Oxidation of aqueous SO₂ by peroxymonosulfate

Recent model calculations suggest that peroxymonosulfate may constitute a significant fraction of the total sulfur budget in remote tropospheric water droplets such as cloud, fog, and rain. However, little is known about the oxidation of dissolved SO₂ by peroxymonosulfate (HSO₅^-). We have found in aqueous solution that the rate of S(IV) oxidation is comparable to the rate of oxidation of S(IV) by hydrogen peroxide and that HSO₄^- is the only detectable oxidation product. We propose a mechanism in which the rate-determining step involves the acid-catalyzed decomposition of a peroxide-bisulfite intermediate to disulfate ion, S₂O₇^2-, and ultimately to sulfuric acid. The rate equation for this mechanism is -d[HSO₃^-]/dt = k₁(k₂/k_-1)K_a1{H ⁺}[HSO₅^-][S(IV)]/((1 + (k₂/k_-1){H⁺})(K_a1 + {H⁺})), where k₁ = 1.21 × 10⁶ M^-1 s^-1, k₂/k^-1 = 5.9 M^-1, k₁k₂/k_-1 = 7.14 × 10⁶ M^-2 s^-1, K_a1 = 2.64 × 10^-2 M at 5°C, and μ = 0.2 M. The activation parameters are ΔH_k1^≠ = 25.74 ± 0.77 kJ mol^-1 and ΔS_k1^≠ = -88.1 ± 2.7 J mol^-1 K^-1.