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

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 cm 3 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, HO 2 ·, and O 2 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.