Mesoscale spatial variability of sulfate in air and rainwater at St. Louis

Sulfate concentrations in rainwater and in air measured on four summer days at St. Louis were highly variable, both spatially or temporally. Maximum/minimum ratios of aerosol SO _inf4 ^sup? varied by up to a factor of 9, and those in rainwater by a factor of 3 on the average. Generally, SO _inf4 ^sup? concentration patterns in air and rainwater were similar, and consistent with wind direction and the location of sources. Direct relationships between SO _inf4 ^sup? in air and in water were evident on two of the individual days, but not for all days together. The non-uniformity of the SO _inf4 ^sup? pattern plus consideration of possible sources of SO _inf4 ^sup? suggests that nucleation of SO _inf4 ^sup? particles must be a major cause of S scavenging, with some possible influence from sub-cloud impaction.