Atmospheric oxidation mechanism and kinetics of indole initiated by •OH and •Cl: a computational study

The atmospheric chemistry of organic nitrogen compounds (ONCs) is of great importance for understanding the formation of carcinogenic nitrosamines, and ONC oxidation products might influence atmospheric aerosol particle formation and growth. Indole is a polyfunctional heterocyclic secondary amine with a global emission quantity almost equivalent to that of trimethylamine, the amine with the highest atmospheric emission. However, the atmospheric chemistry of indole remains unclear. Herein, the reactions of indole with center dot OH and center dot Cl, and subsequent reactions of resulting indole radicals with O-2 under 200 ppt NO and 50 ppt HO2 center dot conditions, were investigated by a combination of quantum chemical calculations and kinetics modeling. The results indicate that center dot OH addition is the dominant pathway for the reaction of center dot OH with indole. However, both center dot Cl addition and H abstraction are feasible for the corresponding reaction with center dot Cl. All favorably formed indole radicals further react with O-2 to produce peroxy radicals, which mainly react with NO and HO2. to form organonitrates, alkoxy radicals and hydroperoxide products. Therefore, the oxidation mechanism of indole is distinct from that of previously reported amines, which primarily form highly oxidized multifunctional compounds, imines or carcinogenic nitrosamines. In addition, the peroxy radicals from the center dot OH reaction can form N-(2-formylphenyl)formamide (C8H7NO2), for the first time providing evidence for the chemical identity of the C8H7NO2 mass peak observed in the center dot OH + indole experiments. More importantly, this study is the first to demonstrate that despite forming radicals by abstracting an H atom at the N site, carcinogenic nitrosamines were not produced in the indole oxidation reaction.

Automated summary

This study investigated the oxidation mechanism of indole in the atmosphere, revealing the reaction pathways of indole with reactive oxygen radicals and confirming that these reactions do not produce carcinogenic nitrosamines.