Mechanism of HOx formation in the gas-phase ozone-alkene reaction. 1. Direct, pressure-dependent measurements of prompt OH yields

The gas-phase reaction of ozone with alkenes is known to be a dark source o f HOchi radicals (such as OH, H, and R) in the troposphere, though the reac tion mechanism is currently under debate. It is understood that a key inter mediate in the reaction is the carbonyl oxide, which is formed with an exce ss of vibrational energy, The branching ratios of the ozone-alkene reaction products (and thus HOchi yields) depend critically on the fate of this int ermediate: it may undergo unimolecular reaction (forming either OH or dioxi rane) or be collisionally stabilized by the bath gas. To investigate this c ompetition between reaction and quenching, we present direct, pressure-depe ndent measurements of hydroxyl radical (OH) yields for a number of gas-phas e ozone-alkene reactions. Experiments are carried out in a high-pressure fl ow system (HPFS) equipped to detect OH using laser-induced fluorescence (LI F), Hydroxyl radicals are measured in steady state, formed from the ozone-a lkene reaction and lost to reaction with the alkene. Short reaction times ( usually similar to 10 ms) ensure negligible interference from secondary and heterogeneous reactions. For all substituted alkenes covered in this study , low-pressure yields are large but decrease rapidly with pressure, resulti ng in yields at 1 atm which are significantly lower than current recommenda tions and indicating the important role of collisional stabilization in det ermining 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 reaction mechanism.