Abstract
In order to determine the rate controlling step for sulfate formation in a heterogeneous droplet system. SO 2-transfer within the gasphase towards and within droplets are calculated, equilibrium between SO 2 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 SO 2-oxidation are discussed: 1. (a) SO 2-oxidation by O 2 in the absence of catalysts; 2. (b) SO 2-oxidation by O 2 in the presence of catalysts and 3. (c) SO 2-oxidation by strongly oxidizing agents. Mechanism 1. (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 SO 2-NH 3-H 2O-system is not the oxidation of SO 2 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 SO 2-oxidation by mechanism 2. (b) seems to be of little importance. The oxidation by strongly oxidizing agents (mechanism 3. (c)) appears to be the dominant mechanism although some experimental discrepancies have to be resolved.
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