration–rotation–tunneling (VRT) spectra of weakly bound complexes, and vibration–rotation spectra of linear carbon clusters are presented. A detailed description of the Berkeley tunable FIR laserspectrometers is presented in the following article.
Abstract: A detailed description is presented for a tunable far infrared laserspectrometer based on frequency mixing of an optically pumped molecular gas laser with tunable microwave radiation in a Schottkypoint contactdiode. The system has been operated on over 30 laser lines in the range 10–100 cm−1 and exhibits a maximum absorption sensitivity near one part in 106. Each laser line can be tuned by ±110 GHz with first–order sidebands. Applications of this instrument are detailed in the preceding paper.
Abstract: Two new intermolecular vibration–rotation–tunneling (VRT) bands of Ar–NH3 have been measured using tunable far infrared laserspectroscopy. We have unambiguously assigned these and a previously measured FIR band [Gwo e t a l., Mol. Phys. 7 1, 453 (1990)] as Π(10, n=0)←Σ(00, n=0), Σ(10, n=0)←Σ(00, n=0), and Σ(00, n=1)←Σ(00, n=0). The three upper states of these are found to be strongly mixed by anisotropy and Coriolis effects. A simultaneous least squares fit of all transitions has yielded vibrational frequencies, rotational and centrifugal distortion constants, and a Coriolis parameter as well as quadrupole hyperfine coupling constants for the upper states. An effective angular potential energy surface for Ar–NH3 in its lowest stretching state has been determined from these data, after explicitly accounting for the effects of bend stretch interactions. Features of the surface include a global minimum at the near T–shaped configuration (θ=90°), a 30 cm−1 to 60 cm−1 barrier to rotation at θ=180° (or 0°), and a very low barrier or possibly a secondary minimum at θ=0° (or 180°). Both attractive and repulsive interactions are shown to contribute significantly to the anisotropic forces in the complex. Comparison with ab initio calculations are presented.
Abstract: Over 150 lines in six tunneling subbands of an intermolecular vibration located near 25 cm−1 have been measured with partial hyperfine resolution and assigned to (NH3)2. The transitions sample all three types of tunneling states (A, G, E) and are consistent with the following assumptions: (1) G36 is the appropriate molecular symmetry group; (2) the equilibrium structure contains a plane of symmetry; (3) interchange tunneling of inequivalent monomers occurs via a t r a n s path; (4) the 2C3+I limit of hydrogen exchange tunneling is appropriate; (5) tunneling and rotational motions are separable. A qualitative vibration–rotation tunneling energy level diagram is presented. Strong perturbations are observed among the states of E symmetry. This work supports the conclusions of Nelson e t a l. [J. Chem. Phys. 8 7, 6365 (1987)].
Abstract: Hyperfine resolved spectra have been measured for the Ar-NH3 complex over the range 21–28 cm-1 using a tunable far-infrared laser in combination with a continuous planar supersonic jet. Twenty-three transitions are assigned to the lowest Σ-bending vibration (v 0 = 26·470633(77)cm-1) from the (asymmetric) inversion level of the Σ, j k c = 00 internal rotor state of NH3 to the Σ, (10) symmetric inversion level. A nearly free-rotor model is used to deduce the zeroth-order intermolecular vibrational energy-level diagram and corresponding selection rules, which are used in assigning the spectra. All evidence obtained in this study supports the contention of Nelson et al. that NH3 is an inverting nearly free rotor in this T-shaped complex.
Abstract: We present an extension of the collocation method developed by Peet and Yang (J. Chem. Phys. 1989,91,6598) for calculating the bound states of rotating atom-diatom systems to atom-polyatom complexes. The method is shown to be general, accurate, efficient, and straightforward to implement. The collocation algorithm is incorporated into a nonlinear least-squares program, which is used in a direct fit of far-infrared vibration-rotation spectra of the Ar-H2O complex to a detailed analytical model for the anisotropic intermolecular potential energy surface. The surface denoted AW 1 was obtained without any dynamical approximations. The minimum (De, = 174.7 cm-1, Re = 3.598 Å) in the intermolecular potential surface occurs for the argon located in the plane of the H2O, nearly perpendicular to the symmetry axis.
Abstract: Seven rovibrational transitions of the (01(1)0) ← (00(0)0)fundamental bending band of C3 have been measured with high precisionwith the use of a tunable far-infrared laser spectrometer. The C3molecules were produced by laser vaporization of a graphite rod andcooled in a supersonic expansion. The astrophysically important nu 2fundamental frequency is determined to be 63.416529(40) cm-1. Thesemeasurements provide the basis for studies of C3 in the interstellarmedium with far-infrared astronomy.
