Product study and mechanisms of the reactions of α‐pinene and of pinonaldehyde with OH radicals

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Abstract
The reactions of α‐pinene and of its main oxidation product, pinonaldehyde (3‐acetyl‐2,2‐dimethyl‐cyclobutyl‐ethanal), with OH radicals have been studied in the laboratory using Fourier transform infrared spectroscopy for real‐time monitoring of the gas‐phase chemical species and a Scanning Mobility Particle Sizer system (3071 A, TSI) for the study of the secondary aerosol formation. All gas‐phase molar yields were quantified using calibrated reference of the pure compound, except for the nitrates products. The results were: for the α‐pinene experiments in the presence of NOx, pinonaldehyde, (87±20)%; total nitrates (18±9)%; formaldehyde, (23±9)%; acetone (9±6)%; for the α‐pinene experiments in the absence of NOx: pinonaldehyde, (37±7)%; formaldehyde, (8±1)%; acetone, (7±2)%; for the pinonaldehyde experiments in the presence of NO, formaldehyde (152±56)% and acetone (15±7)%. The aerosol measurements showed that the condensed products accounted for the missing carbon in the gas‐phase balance. The partitioning of the products into the condensed phase was found to be potentially significant under experimental conditions but less than 10% for initial α‐pinene concentrations lower than 1013 molecule cm−3 and 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 α‐pinene with OH in the presence of NOx has been proposed, including quantitative values for all the involved branching ratios.