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. Atmospheric oxidation chemistry and, more specifically, photooxidation show that the long-term oxidation of organic aerosol (OA) progressively erases theinitial signature of the chemical compounds and can lead to a relativelyuniform character of oxygenated organic aerosol (OOA). This uniformitycharacter observed after a long reaction time seems to contrast with the greatdiversity of reaction mechanisms observed in the early stages of oxidation.The numerous studies carried out on the oxidation of terpenes, and moreparticularly on limonene for its diversity of reaction sites (endo- and oxocyclic), allow this evolution to be studied. We have selected, for their diversityof experimental conditions, nine studies of limonene oxidation at roomtemperature over long reaction times to be compared to the present data setobtained at elevated temperature and short reaction time in order toinvestigate the similarities in terms of reaction mechanisms and chemicalspecies formed. Here, the oxidation of limonene–oxygen–nitrogen mixtures wasstudied using a jet-stirred reactor at elevated temperature and atmosphericpressure. Samples of the reacting mixtures were collected and analyzed byhigh-resolution mass spectrometry (Orbitrap) after direct injection or afterseparation by reverse-phase ultra-high-pressure liquid chromatography andsoft ionization, i.e., (+/-) HESI and (+/-) APCI. Unexpectedly,because of the diversity of experimental conditions in terms of continuous-flowtank reactor, concentration of reactants, temperature, reaction time, massspectrometry techniques, and analysis conditions, the results indicate thatamong the 1138 presently detected molecular formulae, many oxygenates foundin earlier studies of limonene oxidation by OH and/or ozone are alsoproduced 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 theinitial formation of peroxy radicals (RO2), isomerization reactionsyielding keto-hydroperoxides, and other oxygenated intermediates and productsup to C25H32O17, products which could derive from RO2 autoxidation via sequential H shift and O2 addition(C10H14O3,5,7,9,11) and products deriving from the oxidationof alkoxy radicals (produced by RO2 self-reaction or reaction withHO2) through multiple H shifts and O2 additions(C10H14O2,4,6,8,10). The oxidation of RO2, with possible occurrence of the Waddington mechanism and of the Korcek mechanism, involving H shifts is also discussed. The present work demonstratessimilitude between the oxidation products and oxidation pathways of limoneneunder simulated atmospheric conditions and in those encountered during theself-ignition of hydrocarbons at elevated temperatures. These resultscomplement those recently reported by Vereecken and Nozière and confirmfor limonene the existence of an oxidative chemistry of the alkylperoxyradical beyond 450 K based on the H shift (Nozière and Vereecken, 2019; Vereecken and Nozière, 2020).

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