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

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 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 (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 NO_x 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 NO_x 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 NO_x conditions. Hierarchical cluster analysis finds four cluster of which two describe photo-oxidation. Cluster 2 shows a negative dependency on the NO₂/NO_x 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 (OS_c) 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/NO_x 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 OS_c 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/NO_x perturbations, coupled with a priori knowledge, can provide insight into identification of potential functionality.