Products and mechanism of the gas phase reaction of ozone with beta-pinene

Gas phase ozonolysis of beta-pinene was performed in a 570 l static reactor at 730 Torr and 296 K in synthetic air and the products were analysed by a combination of gas phase FTIR spectroscopy, HPLC and IC analyses of gas ph ase and aerosol samples, respectively. The reaction mechanism was investiga ted by adding HCHO, HCOOH and H2O as Criegee intermediate scavenger and cyc lohexane as OH radical scavenger. Main identified products (yields in paren theses) in the presence of cyclohexane as OH radical scavenger were HCHO (0 .65 +/- 0.04), nopinone (0.16 +/- 0.04), 3-hydroxy-nopinone (0.15 +/- 0.05) , CO2 (0.20 +/- 0.04), CO (0.030 +/- 0.002), HCOOH (0.020 +/- 0.002), the s econdary ozonide of beta-pinene (0.16 +/- 0.05), and cis-pinic acid (0.02 /- 0.01). The decomposition of the primary ozonide was found to yield predo minantly the excited C-9-Criegee intermediate and HCHO (0.84 +/- 0.04) and to a minor extent the excited CH2OO intermediate and nopinone (0.16 +/- 0.0 4). Roughly 40% of the excited C-9-Criegee intermediate becomes stabilised and could be shown to react with HCHO, HCOOH and H2O. The atmospherically i mportant reaction of the stabilised C-9-Criegee intermediate with H2O was f ound to result in a nopinone increase of (0.35 +/- 0.05) and in the formati on of H2O2 (0.24 +/- 0.03). Based on the observed products, the unimolecula r decomposition/isomerisation channels of the C-9-Criegee intermediate are discussed in terms of the hydroperoxide and ester channels. Subsequent reac tions of the nopinonyl radical, formed in the hydroperoxide channel, lead t o major products like 3-hydroxy-nopinone but also to minor products like ci s-pinic acid. A mechanism for the formation of this dicarboxylic acid is pr oposed and its possible role in aerosol formation processes discussed.