A smog chamber study of mechanisms for beta-pinene oxidation by hydroxyl radicals

A study has been undertaken to investigate the formation mechanisms of the gas phase organic products formed by beta-pinene oxidation by hydroxyl radicals. Ten beta-pinene experiments and eleven pinaketone experiments have been carried out in the York University Smog Chamber. A suite of instruments have been used to characterize each experiment by acquiring real-time, online quantitative time profiles for temperature, relative humidity, the decay of beta-pinene, NO, NOx (NO + NO2), and particulate size distributions. An atmospheric pressure chemical ionization triple quadrupole mass spectrometer has been used for real-time, online identification and time profiling of the gas phase oxidation products. Oxidation product target ions of interest were first distinguished using observed signal increases in the full mass spectra acquired as the reaction proceeded. Structural information was obtained by examining their fragment ions generated through collision induced dissociation. Time profiles for target→fragment ion pairs of identified products were acquired in real-time, online to monitor product evolution over the course of each experiment. Sixteen products have been distinguished as forming in the gas phase, eight of which have been identified including pinaketone, hydroxy-pinaketone, C10H16O2 hydroxy aldehyde, pinalic 3-acid, norpinic acid, pinalic peroxy acid, nitrooxy-pinaketone and C10H17NO5 dihydroxy nitrate. The formation mechanisms of these products have been probed using the pinaketone experiments to isolate oxidation pathways and by varying the initial NOx concentration to examine the sensitivity of formation pathways. The observed gas phase formation profiles have been compared with current mechanisms to explore correlations and weaknesses in the current understanding of the oxidation pathways.