On the similarities and differences between the products of oxidation of hydrocarbons under simulated atmospheric conditions and cool flames

Benoit, Roland; Belhadj, Nesrine; Lailliau, Maxence; Dagaut, Philippe

Atmospheric oxidation chemistry and, more specifically, photooxidation show that the long-term oxidation of organic aerosol (OA) progressively erases the initial signature of the chemical compounds and can lead to a relatively uniform character of oxygenated organic aerosol (OOA). This uniformity character observed after a long reaction time seems to contrast with the great diversity of reaction mechanisms observed in the early stages of oxidation. The numerous studies carried out on the oxidation of terpenes, and more particularly on limonene for its diversity of reaction sites (endo- and oxocyclic), allow this evolution to be studied. We have selected, for their diversity of experimental conditions, nine studies of limonene oxidation at room temperature over long reaction times to be compared to the present data set obtained at elevated temperature and short reaction time in order to investigate the similarities in terms of reaction mechanisms and chemical species formed. Here, the oxidation of limonene–oxygen–nitrogen mixtures was studied using a jet-stirred reactor at elevated temperature and atmospheric pressure. Samples of the reacting mixtures were collected and analyzed by high-resolution mass spectrometry (Orbitrap) after direct injection or after separation by reverse-phase ultra-high-pressure liquid chromatography and soft ionization, i.e., (inline-formula M1inlinescrollmathml + / - 25pt14ptsvg-formulamathimg1a4f7b3ecffc20cccaa0f65757a3921c acp-21-7845-2021-ie00001.svg25pt14ptacp-21-7845-2021-ie00001.png ) HESI and (inline-formula M2inlinescrollmathml + / - 25pt14ptsvg-formulamathimg2e2a8c41fcdd0a5da4dc5c02fd133e87 acp-21-7845-2021-ie00002.svg25pt14ptacp-21-7845-2021-ie00002.png ) APCI. Unexpectedly, because of the diversity of experimental conditions in terms of continuous-flow tank reactor, concentration of reactants, temperature, reaction time, mass spectrometry techniques, and analysis conditions, the results indicate that among the 1138 presently detected molecular formulae, many oxygenates found in earlier studies of limonene oxidation by OH and/or ozone are also produced under the present conditions. Among these molecular formulae, highly oxygenated molecules and oligomers were detected in the present work. The results are discussed in terms of reaction pathways involving the initial formation of peroxy radicals (inline-formulaRO2), isomerization reactions yielding keto-hydroperoxides, and other oxygenated intermediates and products up to inline-formulaC25H32O17, products which could derive from inline-formulaRO2 autoxidation via sequential H shift and inline-formulaO2 addition (inline-formula M7inlinescrollmathml chem normal C normal 10 normal H normal 14 normal O normal 3 , normal 5 , normal 7 , normal 9 , normal 11 81pt14ptsvg-formulamathimgba99b05cb0b7aecb3e07d9412d390684 acp-21-7845-2021-ie00003.svg81pt14ptacp-21-7845-2021-ie00003.png ) and products deriving from the oxidation of alkoxy radicals (produced by inline-formulaRO2 self-reaction or reaction with inline-formulaHO2) through multiple H shifts and inline-formulaO2 additions (inline-formula M11inlinescrollmathml chem normal C normal 10 normal H normal 14 normal O normal 2 , normal 4 , normal 6 , normal 8 , normal 10 81pt14ptsvg-formulamathimga1f365db156e1c85422587398cf4e1c0 acp-21-7845-2021-ie00004.svg81pt14ptacp-21-7845-2021-ie00004.png ). The oxidation of inline-formulaRO2, with possible occurrence of the Waddington mechanism and of the Korcek mechanism, involving H shifts is also discussed. The present work demonstrates similitude between the oxidation products and oxidation pathways of limonene under simulated atmospheric conditions and in those encountered during the self-ignition of hydrocarbons at elevated temperatures. These results complement those recently reported by Vereecken and Nozière and confirm for limonene the existence of an oxidative chemistry of the alkylperoxy radical beyond 450 K based on the H shift (Nozière and Vereecken, 2019; Vereecken and Nozière, 2020).

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Benoit, Roland / Belhadj, Nesrine / Lailliau, Maxence / et al: On the similarities and differences between the products of oxidation of hydrocarbons under simulated atmospheric conditions and cool flames. 2021. Copernicus Publications.

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