| Abstract
| - The gas-phase reaction of ozone with alkenes is known to be a dark source of HOx radicals (such as OH, H,and R) in the troposphere, though the reaction mechanism is currently under debate. It is understood that akey intermediate in the reaction is the carbonyl oxide, which is formed with an excess of vibrational energy.The branching ratios of the ozone−alkene reaction products (and thus HOx yields) depend critically on thefate of this intermediate: it may undergo unimolecular reaction (forming either OH or dioxirane) or becollisionally stabilized by the bath gas. To investigate this competition between reaction and quenching, wepresent direct, pressure-dependent measurements of hydroxyl radical (OH) yields for a number of gas-phaseozone−alkene reactions. Experiments are carried out in a high-pressure flow system (HPFS) equipped todetect OH using laser-induced fluorescence (LIF). Hydroxyl radicals are measured in steady state, formedfrom the ozone−alkene reaction and lost to reaction with the alkene. Short reaction times (usually ∼10 ms)ensure negligible interference from secondary and heterogeneous reactions. For all substituted alkenes coveredin this study, low-pressure yields are large but decrease rapidly with pressure, resulting in yields at 1 atmwhich are significantly lower than current recommendations and indicating the important role of collisionalstabilization in determining OH yield. The influence of alkene size and degree of substitution on pressure-dependent yield is consistent with the influence of collisional stabilization as well as the accepted reactionmechanism.
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