Abstract

Abstract. Aromatic hydrocarbons are a class of volatile organic compounds associated with anthropogenic activity and make up a significant fraction of urban volatile organic compound (VOC) emissions that contribute to the formation of secondary organic aerosol (SOA). Benzene is one of the most abundant species emitted from vehicles, biomass burning and industry. An iodide time-of-flight chemical ionisation mass spectrometer (ToF-CIMS) and nitrate ToF-CIMS were deployed at the Jülich Plant Atmosphere Chamber as part of a series of experiments examining benzene oxidation by OH under high- and low-NOx conditions, where a range of organic oxidation products were detected. The nitrate scheme detects many oxidation products with high masses, ranging from intermediate volatile organic compounds (IVOCs) to extremely low volatile organic compounds (ELVOCs), including C12 dimers. In comparison, very few species with C≥6 and O≥8 were detected with the iodide scheme, which detected many more IVOCs and semi-volatile organic compounds (SVOCs) but very few ELVOCs and low volatile organic compounds (LVOCs). A total of 132 and 195 CHO and CHON oxidation products are detected by the iodide ToF-CIMS in the low- and high-NOx experiments respectively. Ring-breaking products make up the dominant fraction of detected signal and 21 and 26 of the products listed in the Master Chemical Mechanism (MCM) were detected. The time series of highly oxidised (O≥6) and ring-retaining oxidation products (C6 and double-bond equivalent = 4) equilibrate quickly, characterised by a square form profile, compared to MCM and ring-breaking products which increase throughout oxidation, exhibiting sawtooth profiles. Under low-NOx conditions, all CHO formulae attributed to radical termination reactions of first-generation benzene products, and first-generation auto-oxidation products are observed. Several N-containing species that are either first-generation benzene products or first-generation auto-oxidation products are also observed under high-NOx conditions. Hierarchical cluster analysis finds four clusters, of which two describe photo-oxidation. Cluster 2 shows a negative dependency on the NO2/NOx ratio, indicating it is sensitive to NO concentration and thus likely to contain NO addition products and alkoxy-derived termination products. This cluster has the highest average carbon oxidation state (OSC‾) and the lowest average carbon number. Where nitrogen is present in a cluster member of cluster 2, the oxygen number is even, as expected for alkoxy-derived products. In contrast, cluster 1 shows no dependency on the NO2/NOx ratio and so is likely to contain more NO2 addition and peroxy-derived termination products. This cluster contains fewer fragmented species, as the average carbon number is higher and OSC‾ lower than cluster 2, and more species with an odd number of oxygen atoms. This suggests that clustering of time series which have features pertaining to distinct chemical regimes, for example, NO2/NOx perturbations, coupled with a priori knowledge, can provide insight into identification of potential functionality.

Highlights

  • Benzene is an aromatic volatile organic compound (VOC) commonly used as a vehicular fuel additive (Verma and Des Tombe, 2002) and as a chemical intermediate in the manufacture of a range of products, e.g. detergents (Oyoroko and Ogamba, 2017), lubricants (Rodriguez et al, 2018), dyes (Guo et al, 2018), and pesticides (Wang et al, 2014)

  • Whilst current global estimates of benzenoid emission to the atmosphere by vegetation are of a comparable order to those of anthropogenic activity (∼ 5 times lower; Misztal et al, 2015), and background concentrations of benzene are enhanced by increased biomass burning (Archibald et al, 2015), emission of benzene to the urban atmosphere is dominated by vehicle exhausts (Gentner et al, 2012) and solvent evaporation (Hoyt and Raun, 2015)

  • We detect a range of benzene oxidation products, including many with the same formula as those listed in the MCM, as well as highly oxidised species with both ToF-CIMS instruments

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Summary

Introduction

Benzene is an aromatic volatile organic compound (VOC) commonly used as a vehicular fuel additive (Verma and Des Tombe, 2002) and as a chemical intermediate in the manufacture of a range of products, e.g. detergents (Oyoroko and Ogamba, 2017), lubricants (Rodriguez et al, 2018), dyes (Guo et al, 2018), and pesticides (Wang et al, 2014). SOA formation from benzene has previously been quantified with a focus on the contribution from smaller mass, ring-breaking reaction products such as epoxides (Glowacki et al, 2009) and dicarbonyl aldehydes (e.g. O’Brien et al, 1983; Johnson et al, 2005). 37 % of the glyoxal formed in the LA basin is estimated to derive from aromatic sources (Knote et al, 2014). These smaller ring-breaking products typically make up a larger fraction of SOA mass than the ring-retaining products (Borrás and Tortajada-Genaro, 2012) and so have traditionally been the main focus for SOA quantification. Toxic benzene oxidation products such as catechol, nitrophenol and maleic anhydride are known components of SOA (Borrás and Tortajada-Genaro, 2012)

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