Competitive formation of HSO₄^- and HSO₅^- from ion-induced SO₂ oxidation: Implication in atmospheric aerosol formation

This study presents a theoretical investigation of the ion-induced SO₂ oxidation by the hydroxyl (OH) and hydroperoxyl (HO₂) radicals. Initiated by the superoxide ion (O₂^-), the reaction with HO₂ to form the SO₅^- and HSO₄⁻ ions from two different mechanisms is prevented by high energy barriers, 30.20 and 33.42 kcal mol⁻¹, respectively, whereas the reaction with OH to form HSO₅^- is highly exergonic and takes place through a barrierless process. The latter reaction, hereby, not only highlights the formation mechanism of the HSO₅^- ion in the gas-phase, but closes a mechanism for SO₂ oxidation initiated by O₂^-. Giving the suspected role of HSO₅^- to drive aerosol formation besides HSO₄^- that was brought up in earlier studies, we calculated the formation energies and the thermodynamics of its clustering with 1–3 sulfuric acid molecules with and without one ammonia molecule. The results showed that the formation of HSO₅^--based clusters is highly exergonic and that they are more stabilized by ammonia than HSO₄^--based clusters in the cluster size range considered. Assuming thermal equilibrium, the abundance of the largest HSO₅^--containing cluster, namely NH₃⋅(H₂SO₄)₃⋅HSO₅⁻, is determined to be higher than that of the corresponding HSO₄^--containing cluster at ambient conditions, in agreement with a previous experimental result. This indicates the important role the NH₃⋅(H₂SO₄)₃⋅HSO₅⁻ cluster might play in the early stages of aerosol formation, and highlights the necessity to consider larger HSO₅⁻ – sulfuric acid – bases clusters not only to clarify their implication in the mechanism of atmospheric particle formation but also to quantify their effect on aerosol formation rates.