CONTRASTING BEHAVIOR OF GAS AND AEROSOL SCAVENGING IN CONVECTIVE RAIN - A NUMERICAL AND EXPERIMENTAL-STUDY IN THE AFRICAN EQUATORIAL FOREST

A two-dimensional convective cloud model has been coupled with a chemical model consisting of the explicit prediction of five chemical species: SO2, SO42-, NH4+, O3, and H2O2. The model takes the scavenging processes into account. We examine the relationship between the liquid water content (LWC) and the chemical concentrations of atmospheric trace elements in convective precipitation. The model results compare favorably with observations (ABLE 2B and DECAFE experiments). The modeled dilution curves were found to be nonlinear, in agreement with the DECAFE data. The model also accounts for the large differences in dilution effects that exist between gases and aerosols. More generally, this study shows that within the African equatorial forest there are (1) a reduction of aerosol scavenging efficiency with increasing rain intensity (or LWCg); (2) a strong impact of vertical profiles of atmospheric trace elements on ground rain concentrations; (3) a difference in scavenging efficiencies according to the origin of the elements (gas or aerosol); and (4) a depletion of atmospheric concentrations during rainfall.