On the similarities and differences between the products of oxidation of hydrocarbons under simulated atmospheric conditions and cool flames
Authors: Benoit Roland , Belhadj Nesrine , Lailliau Maxence , Dagaut Philippe 
Published: May 2021

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).