The oxidation of SO2 and the resultant aerosol growht kinetics are inferred from airborne measurements of the number concentrations and size distributions of pure sulfate and mixed particles within the coal-fired Keystone plant plume in western Pennsylvania. Measurements of aerosol concentrations in nine distinct size ranges between 0.01--2.0 μm diameter were coupled with impactor sampling and quantitative sulfate analysis of individual particles at various plume travel times. Cross-sectional plume mappings of the horizontal and vertical concentration profiles of SO2 and sulfate mass provided a means of calculating SO2 conversion rates. Thirteen flights encompassing a wide range of meteorological conditions and plume behavior were compared in an attempt to isolate the various gas-to-particle conversion mechanisms. Both gas and droplet phase reactions involving SO2 have been identified, and their importance for atmospheric sulfate formation are assessed. Total sulfate particle concentration was found to be related directly to solar radiative flux. Sulfate particles in the plume achieved maximum size at very high relative humidities during plume merger with cloud and fog layers.