Hydration increases the lifetime of HSOsub5/sub and enhances its ability to act as a nucleation precursor – a computational study

摘要

Recent experimental findings indicate that HSO₅ radicals mayplay a key role in the nucleation of atmospheric SO₂ oxidationproducts. HSO₅ radicals are metastable intermediates formed in theSO₂ oxidation process, and their stability and lifetime are, atpresent, highly uncertain. Previous high-level computational studies havepredicted rather low stabilities for HSO₅ with respect to dissociationinto SO₃+HO₂, and have predicted the net reaction HSO₃+OH→SO₃+HO₂to be slightly exothermal. However, these studieshave not accounted for hydration of HSO₅ or its precursor HSO₃. Inthis study, we have estimated the effect of hydration on the stability andlifetime 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 alsoreanalyzed the individual steps of the HSO₃+O₂→HSO₅→SO₃+HO₂ reaction at a higher level of theory thanpreviously published. Our results indicate that hydration is likely tosignificantly prolong the lifetime of the HSO₅ intermediate inatmospheric conditions, thus increasing the probability of reactions thatform products with more than one sulfur atom. Kinetic modeling indicatesthat these results may help explain the experimental observations that amixture of sulfur-containing products formed from SO₂ oxidation by OHradicals nucleates much more effectively than sulfuric acid taken from aliquid reservoir.