Atmos. Chem. Phys. Discuss., 11, 20375-20387, 2011
© Author(s) 2011. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
On the discrepancies between theoretical and measured below-cloud particle scavenging coefficients for rain – a numerical study
X. Wang1, L. Zhang2, and M. D. Moran2
1Kellys Environmental Services, Toronto, Canada
2Air Quality Research Division, Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, M3H 5T4, Canada

Abstract. Existing theoretical formulations for size-resolved scavenging coefficient Λ (r) for atmospheric aerosol particles scavenged by rain predict values lower by one to two orders of magnitude than those estimated from field measurements of particle-concentration changes for particles smaller than 3 μm in diameter. Vertical turbulence does not influence the theoretical formulation of Λ (r), but contributed to the field-generated Λ (r) due to its influence on the overall concentration changes of aerosol particles in the layers undergoing impaction scavenging. A detailed one-dimensional cloud microphysics model is used to simulate rain production and below-cloud particle scavenging, and to quantify the contribution of turbulent diffusion to the overall Λ (r) generated from concentration changes. The relative contribution of vertical diffusion to below-cloud scavenging is found to be largest for submicron particles under weak precipitation conditions. The discrepancies between theoretical and field Λ (r) values can largely be explained by the contribution of vertical diffusion for all particles larger than 0.01 μm in diameter for which field data were available. The results presented here suggest that the current theoretical framework for Λ (r) can provide a reasonable approximation of below-cloud aerosol particle scavenging by rain in size-resolved aerosol transport models if vertical diffusion is also considered.

Citation: Wang, X., Zhang, L., and Moran, M. D.: On the discrepancies between theoretical and measured below-cloud particle scavenging coefficients for rain – a numerical study, Atmos. Chem. Phys. Discuss., 11, 20375-20387, doi:10.5194/acpd-11-20375-2011, 2011.
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