Formation of OH radicals in the gas-phase reaction of propene, isobutene, and isoprene with O-3: Yields and mechanistic implications

OH radicals originating from the alkene-ozone reaction have been proposed a s a relevant source of OH radicals in the lower troposphere. Since the repo rted yields of OH radicals differ considerably, we redetermined the OH radi cal yield for three terminal alkenes by performing a series of pseudo-first -order experiments. Ozonolysis studies were carried out under excess ozone conditions in the presence of different cyclohexane concentrations. The dec ay rate of the alkene (k(obs)) was followed by long-path FTIR spectroscopy. From the decrease of the effective rate constant (k(obs) = k(eff)[O-3]) up on addition of cyclohexane, the OH radical yield was determined. The OH rad ical yields were found to be independent of the concentration of reactants for the Criegee intermediates, which are formed in ozonolysis systems. From these results we conclude that OH radicals are formed in a unimolecular pr ocess, presumably from the decomposition of the excited Criegee intermediat e. Determined yields of OH radical formation in the ozonolysis of propene, isobutene, and isoprene were 0.34, 0.60, and 0.26, respectively. Detailed p roduct studies were performed to verify if the observed stable products can be explained by the assumption that OH radicals are formed via the hydrope roxide channel as proposed by Niki et al. [J. Phys. Chem. 1987, 91, 941-946 ]. For the isobutene-ozone system, experimental product yields were found t o agree well with predictions from a chemical mechanism based on the chemis try of the acetonylperoxy radical CH3C(O)CH2O2, which is formed as an impor tant radical product from the decomposition of the (CH3)(2)COO Criegee inte rmediate.