In previous publications (McMurry and Wilson, 1982: McMurry et al., 1981), techniques for determining the relative contributions of gas phase and liquid phase reactions to secondary ambient aerosol formation have been described. In this paper these methods are applied to more recent data acquired during the 1980 Environmental Protection Agency PEPE-NEROS study near Columbus, Ohio. It is shown that liquid phase reactions tend to be the most important mechanism of sulfate aerosol formation for relative humidities greater than 75%, while gas phase reactions are predominant at lower humidities. In either case, although measurements have been made both during daylight and nightime hours, aerosol formation has been observed only in air parcels exposed to sunlight. Gas phase conversion rates typically varied between 0 and 5% hr-1, but liquid phase contributions as high as 12% hr-1 were observed in the Columbus urban plume. Liquid phase reaction rates (moles of SO2 oxidized (volume of liquid)-1 ppb SO2-1 time-1) are determined and are shown to increase with increasing relative humidities.