The impact of collision removal of rainfall on aerosol particles of different sizes was analyzed through the calculation of Stokes number, combining with the hourly PM2.5 concentrations and meteorological data in Haidian from October 2012 to October 2014, and also the size distribution data in a selected rainfall process. The calculation results of Stokes number showed that the raindrops had little effect on direct collision removal of aerosol particles of smaller than 2 μm, and had more effect on aerosol particles of larger than 2 μm. Based on the statistical analysis of the observation data, the precipitation processes or the precipitation hours with significantly decreased PM2.5 were quite limited. However, PM2.5 concentrations were increased in 43.2% of the precipitation hours. By analyzing the size distribution data of aerosol particles during a typical precipitation process, we found that the precipitation had significant scavenging effect on Aitken mode particles (<0.1 μm) and coarse mode particles (>1.0 μm), except for the accumulation mode particles. Since the accumulation mode aerosols contributed most of the mass of PM2.5, the rainfall processes only had minor influence on the collision scavenging of PM2.5.
Keywords: PM2.5; Stokes number; inertial collision; size distribution; wet scavenging.