Hydration increases the lifetime of HSO₅ and enhances its ability to act as a nucleation precursor – a computational study

Abstract. Recent experimental findings indicate that HSO₅ radicals may play a key role in the nucleation of atmospheric SO₂ oxidation products. HSO₅ radicals are metastable intermediates formed in the SO₂ oxidation process, and their stability and lifetime are, at present, highly uncertain. Previous high-level computational studies have predicted rather low stabilities for HSO₅ with respect to dissociation into SO₃+HO₂, and have predicted the net reaction HSO₃+OH→SO₃+HO₂ to be slightly exothermal. However, these studies have not accounted for hydration of HSO₅ or its precursor HSO₃. In this study, we have estimated the effect of hydration on the stability and lifetime of HSO₅ using the advanced quantum chemical methods CCSD(T) and G3B3. We have computed formation energies and free energies for mono- and dihydrates of OH, HSO₃, HSO₅, SO₃ and HO₂, and also reanalyzed the individual steps of the HSO₃+O₂→HSO₅→SO₃+HO₂ reaction at a higher level of theory than previously published. Our results indicate that hydration is likely to significantly prolong the lifetime of the HSO₅ intermediate in atmospheric conditions, thus increasing the probability of reactions that form products with more than one sulfur atom. Kinetic modeling indicates that these results may help explain the experimental observations that a mixture of sulfur-containing products formed from SO₂ oxidation by OH radicals nucleates much more effectively than sulfuric acid taken from a liquid reservoir.