Stabilized Criegee Intermediates (sCIs) have been identified as oxidants of atmospheric trace gases such as SO2, NO2, 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, CH2OO, 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. CH2OO was produced by the reaction of ozone with C2H4, and CH2OO concentrations were probed indirectly by detecting H2SO4 after titration with SO2. 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 CH2OO steady-state conditions. A rate coefficient of k(CH2OO+H2O) = (3.2 ± 1.2) × 10(-16) cm(3) molecule(-1) s(-1) has been determined for sufficiently low H2O concentrations (<10(15) molecule cm(-3)) that allow separation from the CH2OO reaction with the water dimer. In order to evaluate the accuracy of the experimental approach, the rate coefficients of the reactions with acetaldehyde and acetone were reinvestigated. The obtained rate coefficients k(CH2OO+acetald) = (1.7 ± 0.5) × 10(-12) and k(CH2OO+acetone) = (3.4 ± 0.9) × 10(-13) cm(3) molecule(-1) s(-1) are in good agreement with literature data.

 Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Recently, direct kinetic experiments have shown that the oxidation of sulfur dioxide to sulfur trioxide by reaction with stabilized Criegee intermediates (CIs) is an important source of sulfuric acid in the atmosphere. So far, only small CIs, generated in photolysis experiments, have been directly detected. Herein, it is shown that large, stabilized CIs can be detected in the gas phase by FTIR spectroscopy during the ozonolysis of β-pinene. Their transient absorption bands between 930 and 830 cm(-1) appear only in the initial phase of the ozonolysis reaction when the scavenging of stabilized CIs by the reaction products is slow. The large CIs react with sulfur dioxide to give sulfur trioxide and nopinone with a yield exceeding 80%. Reactant consumption and product formation in time-resolved β-pinene ozonolysis experiments in the presence of sulfur dioxide have been kinetically modeled. The results suggest a fast reaction of sulfur dioxide with CIs arising from β-pinene ozonolysis.

Ozonolysis produces chemically activated carbonyl oxides (Criegee intermediates, CIs) that are either stabilized or decompose directly. This branching has an important impact on atmospheric chemistry. Prior theoretical studies have employed statistical models for energy partitioning to the CI arising from dissociation of the initially formed primary ozonide (POZ). Here, we used direct dynamics simulations to explore this partitioning for decomposition of c-CHO, the POZ in ethylene ozonolysis. A priori estimates for the overall stabilization probability were then obtained by coupling the direct dynamics results with master equation simulations. Trajectories were initiated at the concerted cycloreversion transition state, as well as the second transition state of a stepwise dissociation pathway, both leading to a CI (HCOO) and formaldehyde (HCO). The resulting CI energy distributions were incorporated in master equation simulations of CI decomposition to obtain channel-specific stabilized CI (sCI) yields. Master equation simulations of POZ formation and decomposition, based on new high-level electronic structure calculations, were used to predict yields for the different POZ decomposition channels. A non-negligible contribution of stepwise POZ dissociation was found, and new mechanistic aspects of this pathway were elucidated. By combining the trajectory-based channel-specific sCI yields with the channel branching fractions, an overall sCI yield of (48 ± 5)% was obtained. Non-statistical energy release was shown to measurably affect sCI formation, with statistical models predicting significantly lower overall sCI yields (∼30%). Within the range of experimental literature values (35%-54%), our trajectory-based calculations favor those clustered at the upper end of the spectrum.