Atmospheric Environment 42 (2008) 3076–3086 Scavenging of soluble gases by evaporating and growing cloud droplets in the presence of aqueous-phase dissociation reaction Tov Elperin à , Andrew Fominykh, Boris Krasovitov Department of Mechanical Engineering, The Pearlstone Center for Aeronautical Engineering Studies, Ben-Gurion University of the Negev, P.O. Box 653, Beer Sheva 84105, Israel Received 11 September 2007; received in revised form 13 December 2007; accepted 13 December 2007 Abstract In this study we performed numerical analysis of simultaneous heat and mass transfer during evaporation and condensation of a cloud droplet in the presence of soluble gases. It is assumed that gas absorption is accompanied by subsequent aqueous-phase equilibrium dissociation reactions. The system of transient conjugate nonlinear energy and mass conservation equations was solved using anelastic approximation and taking into account thermal effect of gas absorption. It was shown that nonlinear behavior of different parameters, such as temperature and absorbate concentration at the droplet surface stems from the interaction of heat and mass transfer processes. We found that thermal effect of absorption and Stefan flow result in the maximum of droplet surface temperature during the transient period of droplet evaporation. It was shown that heat and mass transfer rates in water droplet–air–water vapor system at short times are considerably enhanced under the effects of Stefan flow, heat of absorption and dissociation reactions within the droplet. Comparison of the results obtained using the model of physical absorption of sulfur dioxide in water droplet with the predictions of the present model that takes into account the subsequent equilibrium dissociation reactions showed that the model of physical absorption underestimates the value of droplet surface temperature and overestimates the average concentration of [SO 2  H 2 O] at the transient stage of gas absorption. The performed calculations showed that the value of pH increase with the increasing relative humidity (RH). r 2007 Elsevier Ltd. All rights reserved. Keywords: Heat transfer; Mass transfer; Droplet evaporation; Gas absorption 1. Introduction Clouds play an important role in removing gaseous pollutants from atmosphere. Scavenging of atmospheric gaseous pollutants by cloud droplets is a result of gas absorption mechanism (Pruppacher and Klett, 1997; Flossmann, 1998). Presence of soluble gas in the atmosphere can affect the dynamics of evaporation and condensation of water droplets in the atmospheric clouds. Comprehensive study of coupled heat and mass transfer during gas absorption by liquid droplets and droplets evapora- tion and growth is a necessary step for adequate predicting of atmospheric changes under the influ- ence of hazardous gases. Gas absorption by non-evaporating droplets accompanied by subsequent dissociation was dis- cussed by Baboolal et al. (1981), Walcek and ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.12.036 à Corresponding author. Tel.: +972 8 6477078; fax: +972 8 6472813. E-mail address: elperin@bgu.ac.il (T. Elperin).