Mechanistic insights into the tropospheric ozonolysis of alkenes: Radical yields and the fate of stabilised Criegee intermediates

The gas-phase reaction of alkenes with ozone has been the subject of considerable research interest over several decades owing to the key role it plays in the Earth's atmosphere. Alkene-ozone reactions are widely recognised as a significant non-photolytic source of radicals, initiating further oxidation and contributing to the overall tropospheric HOx budget, which can be dominant under certain conditions. Central to the ozonolysis reaction mechanism is the formation of energy-rich Criegee Intermediates (CIs). Vibrationally excited CIs can either undergo unimolecular decomposition (to give OH, HO2 and RO2 radicals, yields dependent on CI structure) or can be collisionally stabilised forming stabilised CIs (SCIs), stabilised vinyl hydroperoxides (SVHP) and dioxirane (DIOX) species, which are available to take part in bimolecular reactions. Taatjes and co-workers, directly observing low-pressure CI kinetics for the first time, recently reported that the reactions of the CH2OO SCI (Welz et al., Science, 2012) and the anti-conformer of the CH3CHOO SCI (Taatjes et al., Science, 2013) with SO2 are much faster than originally thought. Therefore, bimolecular SCI reactions can potentially make a substantial contribution to tropospheric SO2 oxidation, implying more rapid formation of SO3, a precursor of sulfate aerosol, with implications for heterogeneous chemistry and direct/indirect climate forcing - an observation substantiated by H2SO4 field measurements (Maudlin et al., Nature, 2012). Here we present results from a series of ozonolysis experiments performed at the EUPHORE atmospheric simulation chamber, Valencia. We report direct radical measurements (OH, HO2 and RO2) for a range of small alkenes (ethene - isoprene). The loss of SO2 in the presence of various important alkene ozonolysis systems, as a function of water vapour, have also been used to derive SCI decomposition rates as well as rate constants for reactions with H2O under atmospheric boundary layer conditions. The results are discussed in terms of mechanistic insight and atmospheric implications.