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 chemi cal model consisting of the explicit prediction of five chemical speci es: SO2, SO42-, NH4+, O3, and H2O2. The model takes the scavenging pro cesses into account. We examine the relationship between the liquid wa ter content (LWC) and the chemical concentrations of atmospheric trace elements in convective precipitation. The model results compare favor ably with observations (ABLE 2B and DECAFE experiments). The modeled d ilution curves were found to be nonlinear, in agreement with the DECAF E data. The model also accounts for the large differences in dilution effects that exist between gases and aerosols. More generally, this st udy shows that within the African equatorial forest there are (1) a re duction of aerosol scavenging efficiency with increasing rain intensit y (or LWCg); (2) a strong impact of vertical profiles of atmospheric t race elements on ground rain concentrations; (3) a difference in scave nging efficiencies according to the origin of the elements (gas or aer osol); and (4) a depletion of atmospheric concentrations during rainfa ll.