ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH G̦ttingen, Germany 10.5194/acpd-11-20375-2011 On the discrepancies between theoretical and measured below-cloud particle scavenging coefficients for rain Рa numerical study Wang X. 1 Zhang L. 2 Moran M. D. 2 Kellys Environmental Services, Toronto, Canada Air Quality Research Division, Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, M3H 5T4, Canada 18 07 2011 11 7 20375 20387 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/11/20375/2011/acpd-11-20375-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/11/20375/2011/acpd-11-20375-2011.pdf

Existing theoretical formulations for size-resolved scavenging coefficient &Lambda; (<i>r</i>) 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 &Lambda; (<i>r</i>), but contributed to the field-generated &Lambda; (<i>r</i>) 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 &Lambda; (<i>r</i>) 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 &Lambda; (<i>r</i>) 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 &Lambda; (<i>r</i>) 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.

 References Ackerman, A. S., Toon, O. B., and Hobbs P. V.: A model for particle microphysics, turbulent mixing, and radiative transfer in the stratocumulus-topped marine boundary layer and comparisons with measurements, J. Atmos. Sci., 52, 1204–1236, 1995. Andronache, C., Grönholm, T., Laakso, L., Phillips, V., and Venäläinen, A.: Scavenging of ultrafine particles by rainfall at a boreal site: observations and model estimations, Atmos. Chem. Phys., 6, 4739–4754, http://dx.doi.org/10.5194/acp-6-4739-2006doi:10.5194/acp-6-4739-2006, 2006. Brost, R. and Wyngaard, J. C.: A model study of the stably stratified planetary boundary layer, J. Atmos. Sci., 35, 1427–1440, 1978. Chate, D. M.: Study of scavenging of submicron-sized aerosol particles by thunderstorm rain events, Atmos. Environ., 39, 6608–6619, 2005. Croft, B., Lohmann, U., Martin, R. V., Stier, P., Wurzler, S., Feichter, J., Posselt, R., and Ferrachat, S.: Aerosol size-dependent below-cloud scavenging by rain and snow in the ECHAM5-HAM, Atmos. Chem. Phys., 9, 4653-4675, http://dx.doi.org/10.5194/acp-9-4653-2009doi:10.5194/acp-9-4653-2009, 2009. Feng, J.: A 3-mode parameterization of below–cloud scavenging of aerosols for use in atmospheric dispersion models, Atmos. Environ., 41, 6808–6822, 2007. Grover, S. N. and Pruppacher, H. R.: The effect of vertical turbulent fluctuations in the atmosphere on the collection of aerosol particles by cloud drops, J. Atmos. Sci., 42, 2305–2318, 1985. Henzing, J. S., Olivié, D. J. L., and van Velthoven, P. F. J.: A parameterization of size resolved below cloud scavenging of aerosols by rain, Atmos. Chem. Phys., 6, 3363–3375, http://dx.doi.org/10.5194/acp-6-3363-2006doi:10.5194/acp-6-3363-2006, 2006. Khain, A. P. and Pinsky, M. B.: Turbulence effects on the collision kernel, II: Increase of the swept volume of colliding drops, Quart. J. Roy. Meteor. Soc., 123, 1543–1560, 1997. Loosmore, G. A. and Cederwall R. T.: Precipitation scavenging of atmospheric aerosols for emergency response applications: testing an updated model with new real–time data, Atmos. Environ., 38, 993–1003, 2004. Mircea, M., Stefan, S., and Fuzzi, S.: Precipitation scavenging coefficient: influence of measured aerosol and raindrop size distributions, Atmos. Environ., 34, 5169–5174, 2000. Park, S. H., Jung, C. H., Jung, K. R., Lee, B. K., and Lee, K. W.: Wet scrubbing of polydisperse aerosols by freely falling droplets, Aerosol Sci., 36, 1444–1458, 2005. Seinfeld, J. H. and Pandis, S. N.: Atmospheric Chemistry And Physics: From Air Pollution To Climate Change, 1203 pp., Wiley & Sons, New Jersey, 2006. Slinn, W. G. N.: Precipitation scavenging, in Atmospheric Sciences and Power Production – 1979, Chap 11, Division of Biomedical Environmental Research, U.S. Department of Energy, Washington, DC, 1983. Sparmacher, H., Fülber, K., and Bonka, H.: Below-cloud scavenging of aerosol particles: Particle-bound radionuclides – Experimental, Atmos. Environ., 27A, 605–618, 1993. Toon, O. B., Turco, R. P., Westphal, D., Malone, R., and Liu M. S.: A multidimensional model for aerosols: Description of computational analogs, J. Atmos. Sci., 45, 2123–2143, 1988. Tost, H., Jöckel, P., Kerkweg, A., Sander, R., and Lelieveld, J.: Technical note: A new comprehensive SCAVenging submodel for global atmospheric chemistry modelling, Atmos. Chem. Phys., 6, 565–574, http://dx.doi.org/10.5194/acp-6-565-2006doi:10.5194/acp-6-565-2006, 2006. Wang, X., Zhang, L., and Moran, M. D.: Uncertainty assessment of current size-resolved parameterizations for below-cloud particle scavenging by rain, Atmos. Chem. Phys., 10, 5685–5705, http://dx.doi.org/10.5194/acp-10-5685-2010doi:10.5194/acp-10-5685-2010, 2010. Zhang, L., Michelangeli, D. V., and Taylor P. A.: Numerical studies of aerosol scavenging by low-level, warm stratiform clouds and precipitation, Atmos. Environ., 38, 4653–4665, 2004.