The role of interfacial processes in the photochemical dynamics of aerosols

The surface layer of an aerosol only comprises a small fraction of the aerosol's total mass. Yet, this region plays an important role in many atmospheric processes. Aerosol surfaces are gateways for chemical species transferring between the gas- and aqueous-phases, are sites where heterogeneous reactions occur, and are locations where photochemistry may occur at a faster rate than in the bulk due to an incomplete solvent-cage. The present work uses a combination of computer simulations and experiments to investigate the dynamics of the physical and chemical processes that occur at aerosol surfaces.;First, the importance of surface processes in halogen production from sea-salt aerosols is quantified through a global sensitivity analysis of a chemical kinetics computer model called MAGIC. Results show that predicted chlorine output from NaCl aerosols irradiated by UV light proceeds primarily though a surface reaction between gaseous OH and surface chloride ions. Bromine production from NaBr aerosols in the dark proceeds primarily though a surface reaction between gaseous ozone and surface bromide ions. However, when NaBr aerosols are irradiated by UV light bromine production proceeds primarily through a mechanism involving gas-phase chemistry, aqueous-phase chemistry, and mass transfer.;Next, MAGIC is used to investigate how aerosol size affects halogen production from NaCl and NaBr aerosols. Aerosol size is varied over three orders of magnitude and the mechanisms of halogen output are analyzed. Results show that the rate of halogen production depends strongly upon the surface area available for heterogeneous reactions and mass transfer.;Finally, enhanced photolysis at aerosol surfaces is investigated with a combination of experiments and Mie theory calculations. Experiments show that Mo(CO)6 in a 1-decene solvent photolyzes many orders of magnitude faster in aerosols than in a bulk solution. Mie theory calculations demonstrate that enhanced photolysis due to morphology dependent resonances cannot explain the experimental observations. Instead, the observations likely are due to the enhanced surface area of aerosols combined with a reduced solvent-cage effect at the surface compared to the bulk. Mie theory calculations also are employed to predict photolysis rate constants for NO-3aq , FeOH2+aq , and H2O2(aq) in water droplets under atmospherically relevant conditions.