Reactivity of syn-CH₃CHOO with H₂O enhanced through a roaming mechanism in the entrance channel

Criegee intermediates are highly reactive species that play a pivotal role in the chemistry of the atmosphere, substantially impacting global climate and air quality. They are formed through the reaction of ozone with alkenes and considerably influence the formation of hydroxyl radicals and aerosols through their unimolecular decomposition and their reaction with key atmospheric components, respectively. However, their interaction with water vapour, a major atmospheric component, remains inadequately characterized. Here, using both time-dependent laser-induced fluorescence experiments and full-dimensional dynamics calculations, we investigate the reaction of syn-CH₃CHOO, a prevalent Criegee intermediate, with water vapour. Our results reveal a much higher reaction rate than previously estimated, challenging the conventional notion that unimolecular decomposition dominates syn-CH₃CHOO removal. Notably, we uncover a complex mechanism involving a roaming process that enhances reactivity. Our findings necessitate a revised assessment of reactions involving syn-mono- and di-substituted Criegee intermediates with water, which are crucial for accurately estimating the OH budget derived from these intermediates.