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

Abstract

Abstract 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 ( d p⩾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 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 α -pinene applicable for H 2 O concentrations of ∼1.5×10 15 and 2.6×10 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 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 [H 2 O]∼1.5×10 15 and 2.6×10 17 molecule cm −3 , respectively. Under the present conditions the pinonaldehyde formation cannot be described via the reaction of a Criegee intermediate with H 2 O. This finding is corroborated by measurements of the co-product H 2 O 2 . 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 3×10 11 molecule cm −3 . The results of the present study suggest that the formation of new particles from the pure O 3 + α -pinene reaction is unlikely under atmospheric conditions.