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

The gas-phase ozonolysis of α-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 (dp 3 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)×10−11 and (6.1±0.9)×10−12 cm3 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 O3 with α-pinene applicable for H2O concentrations of 1.5×1015 and 2.6×1017 moleculecm−3. The formation yield of pinonaldehyde was found to be strongly dependent on the experimental conditions. Generally, the pinonaldehyde yield decreased for increasing α-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] 1.5×1015 and 2.6×1017 moleculecm−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 α-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 α-pinene conversion above 3×1011 moleculecm−3. The results of the present study suggest that the formation of new particles from the pure O3+α-pinene reaction is unlikely under atmospheric conditions.