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

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.

Comment on acp-2021-201

“…Product Distribution. The products that we observed herein (Tables S2-S3) are generally in agreement with those found in previous low-NOx studies (Schwantes et al, 2017;Molteni et al, 2018;Zaytsev et al, 2019;Mehra et al, 2020;Garmash et al, 2020). Differences exist in the relative abundance of species with different oxygen contents within each product category (Molteni et al, 2018;Garmash et al, 2020).…”

“…The presence of NOx results in the formation of nitrogencontaining products with one or two nitrogen atoms, and the signal abundances of all other oxygenated products are much lower than the case of low-NOx experiments. Such a suppression has been reported previously (Tsiligiannis et al, 2019;Garmash et al, 2020;Mehra et al, 2020). The nitrogen-containing products are expected to be organic nitrates or nitrated phenols.…”

“…Effects of NOx Level. The NO or NO2 termination of RO2 radicals competes with the HO2 or RO2 termination and forms nitrogen-containing species at the expense of other highly oxygenated products (Tsiligiannis et al, 2019;Garmash et al, 2020;Mehra et al, 2020). Yet, quantitative understanding in the effects of NOx on the formation of oxygenated products is limited for aromatic precursors compared with those for biogenic VOCs (Nah et al, 2016;Lambe et al, 2017;Sarnela et al, 2018).…”

Chemical characterisation of benzene oxidation products under high- and low-NO<sub><i>x</i></sub> conditions using chemical ionisation mass spectrometry

“…In this study, the oxidation of benzene and its oxidation products by OH under high-and low-NO x conditions are investigated in the Jülich Plant Atmosphere Chamber (JPAC) (Mentel et al, 2009(Mentel et al, , 2015 with two time-of-flight chemical ionisation mass spectrometers (ToF-CIMS) using the iodide and nitrate ionisation schemes. The ionisation schemes are compared to assess the similarities in detected oxidation products in terms of molecular identification and bulk properties.…”

“…The experiments were performed in a 1450 L borosilicate chamber of the Jülich Plant Atmospheric Chamber (JPAC) described elsewhere (Mentel et al, 2009). The chamber is operated as a continuously stirred tank reactor, with modifications as described in Mentel et al (2015).…”