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....
Review status: a revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.
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,aMichael Priestley et al.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
Received: 04 Aug 2020 – Accepted for review: 05 Aug 2020 – Discussion started: 17 Aug 2020
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.
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....