Anthropogenic Control of Biogenic SOA: Salt Interactions of Water Soluble Oxygenated Hydrocarbons (Invited Presentation)

Recent field observations suggest that aerosol sulfate (SO42-) directly controls a significant fraction of secondary organic aerosol (SOA) mass over biogenic environments such as the South Eastern US. Setschenow salting-constants, KS, describe the exponential dependence of the gas- / aqueous phase partitioning (Henry's Law constant) on the salt concentration. Knowledge about KS is of particular importance to predict SOA formation in aerosol water, where the salt concentrations are typically very high. Such salt-induced activity is not typically captured by activity coefficients, and can modify SOA formation rates by altering (1) the effective Henry's Law partitioning coefficient (by several orders of magnitude), (2) changing the multiphase chemical reaction rates; and (3) the volatity of products through the formation of complexes and/or organo-sulfate/nitrate products. This talk discusses laboratory measurements combined with quantum calculations to better understand the values of KS for biogenic OVOC, such as glyoxal, methyl glyoxal and isoprene epoxide in a range of atmospherically relevant salts, incl. (NH4)2SO4, NH4NO3, NaNO3 and NaCl. We find that glyoxal ‘salts-in' (negative KS) consistently, while methyl glyoxal ‘salts-out'. The Gibbs Energy of the chemical bonds of glyoxal, methyl glyoxal and isoprene epoxide with SO42-, NO3-, and Cl- is compared, and used to interrogate their salting-behavior. Our best estimates of KS values for glyoxal, methyl glyoxal and isoprene epoxide have been incorporated into the CMAQ model, and a range of simulations were conducted to predict the effect of salts on their partitioning and potential to form SOA over the continental US.