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

Abstract 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.