As indicated above, quantitative evaluation of the rates of aqueous-phase reactions in clouds are predicated on the assumption that the rate of mass transport processes coupling the gas-phase reservoir of reagent gas to the solution phase within individual cloud droplets is sufficiently fast to maintain the Henry's law equilibrium in competition with the sink of dissolved material by aqueous-phase reaction. The pertinent mass transfer processes are gas-phase diffusion, from the bulk of the gas phase to the gas-liquid interface; transfer across the interface, as governed by the gas-kinetic collision rate and the mass accommodation coefficient, the fraction of collisions resulting in transfer of material across the interface, a property characteristic of individual gases and solutions; and aqueous-phase diffusion of the dissolved gas occurring concomitantly with aqueous-phase reaction. In general, if the reaction is sufficiently slow, mass transport is sufficiently rapid to maintain the solubility equilibria, but departure from equilibrium occurs for sufficiently rapid reaction rates. Criteria for the onset of this "mass transport limitation" of the rate of aqueous-phase reactions in clouds have been developed in terms of drop radius, Henry's law coefficient, effective first-order reaction rate coefficient, diffusion coefficients, and mass accommodation coefficients. For the most part, the rate of reaction of S02 in cloudwater appears only minimally limited by mass transport rates, the exception being the ozone reaction at high pH, under which condition both the solubility and effective first-order rate coefficient are quite large.
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