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

Abstract. 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 O₃. The most important reactions are (a) oxidation to O₂SO₃⁻ and (b) cluster decomposition into SO₃, O₂ and O₃⁻. The former reaction is highly exothermic, and the nascent O₂SO₃⁻ will rapidly decompose into SO₄⁻ and O₂. If the origin of O₂SO₃⁻ is SO₂ oxidation by O₃⁻, 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 NO_x, we roughly estimate that ion-induced catalysis may contribute with several percent of H₂SO₄ levels in typical CO₂-free and low NO_x reaction chambers, e.g. the CLOUD chamber at CERN.