Abstract
The oxidation of monoterpenes under atmospheric conditions has been the subject of numerous studies. They were motivated by the formation of oxidized organic molecules (OOM) which, due to their low vapor pressure, contribute to the formation of secondary organic aerosols (SOA). Among the different reaction mechanisms proposed for the formation of these oxidized chemical compounds, it appears that the autoxidation mechanism, involving successive events of H-migration and O2 addition, common to both low-temperature combustion and atmospheric conditions, is leading to the formation of highly oxidized molecules (HOM). In atmospheric chemistry, the importance of autoxidation compared to other oxidation pathways has been the topic of numerous studies. Conversely, in combustion, autoxidation under cool flame conditions is the main oxidation process commonly taken into account. An analysis of oxidation products detected in both conditions was performed, using the present combustion data and literature data from tropospheric oxidation studies, to investigate possible similarities in terms of observed chemical formulae of products. To carry out this study, we chose two terpenes, α-pinene and limonene (C10H16), among the most abundant biogenic components in the atmosphere, and considered in many previous studies. Also, these two isomers were selected for the diversity of their reaction sites (exo- and endo- carbon-carbon double bonds). We built an experimental database consisting of literature atmospheric oxidation data and presently obtained combustion data for the oxidation of the two selected terpenes. In order to probe the effects of the type of ionization used in mass spectrometry analyses on the detection of oxidation products, we used heated electrospray ionization (HESI) and atmospheric pressure chemical ionization (APCI), in positive and negative modes. The oxidation of limonene-oxygen-nitrogen and α-pinene-oxygen-nitrogen mixtures was performed using a jet-stirred reactor at elevated temperature (590 K), a residence time of 2 s, and atmospheric pressure. Samples of the reacting mixtures were collected in acetonitrile and analyzed by high-resolution mass spectrometry (Orbitrap Q-Exactive) after direct injection and soft ionization, i.e. (+/−) HESI and (+/−) APCI. This work shows a surprisingly similar set of chemical formulae of products, including oligomers, formed in cool flames and under simulated atmospheric conditions. Data analysis showed that a non-negligible subset of chemical formulae is common to all experiments independently of experimental parameters. Finally, this study indicates that more than 40 % of the detected chemical formulae in this full dataset can be ascribed to an autoxidation mechanism.
Highlights
The links between atmospheric and combustion chemistry have often been studied from the point of view of tropospheric reactions of combustion effluents or pollutants, e.g., oxidation of volatile organic compounds, nitrogen oxides reactions, sulfur chemistry (Barsanti et al, 2017;Shrivastava et al, 2017;Zhao et al, 2018;Bianchi et al, 2019)
We have studied the impact of atmospheric pressure chemical ionization (APCI) and heated electrospray ionization (HESI) sources, in positive and negative modes, on the chemical formulae detected
This work is in the continuity of recently published studies which established the importance of autoxidation under tropospheric oxidation and low-temperature combustion conditions
Summary
The links between atmospheric and combustion chemistry have often been studied from the point of view of tropospheric reactions of combustion effluents or pollutants, e.g., oxidation of volatile organic compounds, nitrogen oxides reactions, sulfur chemistry (Barsanti et al, 2017;Shrivastava et al, 2017;Zhao et al, 2018;Bianchi et al, 2019). When studying the oxidation of chemicals and the formation of SOA in the atmosphere, it becomes necessary to determine the contribution of different oxidation pathways pertaining to atmospheric chemistry, combustion chemistry, or both. In low-temperature combustion (cool flame) the formation of oxidized organic molecules (OOM) is mainly attributed to autoxidation reactions (Belhadj et al, 2021;Benoit et al, 2021), whereas in atmospheric chemistry, it is only relatively recently that this pathway has been considered (Vereecken et al, 2007;Crounse et al., 2013;Jokinen et al, 2014a;Berndt et al, 2015;Jokinen et al, 2015a;Berndt et al, 2016;Iyer et al, 2021). Modeling studies complemented by laboratory experiments showed that autoxidation mechanisms proceed simultaneously on different RO2 ̇ radicals and leads to, through isomerization and addition of O2, the production of a wide range of oxidized compounds in a few hundredths of a second (Jokinen et al, 2014a;Berndt et al, 2016;Bianchi et al., 2019). Autoxidation is based on an H-shift and oxygen addition which starts with the initial production of RO 2 ̇
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