On the scavenging of SO2 by cloud and raindrops: I. A theoretical study of SO2 absorption and desorption for water drops in air |
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| Authors: | C J Walcek H R Pruppacher |
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| Institution: | (1) Department of Atmospheric Science, University of California, 90024 Los Angeles, CA, U.S.A. |
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| Abstract: | An extention of our previous theory for trace gas absorption into freely-falling cloud and raindrops is presented. This theory describes the convective diffusion of a trace gas through air and into a water drop with internal circulation, the drop falling at its terminal velocity. Using flow fields for the circulating water inside and for the moving air outside the drop, obtained by numerical solutions to the Navier—Stokes equation of motion, we numerically solved the convective diffusion equation to determine the uptake of SO2 by water drops of various sizes, time exposure to the gas phase, and concentration of SO2 in the gas phase. It was found that for drops of radius larger than 1 mm and relatively low gas concentrations ( 10 ppb (v)), resistance to gas diffusion lies mainly in the gas phase; while for drops of radius less than 500  m and gas concentrations larger than those found in the atmosphere ( 1% (v)), the resistance to diffusion lies primarily in the liquid phase. With drop sizes and gas concentrations between these limits, the rate of SO2 uptake is controlled by a coupled resistance to diffusion inside and outside the drop. In addition to our general model, a simplified version was formulated which allows considerable savings in computer time for evaluation and improved ease of handling without significant loss of accuracy. A comparison between our simplified model and that of Barrie (1978) shows that the boundary-layer approach of Barrie may be a useful alternate approach to estimating trace gas absorption by water drops, provided appropriate values are chosen for the thickness of the boundary layers involved. |
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| Keywords: | Acid rain convective diffusion equation gas desorption gas scavenging mass transfer reversible washout sulfur dioxide trace gas absorption wet deposition |
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