Development of a new semi-empirical parameterization for below-cloud scavenging of size-resolved aerosol particles by both rain and snow

Abstract

A parameter called the scavenging coefficient λ is widely used in aerosol chemical transport models (CTMs) to describe below-cloud scavenging of aerosol particles by rain and snow. However, uncertainties associated with available size-resolved theoretical formulations for λ span one to two orders of magnitude for rain scavenging and nearly three orders of magnitude for snow scavenging. Two recent reviews of below-cloud scavenging of size-resolved particles recommended that the upper range of the available theoretical formulations for λ should be used in CTMs based on uncertainty analyses and comparison with limited field experiments. Following this recommended approach, a new semiempirical parameterization for size-resolved λ has been developed for below-cloud scavenging of atmospheric aerosol particles by both rain (λrain) and snow (λsnow). The new parameterization is based on the 90th percentile of λ values from an ensemble data set calculated using all possible "realizations" of available theoreticalλformulas and covering a large range of aerosol particle sizes and precipitation intensities (R). For any aerosol particle size of diameter d, a strong linear relationship between the 90th-percentile log10(λ) and log10(R), which is equivalent to a power-law relationship between λ and R, is identified. The log-linear relationship, which is characterized by two parameters (slope and y intercept), is then further parameterized by fitting these two parameters as polynomial functions of aerosol size d. A comparison of the new parameterization with limited measurements in the literature in terms of the magnitude of λ and the relative magnitudes of λrain and λsnow suggests that it is a reasonable approximation. Advantages of this new semiempirical parameterization compared to traditional theoretical formulations for λ include its applicability to belowcloud scavenging by both rain and snow over a wide range of particle sizes and precipitation intensities, ease of implementation in any CTM with a representation of size-distributed particulate matter, and a known representativeness, based on the consideration in its development, of all available theoretical formulations and field-derived estimates for λ(d) and their associated uncertainties. © 2014 Author(s).