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

Abstract

Gas-phase hydrocarbon autoxidation is a rapid pathway for the production of in situ aerosol precursor compounds. It is a highway to molecular growth and lowering of vapor pressure, and it produces hydrogen-bonding functional groups that allow a molecule to bind into a substrate. It is the crucial process in the formation and growth of atmospheric secondary organic aerosol (SOA). Recently, the rapid gas-phase autoxidation of several volatile organic compounds (VOC) has been shown to yield highly oxygenated organic molecules (HOM). Most of the details on HOM formation have been obtained from biogenic monoterpenes and their surrogates, with cyclic structures and double bonds both found to strongly facilitate HOM formation, especially in ozonolysis reactions. Similar structural features in common aromatic compounds have been observed to facilitate high HOM formation yields, despite the lack of appreciable O 3 reaction rates. Similarly, the recently observed autoxidation and subsequent HOM formation in the oxidation of saturated hydrocarbons cannot be initiated by O 3 and require different mechanistic steps for initiating and propagating the autoxidation sequence. This Perspective reflects on these recent findings in the context of the direct aerosol precursor formation in urban atmospheres.

Analytical methodologies for oxidized organic compounds in the atmosphere

“…For the time being, HOM analytics mainly rely on MS methods where they are analyzed from the gas phase since the reactivity of their supposed peroxide groups makes them unstable and causes analytical problems. 73,76 Peroxide compounds are challenging as they may degrade in SOAs within a few hours. 127 Analytics are also limited because the exact structures of HOM compounds are still unknown.…”

“…127 Analytics are also limited because the exact structures of HOM compounds are still unknown. 76 Mutzel et al 72 have shown by LC-MS, employing DNPH derivatization, that HOMs found in the gas phase occurred also in the particle phase, and they contained carbonyl groups. Zhao et al 128 applied iodometry (discussed later) to LC-ESI-MS, in which samples were qualitatively analyzed for peroxide functionalities containing species before and after iodometry.…”

“…HOMs have been found to form especially from VOC precursors with a cyclic structure and double bonds, such as monoterpenes and anthropogenic aromatic hydrocarbons. 76 Wang et al 77 demonstrated in a recent study that even saturated hydrocarbons are able to form highly oxidized species by autoxidation under atmospheric and combustion engine's conditions. Also, the aldehydes employed as surrogates representing the first-generation oxidation products of alkanes were found to produce high yields of HOMs.…”

“…72 HOMs occur as monomers or form dimers with their peroxide groups. 76 The current criteria previously presented were determined by researchers specializing in HOM research, but due to the novelty of the compound group and the lack of experimental evidence, the criteria on the basis of which the classification has been made must be clearly defined for the time being. 73 As an example, Jaoui et al 9 have recently presented that 2-methyltartaric acids formed from isoprene could be classified as HOMs.…”

Emissions of Carbonaceous Particulate Matter and Ultrafine Particles from Vehicles—A Scientific Review in a Cross-Cutting Context of Air Pollution and Climate Change

“…This pathway produces hydrogen-bonding functional groups that allow a molecule to bind into a substrate. As stated by Rissanen [ 155 ], it is the crucial process in the formation and growth of atmospheric SOA.…”