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 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., (+/-) HESI and (+/-) 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 (RO2), isomerization reactions yielding keto-hydroperoxides, and other oxygenated intermediates and products up 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 oxidation of alkoxy radicals (produced by RO2 self-reaction or reaction with HO2) 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 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).

Towards a Comprehensive Characterization of the Low-Temperature Autoxidation of Di-n-Butyl Ether

“…For 8 < nC < 16, we observed products of addition or condensation, and for nC > 16, we observed products of oligomerization. A comparison of Figure 10 with results published earlier for the oxidation of limonene in a JSR and with a range of atmospheric oxidation studies [ 25 ] indicates unexpected similar trends regarding the presence of LV-OOA, SV-OOA, and WSOC for the two fuels, namely DBE and limonene (see Supplementary Material, Figure S8 ).…”

“…Unfortunately, no data are available for the atmospheric oxidation of DBE to be compared to the present results. However, in a previous study of ours concerning limonene oxidation in a JSR at atmospheric pressure and 590 K [ 25 ], we showed that products of combustion and atmospheric oxidation presented strong similitude in terms of the chemical formulae detected and the variation of the oxidation state of carbon atoms as a function of the number of carbon atoms in the detected formulae. The oxidation state of carbon atoms (OSc ≈ 2O/C–H/C) allows for evaluating the degree of oxidation of a large range of organic species (alcohols, carbonyls, carboxylic acids, esters, and ethers, but not peroxides) [ 26 ].…”

Autoxidation of terpenes, a common pathway in tropospheric and low temperature combustion conditions: the case of limonene and α-pinene

“…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). Also, it has been identified that highly oxygenated molecules (HOMs), a source of secondary organic aerosols (SOA), can result from autoxidation processes (Wang et al, 2021;Tomaz et al, 2021;Bianchi et al, 2019).…”

“…Similarly, the experiments of Huang et al performed at different temperatures (223 K and 296 K) and precursor concentration (α-pinene 0.714 and 2.2 ppm) suggested that the physicochemical properties, such as the composition of the oligomers, can be affected by a variation of temperature (Huang et al, 2018). Both in combustion and atmospheric chemistry, autoxidation can yield highly oxygenated molecules (HOMs), e.g., compounds containing more than 7 oxygen atoms (Benoit et al, 2021;Bianchi et al, 2019). The term 'HOM' is generally associated with atmospheric chemistry (Bianchi et al, 2019), but this nomenclature does not specify the chemical properties of a compound.…”

“…Therefore, temperature has a measurable effect on the amount of HOMs formed (Wang et al, 2021). In low-temperature combustion chemistry as in atmospheric chemistry, the oxidation of a chemical compound leads to the formation of several thousands of chemical products which result from successive additions of oxygen, isomerization, accretion, fragmentation, and oligomerization (Benoit et al, 2021). The exhaustive analysis of chemical species remains, in the current state of instrumental limitations, impossible.…”