Gas-phase ozonolysis of α-pinene:: gaseous products and particle formation

The gas-phase ozonblysis of alpha-pinene has been studied in a stopped-flow system at 295 +/- 2 K and 950 mbar of synthetic air as well as under flow conditions at 295+/-0.5 K and 1000 mbar of synthetic air. Gaseous products were analyzed using on-line GC-MS/FID and FT-IR measurements. The formation of new particles (dpgreater than or equal to3 nm) was followed by means of a differential mobility particle sizer system and an ultra-fine condensation particle counter. First, the reaction of OH radicals with c-hexane was reinvestigated. Products were c-hexanol with a yield of 0.35 +/- 0.06 and c-hexanone with a yield of 0.53 +/- 0.06. The rate coefficients of the consecutive reaction of OH radicals with c-hexanol and c-hexanone of (2.7+/-0.4) x 10(-11) and (6.1+/-0.9) x 10(-11) cm(3) molecule(-1) s(-1), respectively, were obtained. From the reaction of OH radicals with c-hexanol, a c-hexanone yield of 0.58 +/- 0.07 was observed. Using the c-hexane scavenger technique, an OH radical yield of 0.68 +/- 0.10 was measured for the reaction of O-3 with alpha-pinene applicable for H2O concentrations of similar to 1.5 x 10(15) and 2.6 x 10(17) molecule cm(3). The formation yield of pinonaldehyde was found to be strongly dependent on the experimental conditions. Generally, the pinonaldehyde yield decreased for increasing a-pinene conversion. In the presence of an OH radical scavenger, maximum pinonaldehyde yields were 0.42+/-0.05 and 0.32+/-0.04 for [H2O]similar to1.5 x 10(15) and 2.6 x 10(17) molecule cm(-3), respectively. Under the present conditions the pinonaldehyde formation cannot be described via the reaction of a Criegee intermediate with H2O. This finding is corroborated by measurements of the co-product H2O2. The formation yield of alpha-pinene oxide of 0.03 +/- 0.015 was unaffected by experimental conditions. Newly formed particles were measured for a relatively wide range of experimental conditions. Particle formation was only detectable for an a-pinene conversion above 3 x 10(11) molecule cm(-3). The results of the present study suggest that the formation of new particles from the pure O-3 + alpha-pinene reaction is unlikely under atmospheric conditions. (C) 2003 Elsevier Ltd. All rights reserved.