Aerosol Solute Strength Drives Transition-Metal-Ion-Catalyzed Sulfate Aerosol Formation in Polluted Environments.

Atmospheric sulfate aerosols significantly contribute to air pollution and climate forcing, especially in regions that are impacted by coal combustion. Catalyzed by iron and manganese transition metal ions (TMI) in aqueous aerosol particles, multiphase oxidation of SO2 by O2 is an important yet poorly constrained source of sulfate aerosols in polluted atmospheres. Large uncertainties arise from the poorly characterized effects of aerosol solute strength, interfacial chemistry, and Fe(III)-Mn(II) interactions. Decoupling these factors in aerosol flow reactor experiments with atmospherically relevant aqueous aerosols, we demonstrate that aerosol solute strength drives sulfate formation from Mn(II) aerosol chemistry to be more than 2 orders of magnitude faster than predicted by models that assume dilute, cloudwater chemistry. The presence of a considerable quantity of insoluble Fe(III) serves to further inhibit the rate under conditions of low ionic strength, with a threshold of 16 mol kg-1. When incorporated into an atmospheric model, this new understanding of aerosol chemistry increases the abundance of sulfate aerosols during polluted haze events by more than 50%. Our findings underscore that aerosol solute strength exerts a profound influence on multiphase sulfate production in the atmosphere, bridging the gap between modeled and measured sulfate aerosol abundance.