A study of the spectra and reactivity of oxysulphur-radical anions involved in the chain oxidation of S(IV): A pulse and γ-radiolysis study

• Published in: Atmospheric environment (1994) 1996, Vol.30 (14), p.2483-2493
• Main Authors: Buxton, G.V., Mcgowan, S., Salmon, G.A., Williams, J.E., Wood, N.D.
• Format: Article
• Language: English
• Subjects: Absorption spectroscopy; Clouds; Ions; Oxidation; Q1; Sulfur; Sulfur compounds
• ISSN: 1352-2310; 1873-2844
• DOI: 10.1016/1352-2310(95)00473-4

Absorption spectra of the radicals SO 3 −., SO 4 −. and SO 5 −. have been redetermined and their molar absorptivities at λ max evaluated as: ϵ(SO 3 −.) 250 = (114±2) m 2mol −1, ϵ(SO 4 −.) 450 = (163 ± 5) m 2 mol −1 and ϵ(SO 5 −.) 260 = (79 + 2) m 2 mol −. The rate constants for each of the steps in the freeradical-chain oxidation mechanism at pH 4 and 9 have been determined either by pulse radiolysis or a combination of pulse and steady-state radiolysis. The main chain terminating reaction is SO 5 −. + SO 5 −. → S 2O 8 2− + O 2 for which 2 κ 16 = (9.6 ± 0.3) x 10 7 dm 3mol −1 s −1 The rate determining propagation steps at pH 4 are SO 5 −. + HSO 3 − → HSO 5 −. + SO 3 −. (reaction 7) and SO 5 −. + HSO 3 −. → SO 4 −. + H + + SO 4 2− (reaction 9) with κ 7 = (8.3 ± 1.0) x 10 3 dm 3 mol −1 s −1 and κ 9 = (3.4 ± 0.4) x 10 2 dm 3 mol −1 s −1 and at pH 9 SO 5 −. + SO 3 2− (+ H +) → HSO 5 − + SO 3 −. (reaction 8) and SO 5 −. + SO 3 2− → SO 4 − + SO 4 2− (reaction 10) for which κ 8 = (3.6 ± 0.8) × 10 5 dm 3 mol −1 s −1 and κ 10 = (1.4 ± 0.8) × 10 5 dm 3 mol −1 s −1 A previous report (Huie and Neta, 1987, Atmospheric Environment 21, 1743–1747) gave ( κ 7 + κ 9) ≤ 3 x 10 5 dm 3 mol −1 s − and ( κ 8 + κ 10) = 1.3 x 10 7 dm 3 mol −1s −1. The much lower values reported here suggest that the simple free-radical-chain oxidation of S(IV) may be of less significance in the oxidation of S(IV) in cloud droplets than previously estimated.