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. Aromatic hydrocarbons are a class of volatile organic compounds associatedwith 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 ionisationmass spectrometer (ToF-CIMS) and nitrate ToF-CIMS were deployed at theJülich Plant Atmosphere Chamber as part of a series of experimentsexamining benzene oxidation by OH under high- and low-NOx conditions,where a range of organic oxidation products were detected. The nitratescheme detects many oxidation products with high masses, ranging fromintermediate volatile organic compounds (IVOCs) to extremely low volatileorganic compounds (ELVOCs), including C12 dimers. In comparison, veryfew species with C≥6 and O≥8 were detected withthe iodide scheme, which detected many more IVOCs and semi-volatile organiccompounds (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 theiodide ToF-CIMS in the low- and high-NOx experiments respectively. Ring-breaking products make up the dominant fraction of detected signal and 21and 26 of the products listed in the Master Chemical Mechanism (MCM) weredetected. 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 MCMand ring-breaking products which increase throughout oxidation, exhibitingsawtooth profiles. Under low-NOx conditions, all CHO formulaeattributed to radical termination reactions of first-generation benzeneproducts, and first-generation auto-oxidation products are observed. SeveralN-containing species that are either first-generation benzene products orfirst-generation auto-oxidation products are also observed under high-NOx conditions. Hierarchical cluster analysis finds four clusters, ofwhich two describe photo-oxidation. Cluster 2 shows a negative dependency onthe NO2/NOx ratio, indicating it is sensitive to NO concentrationand thus likely to contain NO addition products and alkoxy-derivedtermination products. This cluster has the highest average carbon oxidationstate (OSC‾) and the lowest average carbon number.Where nitrogen is present in a cluster member of cluster 2, the oxygennumber is even, as expected for alkoxy-derived products. In contrast,cluster 1 shows no dependency on the NO2/NOx ratio and so islikely to contain more NO2 addition and peroxy-derived terminationproducts. This cluster contains fewer fragmented species, as the averagecarbon number is higher and OSC‾ lower thancluster 2, and more species with an odd number of oxygen atoms. Thissuggests that clustering of time series which have features pertaining todistinct chemical regimes, for example, NO2/NOx perturbations, coupledwith a priori knowledge, can provide insight into identification of potentialfunctionality.

atmospheric chemistry and physics

Chemical characterisation of benzene oxidation products under high- and low-NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; conditions using chemical ionisation mass spectrometry