Uncertainty assessment of current size-resolved parameterizations for below-cloud particle scavenging by rain

Current theoretical and empirical size-resolved parameterizations of the scavenging coefficient (3), 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 raindrop-particle collection efficiency can cause uncertainties in size-resolved 3 values of one to two orders of magnitude for particles in the 0.01-3 mu m diameter range. Use of different formulations of raindrop number size distribution can cause 3 values to vary by a factor of 3 to 5 for all particle sizes. The uncertainty in 3 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 3 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 3 parameterizations. The predicted size-resolved particle concentrations using different theoretical 3 parameterization can differ by up to a factor of 2 for particles smaller than 0.01 mu m and by a factor of > 10 for particles larger than 3 mu m 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 3 parameterizations after 2-5 mm of moderate intensity rainfall.