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 SO 2 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 SO 2 can occur at substantial rates through these homogenous reaction paths. The direct photo-oxidation of SO 2 by way of the electronically excited states of SO 2 is relatively unimportant for most conditions which occur within the troposphere. The oxidation of SO 2 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 + SO 2 (+ M) → HOSO 2 ( + M) (39); HO 2 + SO 2 → HO + SO 3 (31); CH 3O 2 + SO 2→CH 3O + SO 3 (33). By combining our kinetic estimates with the Crutzen and Fishman calculation of [HO], [HO 2] and [CH 3O 2] for the troposphere, we estimate that the total rates of SO 2 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 SO 2, NO x, hydrocarbon, CO, aldehyde-polluted lower troposphere, it is predicted that the three reactions, 39. 31 and 33 occur with about equal rates; SO 2 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 SO 2 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 NO 2 and HONO photolysis followed by reaction 39 and the reaction. O ( 3P) + SO 2 (+ M)→ SO 3 (+ M). After the extensive dilution of the stack gases by polluted urban air, the rate of SO 2 homogeneous oxidation is expected to approach that for a typical polluted urban atmosphere (~4%h −1).