Nitrogen (N) moves through the biosphere and atmosphere through several complex processes. Our work primarily focuses on understanding the role of nitrogen oxides (NOx = NO + NO2) in N cycling, as NOx is a pollutant emitted from both anthropogenic (through fossil fuel combustion processes) as well as biogenic sources such as wildfires and soils, and participates in oxidative chemistry that can result in other pollutants that also have important climate effects, such as aerosol and tropospheric ozone. We aim to improve understanding of the processes that govern NOx emission, the subsequent chemistry that results in formation of ozone and aerosol, and the pathways by which NOx is temporarily or permanently removed from the atmosphere.

Anthropogenic emissions

Anthropogenic activities represent the largest source of NO_x emissions in today’s world, with approximately 50% of NO_x coming from fossil fuel burning. NO_x is formed during combustion of fossil fuels by oxidation of N in the fuel (e.g. coal), and/or as a product of dissociation of N₂ and O₂ at high temperatures. Thus NO_x concentrations in most locations are directly impacted by energy consumption; we can use this relationship to help inform policy decisions on regulations regarding air quality and energy efficiency. Subsequent chemistry of NO_x is also of interest, as NO_x impacts ozone and aerosol formation (both of which are air quality concerns and have important climate effects), helps regulate atmospheric oxidation, and can have far-reaching impacts through chemical transport of NO_x reservoir species like peroxy nitrates. We have participated in several measurement campaigns examining how NO_x emitted by human activities is chemically processed and transported through the atmosphere; please check out our publications page to learn more.

Soil Emissions

Soils are responsible for approximately 20% of global NO_x emissions. These emissions are the result of microbial activity within the soil (e.g. denitrification). The processes governing this emission are not well understood, but emissions seem to correlate best with water-filled pore space, temperature, and N availability. We have synthesized information from satellite data and models of soil NO_x emissions, demonstrated that soil NO_x emissions can be observed from space, and developed new mechanistic models for soil NO_x emissions that agree better with observations.

Reference:
R. C. Hudman, N. E. Moore, A. K. Mebust, R. V. Martin, A. R. Russell, L. C. Valin, and R. C. Cohen, Steps towards a mechanistic model of global soil nitric oxide emissions: implementation and space based-constraints, Atmos. Chem. Phys. 12, 7779-7795, 2012.

Biomass Burning

Biomass burning contributes approximately 15% of NO_x emission to the atmosphere. This source is partly natural and partly anthropogenic in nature, as fires are often due to human activity. Biomass burning NO_x emissions are highly variable from fire to fire depending on several properties, including the temperature of combustion within the fire and the N content of the fuel (as most NO_x from biomass burning is formed via oxidation of this fuel-based N). This variability is poorly constrained, owing to the difficulties of measuring large-scale wildfires and of reproducing these emissions in a laboratory setting. We use the large collection of observations of fire emissions of NO_x obtained from satellite instruments to perform systematic, statistically rigorous analyses of the origins of this variability. Using observations in California and Nevada, we developed a technique that can distinguish relative differences in emissions between land types.

Reference:
A. K. Mebust, A. R. Russell, R. C. Hudman, L. C. Valin, and R. C. Cohen, Characterization of wildfire NO_x emissions using MODIS fire radiative power and OMI tropospheric NO₂ columns, Atmos. Chem. Phys. 11, 5839-5851, 2011.

Atmosphere-Forest Exchange

Quantifying the magnitude and direction of the exchange of nitrogen oxides between the atmosphere and forest is key to understanding low-NO_x oxidative chemistry, nitrogen oxide and O₃ transport and deposition, and N nutrient cycling. In one example, there is conflicting evidence about the role of the trees in the forest-atmosphere exchange NO₂. For example, top-down studies have convincingly established that forests constitute a NO_x sink due to direct foliar NO₂ uptake, attenuating the escape of soil emissions from forest canopies. In contrast, leaf-scale studies suggest that fluxes may be bi-directional and that at very low ambient NO_x concentrations trees act as NO_x sources. In the Cohen Research Team, we study atmosphere-forest exchange through a combination of (1) canopy-scale field observations of eddy covariance fluxes and concentration gradients of NO, NO₂, peroxy nitrates (RO₂NO₂), alkyl nitrates (RONO₂), and HNO₃, and (2) lab-based experiments of the rate of NO and NO₂ exchange at the leaf scale. More information about one experiment, the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX), can be found in this ACP special issue.