the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Atmospheric Oxidation Mechanism and Kinetics of Indole Initiated by ·OH and ·Cl: A Computational Study
- 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
- 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.
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Jingwen Xue et al.
Status: open (until 07 Jun 2022)
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RC1: 'Comment on acp-2022-88', Anonymous Referee #2, 17 May 2022
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This manuscript presents theoretical calculations on the mechanisms and kinetics of the reaction systems initiated by the indole + OH and the indole + Cl reaction under atmospheric conditions. In particular, consecutive reactions of the produced intermediate radicals with O2 were included, and the unimolecular reactions of the subsequent radical-O2 adducts were studied in their competition to bimolecular reactions with NO and HO2. Relative yields for the most important reaction channels were derived. An important finding is that the N-centered radicals produced by hydrogen abstraction from the indole nitrogen react much faster with O2 than with NO under typical tropospheric conditions. It is concluded that the formation of carcinogenic nitrosamines appears less important for indole as for aliphatic amines (at least via this reaction pathway). The detailed theoretical characterization of several very complex reaction mechanisms with advanced quantum chemical calculations and statistical rate theory must have been really painstaking work. As the problem addressed in this paper is timely, and the methods applied appear adequate, the manuscript merits publication. However, before final acceptance, the authors could further improve the quality by considering a few minor points. They should make a bit more clear (in section 2.2) why two different software packages (MultiWell and MESMER) were used. Which one was used for which reaction, and why? Please explicitly state whether you included all the channels shown in Fig. 1(A) for OH and in Fig. 1(B) for Cl in the respective master equations. In other words, did you use a full multichannel approach coupling the entire reaction system or did you solve a corresponding number of one(few)-channel master equations? Furthermore, as far as this reviewer understands, the calculations were obviously performed for a single pressure of 1 atm. Does pressure have any effect on the relative yields in the tropospherically relevant range down to say 100 mbar? And if so, what about the energy transfer parameters (e.g. the average energy transferred per collision)? After all, how do the time-dependent results from the master equation calculations (like those illustrated in Fig. 3) translate to steady-state situations in the atmosphere. Here a brief discussion would also be useful.
Some minor technical issues are:
line 9: please insert comma after nitrosamines
line 14: please insert ‘the’ before dominant
line 21: please insert ‘that’ after demonstrate
line 34: ‘10% of total gas phase nitrogen’ – probably excluding N2
line 60: ‘the kOH value’ should be correctly termed rate constant
line 78: ‘reactions’ should better read ‘reaction’
line 123: ‘phenyl group’ should probably better read ‘the benzene ring’ or ‘the C6 ring’
line 126: The authors should mention at this point that the numbering of the atoms is given in Fig. 1.
line 239: ‘biomolecular’ read ‘bimolecular’
line 283: please insert ‘that’ after reveal (also line 250)
line 292: If you specify author contributions then please also include Jonas Elm.
Fig. S1: Please mention the pressure in the figure caption.
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RC2: 'Reply on RC1', Anonymous Referee #1, 18 May 2022
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I have no further comments.
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RC2: 'Reply on RC1', Anonymous Referee #1, 18 May 2022
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Jingwen Xue et al.
Jingwen Xue et al.
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