Abstract
In order to determine the rate controlling step for sulfate formation in a heterogeneous droplet system. SO2-transfer within the gasphase towards and within droplets are calculated. equilibrium between SO2 in the gasphase and sulfur (IV) in cloud and fog droplets is reached within less than 1 second. Oxidation of sulfur (IV) to sulfate in the droplet phase proceeds slower by orders of magnitutde. Three mechanisms of SO2-oxidation are discussed: (a) SO2-oxidation by O2 in the absence of catalysts; (b) SO2-oxidation by O2 in the presence of catalysts, and (c) SO2-oxidation by strongly oxidizing agents. Mechanism (a) contributes only to a negligible extent to sulfate formation in droplets even in the presence of typical concentrations of ammonia. Indications are strong that the major function of the SO2–NH3–H2O-system is not the oxidation of SO2 to sulfate. Oxidation mechanism (b) may contribute to a significant extent to sulfate formation in urban fogs in which case the concentrations of catalysts can be sufficiently high. For clouds in remote areas with much lower catalyst concentrations SO2-oxidation by mechanism (b) seems to be of little importance. The oxidation by strongly oxidizing agents (mechanism (c)) appears to be the dominant mechanism although some experimental discrepancies have to be resolved.
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