From the journal Environmental Science: Atmospheres Peer review history

19-Oct-2020

Dear Dr Nizkorodov:

Manuscript ID: EA-ART-09-2020-000007
TITLE: Effect of Aromatic Ring Substituents on the Ability of Catechol to Produce Brown Carbon in Iron(III)-Catalyzed Reactions

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

I have carefully evaluated your manuscript and the reviewers’ reports, and the reports indicate that major revisions are necessary.

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Dr Claudia Mohr

Associate Editor, Environmental Science: Atmospheres

************

Reviewer 1

This manuscript explored the reaction between 7 catechol derivatives (including catechol) and Fe(III). The absorption characteristic, the formed particles and the main products of the reactions were studied. It was found that the substituents affect the reaction. However, some problems were not dealt with, which were listed below. Generally speaking, this work meets the requirement of Environmental Science: Atmospheres, experiments, results, and writing. I would recommend the manuscript for publication after major revisions.
1. In Page 4, Line 103, it was mentioned that 7 different catechol derivatives were chosen for the study. However, what are the reasons for choosing them? Why were nitro groups mainly considered?
2. From Page 9 to 11, the characteristics and differences of the absorption spectra of the 7 catechol derivatives were described in detail. Could you conclude the effects of different substituents and substituting positions on their absorption spectra after reaction?
3. In Page 12, Line 322, it was mentioned that whether CON, 3-HC and 4-HC particles contain Fe has not been verified. I hope you can use XRF to confirm that and show the result in this paper. In addition, the particles could also be digested and the Fe could be detected by ICP-MS.
4. In Page 12, Line 329, “The particle count in both NC and MC remained low count throughout the experiment, confirming the absence of the particulate reaction products.” If the concentration of NC, MC, Fe and even KCl increases, whether could the conclusion be different?
5. In Part 3.5, the MS analysis of the products are not very clear, could you improve it?
6. In Page 15, Line 422, it is hasty to consider the lack of guaiacol and catechol in Sample #1.When some other organic substances with high complexing ability exist, the reaction might not occur. Hence, it is hard to confirm the existence of guaiacol or guaiacol through this reaction.

Reviewer 2

In the study presented here the authors conducted experiments on the Fe3+ catalyzed reactions of a selection of aromatic diols and related compounds, as well as biomass burning organic aerosol (BBOA) samples, to investigate the chemistry involved in the formation of brown carbon. Reactions were conducted in solutions containing a mixture of Fe3+ and the aromatic compound or BBOA, adjusted to a typical atmospheric pH of 3. Reactions were monitored with UV-vis spectroscopy and dynamic light scattering, and filter samples were collected to quantify the mass yields of insoluble particles and to analyze the composition of extracts using liquid chromatography and electrospray ionization mass spectrometry. Photographs were also taken of the solutions to provide visual information on the optical changes. Reaction mechanisms were developed to explain the changes in the optical properties of the solutions and particle composition, which consisted of oligomeric products.

The work was carefully done, and the data analysis and interpretation seem convincing. The authors clearly understand how to interpret changes in spectra based on electronic structure, thus extracting optimal information. Optical spectroscopy studies are somewhat rare in aerosol chemistry, so I find the mechanistic information gained from those measurements, when combined with the mass spectrometry results, to be impressive. The paper seems technically sound, and provides important new insights into the effects of aromatic compound structure on the Fe3+ catalyzed reactions of these compounds. Such reactions could occur in atmospheric aerosol particles and be an important source of brown carbon, which can impact radiation and therefore climate. The paper is also concise and well written. I think will be suitable for publication in ES-Atmospheres once the following minor comments are addressed.

Technical Comments

1. More information should be provided on the relevance of these experiments to the atmosphere. In particular, I suggest adding information on the concentrations of Fe3+ one might expect to find in atmospheric particles, and how those compare with the concentrations used in these experiments? If very different, then how should the results presented here be extrapolated to the atmosphere?

