| Abstract
| - The gas-phase reaction of ozone with alkenes is known to be a significant source of OH radicals in thetroposphere. The pressure dependence of the OH yield in ozone−alkene reactions is both important andcontroversial; the poor understanding of the pressure-dependent OH yield for different ozone−alkene reactionsis a major obstacle to developing an accurate simulation of tropospheric chemistry. Using a high-pressureflow reactor, we have investigated the ozonolysis of a series of alkenes in the presence of NO2. The fouralkenes studied were 2,3-dimethyl-2-butene (TME), trans-5-decene, cyclohexene, and α-pinene, which providesignificant differences in size (C6 vs C10) and structure (linear vs cyclic) to elucidate the influence of thesecompeting effects on OH formation. OH yields from TME and trans-5-decene ozonolysis decrease withincreasing pressure, but OH yields from cyclohexene (0.64 ± 0.20) and α-pinene (0.89 ± 0.20) are pressure-independent and consistent with the literature. Acetone production increases relative to TME consumption aspressure increases; this observation, supported by density functional calculations, is consistent with acetoneand nitrate radical production from the SCI + NO2 reaction. Both the pressure dependence of OH formationfrom the linear alkenes (TME and trans-5-decene) and the pressure-independent OH yields observed forcyclohexene and α-pinene can be explained by changes in the extent of collisional stabilization of the carbonyloxide (Criegee) intermediate with increasing pressure.
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