The change of the aerosol optical properties during the wet removal process

Scavenging of particles by precipitation or below-cloud scavenging is one of the most efficient particle sinks in the air. Below-cloud atmospheric particles are removed by raindrops via Brownian diffusion, interception, and impaction. Generally, an increase in the particle size will induce an increase in the collection efficiency as a result of interception and inertial impaction mechanisms; whereas, a reduction in the particle size increases the collection efficiency due to Brownian diffusion. These changes of the aerosol size distribution during the precipitation may change the optical properties of atmospheric aerosol such as extinction, visibility and Ångstrom exponent, etc. In this study, the change of the optical properties of polydispersed aerosol was simulated quantitatively. The polydispersed log-normal size distribution is assumed and the rain distribution from Bae et al. (2010) was parameterized. Based on these assumptions, the change of extinction coefficient was simulated as a function of time for various rain intensities, rain distribution and precipitation time with different aerosol size distribution. The results showed that the below-cloud scavenging process scavenged aerosol particles and change the aerosol size distribution, which affect the aerosol optical properties. The degree of change in optical properties during the precipitation was a function of rain and aerosol size distribution and precipitation time.

References

Bae, S. Y., Jung, C. H., and Kim, Y. P. (2010) Derivation and verification of an aerosol dynamics expression for the below-cloud scavenging process using the moment method, Journal of Aerosol Science, 41(3), 266–280.