Manganese-catalysed autoxidation of dissolved sulfur dioxide in the atmospheric aqueous phase

Autoxidation of SO2(aq) in the presence of manganese(II) is one of the important pathways for sulfuric acid formation in atmospheric clouds and fogs. Recent experimental results indicating that the catalyzed reaction takes place via a complex free-radical mechanism are discussed. Previous literature is reviewed in the light of this mechanism. Under atmospheric conditions of low total concentrations of manganese(II) ( < 2 × 10−5 M) and sulfur(IV) ( ≤ 10−5 M) and 2.5 < pH < 5, the rate law for conversion of SO2(aq) to SO3(aq) is reduced to d[S(IV)]/dt = k[Mn(II)][S(IV)], where [S(IV)] denotes the total concentration. A value of the overall rate constant k of 1.4 × 103 M−1s−1 is recommended for use in atmospheric model calculations.