THE DROP SIZE-DEPENDENCE OF IRON AND MANGANESE CONCENTRATIONS IN CLOUDS AND FOGS - IMPLICATIONS FOR SULFATE PRODUCTION

Differences in total iron and manganese concentrations between large ( d > 23 mu m) and small (4 < d < 23 mu m) cloud and fog drops were inve stigated at four locations in the United States. The study examined co astal stratus and stratocumulus clouds in southern California and nort hern Oregon, frontal and orographic clouds at Mt. Mitchell, North Caro lina, and radiation fogs in California's San Joaquin Valley. The speci ation of iron as a function of drop size was also examined in some fog samples from the San Joaquin Valley. Total iron and manganese concent rations were generally higher in large drops than in small drops in cl ouds sampled at Mt. Mitchell and along the southern California coast. These species were typically enriched in small drops at the Oregon coa st and San Joaquin Valley sites. Ratios of dissolved Fe(III) to total dissolved Fe ranged from 0.88 to 0.93 in small fog drops. Non-uniform, distributions of iron and manganese across the drop size spectrum can influence rates of metal catalyzed S(IV) autooxidation. Approximately 50% of the sampled clouds were calculated to experience autooxidation rate enhancements greater than 30% due to variations in drop acidity and catalyst concentrations with drop size.