Review and uncertainty assessment of size-resolved scavenging coefficient formulations for below-cloud snow scavenging of atmospheric aerosols

Abstract. Theoretical parameterizations for the size-resolved scavenging coefficient for atmospheric aerosol particles scavenged by snow (Λ_snow) need assumptions regarding (i) snow particle–aerosol particle collection efficiency E, (ii) snow-particle size distribution N(D_p), (iii) snow-particle terminal velocity V_D, and (iv) snow-particle cross-sectional area A. Existing formulas for these parameters are reviewed in the present study, and uncertainties in Λ_snow caused by various combinations of these parameters are assessed. Different formulations of E can cause uncertainties in Λ_snow of more than one order of magnitude for all aerosol sizes for typical snowfall intensities. E is the largest source of uncertainty among all the input parameters, similar to rain scavenging of atmospheric aerosols (Λ_rain) as was found in a previous study by Wang et al. (2010). However, other parameters can also cause significant uncertainties in Λ_snow, and the uncertainties from these parameters are much larger than for Λ_rain. Specifically, different N(D_p) formulations can cause one-order-of-magnitude uncertainties in Λ_snow for all aerosol sizes, as is also the case for a combination of uncertainties from both V_D and A. Assumptions about dominant snow-particle shape (and thus different V_D and A) will cause an uncertainty of up to one order of magnitude in the calculated scavenging coefficient. In comparison, uncertainties in Λ_rain from N(D_p) are smaller than a factor of 5, and those from V_D are smaller than a factor of 2. As expected, Λ_snow estimated from empirical formulas generated from field measurements falls in the upper range of, or is higher than, the theoretically estimated values, which can be explained by additional processes/mechanisms that influence field-derived Λ_snow but that are not considered in the theoretical Λ_snow formulas. Predicted aerosol concentrations obtained by using upper range vs. lower range of Λ_snow values (a difference of around two orders of magnitude in Λ_snow) can differ by a factor of 2 for just a one-centimetre snowfall (liquid water equivalent of approximately 1 mm). Based on the median and upper range of theoretically generated Λ_snow and Λ_snow values, it is likely that, for typical rain and snow events, the removal of atmospheric aerosol particles by snow is more effective than removal by rain for equivalent precipitation amounts, although a firm conclusion requires much more evidence.