ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-6489-2012 α-pinene photooxidation under controlled chemical conditions – Part 1: Gas-phase composition in low- and high-NO<sub>x</sub> environments Eddingsaas N. C. 1 Loza C. L. 1 Yee L. D. 2 Seinfeld J. H. 1 2 Wennberg P. O. 2 3 Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA 25 07 2012 12 14 6489 6504 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/6489/2012/acp-12-6489-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/6489/2012/acp-12-6489-2012.pdf

The OH oxidation of α-pinene under both low- and high-NO<sub>x</sub> environments was studied in the Caltech atmospheric chambers. Ozone was kept low to ensure OH was the oxidant. The initial α-pinene concentration was 20–50 ppb to ensure that the dominant peroxy radical pathway under low-NO<sub>x</sub> conditions is reaction with HO<sub>2</sub>, produced from reaction of OH with H<sub>2</sub>O<sub>2</sub>, and under high-NO<sub>x</sub> conditions, reactions with NO. Here we present the gas-phase results observed. Under low-NO<sub>x</sub> conditions the main first generation oxidation products are a number of α-pinene hydroxy hydroperoxides and pinonaldehyde, accounting for over 40% of the yield. In all, 65–75% of the carbon can be accounted for in the gas phase; this excludes first-generation products that enter the particle phase. We suggest that pinonaldehyde forms from RO<sub>2</sub> + HO<sub>2</sub> through an alkoxy radical channel that regenerates OH, a mechanism typically associated with acyl peroxy radicals, not alkyl peroxy radicals. The OH oxidation and photolysis of α-pinene hydroxy hydroperoxides leads to further production of pinonaldehyde, resulting in total pinonaldehyde yield from low-NO<sub>x</sub> OH oxidation of ~33%. The low-NO<sub>x</sub> OH oxidation of pinonaldehyde produces a number of carboxylic acids and peroxyacids known to be important secondary organic aerosol components. Under high-NO<sub>x</sub> conditions, pinonaldehyde was also found to be the major first-generation OH oxidation product. The high-NO<sub>x</sub> OH oxidation of pinonaldehyde did not produce carboxylic acids and peroxyacids. A number of organonitrates and peroxyacyl nitrates are observed and identified from α-pinene and pinonaldehyde.

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