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

Alexander Zaytsev,Jesse H Kroll,Frank N Keutsch,Joseph R Roscioli,Joshua Moss,Kevin J Nihill,Martin Breitenlechner,James C Rowe,Manjula R Canagaratna,Christopher Y Lim,Joshua L Cox,Jordan E Krechmer,Douglas R Worsnop,Abigail R Koss

doi:10.5194/acp-19-15117-2019

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 (NO x ~ 10ppb). An extensive suite of instrumentation including two proton-transfer-reaction mass spectrometers (PTR-MS) and two chemical ionisation mass spectrometers ( 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.

Highlights

Aromatic compounds represent a significant fraction of volatile organic compounds (VOCs) in the urban atmosphere and play a substantial role in the formation of tropospheric ozone and secondary organic aerosol (SOA) (Calvert et al, 2002)
Zaytsev et al.: Mechanistic study of the formation of ring-retaining and ring-opening products portant group of aromatic compounds as they tend to have high SOA yields (Li et al, 2016) and account for a significant fraction of non-methane hydrocarbons in the industrialised regions of China (Tang et al, 2007; Zheng et al, 2009). 1,2,4-Trimethylbenzene (1,2,4-TMB) is chosen as a model molecule to study the oxidation of more substituted aromatic compounds
The goal of this work is to identify gas-phase pathways leading to the production of low-volatility compounds, which are important for SOA formation and support these identifications with chemical ionisation mass spectrometry (CIMS) data and a method to characterise the kinetics of an oxidation system

Summary

Introduction

Aromatic compounds represent a significant fraction of volatile organic compounds (VOCs) in the urban atmosphere and play a substantial role in the formation of tropospheric ozone and secondary organic aerosol (SOA) (Calvert et al, 2002). The most abundant alkylbenzene in the atmosphere, is primarily emitted by the aforementioned anthropogenic processes (Wu et al, 2014). A. Zaytsev et al.: Mechanistic study of the formation of ring-retaining and ring-opening products portant group of aromatic compounds as they tend to have high SOA yields (Li et al, 2016) and account for a significant fraction of non-methane hydrocarbons in the industrialised regions of China (Tang et al, 2007; Zheng et al, 2009). 1,2,4-Trimethylbenzene (1,2,4-TMB) is chosen as a model molecule to study the oxidation of more substituted aromatic compounds (i.e. trimethylbenzenes) Zaytsev et al.: Mechanistic study of the formation of ring-retaining and ring-opening products portant group of aromatic compounds as they tend to have high SOA yields (Li et al, 2016) and account for a significant fraction of non-methane hydrocarbons in the industrialised regions of China (Tang et al, 2007; Zheng et al, 2009). 1,2,4-Trimethylbenzene (1,2,4-TMB) is chosen as a model molecule to study the oxidation of more substituted aromatic compounds (i.e. trimethylbenzenes)

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