Kinetic study of isoprene hydroxy hydroperoxide radicals reacting with sulphur dioxide and their global-scale impact on sulphate formation†

Isoprene is the most relevant volatile organic compound emitted during the biosynthesis of metabolism processes. The oxidation of isoprene by a hydroxy radical (OH) is one of the main consumption schemes that generate six isomers of isoprene hydroxy hydroperoxide radicals (ISOPOOs). In this study, the rate constants of ISOPOOs + sulphur dioxide (SO₂) reactions that eventually generate sulphur trioxide (SO₃), the precursor of sulphate aerosol (SO₄²⁻(p)), are determined using microcanonical kinetic theories coupled with molecular structures and energies estimated by quantum chemical calculations. The results show that the reaction rates range from 10⁻²⁷ to 10⁻²⁰ cm³ molecule⁻¹ s⁻¹, depending on the atmospheric temperature and structure of the six ISOPOO isomers. The effect of SO₃ formation from SO₂ oxidation by ISOPOOs on the atmosphere is evaluated by a global chemical transport model, along with the rate constants obtained from microcanonical kinetic theories. The results show that SO₃ formation is enhanced in regions with high SO₂ or low nitrogen oxide (NO), such as China, the Middle East, and Amazon rainforests. However, the production rates of SO₃ formation by ISOPOOs + SO₂ reactions are eight orders of magnitude lower than that from the OH + SO₂ reaction. This is indicative of SO₄²⁻(p) formation from the direct oxidation of SO₂ by ISOPOOs, which is almost negligible in the atmosphere. The results of this study entail a detailed analysis of SO₃ formation from gas-phase reactions of isoprene-derived products.