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

Absorption spectra of the radicals SO3−., SO4−. and SO5−. have been redetermined and their molar absorptivities at λmax evaluated as: ε(SO3−.)250 = (114±2) m2mol−1, ε(SO4−.)450 = (163 ± 5) m2 mol−1 and ε(SO5−.)260 = (79 + 2) m2 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 SO5−. + SO5−. → S2O82− + O2 for which 2κ16 = (9.6 ± 0.3) x 107 dm3mol−1 s−1 The rate determining propagation steps at pH 4 are SO5−. + HSO3− → HSO5−. + SO3−. (reaction 7) and SO5−. + HSO3−. → SO4−. + H+ + SO42− (reaction 9) with κ7 = (8.3 ± 1.0) x 103 dm3 mol−1 s−1 and κ9 = (3.4 ± 0.4) x 102 dm3 mol−1 s−1 and at pH 9 SO5−. + SO32− (+ H+) → HSO5− + SO3−. (reaction 8) and SO5−. + SO32− → SO4− + SO42− (reaction 10) for which κ8 = (3.6 ± 0.8) × 105 dm3 mol−1 s−1 and κ10 = (1.4 ± 0.8) × 105 dm3 mol−1 s−1 A previous report (Huie and Neta, 1987, Atmospheric Environment 21, 1743–1747) gave (κ7 + κ9) ≤ 3 x 105 dm3 mol−1 s− and (κ8 + κ10) = 1.3 x 107 dm3 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.