Formation of OH Radicals in the Gas-Phase Reaction of Propene, Isobutene, and Isoprene with O3: Yields and Mechanistic Implications

Abstract

OH radicals originating from the alkene−ozone reaction have been proposed as a relevant source of OH radicals in the lower troposphere. Since the reported yields of OH radicals differ considerably, we redetermined the OH radical 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 decay rate of the alkene (kobs) was followed by long-path FTIR spectroscopy. From the decrease of the effective rate constant (kobs = keff[O3]) upon addition of cyclohexane, the OH radical yield was determined. The OH radical 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 process, presumably from the decomposition of the excited Criegee intermediate. Determined yields of OH radical formation in the ozonolysis of propene, isobutene, and isoprene were 0.34, 0.60, and 0.26, respectively. Detailed product studies were performed to verify if the observed stable products can be explained by the assumption that OH radicals are formed via the hydroperoxide channel as proposed by Niki et al. [J. Phys. Chem. 1987, 91, 941−946]. For the isobutene−ozone system, experimental product yields were found to agree well with predictions from a chemical mechanism based on the chemistry of the acetonylperoxy radical CH3C(O)CH2O2, which is formed as an important radical product from the decomposition of the (CH3)2COO Criegee intermediate.