Trees, Aerosols, and Clouds

The eastern United States is currently seeing a decrease in sulfate based aerosol concentrations and an increase in biogenic secondary organic aerosol (SOA) concentrations. This study examined how the changing composition of aerosols affects cloud formation and droplet growth processes in these areas. Kohler theory was modified to be relevant to organic based aerosols by implementing the effect of an insoluble aerosol core and treating the aerosol as a slowly dissolving substance with a limited solubility. The modified Kohler theory was introduced into a Lagrangian model for an adiabatic rising air parcel. The model was run for 9 compounds with different solubilities at the vertical updraft speeds: from 0.25 m s−1 to 10 m s−1. Analysis of the Kohler curves and the air parcel model results show that high saturation ratios are needed to activate monoterpenes (e.g. α-pinene and β-pinene) due to their low solubility. For oxidized monoterpenes (e.g. pinic acid, pinonic acid, norpinonic acid), particles smaller than 0.039 μm will not activate at updraft speeds less than 5 m s−1 due to the high saturation ratio required. At updraft speeds of 2.5 m s−1, the percentage of activated droplets was 100%, 99.5%, and 81% for sulfates, oxidized monoterpenes, and monoterpenes respectively. At updraft speeds of 10 m s−1, the percentage of activated droplets was 100%, 99.5%, and 81% for sulfates, oxidized monoterpenes, and monoterpenes respectively. The data shows that oxidized monoterpenes actively play a role in cloud formation and growth processes.