Derivation and verification of an aerosol dynamics expression for the below-cloud scavenging process using the moment method

An aerosol dynamics equation for the below-cloud scavenging process considering phoretic and electric charging effects in addition to the conventional mechanisms (the Brownian diffusion, interception, and impaction) is developed by using the moment method. Then, the dynamics of particle size distribution by the below-cloud scavenging process is calculated by using the developed equation and verified with the measurement data. The calculated particle size distribution changes are quite small compared to the measured changes. The calculated removal rate is smaller by 10−2–10−3 than the measured data when only the conventional mechanisms are considered. With the extended mechanisms, the scavenging coefficient increases upto 20 times, mainly for the particle size range of 0.1 μm<dp<3.0 μm. However, the difference between the calculated and measured scavenging coefficient is still large, especially, for dp<0.1 μm. Other possible scavenging mechanisms that might affect the below-cloud scavenging process such as coagulation and condensational growth of hygroscopic particles, turbulence, and updraft into cloud are discussed. It is recommended that further studies on wet scavenging process are needed.