Products and mechanism of the gas-phase reaction of NO3 radicals with alpha-pinene

The gas-phase reaction of NO3 radicals with alpha-pinene has been studied under flow conditions in the pressure range 20 < p/mbar <200 at 298 K using gas chromatography-mass spectrometry/flame ionisation detection (GC-MS/FID), MS and long-path FTIR spectroscopy as detection techniques. NO, radicals were generated by thermal decomposition of N2O5. He, N-2 or O-2- N-2 mixtures served as carrier gas. In the absence of O-2 in the carrier gas, alpha-pinene oxide (ca. 60%) and an organic nitrate (ca. 40%) were found to be the main products with minor amounts of alpha-campholene aldehyde. The yields were slightly pressure dependent. In the presence of O-2, pinonaldehyde was the predominant product. When NO was added for conversion of the formed peroxyl radicals via RO2 + NO --> RO --> NO2, the following product yields were obtained at p = 200 mbar and [O-2] > 10(17) molecule cm(-3), pinonaldehyde 75 +/- 6%, alpha-pinene oxide 15 +/- 3%, organic nitrates (total) 14 +/- 3%, alpha-campholene aldehyde 3 +/- 1%. In the absence of NO, nitroperoxy-group-containing substances were observed, arising from the reaction RO2 + NO2 --> RO2NO2. A reaction mechanism is proposed and a tropospheric application of the results is discussed.