A Simplified Model of Predicting SO2 Absorption by Single Atmospheric Raindrops with Chemical Dissociation and Internal Circulation

A simplified model of predicting chemical SO2 absorption by single freely falling raindrops with internal circulation in the atmosphere is developed in the present study. By multiplying a modification factor α into the model of interfacial velocity established from creeping flow, it is found that the relative error between the simplified model and the two-phase simulation method is less than 4%. Accordingly, the simplified model enables us to simulate the atmospheric SO2 absorption process with less computational effort and without losing accuracy. The simulated results indicate that the dissociation of H2SO3 governs the mass transfer process and the concentration of HSO3− is by far larger than those of SO32− and H2SO3. As a result, the chemical absorption takes a much longer period of time to achieve the uptake process. Specifically, for the raindrop radius in the range of 200–500 μm, the absorption time of chemical absorption is larger than that of physical absorption by the factors of 70–290. From the perspective of characteristic time, mass diffusion is the controlling mechanism for SO2 absorption. When chemical absorption is carried out, the absorption period is 28–33 folds of the characteristic time of mass diffusion, implying that the former is always larger than the latter by over an order of magnitude.