Mass-transport limitation to the rate of in-cloud oxidation of SO 2: Re-examination in the light of new data

Abstract

Recent measurements of mass-accommodation coefficients of SO 21, O 3, and H 2O 2 on aqueous solution allow definitive examination of mass-transport limitation to the rate of in-cloud aqueous-phase oxidation of SO 2 by O 3 and H 2O 2. Conditions explicitly considered were drop diameter, 10 and 30 μm; temperature, 0 and 25°C; pH, 2–6; reagent partial pressures O 3,30 ppb; H 2O 2, 1 ppb; SO 2, 1 ppb. However, the graphical analysis employed readily permits extension to other conditions. Despite the low massaccommodation coefficient of O 3(5 × 10 −4; Tang and Lee, Chemistry of Acid Rain, 1987) interfacial mass transport of this species presents no limitation to the in-cloud reaction rate. Lower bounds for massaccommodation coefficients of H 2O 2 and SO 2 (⩾ 0.2 and ⩾ 0.08, respectively; Gardner et al, 1987) establish that interfacial mass transport of these species is less restrictive than gas-phase diffusion and is not a limitation to the in-cloud reaction under conditions of interest. Although aqueous-phase limitation of the O 3 reaction and gas-phase limitation of the H 2O 2 reaction are indicated under certain extremes of conditions, for the most part this limitation is absent, ruling out any drop-size dependence to the kinetics of these reactions in most situations. The present results further indicate that the rates of these reactions may be accurately evaluated in many situations of interest by treating cloudwater as saturated in the reagent gases.