Just add water dimers

Alkenes constitute a large fraction of the natural and human-made volatile organic compounds (VOCs) that are emitted into the troposphere. Their oxidation products degrade air quality and contribute to climate warming. Alkene oxidation is thought to involve Criegee intermediates (CIs), highly reactive molecules that form when ozone reacts with alkenes. However, the impact of CIs may be limited if they react rapidly with water. Modelers have found it difficult to quantify the effect of CIs on atmospheric composition, because laboratory data on CI reactions with water have been contradictory. On page 751 of this issue, Chao et al. (1) show that the simplest CI, formaldehyde oxide (CH_2OO), reacts rapidly with the water dimer, (H_2O)_2. Similar results are reported by Lewis et al. (2)