Abstract: Measurements of the fundamental van der Waals stretching vibration Σ(00 0,v s =1) ←Σ(00 0,v s =0) of Ar–H2O [ν0=907 322.08(94) MHz] and a transition from the lowest excited internal rotor state Σ(10 1,v s =0) to the Σ(10 1,v s =1) level [ν0=1019 239.4(1.0) MHz] are presented. A simultaneous rotational analysis of the new stretching data with the internal rotor bands observed by us previously [J. Chem. Phys. 8 9, 4494 (1988)], including the effects of Coriolis interactions, provides experimental evidence for the new assignment of the internal rotor transitions suggested by Hutson in the accompanying paper. Fits to the rotational term values for the v s =0 states are used to derive effective radial potential energy surfaces for each of the Σ internal rotor states. The results show the well depth (153.4 cm− 1) of the effective radial potential for the Σ(10 1,v s =0) level to be approximately 25 cm− 1 deeper than that for the Σ(00 0,v s =0) ground state of the complex, indicating that the former is stabilized considerably more by the anisotropic intermolecular potential energy surface than is the ground state.
Abstract: Fifty–six transitions from the K=1 lower→K=2 lower tunneling–rotation band of water dimer have been measured and assigned at 22 cm− 1 by direct absorption spectroscopy in a cw planar supersonic jet expansion using a tunable far infrared laser spectrometer. Two different models were used to fit the data and several spectroscopic constants were determined for the upper and lower states. This work supports the local IAM model recently proposed by Coudert and Hougen for the hydrogen bond tunneling dynamics of the water dimer. This model includes four different tunneling motions, all of which contribute to the observed tunneling splittings. This is the most complicated hydrogen bonded system considered to be well understood at this time, at least in the lowest few K states.
Abstract: An improved value for the dipole moment of ArH+ has been obtained from new measurements of the rotational g factors of ArH+ and ArD+ made with tunable far–IR laser spectroscopy. Systematic errors present in earlier measurements have been eliminated. The new result (μ=3.0±0.6 D) is slightly higher than the ab initio value of Rosmus (2.2 D) at the 2σ limits of precision.
Abstract: We report a new high-resolution spectroscopic technique designed for the study of short-lived free radicals and clusters containing free radicals. Excimer laser photolysis of a suitable precursor during the initial stages of a planar supersonic expansion is used to generate ultracold free radicals which are subsequently probed by a tunable far infrared laser. A detection limit of 108 molecules/cm3 for light hydrides is demonstrated and prospects for 2 to 3 orders of magnitude improvement are discussed.
Abstract: The ground state Ka=0(u)→1(g) b–type subband of the rotation–tunneling spectrum of the symmetric 35Cl–35Cl, 37Cl–37Cl, and the mixed 35Cl–37Cl hydrogen chloride dimers have been recorded near 26.3 cm− 1 with sub–Doppler resolution in a continuous two–dimensional supersonic jet with a tunable far–infrared laser spectrometer. Quadrupole hyperfine structure from the chlorine nuclei has been resolved. From the fitted rotational constants a (H35Cl)2 center–of–mass separation of 3.81 Å is derived for the K a =1(g) levels, while the nuclear quadrupole coupling constants yield a vibrationally averaged angular structure for both tunneling states of approximately 20–25 deg for the hydrogen bonded proton and at least 70–75 deg for the external proton. This nearly orthogonal structure agrees well with that predicted by ab initio theoretical calculations, but the observed splittings and intensity alterations of the lines indicate that the chlorine nuclei are made equivalent by a large amplitude tunneling motion of the HCl monomers. A similar geared internal rotation tunneling motion has been found for the HF dimer, but here the effect is much greater. The ground state tunneling splittings are estimated to lie between 15–18 cm− 1, and the selection rules observed indicate that the transtunneling path dominates the large amplitude motion, as expected, provided the dimer remains planar. From the observed hyperfine constants, we judge the dimer and its associated tunneling motion to be planar to within 10°.
Abstract: The first high resolution spectra of a rare gas–H2O cluster have been observed using a tunable far infrared laser to probe the vibration–rotation–tunneling levels of Ar–H2O formed in a continuous planar supersonic jet. The high sensitivity of this spectrometer facilitated extensive measurements of two perpendicular subbands which are assigned to transitions from the ground state to the upper component of a hydrogen exchange tunneling doublet (c–type) at 21 cm− 1, and to v b1 =1+ (b–type) at 25 cm− 1, the lower tunneling component of a bending vibration which is perpendicular to the tunneling coordinate. The tunneling splitting is shown to be in the range 2.5–7 cm− 1 and the lower tunneling component of the excited bending vibration lies between 39 and 43 cm− 1 above the ground state of the complex. The experimentally determined center–of–mass separation (R c.m. =3.75 Å) and harmonic stretching force constant (k s =0.0134 mdyn/Å) are compared to those of related first and second row hydrides. The large amplitude motions occurring within this complex make it difficult to establish its structure.
