Ozonolysis fragment quenching by nitrate formation: The pressure dependence of prompt OH radical formation

The gas-phase reaction of ozone with alkenes is known to be a significant source of OH radicals in the troposphere. The pressure dependence of the OH yield in ozone-alkene reactions is both important and controversial; the poor understanding of the pressure-dependent OH yield for different ozone-alkene reactions is a major obstacle to developing an accurate simulation of tropospheric chemistry. Using a high-pressure flow reactor, we have investigated the ozonolysis of a series of alkenes in the presence of NO2. The four alkenes studied were 2,3-dimethyl-2-butene (TME), trans-5-decene, cyclohexene, and alpha-pinene, which provide significant differences in size (C6 vs C10) and structure (linear vs cyclic) to elucidate the influence of these competing effects on OH formation. OH yields from TME and trans-5-decene ozonolysis decrease with increasing pressure, but OH yields from cyclohexene (0.64 +/- 0.20) and alpha-pinene (0.89 +/- 0.20) are pressure-independent and consistent with the literature. Acetone production increases relative to TME consumption as pressure increases; this observation, supported by density functional calculations, is consistent with acetone and nitrate radical production from the SCI + NO2 reaction. Both the pressure dependence of OH formation from the linear alkenes (TME and trans-5-decene) and the pressure-independent OH yields observed for cyclohexene and alpha-pinene can be explained by changes in the extent of collisional stabilization of the carbonyl oxide (Criegee) intermediate with increasing pressure.