A 2-D spectral model simulation of the scavenging of gaseous and particulate sulfate by a warm marine cloud

Our 2-D dynamic model including spectral microphysics and a detailed treatment of the scavenging and processing of (NH4)(2)SO4 and NaCl aerosol particles as well as SO2, H2O2, and O-3 has been evaluated for a warm precipitating convective cloud at Day 261 (September 18, 1974) of the GATE campaign. The model determined the rate at which S (IV) was converted to S (VI) inside the cloud and rain water and the rate at which its pH changed. The rate of drop sulfate formation due to scavenging of particles was compared with the rate of sulfate formation due to the uptake and oxidation of SO2. It was found that of the sulfate processed by the cloud 87% was scavenged by nucleation of sulfate containing aerosol particles, 6% of sulfate entered the drop through impaction scavenging below cloud base, almost 7% was produced by oxidation by H2O2, but only 0.2% by oxidation of O-3. Averaging over space and time we derived from the model that about 2 mg/l in the rain was due to particulate sulfate and 1 mg/l was due to sulfate from oxidized SO2 in the drops. Oxidation of SO2 was accompanied by an average pH of 4.4 in the rain water. For sulfur a washout ratio of 600 and a scavenging coefficient of about 10(-4)s(-1) was calculated. In addition, a comparison of the aerosol particle and CCN spectrum at one point in the atmosphere before and after the cloud event showed that the size of the aerosol particles had increased and their number decreased such that the new CCN spectrum activates the particles at lower supersaturations.