Chemical characterisation of benzene oxidation products under high- and low-NO<sub><i>x</i></sub> conditions using chemical ionisation mass spectrometry

Abstract

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.

Advances on Atmospheric Oxidation Mechanism of Typical Aromatic Hydrocarbons

“…首先, 温度是影响芳香烃大气氧化初始反应的关键因 素, 但在常温常压下, OH 自由基与芳香烃的加成途径 占主导地位 [4,62,66] . 而多项研究发现芳香烃 OH 加成产 物的后续反应主要受 O 2 浓度, OH 浓度和 NO x 浓度的影 响 [27,34,60,[126][127] . 在实际大气条件下, NO x 浓度是影响芳 香烃氧化过程的主导因素.…”

“…ETHBEN 代表乙苯, a 代表 MCM 机 制的结果(http://mcm.york.ac.uk/), b 代表 Birdsall 和 Elrod 等 [34] 的研究结果, c 代表 Wang 等 [24] 的研究结果, d 代表 Xu 等 [27] 的研究结果, e 代表 Wu 等 [62] 的研究结果, f 代表 Ji 等 [38] 的研究结果, g 代表 Li 和 Wang 等 [133] 的研究结果 Figure 6 Summary of the proportion of the main oxidation pathways of aromatic hydrocarbons and the yield of intermediate products in theoretical calculation and laboratory research [24][25]27,[30][31][32][33][34][35][36][37][38][39][40][41][44][45][46][47][48]50,62,119,133,135] . ETHBEN: ethyl benzene, a represent the result of MCM mechanism (http://mcm.york.ac.uk/), b represent the study of Birdsall and Elrod [34] , c represent the study of Wang et al [24] , d represent the study of Xu et al [27] , e represent the study of Wu et al [62] , f represent the study of Ji et al [38] , g represent the study of Li and Wang [133] [27,34,60,[126][127] . 其中, 实际大气条件下, NO x 浓度 是影响芳香烃氧化过程的主导因素.…”

Exploring the composition and volatility of secondary organic aerosols in mixed anthropogenic and biogenic precursor systems

“…Aerosol particles are ubiquitous in the atmosphere, with substantial impacts on climate (Ramanathan et al, 2001) and human health (Lelieveld et al, 2015;Brunekreef and Holgate, 2002). These particles may contain a wide variety of com-Published by Copernicus Publications on behalf of the European Geosciences Union.…”

Combined application of Online FIGAERO-CIMS and Offline LC-Orbitrap MS to Characterize the Chemical Composition of SOA in Smog Chamber Studies

“…Generally, there are two main hierarchical clustering analysis methods: divisive hierarchical clustering (working in a topdown method) which describes if a cluster needs to be split and agglomerative hierarchical clustering (working from the bottom-up) which describes if clusters should be combined (Bar-270 Joseph et al, 2001;Müllner, 2011;Nielsen, 2016). Compared to divisive hierarchical clustering, agglomerative hierarchical clustering is commonly used to cluster measurements with distinct time-series behaviours from mass spectrometry datasets and describe the degree of similarity between any two measurements, reducing the dimensionality of mass spectrometry datasets and improving understanding of bulk properties of the chemical processes (Sánchez-López et 275 al., 2014;Rosati et al, 2019;Koss et al, 2020;Priestley et al, 2021). HCA is independent of calibration or instrumental sensitivity since it relies on the relative differences between time series shapes.…”