Evidence for a New Oxidation Mechanism for Sulfur Dioxide from Laboratory Measurements

The oxidization of sulfur dioxide (SO2) occurs in the gas and liquid phase and this oxidation contributes to particulate matter and acid precipitation. The production of sulfate particles is significant because of their impact on climate, precipitation acidification, and human health. In this paper, the focus is on the oxidation of SO2 and on the possibility of unknown heterogeneous reactions that may occur on sulfate aerosol surfaces. These results are based on the reanalysis of a foundational set of SO2 laboratory oxidation measurements. The experiments involved two sets of photochemical studies of nitrous acid (HONO), nitrogen oxides (NOx = NO + NO2), SO2, carbon monoxide (CO), and water vapor (H2O) mixtures made in molecular nitrogen (N2) with traces of molecular oxygen or in synthetic air. The reanalysis strongly suggests that there are uncharacterized processes for the oxidation of SO2 that are nearly three times faster than the known gas-phase reactions. The uncharacterized processes may involve sulfate aerosol surface reactions in the presence of nitrogen oxides. If these processes can be included in current atmospheric chemistry models, greater conversion rates of SO2 to sulfate aerosol will be calculated and this may reduce modeling bias.