Review and uncertainty assessment of size-resolved scavenging coefficient formulations for below-cloud snow scavenging of atmospheric aerosols
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(Dp), (iii) snow-particle terminal velocity VD, 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(Dp) 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 VD and A. Assumptions about dominant snow-particle shape (and thus different VD 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(Dp) are smaller than a factor of 5, and those from VD 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.