A potential source of atmospheric sulfate from O2--induced SO2 oxidation by ozone

It was formerly demonstrated that O2SOO- forms at collisions rate in the gas phase as a result of SO2 reaction with O-2(-). Here, we present a theoretical investigation of the chemical fate of O2SOO- by reaction with O-3 in the gas phase, based on ab initio calculations. Two main mechanisms were found for the title reaction, with fundamentally different products: (i) formation of a van der Waals complex followed by electron transfer and further decomposition to O-2 + SO2 + O-3(-) and (ii) formation of a molecular complex from O-2 switching by O-3, followed by SO2 oxidation to SO3- within the complex. Both reactions are exergonic, but separated by relatively low energy barriers. The products in the former mechanism would likely initiate other SO2 oxidations as shown in previous studies, whereas the latter mechanism closes a path wherein SO2 is oxidized to SO3-. The latter reaction is atmospherically relevant since it forms the SO3- ion, hereby closing the SO2 oxidation path initiated by O-2. The main atmospheric fate of SO3- is nothing but sulfate formation. Exploration of the reactions kinetics indicates that the path of reaction (ii) is highly facilitated by humidity. For this path, we found an overall rate constant of 4.0 x10(-11) cm(3) molecule(-1) s(-1) at 298 K and 50% relative humidity. The title reaction provides a new mechanism for sulfate formation from ion-induced SO2 oxidation in the gas phase and highlights the importance of including such a mechanism in modeling sulfate-based aerosol formation rates.