A potential source of atmospheric sulfate from O 2 - -induced SO 2 oxidation by ozone

Abstract

Abstract. It was formerly demonstrated that O2SOO− forms at collisions\nrate in the gas phase as a result of SO2 reaction with\n O 2 - . Here, we present a theoretical investigation of the\nchemical fate of O2SOO− by reaction with O3 in the\ngas phase, based on ab\xa0initio calculations. Two main mechanisms were found\nfor the title reaction, with fundamentally different products: (i)\xa0formation\nof a van der Waals complex followed by electron transfer and further\ndecomposition to O2 + SO2 + O 3 - and\n(ii)\xa0formation of a molecular complex from O2 switching by\n O3 , followed by SO2 oxidation to SO 3 - within\nthe complex. Both reactions are exergonic, but separated by relatively low\nenergy barriers. The products in the former mechanism would likely initiate\nother SO2 oxidations as shown in previous studies, whereas the\nlatter mechanism closes a path wherein SO2 is oxidized to\n SO 3 - . The latter reaction is atmospherically relevant since it\nforms the SO 3 - ion, hereby closing the SO2 oxidation\npath initiated by O 2 - . The main atmospheric fate of\n SO 3 - is nothing but sulfate formation. Exploration of the\nreactions kinetics indicates that the path of reaction (ii)\xa0is highly\nfacilitated by humidity. For this path, we found an overall rate constant of\n 4.0 × 10 - 11 \u2009cm 3 \u2009molecule −1 \u2009s −1 at 298\u2009K and 50\u2009%\nrelative humidity. The title reaction provides a new mechanism for sulfate\nformation from ion-induced SO2 oxidation in the gas phase and\nhighlights the importance of including such a mechanism in modeling\nsulfate-based aerosol formation rates.