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

Biogenic volatile organic compounds (BVOCs) are intensely emitted by forests and crops into the atmosphere. During the night, they react very rapidly with the nitrate radical (inline-formulaNO₃), leading to the formation of a variety of functionalized products including organic nitrates and to large amounts of secondary organic aerosols (SOAs). Organic nitrates (ONs) have been shown not only to play a key role in the transport of reactive nitrogen and consequently in the ozone budget but also to be important components of the total organic-aerosol mass, while SOAs are known to play a direct and indirect role in the climate. However, the reactivity of BVOCs with inline-formulaNO₃ remains poorly studied. The aim of this work is to provide new kinetic and mechanistic data for two monoterpenes (inline-formulaC₁₀H₁₆), inline-formulaα- and inline-formulaγ-terpinene, through experiments in simulation chambers. These two compounds, which have very similar chemical structures, have been chosen in order not only to overcome the lack of experimental data but also to highlight the influence of the chemical structure on the reactivity.

Rate constants have been measured using both relative and absolute methods. They were found to be inline-formula $M9inlinescrollmathml(\mathrm{normal\; 1.2}\pm \mathrm{normal\; 0.5})\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 10}}$ 89pt15ptsvg-formulamathimge7da5de7f6d22f350852b4d8faf07c17 acp-20-15167-2020-ie00001.svg89pt15ptacp-20-15167-2020-ie00001.png and (inline-formula $M10inlinescrollmathml\mathrm{normal\; 2.9}\pm \mathrm{normal\; 1.1})\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 11}}$ 85pt15ptsvg-formulamathimg12768fc4778aa761021ecc4c03fcad0a acp-20-15167-2020-ie00002.svg85pt15ptacp-20-15167-2020-ie00002.png  cminline-formula³ moleculeinline-formula⁻¹ sinline-formula⁻¹ for inline-formulaα- and inline-formulaγ-terpinene respectively. Mechanistic studies have also been conducted in order to identify and quantify the main reaction products. Total organic nitrate and SOA yields have been determined. While organic nitrate formation yields appear to be similar, SOA yields exhibit large differences with inline-formulaγ-terpinene being a much more efficient precursor 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.