Potential contribution to secondary aerosols from benzothiazoles in the atmospheric aqueous phase based on oxidation and oligomerization mechanisms

Benzothiazoles (BTs), widely used as vulcanization accelerators in the rubber industry, have frequently been identified in the atmosphere, especially in areas with heavy traffic. BTs can undergo gas-phase oxidation in the atmosphere, which contributes to secondary aerosol mass. However, given their certain water solubility, the atmospheric fate of BTs associated with aqueous-phase transformations is unclear. In this study, the reactions of benzothiazole (BT), 2-methylbenzothiazole (MBT), and 2-chlorobenzothiazole (CBT) with hydroxyl radicals (OH) were investigated. The rate constants of BT, MBT, and CBT reacted with OH radicals were determined to be (8.0 ± 1.8), (7.6 ± 1.7), and (7.6 ± 1.9) × 10⁹ M⁻¹ s⁻¹ at initial pH 2 and (9.7 ± 2.7), (9.8 ± 2.7), and (9.4 ± 2.7) × 10⁹ M⁻¹ s⁻¹ at initial pH 10, respectively. Lifetimes ranging from several minutes to several hours were estimated under mean OH concentrations in various atmospheric aqueous phases, which are significantly shorter than those estimated under mean OH concentrations in the gas phase. The nanoparticle tracing analysis (NTA) directly shows the formation of nanoparticles from the aqueous-phase photooxidation of the selected BTs. Data analysis of liquid chromatography Orbitrap mass spectrometry (LC-Orbitrap MS) identifies many multifunctional oligomers. Changes in optical property support the formation of oligomers and suggest that the products have the potential to contribute to the atmospheric brown carbon. In addition, higher yields of sulfate are formed after the reactions. It is highlighted that the aqueous-phase oxidation of BTs can contribute to the secondary aerosol mass in the ambient atmosphere, particularly in polluted regions where concentrations of BTs are comparable to those of benzenes, potentially altering the chemical composition and optical properties of atmospheric particles.