On the Concentration of Sulfate in Precipitation

Abstract

Two case studies are used to examine the relationship of the sulfate concentration in surface precipitation to the microphysical characteristics of the precipitating cloud systems. The data from one case study support the contention that existing sulfate aerosol was incorporated into cloud water by the nucleation process and accounted for nearly all of the observed cloud and precipitation water sulfate concentration. These activated sulfate particles comprised nearly 60% of the clear-air sulfate mass concentration. Once nucleated, the sulfate particles accumulated water through the condensation process and were subsequently deposited at the surface after accretion on large snowflakes. The presumption of aqueous phase sulfate oxidation of SO2 was not necessary to account for the observed sulfate concentrations. The data from the second case study are more limited and difficult to interpret. Nucleation and below cloud washout appeared to be the main contributors to the surface sulfate concentration in precipitation water.