Estimated variability of below-cloud aerosol removal by rainfall for observed aerosol size distributions

[摘要] Below-cloud scavenging (BCS) coefficients of aerosols by rainfall are estimated forreported aerosol size distributions measured during field experiments in variousenvironments. The method employed is based on explicit calculations of the efficiency ofcollision between a raindrop and aerosol particles. Such BCS coefficients can beused in numerical models that describe: 1) the detailed evolution of aerosol sizedistribution and, 2) the evolution of total aerosol mass concentration. The effectsof raindrop size distribution and aerosol size distribution variability onBCS coefficients are illustrated using observed data. Results show that BCS coefficientincreases with rainfall rate and has a significant dependence on aerosol size distributionparameters. Thus, BCS is important for very small particles (with diameters less than0.01 $\mu$m) and for coarse particles (with diameters larger than 2 µm). For rainfall rateR ~ 1 mm hr^-1, the 0.5-folding time of these particles is of the order of one hour. It is shownthat BCS is negligible for aerosol particles in the range [0.1-1] µm if compared with in-cloudscavenging rates for low and moderate rainfall rates ( R ~ 0.1-10 mm hr^-1). Theresults indicate that a boundary layer aerosol size distribution with coarse mode isdrastically affected very shortly after rain starts (in a fraction of one hour) andconsequently, the below-cloud aerosol size distribution becomes dominated by particlesin the accumulation mode.

[学科分类] 大气科学