Reactivity and Role of SO₅•- Radical in Aqueous Medium Chain Oxidation of Sulfite to Sulfate and Atmospheric Sulfuric Acid Generation

This study reevaluates the role of peroxymonosulfate anion radical ( - O 3 SOO • or SO 5 •- ) intermediate during radical-induced chain oxidation of HSO 3 - /SO 3 2- in oxygenated aqueous solution. The SO 5 •- radical absorption band in the UV is weak: ε = 1065 ± 80 M -1 cm -1 at λ max (260−265 nm). The SO 5 •- radical takes part in two radical−radical and four radical−solute reactions, partially producing the other chain carrier, the SO 4 •- radical, in either case. In this study, employing the pulse-radiolysis technique but adopting a new approach, these two types of reactions of the SO 5 •- radical have been separately quantified (at room temperature). For example, over pH 3.5−12, the branching ratio of (SO 5 •- + SO 5 •- ) reactions giving rise to either the SO 4 •- radical or S 2 O 8 2- is found to remain ∼1. The respective reaction rate constants for I → 0 are (2.2 ± 0.3) and (2.1 ± 0.3) × 10 8 M -1 s -1 . The (SO 5 •- + HSO 3 - ) reactions in acid pH follow two paths, forming the SO 4 •- radical in one and regenerating the SO 3 •- radical in the other, with respective rates of ca. (6.0 ± 0.4) and (3.0 ± 0.3) × 10 7 M -1 s -1 . In alkaline pH (for SO 5 •- + SO 3 2- reactions), the rates for similar reactions are ca. (5.6 ± 0.6) and (1.0 ± 0.1) × 10 8 M -1 s -1 . From only these results, the earlier prediction of chain length reaching a few thousands could be supported in simulation studies (Bigelow, S. L. Z. Phys. Chem. 1898, 28, 493. Young, S. W. J. Am. Chem. Soc. 1902, 24, 297. Titoff, A. Z. Phys. Chem. 1903, 45, 641. Bäckström, H. L. J. J. Am. Chem. Soc. 1927, 49, 1460. Alyea, H. N.; Bäckström, H. L. J. J. Am. Chem. Soc. 1929, 51, 90). To explore the feasibility of controlling S(IV) chain oxidation to sulfuric acid in liquid hydrometeors, the effect of radical scavenging on each SO x •- radical ( x = 3, 4, 5) was simulated. The results show that for the SO 5 •- radical a scavenger reactivity of ∼100 s -1 may be enough to reduce the chain length by >98%. However, in the case SO 4 •- radical scavenging under similar conditions, only ∼75−80% reduction in acid production was observed. These results suggest a fresh modeling of sulfuric acid generation in atmospheric liquid hydrometeors.