Abstract. The scavenging of aerosols by precipitation is a major removal mechanism for atmospheric particles. This process involves complex interactions between aerosols and falling hydrometeors and it is strongly dependent on precipitation characteristics and to some extent on aerosol properties. Aerosol particles enter into cloud through nucleation scavenging and undergo rapid growth by condensation and are efficiently collected by falling raindrops inside the cloud. This is the in-cloud scavenging (ICS) process, which is strongly dependent on precipitation characteristics. Raindrops that fall below the cloud base continue to collect aerosol particles from the boundary layer (BL). This is the below-cloud scavenging (BCS) process that is dependent on both precipitation and aerosol characteristics. For practical purposes, a wet-removal rate or scavenging coefficient is defined (in units of inverse time) to be used in the aerosol mass continuity equation. Available model and experimental results show that ICS is generally the dominant wet removal process and it is strongly dependent on precipitation type and intensity. This study provides a statistical description of ICS and BCS coefficients for representative locations in the United States. The wet scavenging coefficient is calculated based on the efficiency of collision between aerosol particles and raindrops within cloud and below-cloud and it is linked to precipitation rate. Results are shown based on measured hourly precipitation rates and on aerosol distributions available from observations. The study illustrates the structure of wet scavenging variability within various locations and times during the year. Comparisons between ICS and BCS coefficients are shown to illustrate cases when BCS might be important. Thus in the case of BCS, the fine and coarse particles can be effectively removed from the BL shortly after rain starts and the accumulation mode particles tends to be more persistent. The study documents the role of precipitation field in determination of wet removal of aerosols and discusses the implications for numerical modeling and pollution studies.
Copyright 2002 by the American Geophysical Union.