Abstract: A tunable far infrared laser system has been used to measure the vibration–rotation spectrum of the lowest Σ bending state of ArHCl near 24 cm− 1 in a cw planar jet operating with a terminal jet temperature near 3 K. Over 60 transitions have been observed for both 3 5Cl and 3 7Cl isotopes with resolution of the quadrupolehyperfine structure. An improved set of molecular parameters was determined, including B, D, H, and e q Q for both upper and lower states. Very narrow linewidths (approximately 300 kHz) resulting in high resolution and sensitivity make this technique a powerful new method for the detailed investigation of intermolecular forces.
Abstract: The details of the first experimental determination of the dipole moment of a molecular ion from the rotational Zeeman effect are presented, along with an assessment of the ultimate accuracy of the technique.
Abstract: The lowest Σ bending vibration (0200) of ArH35Cl has been observed near 23.6 cm−1 from P(23) to R(17), in a continuous planar jet expansion using a tunable far-infrared laser. A least-squares analysis has been performed to determine the centrifugal distortion constants for the ground and excited states, which will greatly facilitate the determination of an accurate intermolecular potential surface.
Abstract: Rotational collision-induced absorbance has been observed in the far I.R. spectra of cyclopropane samples and cyclopropane-argon mixtures. The part of the spectrum due to the polarizability and quadrupolar and octapolar field is calculated using the literature values of the molecular properties for this oblate symmetric top. Prolate and oblate top molecules are shown to have excess observed absorbance at different frequencies. The excess absorbance is analysed in terms of violations of the multipolar selection rule, ΔK = 0, which is caused by asymmetrical interactions during collisions.
Abstract: The theory for collision induced rotational absorption between nonpolar molecules is extended to near-symmetric top molecules using Wigner rotational matrices to represent the field and polarizability in the induced dipole moment. The resulting theory is applied to the collision induced spectrum of ethylene-ethylene. This spectrum is shown to be dominated by absorption due to the off-diagonal quadrupole moment in spherical coordinates, Q¯ 2 2. Fitting the calculated to observed absorption coefficient at 140 cm-1 leads to an accurate value of the quadrupole moment difference, Q¯ xx - Q¯ yy = -4·33 × 10-26 esu·cm2 in molecule fixed cartesian coordinates.
Abstract: Laboratory observations of the pure rotational spectrum of HCO+ in its lowest excited bending state are reported. Because of their severe excitation requirements, such vibrational satellites and the high-J ground-state lines, also measured here, sample only hot, dense regions of matter in active molecular cloud cores and circumstellar envelopes. As the HCO+ abundance is tied directly to the gas fractional ionization, it is probable that the vibrationally excited formyl ion transitions will provide high-contrast observations of shocked molecular material, rather than the more quiescent, radiatively heated gas surrounding stellar sources detected with the few vibrationally excited neutral species observed to date.
Abstract: The first determination of the electric dipole moment of a molecular ion is reported. A tunable far- infrared laser was used to measure the Zeeman effect in low-J rotational transitions of ArH+ and ArD+, and the dipole moment was determined from the isotopic dependence of gr. The result (μ =1.4±0.4 D) is in marginal agreement with ab initio calculations (2.2 D). It is shown that this method can ultimately provide molecular-ion dipole moments accurate to within a few percent. The adi- abatic internuclear separations of both isotopes were also determined from the gr values.
Abstract: Analysis of observed far i.r. collision-induced absorption in ethane and ethane—rare gas mixtures indicates that only a small fraction is due to the canonical multipole-induced and overlap-induced dipole moments. Nevertheless, multipole-induced dipole spectra calculated with no adjustable parameters are shown to fit the low-frequency wing ( < 60 cm−1) of the observed ethane collision-induced spectrum to within 10%. More than half of the observed intensity in the ethane and ethane—rare gas far i.r. spectra has been shown to be due to rotational and vibrational modulation of dipoles induced by electric fields which do not reflect the full symmetry of the inducing molecule but depend strongly on local symmetry of the inducing charge distribution. The ethane C-C stretch, ν3 at 993 cm−1, has been observed to have a collision-induced absorption which is also assigned to effects of local symmetry. In addition to the non-resonant effects due to overlap and frame distortion, we present a new theoretical analysis of a resonant intermolecular effect, also due to frame distortion, at the frequency of the torsional mode, ν4.