A size-resolved model for below-cloud scavenging of aerosols by snowfall

A size-resolved model of below-cloud scavenging is required in air quality and atmospheric dispersion modeling. Explicit calculation of the below-cloud snow scavenging coefficient requires determinations of (1) cross section, (2) terminal velocity, and (3) number size distribution of snow particles; and (4) spectral collision efficiency of aerosol particles with snow particles. In this study, we present a size-resolved model for below-cloud snow scavenging, based on current knowledge of each element of the above. In particular, a new scheme is proposed to specify the spectral collision efficiency of aerosol particles for the whole size range of snow particles. The proposed scheme specifies the collision efficiencies for planar and columnar ice crystals until their specific critical sizes, according to the theoretical calculations by Martin et al. (1980) and Miller and Wang (1989), respectively; uses the parameterization of Murakami et al. (1985a) to calculate the spectral collision efficiency for snow particles with maximum dimension D-m equal to 5 mm; and then determines the collision efficiency for D-m between the critical size and 5 mm through exponential interpolation. For D-m greater than 5 mm, the collision efficiency is set to be equal to that for D-m = 5 mm. Compared with the measured one in the literature, the current model generally captures well the trend and the variation range of the scavenging coefficient.