Abstract. Current theoretical and empirical size-resolved parameterizations of the scavenging coefficient ( ), a parameter commonly used in aerosol transport models to describe below-cloud particle scavenging by rain, have been reviewed in detail and compared with available field and laboratory measurements. Use of different formulations for raindropparticle collection efficiency can cause uncertainties in sizeresolved values of one to two orders of magnitude for particles in the 0.01-3 µm diameter range. Use of different formulations of raindrop number size distribution can cause values to vary by a factor of 3 to 5 for all particle sizes. The uncertainty in caused by the use of different droplet terminal velocity formulations is generally small than a factor of 2. The combined uncertainty due to the use of different formulations of raindrop-particle collection efficiency, raindrop size spectrum, and raindrop terminal velocity in the current theoretical framework is not sufficient to explain the one to two order of magnitude under-prediction of for the theoretical calculations relative to the majority of field measurements. These large discrepancies are likely caused by additional known physical processes (i.e, turbulent transport and mixing, cloud and aerosol microphysics) that influence field data but that are not considered in current theoretical parameterizations. The predicted size-resolved particle concentrations using different theoretical parameterization can differ by up to a factor of 2 for particles smaller than 0.01 µm and by a factor of >10 for particles larger than 3 µm after 2-5 mm of rain. The predicted bulk mass and number concentrations (integrated over the particle size distribution) can differ by a factor of 2 between theoretical and empirical parameterizations after 2-5 mm of moderate intensity rainfall.

Abstract. Theoretical parameterizations for the sizeresolved scavenging coefficient for atmospheric aerosol particles scavenged by snow ( 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 snow caused by various combinations of these parameters are assessed. Different formulations of E can cause uncertainties in 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 ( rain ) as was found in a previous study by Wang et al. (2010). However, other parameters can also cause significant uncertainties in snow , and the uncertainties from these parameters are much larger than for rain . Specifically, different N(D p ) formulations can cause one-order-of-magnitude uncertainties in snow for all aerosol sizes, as is also the case for a combination of uncertainties from both V D and A. Assumptions about dominant snowparticle 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 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, 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 snow but that are not considered in the theoretical snow formulas. Predicted aerosol concentrations obtained by using upper range vs. lower range of snow values (a difference of around two orders of magnitude in 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 snow and 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.