1. Chemical characterisation of benzene oxidation products under high- and low-NOx conditions using chemical ionisation mass spectrometry
- Author
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Asan Bacak, Sebastian Schmitt, Stephen D. Worrall, Ralf Tillmann, Thomas J. Bannan, Jürgen Wildt, Sungah Kang, Thomas F. Mentel, Carl J. Percival, Hugh Coe, Mattias Hallquist, Michael Le Breton, Gordon McFiggans, Mikael Ehn, Olga Garmash, Dudley E. Shallcross, Einhard Kleist, Michael Priestley, Astrid Kiendler-Scharr, Defeng Zhao, Archit Mehra, Iida Pullinen, and Åsa M. Hallquist
- Subjects
chemistry.chemical_classification ,Atmospheric Science ,010504 meteorology & atmospheric sciences ,Iodide ,Analytical chemistry ,chemistry.chemical_element ,010501 environmental sciences ,01 natural sciences ,Nitrogen ,Oxygen ,chemistry.chemical_compound ,chemistry ,13. Climate action ,Oxidation state ,Volatile organic compound ,Benzene ,Carbon ,NOx ,0105 earth and related environmental sciences - 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 Julich Plant Atmosphere Chamber as part of a series of experiments examining benzene oxidation by OH under high- and low-NO x 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 C 12 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-NO x 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 (C 6 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-NO x 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-NO x conditions. Hierarchical cluster analysis finds four clusters, of which two describe photo-oxidation. Cluster 2 shows a negative dependency on the NO 2 / NO x 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 ( OS C ‾ ) 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 NO 2 / NO x ratio and so is likely to contain more NO 2 addition and peroxy-derived termination products. This cluster contains fewer fragmented species, as the average carbon number is higher and OS C ‾ 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, NO 2 / NO x perturbations, coupled with a priori knowledge, can provide insight into identification of potential functionality.
- Published
- 2021