Product study and mechanisms of the reactions of alpha-pinene and of pinonaldehyde with OH radicals

The reactions of alpha-pinene and of its main oxidation product, pinonaldeh yde (3-acetyl-2,2-dimethyl-cyclobutyl-ethanal) with OH radicals have been s tudied in the laboratory using Fourier transform infrared spectroscopy for real-time monitoring of the gas-phase chemical species and a Scanning Mobil ity Particle Sizer system (3071 A, TSI) for the study of the secondary aero sol formation. All gas-phase molar yields were quantified using calibrated reference of the pure compound, except for the nitrates products. The resul ts were: for the alpha-pinene experiments in the presence of NOx, pinonalde hyde, (87 +/- 20)%; total nitrates (18 +/- 9)%; formaldehyde, (23 +/- 9)%; acetone (9 +/- 6)%; for the alpha-pinene experiments in the absence of NO,: pinonaldehyde, (37 +/- 7)%; formaldehyde, (8 +/- 1)%; acetone, (7 +/- 2)%; for the pinonaldehyde experiments in the presence of NOx formaldehyde (152 +/- 56)% and acetone (15 +/- 7)%. The aerosol measurements showed that the condensed products accounted for the missing carbon in the gas-phase balan ce. The partitioning of the products into the condensed phase was found to be potentially significant under experimental conditions but less than 10% for initial alpha-pinene concentrations lower than 10(13) molecule cm(-3) a nd hence negligible under atmospheric conditions in the absence of aerosol seeds. On the basis of these results a comprehensive mechanism for the gas- phase reaction of alpha-pinene with OH in the presence of NOx has been prop osed, including quantitative values for all the involved branching ratios.