Chemical characterisation of benzene oxidation products under high- and low-NOx conditions using chemical ionisation mass spectrometry
Authors: Priestley Michael , Bannan Thomas J. , Le Breton Michael , Worrall Stephen D. , Kang Sungah , Pullinen Iida , Schmitt Sebastian , Tillmann Ralf , Kleist Einhard , Zhao Defeng , Wildt Jürgen , Garmash Olga , Mehra Archit , Bacak Asan , Shallcross Dudley E. , Kiendler-Scharr Astrid , Hallquist Åsa M. , Ehn Mikael , Coe Hugh , Percival Carl J. , Hallquist Mattias , Mentel Thomas F. , McFiggans Gordon 
Published: March 2021

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
Jülich Plant Atmosphere 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 (IVOCs) to extremely low volatile
organic compounds (ELVOCs), including C12 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-NOx 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 (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
sawtooth profiles. Under low-NOx 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-NOx conditions. Hierarchical cluster analysis finds four clusters, of
which two describe photo-oxidation. Cluster 2 shows a negative dependency on
the NO2/NOx 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 (OSC‾) 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 NO2/NOx ratio and so is
likely to contain more NO2 addition and peroxy-derived termination
products. This cluster contains fewer 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 that clustering of time series which have features pertaining to
distinct chemical regimes, for example, NO2/NOx perturbations, coupled
with a priori knowledge, can provide insight into identification of potential
functionality.