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

Abstract

The gas-phase reaction of NO ₃ radicals with α-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 ₃ radicals were generated by thermal decomposition of N ₂ O ₅ . He, N ₂ or O ₂ –N ₂ mixtures served as carrier gas. In the absence of O ₂ in the carrier gas, α-pinene oxide (ca. 60%) and an organic nitrate (ca. 40%) were found to be the main products with minor amounts of α-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 RO ₂ +NO→RO→NO ₂ , the following product yields were obtained at p=200 mbar and [O ₂ ]>10 ¹⁷ molecule cm ^-3 , pinonaldehyde 75±6%, α-pinene oxide 15±3%, organic nitrates (total) 14±3%, α-campholene aldehyde 3±1%. In the absence of NO, nitroperoxy-group-containing substances were observed, arising from the reaction RO ₂ +NO ₂ →RO ₂ NO ₂ . A reaction mechanism is proposed and a tropospheric application of the results is discussed.