26 Apr 2022
26 Apr 2022
Status: this preprint is currently under review for the journal ACP.

Atmospheric Oxidation Mechanism and Kinetics of Indole Initiated by ·OH and ·Cl: A Computational Study

Jingwen Xue1, Fangfang Ma1, Jonas Elm2, Jingwen Chen1, and Hong-Bin Xie1 Jingwen Xue et al.
  • 1Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China
  • 2Department of Chemistry and iClimate, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark

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 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/·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 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 despite forming radicals by abstracting an H-atom at the N-site, carcinogenic nitrosamines were not produced in the indole oxidation reaction.

Jingwen Xue et al.

Status: open (until 07 Jun 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-88', Anonymous Referee #2, 17 May 2022 reply
    • RC2: 'Reply on RC1', Anonymous Referee #1, 18 May 2022 reply

Jingwen Xue et al.

Jingwen Xue et al.


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Short summary
·OH/·Cl initiated indole reactions mainly form organonitrates, alkoxy radicals and hydroperoxide products, showing a varying mechanism from previously reported amines reactions. This study firstly revealed carcinogenic nitrosamines can’t formed in indole oxidation reaction despite radicals formed from N-H H-abstraction. The results are important to understand the atmospheric impact of indole oxidation and extend current understanding on the atmospheric chemistry of organic nitrogen compounds.