Evaluation of the Chemical Composition of Gas and Particle Phase Products of Aromatic Oxidation

Abstract

Abstract. Aromatic volatile organic compounds (VOC) 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 formed during the hydroxyl radical oxidation of substituted C9-aromatic isomers (1,3,5-trimethyl benzene, 1,2,4-trimethyl benzene, propyl benzene and isopropyl benzene) and a substituted polyaromatic hydrocarbon (1-methyl naphthalene) under low and medium NOx conditions. The majority of product signal in both 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, and comparison with the near explicit gas phase Master Chemical Mechanism (MCMv3.3.1) highlights a range of missing highly oxidised products in the pathways. In the particle phase, the bulk of product signal from all precursors comes from ring scission ions, many of which have undergone further oxidation to form HOMs. Under 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 25 % of product signal comes from ring-retaining ions including highly oxygenated organic molecules (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.

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

“…As discussed in Section 4.2.1, these lightly oxygenated molecules can be directly emitted from anthropogenic and biogenic sources or come from oxidation processes of various VOC precursors (Conley et al, 2005;Pandya et al, 2006;Rantala et al, 2015;Hartikainen et al, 2018). For instance, C7H10O has been found from direct soil emissions (Abis et al, 2020) or oxidation processes of 1,2,4-trimethyl benzene (Mehra et al, 2020). Therefore, we expect the molecules in this factor to be either directly emitted or as oxidation products of forest emissions.…”

Using highly time-resolved online mass spectrometry to examine biogenic and anthropogenic contributions to organic aerosol in Beijing

“…3 shows the proportion of mass spectral signal of each factor attributable to ion formulae measured during the aromatic oxidation experiments classied as ring-retaining (C 6 -C 9 and double bound equivalent (DBE)$ 4). 1 This analysis indicates that the PM-NOA has the largest potential contribution from aromatic SOA products (26%), followed by AM-NOA (19%) and NT-NOA (16%). It should be noted that these contributions do not include nitro-aromatics which were not observed by Mehra et al (2020) 1 due to the lower HO 2 : NO ratios employed in these experiments.…”

“…77 Recently, we studied the composition of SOA from several aromatic hydrocarbons using a FIGAERO-CIMS and identied a suite of ring-retaining products, including HOM, which contributed up to 43% of observed signal from single component aromatic experiments. 1 Here we compare the ions observed in the ambient factors with those from this aromatic study to estimate the potential contribution that aromatics could have to measured SOA in Beijing. As discussed in Mehra et al (2020), 1 many ions observed from aromatics have ion formulas concurrent with products from oxidation of other precursors.…”

“…Due to the potential for ring-scission products, in particular, to be attributed to many different sources, here we focus upon the ring-retaining products identied from aromatic oxidation in Mehra et al (2020). 1 Using these ions as markers provides a lower constraint on the contribution of aromatic SOA to the observed factor mass spectra, as many of the small oxidised products in the factors are likely to have an aromatic contribution from ring-scission. Fig.…”

“…31,32 Recently, highly-time resolved online chemical ionisation mass spectrometry (CIMS) techniques have been applied to in situ measurements of trace gas [33][34][35][36] and aerosol composition 37,38 providing sensitive and selective detection of molecules spanning an atmospherically relevant oxidation range. [39][40][41] These have been employed for the study of VOC oxidation, driving novel insights into gas-phase and SOA chemistry, 1,[42][43][44][45][46] including detailed studies of highly oxygenated organic molecules (HOM), 42,47,48 oligomers 49 and new particle formation (NPF). 50,51 Their application in ambient measurements have mostly focused on gas phase oxidised organic compounds, [52][53][54] with fewer studies measuring OA composition directly.…”