Yanan Zhan, Dan Dan Huang, Yaqin Gao, Liyuan Zhou, Jincan Shen, Wei Yuan, Jing Duan, Xue Li, Zhifeng Tang, Haobin Zhong, Cheng Huang, Hongli Wang, Ru-Jin Huang; Molecular Tracking the Formation and Aging of Secondary Organic Aerosol from Benzothiazole Photooxidation. Environ. Sci. Technol. 21 April 2026; 60 (15): 11597–11607. https://doi.org/10.1021/acs.est.5c17162

Benzothiazoles (BTHs) are emerging pollutants mainly released from tire wear and are ubiquitous in the atmosphere. BTHs can react with OH radicals to form secondary organic aerosol (SOA), but the underlying reaction pathways and molecular products remain poorly understood. A team led by Prof. Dr. Ru-Jin Huang of the Institute of Earth Environment, Chinese Academy of Sciences investigated the photooxidation of benzothiazole (BTH, the parent compound of BTHs) using an oxidation flow reactor coupled with multiple advanced mass spectrometers, identifying 137 gaseous and 129 particulate products. Thiazole ring-retaining CHONS compounds were major products, e.g., accounting for 83-93% of the mass spectrometric signal intensity of particulate products in positive ion mode, which indicates a preferential OH attack on the benzene ring. BTH photooxidation was initiated by OH addition and proceeded via phenolic pathway (i.e., HO₂ elimination) and peroxide-bicyclic intermediate pathway to yield first-generation products (e.g., C₇H₅ONS, C₇H₇O₅NS and C₅H₅O₂NS). Multi-generation OH oxidation and autoxidation further transformed these first-generation products into low-volatility CHONS compounds, driving their gas-particle partitioning. Thiazole ring-retaining compounds acted as chromophores and contributed to light absorption of BTH-derived SOA, with a mass absorption efficiency at 365 nm (MAE₃₆₅) of 0.37 m² g^-1 at an OH exposure of 9 × 10¹¹ molecules cm^-3 s, comparable to the MAE₃₆₅ value of fresh SOA from benzene or ethylbenzene oxidation. However, enhanced atmospheric aging (e.g., OH exposure ≥1.6 × 10¹² molecules cm^-3 s) fragmented thiazole ring-retaining compounds into low-molecular-weight CHO/CHON compounds (e.g., carbonyls and amides), resulting in a decrease in mass and light absorption of BTH SOA. Notably, nitrogen was found to be retained in reduced organic forms during oxidative processing, distinct from sulfur which was oxidized to inorganic sulfate. This study elucidates the BTH photooxidation at the molecular level, providing a mechanistic understanding of its evolution pathways and optical properties.