2. Scheme 2: Did the authors attempt any experiments in which an OH scavenger was added? This would be a valuable way to test the proposed mechanism.

3. Are there other species present in atmospheric aerosol particles that are not present in the experimental solutions that could alter the chemistry observed here, such as by reacting with Fe3+, OH radicals, or other reactive intermediates? If so, how does this impact the extrapolation of these results to the atmosphere?

Editorial Comments

1. Lines 27 and 95: I suggest deleting “a limited number of”, since this suggests that one could instead choose to analyze an unlimited number.

2. Line 51: Something wrong with “… formation of colored produced on …”

3. Line 83: Add “of” (or something else) before “radicals”.

4. Line 124: “were” should be “was”.

5. Line 160: “… 30 min intervals.”?

6. Line 176: Insert space after “mL”.

7. Line 214: Should be “time”.

8. Scheme 2 caption: Define LMCT.

9. Scheme 3: I suggest drawing arrows to show the H-shifts that restores aromaticity to the ring.

10. Line 458: Should be “suspended”.

This text has been copied from the PDF response to reviewers and does not include any figures, images or special characters.

Thank you for the speedy review. We will upload a point-by-point response to the reviewers' comments as well as a track-changed version of the manuscript in a PDF file.

Response to Reviewer
Comments
Journal: Environmental Science: Atmospheres
Manuscript ID: EA-ART-09-2020-000007
Title: Effect of Aromatic Ring Substituents on the Ability of Catechol to Produce Brown Carbon in Iron(III)-Catalyzed Reactions
Author(s): Chin, Henry; Hopstock, Katherine; Fleming, Lauren; Nizkorodov, Sergey; Al-Abadleh, Hind
The reviewer suggestions and questions are reproduced, followed by our responses in a different style font.
We have numbered the reviewer comments and inserted comment numbers in the main text next to each revision.
Reviewer 1
1. In Page 4, Line 103, it was mentioned that 7 different catechol derivatives were chosen for the study. However, what are the reasons for choosing them? Why were nitro groups mainly considered? Some of the reasons we selected the 7 catechol derivatives are described in the last paragraph of the introduction. We have inserted a sentence there explaining that these compounds are common constituents of BBOA. 2. From Page 9 to 11, the characteristics and differences of the absorption spectra of the 7 catechol derivatives were described in detail. Could you conclude the effects of different substituents and substituting positions on their absorption spectra after reaction? In response to this suggestion, we have inserted a brief summary statement for the UV/Vis portion of the work, and added Table 1 to simplify the presentation. 3. In Page 12, Line 322, it was mentioned that whether CON, 3-HC and 4-HC particles contain Fe has not been verified. I hope you can use XRF to confirm that and show the result in this paper. In addition, the particles could also be digested and the Fe could be detected by ICP-MS. Thank you for this suggestion. We wished to have completed XRF and ICP-MS analysis but, due to the COVID19 shutdown, we were unable to carry out these experiments. An additional sentence was added to address this. 4. In Page 12, Line 329, “The particle count in both NC and MC remained low count throughout the experiment, confirming the absence of the particulate reaction products.” If the concentration of NC, MC, Fe and even KCl increases, whether could the conclusion be different? This is a good question. In our previous study on CA+Fe(III) reactions we examine the effect of the ratio of CA to Fe(III) on the particle yield. We have not examined the concentration dependence in this study as we were Environmental Science: Atmospheres Page 2 of 81 focusing on the effect of the aromatic ring substituents. We inserted a statement about this in the revised manuscript. 5. In Part 3.5, the MS analysis of the products are not very clear, could you improve it? We agree that the analysis could be more comprehensive. But due to the complexity of the mechanism of particle formation we elected to focus on just a few major light-absorbing species. After the pandemic, we hope to be able to analyze both the particles and filtrates more comprehensively for one selected catechol derivative. 6. In Page 15, Line 422, it is hasty to consider the lack of guaiacol and catechol in Sample #1.When some other organic substances with high complexing ability exist, the reaction might not occur. Hence, it is hard to confirm the existence of guaiacol or guaiacol through this reaction. We agree. We have inserted a possibility suggested by the reviewer in the text.
Reviewer 2

