Mechanistic study of the formation of ring-retaining and ring-opening products from the oxidation of aromatic compounds under urban atmospheric conditions

Abstract

Abstract. Aromatic hydrocarbons make up a large fraction of anthropogenic volatile organic compounds and contribute significantly to the production of tropospheric ozone and secondary organic aerosol (SOA). Four toluene and four 1,2,4-trimethylbenzene (1,2,4-TMB) photooxidation experiments were performed in an environmental chamber under relevant polluted conditions (NOx∼10 ppb). An extensive suite of instrumentation including two proton-transfer-reaction mass spectrometers (PTR-MS) and two chemical ionisation mass spectrometers (NH4+ CIMS and I− CIMS) allowed for quantification of reactive carbon in multiple generations of hydroxyl radical (OH)-initiated oxidation. Oxidation of both species produces ring-retaining products such as cresols, benzaldehydes, and bicyclic intermediate compounds, as well as ring-scission products such as epoxides and dicarbonyls. We show that the oxidation of bicyclic intermediate products leads to the formation of compounds with high oxygen content (an O:C ratio of up to 1.1). These compounds, previously identified as highly oxygenated molecules (HOMs), are produced by more than one pathway with differing numbers of reaction steps with OH, including both auto-oxidation and phenolic pathways. We report the elemental composition of these compounds formed under relevant urban high-NO conditions. We show that ring-retaining products for these two precursors are more diverse and abundant than predicted by current mechanisms. We present the speciated elemental composition of SOA for both precursors and confirm that highly oxygenated products make up a significant fraction of SOA. Ring-scission products are also detected in both the gas and particle phases, and their yields and speciation generally agree with the kinetic model prediction.

Atmospheric organic vapors in two European pine forests measured by a Vocus PTR-TOF: insights into monoterpene and sesquiterpene oxidation processes

“…These findings have not been observed by previous studies, and the underlying reasons remain unclear. High-RH conditions typically occur during nights with temperature inversion (Zha et al, 2018), while RH below 40 % generally only occurs at the station during sunny days. The controlling role of temperature can be ruled out because temperature is strongly anticorrelated with RH and is known to influence terpene emissions and terpene reaction rates.…”

Source identification of atmospheric organic vapors in two European pine forests: Results from Vocus PTR-TOF observations

“…The C6 oxygenated compounds have been observed during the oxidation processes of benzene and C7 oxygenated compounds from toluene oxidations (Sato et al, 2012;Zaytsev et al, 2019). These compounds can also be directly emitted from biogenic or anthropogenic sources (Conley et al, 2005;Pandya et al, 2006;Rantala et al, 2015).…”

Oxygenated products formed from OH-initiated reactions of trimethylbenzene: Autoxidation and accretion

“…Identities and yields of HOM products from different aromatics were systematically measured and the determined molar HOM yields were in the range of 0.1 % to 2.5 %, which are similar to the molar HOM yields of OH-initiated reactions of BVOCs (Jokinen et al, 2015;Molteni et al, 2018). Currently, aromaticsderived HOMs are believed to be formed via many reaction pathways, including accretion, bicyclic intermediate reactions, and multi-generation OH reactions (Berndt et al, 2018b;Garmash et al, 2019;Zaytsev et al, 2019). The unimolecular isomerization and autoxidation reactions of aromatic peroxy https://doi.org/10.5194/acp-2020-165 Preprint.…”

“…Recent studies have emphasized on the importance of the peroxide-bicyclic pathway in producing highly oxygenated compounds in the oxidation of alkylbenzenes (Wang et al, 2017;Zaytsev et al, 2019), which leads to the formation of ring-retaining products. Therefore, here we further investigated C9 products of TMB oxidation detected by the three MSs (Figure 3).…”