Kinetics and mechanism of the oxidation of S(IV) by ozone in aqueous solution with particular reference to SO₂ conversion in nonurban tropospheric clouds

Abstract

The kinetics of S(IV) oxidation by ozone in aqueous solution have been studied at 10°C and 25°C in the pH range 3.0 to 6.2 by using stopped‐flow spectrophotometry. The rate data are empirically correlated by r = ( k 1 + k 2 10 pH )[S(IV)][O 3 ] with k 1 = 4.39 × 10 11 e −4131/ T (±3 × 10 4 ) M −1 S −1 , and k 2 = 2.56 × 10 3 e −966/ T (±15)s −1 at an ionic strength of 0.01 M . Reaction rate is unaffected by incident strong UV radiation, or the presence of Mn(II) or Fe(III), but is weakly dependent on ionic strength. The present results, in comparison with reported rates of oxidation of S(IV) by hydrogen peroxide, indicate that at typical pH levels encountered in nonurban tropospheric cloud water and at representative concentrations of O 3 and H 2 O 2 in the troposphere, oxidation by O 3 can be competitive with that by H 2 O 2 , or possibly even dominate. The reaction appears to proceed by other than a free‐radical process, and a reaction scheme involving direct ozone attack catalyzed by hydroxide ion is proposed. A rate expression consistent with this scheme is r = ( k a + k b [OH − ])[HSO 3 − ][O 3 ] where k a = 3.8 × 10 5 M −1 s −1 and k b = 1.05 × 10 16 M −2 s −1 at 25°C. This expression correlates the present data with literature data over the pH range 1.0 to 6.2.