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 (SO 2 ) reactions that eventually generate sulphur trioxide (SO 3 ), the precursor of sulphate aerosol (SO 4 2- (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 cm 3 molecule -1 s -1 , depending on the atmospheric temperature and structure of the six ISOPOO isomers. The effect of SO 3 formation from SO 2 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 SO 3 formation is enhanced in regions with high SO 2 or low nitrogen oxide (NO), such as China, the Middle East, and Amazon rainforests. However, the production rates of SO 3 formation by ISOPOOs + SO 2 reactions are eight orders of magnitude lower than that from the OH + SO 2 reaction. This is indicative of SO 4 2- (p) formation from the direct oxidation of SO 2 by ISOPOOs, which is almost negligible in the atmosphere. The results of this study entail a detailed analysis of SO 3 formation from gas-phase reactions of isoprene-derived products.