A study of the spectra and reactivity of oxysulfur-radical anions involved in the chain oxidation of S(IV) - a pulse and gamma-radiolysis study

Absorption spectra of the radicals SO3-., SO4-. and SO5-. have been redetermined and their molar absorptivities at lambda(max) evaluated as: epsilon(SO3-.)(250) = (114 +/-2)m2mol-1, epsilon(SO4-.)(450) (163 +/- 5)m2 mol-1 and epsilon(SO5-.)(260) = (79 +/- 2)m2mol -1. The rate constants for each of the steps in the free-radical-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-. --> S2082- + 0-2 for which 2k(16) = (9.6 +/- 0.3) x 107 dm3 mol-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 k7 = (8.3 +/- 1.0) x 103 dm3 mol-1 s-1 and k9 = (3.4 +/- 0.4) x 10(2) dm3 mol-1 s(-1) and at pH9 SO5-. +SO32-(+ H +)--> HSO5- + SO3-. (reaction 8) and SO5-. + SO32- --> SO4-. + SO42-(reaction 10) for which k(8) = (3.6 +/-0.8) x 105 dm3 mol-1 s-1 and k10 = (1.4 +/- 0.8) x 105 dm3 mol-1 s-1. A previous report (Huie and Neta, 1987, Atmospheric Environment 21, 1743-1747) gave (k(7) + k(9)) less than or equal to 3 x 105 dm3 mol-1 s-1 and (k(8) + k(10)) = 1.3 x 107 dm3 mol-1 s-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. (Author)