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 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.