From the journal Environmental Science: Atmospheres Peer review history

27-Feb-2023

Dear Dr Murphy:

Manuscript ID: EA-ART-02-2023-000020
TITLE: Accretion Product Formation in the Self-Reaction of Ethene-Derived Hydroxy Peroxy Radicals

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.

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Dr. Stephen Klippenstein
Associate Editor, Environmental Science: Atmospheres

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Reviewer 1

This is a well-designed and well-written study on a reaction of great atmospheric interest. I'm happy to recommend publication of the study as is.

Reviewer 2

Murphy et al. revisit a significant question related to SOA formation by examining product formation from photolytically generated OH-substituted ethylperoxy radicals.
The authors conclude, convincingly, using a process of elimination from combination of speciation data and possible reaction pathways that a non-negligible fraction of the peroxy radical self-reaction leads to a previously disregarded accretion product formation pathway. The authors provide, for the first time, molecular structure assignment for this type of product and experimentally derived rate analysis as well. Quantitatively, their results show that the channel is more than one order of magnitude larger than the previously assumed value of ~1%, attributed to wall loss in previous experiments by Barnes.

This work has significant implications on tropospheric chemistry and may also contribute to the recent discovery of brown carbon formation during low-temperature combustion given the importance of ROO radicals in combustion at temperatures from 500 – 800 K. As such, this paper will be highly cited.

The experimental approach and analytical process by which the accretion product is proposed is very clear and impressive, but is there any importance to the sequence of reactions HO-ROO + HO-ROO = ROOH + R-H(=O)OO? This may impact the kinetics interpretation and would occur if one of the HO-ROO radicals abstracted the secondary H atom located on the carbon to which the OH is bound (i.e. the ‘alpha’ carbon). The formula for that product, HO-ROOH, would be C2H6O3 and would have a nominal mass of m/z 78. Is there any evidence for this species? The co-product would be HO(CH)CH2OO that may decompose into vinyl alcohol (or via tautomerization, acetaldehyde) + O2. Perhaps this would be in the category of ‘secondary losses’ for eqn. (11).

One general comment is that it would be very helpful if the authors could provide a non-equation type of reaction scheme to show, pictorially using arrow-pushing, how the mechanism to the dihydroxy diethyl peroxide accretion product occurs.

We thank the reviewers for their helpful reviews of the manuscript. Responses to these reviews are below, with the original comment in italics. In addition to the revisions described below, we have also added the following reference, which was published while this paper was in review: H. Yue, C. Zhang, X. Lin, Z. Wen, W. Zhang, S. Mostafa, P. Luo, Z. Zhang, P. Hemberger, C. Fittschen and X. Tang, International Journal of Molecular Sciences, 2023, 24, 3731. We have also made some minor edits to the manuscript, which are clearly marked in the requested edited PDF.

1. The experimental approach and analytical process by which the accretion product is proposed is very clear and impressive, but is there any importance to the sequence of reactions HO-ROO + HO-ROO = ROOH + R-H(=O)OO? This may impact the kinetics interpretation and would occur if one of the HO-ROO radicals abstracted the secondary H atom located on the carbon to which the OH is bound (i.e. the ‘alpha’ carbon). The formula for that product, HO-ROOH, would be C2H6O3 and would have a nominal mass of m/z 78. Is there any evidence for this species? The co-product would be HO(CH)CH2OO that may decompose into vinyl alcohol (or via tautomerization, acetaldehyde) + O2. Perhaps this would be in the category of ‘secondary losses’ for eqn. (11).

Reviewer 2 suggests the possibility of an H-atom abstraction mechanism in the peroxy radical self-reaction. We evaluated this possibility using computational chemistry and find the barrier to abstraction is greater than 10 kcal/mol. While this reaction may be important at typical combustion temperatures, it will not be important in the laboratory or in the atmosphere.

2. One general comment is that it would be very helpful if the authors could provide a non-equation type of reaction scheme to show, pictorially using arrow-pushing, how the mechanism to the dihydroxy diethyl peroxide accretion product occurs.

We have added the suggested reaction scheme to the SI (see SI Appendix A).

19-Mar-2023

Dear Dr Murphy:

Manuscript ID: EA-ART-02-2023-000020.R1
TITLE: Accretion Product Formation in the Self-Reaction of Ethene-Derived Hydroxy Peroxy Radicals

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. 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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Dr. Stephen Klippenstein
Associate Editor, Environmental Science: Atmospheres