Reactivity and role of SO5•- radical in aqueous medium chain oxidation of sulfite to sulfate and atmospheric sulfuric acid generation

Abstract

This study reevaluates the role of peroxymonosulfate anion radical (-O3SOO• or SO5•-) intermediate during radical-induced chain oxidation of HSO3-/SO32- in oxygenated aqueous solution. The SO5•- radical absorption band in the UV is weak: ε = 1065 ± 80 M-1 cm-1 at λmax (260-265 nm). The SO5•- radical takes part in two radical-radical and four radical-solute reactions, partially producing the other chain carrier, the SO4•- radical, in either case. In this study, employing the pulse-radiolysis technique but adopting a new approach, these two types of reactions of the SO5•- radical have been separately quantified (at room temperature). For example, over pH 3.5-12, the branching ratio of (SO5•- + SO5•-) reactions giving rise to either the SO4•- radical or S2O82- is found to remain ∼1. The respective reaction rate constants for I → 0 are (2.2 ± 0.3) and (2.1 ± 0.3) × 108 M-1 s-1. The (SO5•- + HSO3-) reactions in acid pH follow two paths, forming the SO4•- radical in one and regenerating the SO3•- radical in the other, with respective rates of ca. (6.0 ± 0.4) and (3.0 ± 0.3) × 107 M-1 s-1. In alkaline pH (for SO5•- + SO32- reactions), the rates for similar reactions are ca. (5.6 ± 0.6) and (1.0 ± 0.1) × 108 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 SOx•- radical (x = 3, 4, 5) was simulated. The results show that for the SO5•- radical a scavenger reactivity of ∼100 s-1 may be enough to reduce the chain length by >98%. However, in the case SO4•- 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. © 2001 American Chemical Society.