The net radiative forcing effect (and thus the net effect on climate change) of aerosols is one of the most uncertain factors in our understanding of how climate is changing, in part because of complex processes that govern aerosol formation, as well as the complex relationship between aerosols and cloud formation. We study the relationship between evaporation kinetics and the formation of cloud droplets, as well as the role nitrogen oxides (NO_x) play in the formation of secondary organic aerosol (SOA).

Evaporation Kinetics

The transport of water molecules across vapor-liquid interfaces in the atmosphere is a crucial step in the formation and evolution of cloud droplets. Despite decades of study, the effects of solutes on the mechanism and rate of evaporation and condensation remain poorly characterized. In an ongoing collaboration with the Saykally group, we investigate these effects using a liquid microjet technique. In our experiments, we create a train of micron-sized droplets and measure their temperature via Raman thermometry as they undergo evaporation without condensation. By comparing the measurements to a simple cooling model, we determine the evaporation coefficient (γ) of the system.¹ Our work has shown that inorganic salts have little effect on γ, with surface-adsorbing anions causing only a slight reduction in the coefficient from that measured for pure water.^2,3 Preliminary studies of soluble carboxylic acids show that these molecules, though present at the interface, cause no measurable change in the evaporation coefficient.

Film-forming organic compounds are ubiquitous in atmospheric aerosol. These molecules are expected to impact cloud droplet formation both by significantly impeding the rates of evaporation and condensation and by reducing the surface tension and hence the vapor pressure of micron-sized droplets. The magnitude of these effects and their implications for cloud properties remain unknown. Experiments to directly probe the thermodynamic properties and the kinetics of micron-sized droplets coated with atmospherically-relevant surfactants are forthcoming.

References
1. Smith, J.D., Cappa, C.D., Drisdell, W.S., Cohen, R.C. and Saykally, R.J., Raman thermometry measurements of free evaporation from liquid water droplets, J. Amer. Chem. Soc., 2006, 128, 12892.
2. Drisdell, W.S., Saykally, R.J. and Cohen, R.C., On the evaporation of ammonium sulfate solution, Proc. Natl. Acad. Sci., 2009, 106, 18897.
3. Drisdell, W.S., Saykally, R.J. and Cohen, R.C., Effect of Surface Active Ions on the Rate of Water Evaporation, J. Phys. Chem C, 2010, 114, 11880.

SOA Formation

Secondary organic aerosol (SOA) formation is a complicated and important area of research. SOA forms when organic material present in the gas phase partitions into the particle phase; this can result from gas-phase reactions between organic compounds and NO_x, OH or other oxidizing chemicals, producing oxidized organic compounds with a significantly lower vapor pressure than the precursor organic molecule. SOA formation is expected to have a complicated relationship with NO_x concentration, due to multiple possible reaction pathways and the difference in volatility of resulting products, and remains an active field of research. We have adapted our TD-LIF process to quantify organic nitrates in the particle phase, allowing us to elucidate some of the details this relationship. For example, as part of the CalNex-2010 campaign,we measured organic nitrate content in ambient aerosol in Bakersfield, CA. The results suggest that organic nitrates in the particle phase are formed in the gas phase via reaction of gas phase organic precursors with the nighttime oxidant NO₃ radical. Nitrogen oxides (NO_x) are also responsible for the formation of organic nitrates during the daytime when photooxidation is active. 

Reference:
Rollins, A. W., Browne, E. C., Min, K.-E., Pusede, S. E., Wooldridge, P. J., Gentner, D. R., Goldstein, A. H., Liu, S., Day, D. A., Russell, L. M., and R. C. Cohen, Evidence for NO_x Control over Nighttime SOA Formation, Science, 2012, 337, 1210-1212.