Evaluation of the chemical composition of gas- and particle-phase products of aromatic oxidation

Archit Mehra ,Daniel J Bryant ,Andrew T Lambe ,Yuwei Wang ,Douglas R Worsnop ,Kelly L Pereira ,Jordan E Krechmer ,Andrew R Rickard ,Francesca Majluf ,Michael Priestley ,Manjula R Canagaratna ,Mike J Newland ,Harald Stark ,Philip Croteau ,Thomas J Bannan ,Lin Wang ,Melissa A Morris ,John T Jayne ,Jacqueline F Hamilton ,Hugh Coe

doi:10.5194/acp-20-9783-2020

Abstract

Abstract. Aromatic volatile organic compounds (VOCs) are key anthropogenic pollutants emitted to the atmosphere and are important for both ozone and secondary organic aerosol (SOA) formation in urban areas. Recent studies have indicated that aromatic hydrocarbons may follow previously unknown oxidation chemistry pathways, including autoxidation that can lead to the formation of highly oxidised products. In this study we evaluate the gas- and particle-phase ions measured by online mass spectrometry during the hydroxyl radical oxidation of substituted C9-aromatic isomers (1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene, propylbenzene and isopropylbenzene) and a substituted polyaromatic hydrocarbon (1-methylnaphthalene) under low- and medium-NOx conditions. A time-of-flight chemical ionisation mass spectrometer (ToF-CIMS) with iodide–anion ionisation was used with a filter inlet for gases and aerosols (FIGAERO) for the detection of products in the particle phase, while a Vocus proton-transfer-reaction mass spectrometer (Vocus-PTR-MS) was used for the detection of products in the gas phase. The signal of product ions observed in the mass spectra were compared for the different precursors and experimental conditions. The majority of mass spectral product signal in both the gas and particle phases comes from ions which are common to all precursors, though signal distributions are distinct for different VOCs. Gas- and particle-phase composition are distinct from one another. Ions corresponding to products contained in the near-explicit gas phase Master Chemical Mechanism (MCM version 3.3.1) are utilised as a benchmark of current scientific understanding, and a comparison of these with observations shows that the MCM is missing a range of highly oxidised products from its mechanism. In the particle phase, the bulk of the product signal from all precursors comes from ring scission ions, a large proportion of which are more oxidised than previously reported and have undergone further oxidation to form highly oxygenated organic molecules (HOMs). Under the perturbation of OH oxidation with increased NOx, the contribution of HOM-ion signals to the particle-phase signal remains elevated for more substituted aromatic precursors. Up to 43 % of product signal comes from ring-retaining ions including HOMs; this is most important for the more substituted aromatics. Unique products are a minor component in these systems, and many of the dominant ions have ion formulae concurrent with other systems, highlighting the challenges in utilising marker ions for SOA.

Highlights

Volatile organic compounds (VOCs) are emitted from both natural and anthropogenic sources, and their oxidation in the troposphere is important for reactive chemistry leading to ozone (Atkinson, 2000; Derwent et al, 1998) and secondary organic aerosol (SOA) formation (Ziemann et al, 2012)
Detailed mechanisms of aromatic oxidation have been generated previously from experimental work in simulation chambers (Calvert, 2002), and this chemistry has been included in chemical mechanisms such as the Master Chemical Mechanism (MCM: http://mcm.york.ac.uk, last access: 11 August 2020) and the Statewide Air Pollution Research Center (SAPRC; of the University of California, Riverside) mechanism, whose primary goal is to describe ozone formation (Bloss et al, 2005; Carter, 1988; Carter and Heo, 2013; Metzger et al, 2008; Suh et al, 2003; Volkamer et al, 2002)
The construction protocol developed to allow the building of comprehensive, consistent gas-phase degradation schemes for aromatic VOCs in the MCM is given in Jenkin et al (2003), which was subsequently updated and optimised using an extensive range of chamber experiments in 2005 by Bloss et al (2005a, 2005b)

Summary

Introduction

Volatile organic compounds (VOCs) are emitted from both natural and anthropogenic sources, and their oxidation in the troposphere is important for reactive chemistry leading to ozone (Atkinson, 2000; Derwent et al, 1998) and secondary organic aerosol (SOA) formation (Ziemann et al, 2012). This can have severe air quality, environmental and health impacts (Hallquist et al, 2009). Explicit chemical mechanisms often lead to large discrepancies between modelled and measured SOA formation (Johnson et al, 2006; Khan et al, 2017; Volkamer et al, 2006), which may be associated with both uncertainty in the formation pathways of semi-volatile and low-volatility species and a poor representation of aerosol volatility and partitioning

Methods

Results

Conclusion