A study of the spectra and reactivity of oxysulphur-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)m(2)mol(-1), epsilon(SO4-.)(450) = (163 +/- 5)m(2) mol(-1) and epsilon(SO5-.)(260) = (79 +/- 2)m(2)mol(-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-. --> S2O82- + O-2 for which 2k(16) = (9.6 +/- 0.3) x 10(7) dm(3) 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 k(7) = (8.3 +/- 1.0) x 10(3) dm(3) mol(-1) s(-1) and k(9) = (3.4 +/- 0.4) x 10(2) dm(3) mol(-1) s(-1) and at pH 9 SO5-. + SO32- (+ H+) --> HSO5- + SO3-. (reaction 8) and SO5-. + SO32- --> SO4-. + SO42- (reaction 10) for which k(8) = (3.6 +/- 0.8) x 10(5) dm(3) mol(-1) s(-1) and k(10) = (1.4 +/- 0.8) x 10(5) dm(3) 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 10(5) dm(3) mol(-1) s(-1) and (k(8) + k(10)) = 1.3 x 10(7) dm(3) 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. Copyright (C) 1996 Elsevier Science Ltd