The influence of chemical heterogeneity among cloud drop populations on processing of chemical species in winter clouds

Drop size-resolved measurements of winter cloud composition in the Rocky Mountains of northern Colorado revealed significant variations of cloud drop pH, ion (SO42−, NO3−, NH4+, and Ca2+) concentrations, and concentrations of trace metal catalysts (Fe and Mn) with drop size. Simultaneous measurements of snow chemical composition and the degree of cloud drop capture by snow crystals (riming) revealed a positive correlation between snow composition and the extent of ice crystal riming in two of four cases studied. Observations indicate that the size-dependent chemical composition of the clouds tends to enhance aqueous phase sulfate production rates when the primary oxidant is ozone. Enrichment of accumulation mode aerosol species in small cloud drops, which are inefficiently scavenged by ice crystals, appears to cause small (typically less than 15%) reductions in the efficiency with which these species are scavenged by precipitation during accretional ice crystal growth.