Abstract. 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 ⚫OH and ⚫Cl,
and subsequent reactions of resulting indole radicals with O2 under 200 ppt NO and 50 ppt HO2⚫ conditions, were investigated by a
combination of quantum chemical calculations and kinetics modeling. The
results indicate that ⚫OH addition is the dominant pathway for the
reaction of ⚫OH with indole. However, both ⚫Cl addition and
H abstraction are feasible for the corresponding reaction with ⚫Cl.
All favorably formed indole radicals further react with O2 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 ⚫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
⚫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.