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

Abstract

Abstract. Atmospheric oxidation chemistry and, more specifically, photooxidation show that the long-term oxidation of organic aerosol (OA) progressively erases the\ninitial signature of the chemical compounds and can lead to a relatively\nuniform character of oxygenated organic aerosol (OOA). This uniformity\ncharacter observed after a long reaction time seems to contrast with the great\ndiversity of reaction mechanisms observed in the early stages of oxidation.\nThe numerous studies carried out on the oxidation of terpenes, and more\nparticularly on limonene for its diversity of reaction sites (endo- and oxocyclic), allow this evolution to be studied. We have selected, for their diversity\nof experimental conditions, nine studies of limonene oxidation at room\ntemperature over long reaction times to be compared to the present data set\nobtained at elevated temperature and short reaction time in order to\ninvestigate the similarities in terms of reaction mechanisms and chemical\nspecies formed. Here, the oxidation of limonene–oxygen–nitrogen mixtures was\nstudied using a jet-stirred reactor at elevated temperature and atmospheric\npressure. Samples of the reacting mixtures were collected and analyzed by\nhigh-resolution mass spectrometry (Orbitrap) after direct injection or after\nseparation by reverse-phase ultra-high-pressure liquid chromatography and\nsoft ionization, i.e., ( + / - ) HESI and ( + / - ) APCI. Unexpectedly,\nbecause of the diversity of experimental conditions in terms of continuous-flow\ntank reactor, concentration of reactants, temperature, reaction time, mass\nspectrometry techniques, and analysis conditions, the results indicate that\namong the 1138 presently detected molecular formulae, many oxygenates found\nin earlier studies of limonene oxidation by OH and/or ozone are also\nproduced under the present conditions. Among these molecular formulae,\nhighly oxygenated molecules and oligomers were detected in the present work.\nThe results are discussed in terms of reaction pathways involving the\ninitial formation of peroxy radicals ( RO2 ), isomerization reactions\nyielding keto-hydroperoxides, and other oxygenated intermediates and products\nup to C25H32O17 , products which could derive from RO2 autoxidation via sequential H shift and O2 addition\n( C 10 H 14 O 3 , 5 , 7 , 9 , 11 ) and products deriving from the oxidation\nof alkoxy radicals (produced by RO2 self-reaction or reaction with\n HO2 ) through multiple H shifts and O2 additions\n( C 10 H 14 O 2 , 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 demonstrates\nsimilitude between the oxidation products and oxidation pathways of limonene\nunder simulated atmospheric conditions and in those encountered during the\nself-ignition of hydrocarbons at elevated temperatures. These results\ncomplement those recently reported by Vereecken and Noziere and confirm\nfor limonene the existence of an oxidative chemistry of the alkylperoxy\nradical beyond 450\u2009K based on the H shift (Noziere and Vereecken, 2019; Vereecken and Noziere, 2020).