Exploring the atmospheric chemistry of O2SO3- and assessing the maximum turnover number of ion-catalysed H2SO4 formation

It has recently been demonstrated that the O2SO3- ion forms in the atmosphere as a natural consequence of ionizing radiation. Here, we present a density functional theory-based study of the reactions of O2SO3- with O-3. The most important reactions are (a) oxidation to O3SO3- and (b) cluster decomposition into SO3, O-2 and O-3(-). The former reaction is highly exothermic, and the nascent O3SO3- will rapidly decompose into SO4- and O-2. If the origin of O2SO(3)(-) is SO2 oxidation by O-3(-), the latter reaction closes a catalytic cycle wherein SO2 is oxidized to SO3. The relative rate between the two major sinks for O2SO3- is assessed, thereby providing a measure of the maximum turnover number of ion-catalysed SO2 oxidation, i.e. how many SO2 can be oxidized per free electron. The rate ratio between reactions (a) and (b) is significantly altered by the presence or absence of a single water molecule, but reaction (b) is in general much more probable. Although we are unable to assess the overall importance of this cycle in the real atmosphere due to the unknown influence of CO2 and NOx, we roughly estimate that ion-induced catalysis may contribute with several percent of H2SO4 levels in typical CO2-free and low NOx reaction chambers, e. g. the CLOUD chamber at CERN.