Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 2 6 2002 10.5194/acpd-2-2095-2002 http://www.atmos-chem-phys-discuss.net/2/2095/2002/ http://www.atmos-chem-phys-discuss.net/2/2095/2002/acpd-2-2095-2002.html http://www.atmos-chem-phys-discuss.net/2/2095/2002/acpd-2-2095-2002.pdf 2095 2131 2002-11-14 Estimated variability of below-cloud aerosol removal by rainfall for observed aerosol size distributions C. Andronache Boston College, Chestnut Hill, Massachusetts, USA Below-cloud scavenging (BCS) coefficients of aerosols by rainfall are estimated for reported aerosol size distributions measured during field experiments in various environments. The method employed is based on explicit calculations of the efficiency of collision between a raindrop and aerosol particles. Results show that BCS coefficient increases with rainfall rate and has a significant dependence on aerosol size distribution parameters. Thus, BCS is important for very small particles (with diameters less than 0.01 µm) and for coarse particles (with diameters larger than 2 µm). For rainfall rate R ~ 1 mm hr ^-1, the 0.5-folding time of these particles is of the order of one hour. It is shown that BCS is negligible for aerosol particles in the range [0.1-1] µm if compared with in-cloud scavenging rates for low and moderate rainfall rates (R ~ 0.1-10 mm hr ^-1). The results indicate that a boundary layer aerosol size distribution with coarse mode is drastically affected very shortly after rain starts (in a fraction of one hour) and consequently, the below-cloud aerosol size distribution becomes dominated by particles in the accumulation mode.