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

Abstract

Abstract. 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 (NO3), leading to the formation of a variety of functionalized products including organic nitrates and to large amounts of secondary organic aerosols (SOA). Organic nitrates (ONs) have been shown 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 while SOA are known to play a direct and indirect role on the climate. However, the reactivity of BVOCs with NO3 remains poorly studied. The aim of this work is to provide new kinetic and mechanistic data for two monoterpenes (C10H16), α- and γ-terpinene, through experiments in simulation chambers. These two compounds, which have very similar chemical structures, have been chosen in order to fill 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 (1.2 ± 0.5) x 10-10 and (2.9 ± 1.1) x 10-11 cm3 molecule-1 s-1 for α- and γ-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 γ-terpinene being a much more efficient precursor of aerosols. In order to provide explanations for this difference, chemical analysis of the gas phase products were performed at the molecular scale. Detected products allowed proposing chemical mechanisms and providing explanations through peroxy and alkoxy reaction pathways.

An experimental study of the reactivity of terpinolene and <i>β</i>-caryophyllene with the nitrate radical

“…The terpinolene rate constant can be compared to the values found by Fouqueau et al (2020a) for α-terpinene and γ -terpinene and is shown in Table 2. They indeed have very similar structures, only differing by the position of the double bonds.…”

“…For example, limonene ON yields vary between 30 % and 72 % (Fry et al, 2014;Hallquist et al, 1999;Spittler et al, 2006) and β-pinene between 40 % and 74 % (Fry et al, 2014;Hallquist et al, 1999;Boyd et al, 2015). For αand γ -terpinene (Fouqueau et al, 2020a) very close yields (47 % and 44 % respectively) have been measured. The only exception is αpinene, for which yields vary between 10 % and 30 % (Fry et al, 2014;Hallquist et al, 1999;Spittler et al, 2006).…”

“…As discussed previously in Sect. 4.3 and also in Fouqueau et al (2020a), two mechanism steps are critical for SOA formation: the peroxy and the alkoxy reaction pathways. For the peroxy radicals, this study has shown the hydroxynitrates coming from the reaction RO 2 + RO 2 → ROH + R(O) have low vapor pressures and can contribute to SOA formation.…”

“…In this study, IBI ON (900-820 cm −1 ) = (9.5 ± 2.9) ×10 −18 cm molec. −1 was used (Fouqueau et al, 2020a).…”

“…In this study, we have investigated the reactivity of the NO 3 radical with two BVOCs, terpinolene (a monoterpene) and β-caryophyllene (a sesquiterpene) (see Fig. 1), using simulation chambers for determining both rate constants and mechanisms, with an experimental protocol similar to the one used in Fouqueau et al (2020a). Terpinolene represents 30 % of Sassafras albidum monoterpene emissions, and the global emission is estimated to be 1.3 Tg yr −1 (Guenther et al, 2012).…”