A comparative and experimental study of the reactivity with nitrate radical of two terpenes: α -terpinene and γ -terpinene

Abstract. Biogenic volatile organic compounds (BVOCs) are intensely\nemitted by forests and crops into the atmosphere. During the night, they\nreact very rapidly with the nitrate radical ( NO3 ), leading to the\nformation of a variety of functionalized products including organic nitrates\nand to large amounts of secondary organic aerosols (SOAs). Organic nitrates\n(ONs) have been shown not only to play a key role in the transport of reactive\nnitrogen and consequently in the ozone budget but also to be important\ncomponents of the total organic-aerosol mass, while SOAs are known to play a direct\nand indirect role in the climate. However, the reactivity of BVOCs with\n NO3 remains poorly studied. The aim of this work is to provide new\nkinetic and mechanistic data for two monoterpenes ( C10H16 ),\n α - and γ -terpinene, through experiments in simulation\nchambers. These two compounds, which have very similar chemical structures,\nhave been chosen in order not only to overcome the lack of experimental data but also to\nhighlight the influence of the chemical structure on the reactivity. Rate constants have been measured using both relative and absolute methods.\nThey were found to be ( 1.2 ± 0.5 ) × 10 - 10 and ( 2.9 ± 1.1 ) × 10 - 11  cm 3  molecule −1  s −1 for α - and γ -terpinene respectively. Mechanistic studies have\nalso been conducted in order to identify and quantify the main reaction\nproducts. Total organic nitrate and SOA yields have been determined. While\norganic nitrate formation yields appear to be similar, SOA yields exhibit\nlarge differences with γ -terpinene being a much more efficient\nprecursor of aerosols. In order to provide explanations for this difference, chemical analysis of the gas-phase products was performed at the molecular scale. Detected products allowed for proposing chemical mechanisms and providing explanations through peroxy and alkoxy reaction pathways.