Kinetics of the unimolecular reaction of CH2OO and the bimolecular reactions with the water monomer, acetaldehyde and acetone under atmospheric conditionsElectronic supplementary information (ESI) available. See DOI: 10.1039/c5cp02224j

Stabilized Criegee Intermediates (sCIs) have been identified as oxidants of atmospheric trace gases such as SO 2 , NO 2 , carboxylic acids or carbonyls. The atmospheric sCI concentrations, and accordingly their importance for trace gas oxidation, are controlled by the rate of the most important loss processes, very likely the unimolecular reactions and the reaction with water vapour (monomer and dimer) ubiquitously present at high concentrations in the troposphere. In this study, the rate coefficients of the unimolecular reaction of the simplest sCI, formaldehyde oxide, CH 2 OO, and its bimolecular reaction with the water monomer have been experimentally determined at T = (297 ± 1) K and at atmospheric pressure by using a free-jet flow system. CH 2 OO was produced by the reaction of ozone with C 2 H 4 , and CH 2 OO concentrations were probed indirectly by detecting H 2 SO 4 after titration with SO 2 . Time-resolved experiments yield a rate coefficient of the unimolecular reaction of k (uni) = (0.19 ± 0.07) s −1 , a value that is supported by quantum-chemical and statistical rate theory calculations as well as by additional measurements performed under CH 2 OO steady-state conditions. A rate coefficient of k (CH 2 OO+H 2 O) = (3.2 ± 1.2) × 10 −16 cm 3 molecule −1 s −1 has been determined for sufficiently low H 2 O concentrations (