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

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.