Atmospheric Oxidation Mechanism of Benzimidazole initiated by Hydroxyl Radicals (^•OH): Nitrosamine Formation from...

Atmospheric oxidation of nitrogen-containing volatile chemical products (N-VCPs) significantly affects air quality, with the formation of carcinogenic nitrosamines being particularly concerning. Benzimidazoles (BZIs) are high-production chemicals and a type of N-VCP. However, their atmospheric oxidation mechanisms and subsequent impacts remain unclear. Here, using the simplest benzimidazole (BZI) as a representative compound, we employed theoretical calculations and toxicological modeling to investigate the ^•OH-initiated reaction mechanism of BZI and assess the toxicity of their oxidation products (OPs). The calculated reaction rate constant for the BZI + ^•OH reaction is 1 × 10^-11 cm³ molecule^-1 s^-1 at 298 K and 1 atm, which is comparable to those of other studied N-VCPs. The ^•OH-addition intermediate is dominantly formed in the BZI + ^•OH reaction, which further leads to the formation of organonitrates, alkoxy radicals, hydroperoxides, and carcinogenic nitrosamines under conditions where NO, HO₂·, and O₂ coexist. Toxicity predictions indicate that almost all OPs are more toxic than BZI. This study clarifies a previously unrecognized pathway to the formation of nitrosamines via the ^•OH-addition intermediate, beyond the conventional N-H H-abstraction intermediate, thereby refining the reaction diversity (two distinct pathways) and benefiting the assessment of the atmospheric impact caused by the oxidation of N-VCPs.