Heterogeneous reactions significantly contribute to the atmospheric formation of nitrated aromatic compounds during the haze episode in urban Beijing

• Published in: The Science of the total environment 2024-03, Vol.917, p.170612-170612, Article 170612
• Main Authors: Cheng, Zhen, Qiu, Xinghua, Li, Ailin, Chai, Qianqian, Shi, Xiaodi, Ge, Yanli, Koenig, Theodore K., Zheng, Yan, Chen, Shiyi, Hu, Min, Ye, Chunxiang, Cheung, Rico K.Y., Modini, Robin L., Chen, Qi, Shang, Jing, Zhu, Tong
• Format: Article
• Language: English
• Subjects: Atmospheric transformation; Haze episode; Heterogeneous reaction; Nitrated aromatic compounds; Urban atmosphere
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2024.170612

Nitrated aromatic compounds (NACs) are key components of air pollution; however, due to the presence of complex mixtures of primary and secondary species, especially in urban environments, their atmospheric formation is poorly understood. Here we conducted a field campaign during a winter haze episode in urban Beijing, China to monitor gaseous and particulate NACs at 2-h time resolution. Through a standard-independent non-targeted approach, a total of 238 NACs were screened, of which 127 species were assigned chemical formula and 25 structures were confirmed. Four main classes were identified: nitrated aromatic hydrocarbons, nitrophenols, oxygenated nitrated aromatic compounds, and nitrated heterocyclic aromatic compounds. Hierarchical clustering analysis revealed disparate temporal variances of diurnal or nocturnal elevation, among which different nitration formations were captured, i.e., daytime photochemical oxidation and nighttime heterogeneous reactions. Isomeric information, particularly the substitution position of the nitro group on biphenyl, further demonstrated a potential heterogeneous mechanism of electrophilic nitration by NO2+. Assisted by source apportionment, we found that nighttime heterogeneous reactions significantly contributed to NAC formation, e.g., 31.3 % and 60.8 %, respectively, to 2-nitrofluoranthene and 2-nitropyrene, which were previously considered as classical daytime gas-phase products. This study provides comprehensive information on urban NAC species and highlights the importance of unheeded heterogeneous reactions in the atmosphere. [Display omitted] •238 gaseous and particulate NACs were detected in a bihourly field observation.•Novel oxygenated and heterocyclic NACs were first found in the urban atmosphere.•Daytime photochemical oxidation and nighttime heterogeneous reactions were captured.•Heterogeneous reactions greatly contributed to NAC formation during the haze episode.