Aerosol Mixing State Impacts on Aqueous Phase Chemistry

Chemistry within cloud and fog droplets contributes to atmospheric S(IV) oxidation, the production of secondary organic aerosol, and plays an important role as a source or sink for a variety of trace gaseous species. Detailed mechanisms for atmospheric aqueous-phase chemistry have been developed, and have proven successful at modeling the processing of key atmospheric species in these systems. However, due to the complex interactions between aerosol particles and cloud or fog droplets, large uncertainties remain in modeling the composition and properties of the gas and condensed phases which directly affect their radiative properties and impact on climate.

This presentation focuses on the question of how aerosol mixing state impacts the production of sulfate and secondary organic aerosol in clouds. To this end, we developed an aqueous-phase chemistry module for the stochastic particle-resolved model PartMC-MOSAIC, based on the CAPRAM 2.4-reduced mechanism of Ervens et al. (JGR, 2003). Stochastic particle-resolved models are uniquely suited for this work as they explicitly track the composition of individual aerosol particles and droplets. They can therefore capture the effects of non-linearity in reaction rates that may occur due to differences in droplet composition in the highly externally mixed aerosol systems often found in the real atmosphere. Furthermore, we developed several scenarios representative of urban emission plumes for which the aerosol mixing states varied, and used these aerosol populations as inputs for particle-resolved cloud parcel model simulations. Finally, we discuss the effects of particle mixing state and aqueous-phase chemistry on particle and droplet composition and properties as the aerosol populations undergo cloud processing.