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© Author(s) 2020. This work is distributed under
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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  17 Aug 2020

17 Aug 2020

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This preprint is currently under review for the journal ACP.

Chemical characterisation of benzene oxidation products under high and low NOx conditions using chemical ionisation mass spectrometry

Michael Priestley1, Thomas J. Bannan1, Michael Le Breton1,d, Stephen D. Worrall1,c, Sungah Kang2, Iida Pullinen2,b, Sebastian Schmitt2, Ralf Tillmann2, Einhard Kleist7, Defeng Zhao2,g, Jürgen Wildt2,7, Olga Garmash4, Archit Mehra1,f, Asan Bacak1,e, Dudley E. Shallcross3,5, Åsa Halquist8, Mikael Ehn4, Astrid Kiendler-Scharr2, Thomas F. Mentel2, Gordon McFiggans1, Mattias Halquist6, Hugh Coe1, and Carl J. Percival1,a Michael Priestley et al.
  • 1Centre for Atmospheric Science, Departmentof Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
  • 2Institut für Energie und Klimaforschung, IEK-8, Forschungszentrum Jülich, Jülich, Germany
  • 3School of Chemistry, The University of Bristol, Cantock’s Close BS8 1TS, UK
  • 4Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, 00014 Helsinki, Finland
  • 5Department of Chemistry, University of the Western Cape, Bellville, South Africa
  • 6Department of Chemistry and Molecular Biology, University of Gothenburg, 412 96, Gothenburg, Sweden
  • 7Institut für Bio- und Geowissenschaften, IBG-2: Pflanzenwissenschaften, Forschungszentrum Jülich GmbH, Jülich, Germany
  • 8IVL Swedish Environmental Research Institute, Gothenburg, Sweden
  • anow at: Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109
  • bnow at: Department of Applied Physics, University of Eastern Finland, 702132 Kuopio, Finland
  • cnow at: Aston Institute of Materials Research, School of Engineering and Applied Science, Aston University, Birmingham, B4 7ET, UK
  • dnow at: Volvo Group Trucks Technology, L3 Lundby, Gothenburg, Sweden
  • enow at: Turkish Accelerator & Radiation Laboratory, Ankara University, Institute of Accelerator, Technologies Gölbaşı Campus, 06830 Golbasi/Ankara, Turkey
  • fnow at: Faculty of Science and Engineering, University of Chester, CH2 4NU, UK
  • gnow at: Department of Atmospheric and Oceanic Sciences & Institute of Atmospheric Sciences, Fudan University, Shanghai, China

Abstract. Aromatic hydrocarbons are a class of volatile organic compounds associated with anthropogenic activity and make up a significant fraction of urban 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 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 (IVOC) to extremely low volatile organic compounds (ELVOC), including C12 dimers. In comparison, very few species with C≥6 and O≥8 were detected with the iodide scheme, which detected many more IVOC and semi volatile organic compounds (SVOC) but very few ELVOC and low volatile organic compounds (LVOC). 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 (89–91 %). 21 and 26 of the products listed in the master chemical mechanism (MCM) were detected and account for 6.4–7.3 % of total signal. 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 saw tooth profiles. Under low NOx conditions, all CHO formulae attributed to radical termination reactions of 1st generation benzene products and 1st generation autoxidation products are observed, and one exclusively 2nd generation autoxidation product is also measured (C6H8O8). Several N containing species that are either 1st generation benzene products or 1st generation autoxidation products are also observed under high NOx conditions. Hierarchical cluster analysis finds four cluster of which two describe photo-oxidation. Cluster 2 shows a negative dependency on the NO2/NOx ratio indicating it is sensitive to NO concentration 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 and where nitrogen is present in cluster member, 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 less 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 clustering of time series which have features pertaining to distinct chemical regimes e.g. NO2/NOx perturbations, coupled with a priori knowledge, can provide insight into identification of potential functionality.

Michael Priestley et al.

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Michael Priestley et al.

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Short summary
A significant fraction of emissions from human activity consists of aromatic hydrocarbons e.g. benzene, which oxidise to form new compounds important for particle growth. Characterisation of benzene oxidation products highlights the range of species produced as well as their chemical properties and contextualises them within relevant frameworks e.g. MCM. Cluster analysis of the oxidation product time series' distinguishes behaviours of CHON compounds that can aid identifying functionality.
A significant fraction of emissions from human activity consists of aromatic hydrocarbons e.g....