MECHANISM OF THE HOMOGENEOUS OXIDATION OF SULFUR DIOXIDE IN THE TROPOSPHERE

Abstract

An evaluation has been made of the existing kinetic data related to the elementary, homogeneous reactions of SO2 within the troposphere. A set of preferred values of the rate constants for these reactions is presented. Simulations using these data provide significant new evidence that the oxidation of SO2 can occur at substantial rates through these homogenous reaction paths. The direct photo-oxidation of SO2 by way of the electronically excited states of SO2 is relatively unimportant for most conditions which occur within the troposphere. The oxidation of SO2 within the natural troposphere is expected to occur largely by way of reactions 39, 31, and 33, with reaction 39 being the dominant path: HO + SO2 (+ M) → HOSO2 (+ M) (39); HO2 + SO2 → HO + SO3 (31); CH3O2 + SO2 → CH3O + SO3 (33). By combining our kinetic estimates with the Crutzen and Fishman calculation of [HO], [HO2], and [CH3O2] for the troposphere, we estimate that the total rates of SO2 oxidation as high as 1.5% h–1 are expected at midday in July in the midlatitudes. Theoretical estimates of the monthly rates averaged over the northern hemisphere vary from a low of 0.1% h–1 in January to a maximum of about 0.2% h–1 in July. From our computer simulations of the reactions within an SO2, NOx, hydrocarbon, CO, aldehyde-polluted lower troposphere, it is predicted that the three reactions, 39, 31, and 33 occur with about equal rates; SO2 oxidations for this case can proceed homogeneously at rates as high as 4% h–1. Considerations of the reactions in stack gas plumes suggest that a small maximum in the SO2 photo-oxidation rate may occur during the early stages of the dispersion of a parcel of the stack gases into the air. This should be followed by a short period of slower oxidation. In theory the initial burst is expected to arise from NO2 and HONO photolysis followed by reaction 39 and the reaction, O (3P) + SO2 (+ M) → SO3 (+ M). After the extensive dilution of the stack gases by polluted urban air, the rate of SO2 homogeneous oxidation is expected to approach that for a typical polluted urban atmosphere (~4% h–1).