Efficient alkane oxidation under combustion engine and atmospheric conditions

Abstract

Oxidation chemistry controls both combustion processes and the atmospheric transformation of volatile emissions. In combustion engines, radical species undergo isomerization reactions that allow fast addition of O2. This chain reaction, termed autoxidation, is enabled by high engine temperatures, but has recently been also identified as an important source for highly oxygenated species in the atmosphere, forming organic aerosol. Conventional knowledge suggests that atmospheric autoxidation requires suitable structural features, like double bonds or oxygen-containing moieties, in the precursors. With neither of these functionalities, alkanes, the primary fuel type in combustion engines and an important class of urban trace gases, are thought to have minor susceptibility to extensive autoxidation. Here, utilizing state-of-the-art mass spectrometry, measuring both radicals and oxidation products, we show that alkanes undergo autoxidation much more efficiently than previously thought, both under atmospheric and combustion conditions. Even at high concentrations of NOX, which typically rapidly terminates autoxidation in urban areas, the studied C6–C10 alkanes produce considerable amounts of highly oxygenated products that can contribute to urban organic aerosol. The results of this inter-disciplinary effort provide crucial information on oxidation processes in both combustion engines and the atmosphere, with direct implications for engine efficiency and urban air quality.

Formation of condensable organic vapors from anthropogenic and biogenic volatile organic compounds (VOCs) is strongly perturbed by NO<sub><i>x</i></sub> in eastern China

“…Even at night, the concentration of ozone is rarely lower than 10 ppb, resulting from the weak titration of low NO. At the same time, the reaction between ozone and high concentrations of NO 2 can provide sufficient NO 3 radicals, dominating nocturnal degradation of certain VOCs (Wayne et al, 1991). The elevated mixing level of total aromatic hydrocarbons is one of the main characteristics of the atmosphere in densely populated areas, in addition to which there should be many alkanes and alkenes which cannot be observed by the PTR-TOF-MS (Fu et al, 2013;Xu et al, 2017).…”

Formation of condensable organic vapors from anthropogenic and biogenic VOCs is strongly perturbed by NO<sub>x</sub> in eastern China

“…5). Recent work has showed that autoxidation is more common than previously thought (Wang et al, 2021), and more studies are needed to explore the oxidation mechanisms of anthropogenic aliphatics and to evaluate their contribution to SOA.…”

Anthropogenic Volatile Organic Compound (AVOC) Autoxidation as a Source of Highly Oxygenated Organic Molecules (HOM)

“…Recently, Noziere and Vereecken 36 reported how even the naked alkanes have the potential for H-shift reactions, but they are too slow to provide a significant competition for other RO 2 loss processes. Thus, bimolecular RO 2 reactions yielding RO (e.g., with NO, 81 RO 2 , or even HO 2 , 82 even multiple consecutive 33 ) seem pivotal for the initiation of alkane autoxidation and formation of HOM ( Figure 5 ).…”

“…Note the special role of RO radicals in propagating the oxidation sequence. Reprinted with permission from ref ( 33 ). Copyright 2021 Springer Nature.…”

“… 33 In pure, naked hydrocarbons the initiation of RO 2 H-shift reactions is very slow, 36 but with a functionalization of the molecule, or at elevated temperature, the initiation considerably speeds up. 33 …”