Technical Comments 1. More information should be provided on the relevance of these experiments to the atmosphere. In particular, I suggest adding information on the concentrations of Fe3+ one might expect to find in atmospheric particles, and how those compare with the concentrations used in these experiments? If very different, then how should the results presented here be extrapolated to the atmosphere? Thank you for this question. To the best of our knowledge, there have been no direct measurements of acids or iron concentration in water adsorbed on atmospheric particles. We selected these relatively high concentrations to mimic aqueous surfaces where organic compounds are enriched. We have added few sentences in the experimental section to state this. 2. Scheme 2: Did the authors attempt any experiments in which an OH scavenger was added? This would be a valuable way to test the proposed mechanism. This is a great suggestion. Unfortunately, due to the COVID-19 shutdown, we will be unable to test the OH scavenger reactivity any time soon, but our future experiments with BBOA + Fe(III) chemistry will utilize this method. 3. Are there other species present in atmospheric aerosol particles that are not present in the experimental solutions that could alter the chemistry observed here, such as by reacting with Fe3+, OH radicals, or other reactive intermediates? If so, how does this impact the extrapolation of these results to the atmosphere? Page 3 of 81 Environmental Science: Atmospheres We acknowledge that our experiments only considered a narrow range of potential atmospherically relevant conditions: dark and illuminated reactions of FeCl3 with BBOA precursor dissolved in KCl solutions at pH 3. Atmospheric aerosol composition will have a variety of anions (oxalate, sulfate, etc.) which can potentially affect this chemistry. We have added a paragraph in the atmospheric implication section to address this important point. Editorial Comments 1. Lines 27 and 95: I suggest deleting “a limited number of”, since this suggests that one could instead choose to analyze an unlimited number. Thank you for pointing this out. The correction has been made on lines 27 and 95. 2. Line 51: Something wrong with “… formation of colored produced on …” Thank you for pointing this out. The paper referenced here studied the reaction of Fe(III) modified montmorillonite with guaiacol (used as the model phenol substance). Polymerization of guaiacol was achieved and brown carbon particles were formed. Line 52 was changed: Ling et al. (2020) demonstrated that Fe-containing clay minerals produced colored particles when exposed to guaiacol. 3. Line 83: Add “of” (or something else) before “radicals”. Thank you for pointing this out. The correction has been made. 4. Line 124: “were” should be “was”. Corrected. 5. Line 160: “… 30 min intervals.”? A correction was made to line 164 to clarify this. 6. Line 176: Insert space after “mL”. Space added. 7. Line 214: Should be “time”. Corrected. 8. Scheme 2 caption: Define LMCT. Thank you for pointing this out. We have edited the caption 9. Scheme 3: I suggest drawing arrows to show the H-shifts that restores aromaticity to the ring. We have added this to the mechanism 10. Line 458: Should be “suspended”. Corrected.

24-Nov-2020

Dear Dr Nizkorodov:

Manuscript ID: EA-ART-09-2020-000007.R1
TITLE: Effect of Aromatic Ring Substituents on the Ability of Catechol to Produce Brown Carbon in Iron(III)-Catalyzed Reactions

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. After considering the changes you have made, I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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With best wishes,

Dr Claudia Mohr

Associate Editor, Environmental Science: Atmospheres

Reviewer 1

This manuscript explored the reaction between 7 catechol derivatives (including catechol) and Fe(III). The absorption characteristic, the formed particles and the main products of the reactions were studied. It was found that the substituents affect the reaction.Due to COVID-19, some experiments could not be produced which is a pity for this study. Generally speaking, this work meets the requirement of Environmental Science: Atmospheres. I would recommend the manuscript for publication.