On the scavenging of SO2 by cloud and raindrops: II. An experimental study of SO2 absorption and desorption for water drops in air |
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| Authors: | C J Walcek H R Pruppacher J H Topalian S K Mitra |
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| Institution: | (1) Department of Atmospheric Sciences, University of California, 90024 Los Angeles, CA, U.S.A. |
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| Abstract: | For the purpose of testing our previously described theory of SO2 scavenging a laboratory investigation was carried out in the UCLA 33 m long rainshaft. Drops with radii between 250 and 2500  m were allowed to come to terminal velocity, after which they passed through a chamber of variable length filled with various SO2 concentrations in air. After falling through a gas separating chamber consisting of a fluorocarbon gas the drops were collected and analyzed for their total S content in order to determine the rate of SO
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absorption.The SO2 concentration in air studied ranged between 1 and 60% (v). Such relatively large concentrations were necessary due to the short times the drops were exposed to SO2 in the present setup. The present experimental results were therefore not used to simulate atmospheric conditions but rather to test our previously derived theory which is applicable to any laboratory or atmospheric condition. Comparison of our studies with the results from our theory applied to our laboratory conditions led to predicted values for the S concentration in the drops which agreed well with those observed if the drops had radii smaller than 500 m. In order to obtain agreement between predicted and observed S concentrations in larger drops, an empirically derived eddy diffusivity for SO2 in water had to be included in the theory to take into account the effect of turbulent mixing inside such large drops.In a subsequent set of experiments, drops initially saturated with S (IV) were allowed to fall through S-free air to determine the rate of SO
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desorption. The results of these studies also agreed well with the results of our theoretical model, thus justifying the reversibility assumption made in our theoretical models.In a final set of experiments, the effects of oxidation on SO2 absorption was studied by means of drops containing various amounts of H2O2. For comparable exposure times to SO2, the S concentration in drops with H2O2 was found to be up to 10 times higher than the concentration in drops in which no oxidation occurred. |
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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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