Kinetic Study of the Atmospheric Oxidation of a Series of Epoxy Compounds by OH Radicals by Carmen Maria Tovar et al. presents a series of measurements of epoxides using the relative rate technique with a variety of reference compounds. This work fills in gaps in the kinetic information on these compounds that are used in industrial chemistry and potentially CO₂ capture technology. This work also discusses the failings of the current structure activity relationships (SARs) used to estimate the rate constants of epoxides with OH radicals. This work adequately presents their laboratory results and comparison to the SARs, however it is unclear what actually needs to be adjusted to the current SARs from the presented results, as there was no attempt to create a more representative SAR. Because of the additional knowledge presented in this paper on the rate constants of these chemicals and potential to improve current SARs for epoxides, I recommend that this is reconsidered for publication after major revisions. 

Major Comments:

L285-327: “(Tamres et al., 1954) stated…means of theoretical calculations.” This seems beyond the scope of the paper. I think that it is useful to think of the reactivity of these compounds from a theoretical perspective in some situations. However, structure activity relationships are observationally/experimentally derived. In these paragraphs, there are many facts stated about the electronegativity, ring strain, hybridization, and other parameters of epoxides, but it is unclear how these relate to your results. Are these factors that can be put into a revised SAR? Can you obtain these from just knowing the structure of the molecule? As a concrete example, how does the suggestion “that the bonds are not saturated and can interact with π-electron systems” affect the OH rate constants with the epoxides? If you keep this section, it should be connected to the SAR and reaction rates more clearly. 

Table 1: What is the error of your concentrations? I don’t think the error on the 1ppm starting concentration is the same as the 84ppm, which is what this table implies. Were the initial concentrations assumed from the known amount injected? If so, how do you know it all made it into the chambers without decaying? How did you actually manage to get these chemicals into the gas-phase consistently?

Table 2: The final errors on the rate constants do not seem to make sense. The error on the Ethylene rate constant is ~1.5% and the error of your measured slope is ~5% (is this just the error on the slope? Do you force the fit through zero? Does that change the error?). How does the reported rate constant error for CHO come out to be ~16%? You say “up to 35% contribution in the recommended values of the rate coefficients for reference compounds” is accounted for in the error, but not all of your reference compounds have this large of errors. Also, unless you have a valid justification, it does not make sense for reported errors to have more than one significant figure. On a similar note, the error should also determine the significant figures of the reported value. This might end up affecting your claim that the trans- and cis- isomers have duifferent rates, since the reported rates are within each other’s errors. 

Table 4 (and corresponding discussion): There should be more discussion about what a homologous compound is. Potentially having the structures drawn would help with this. From my perspective, I just see that CHO is C6H10O, methylcyclohexane is C7H14, and di-n-propyl ether is C6H14O which have different numbers of total atoms. I expect the rate constant for these other compounds to be faster because there are more hydrogens to abstract, so I am not sure if your point is coming across with this table and discussion. It is difficult to see how the addition of the 3-membered ring or additional oxygen actually affects the rates when the molecules could have other factors changing their rate constants. 

Minor Comments:

L162: Why are only some reference compounds used for some of the epoxides? Why did you not use all 6 reference compounds? 

L169: Keep names the same for compounds (e.g. propene vs propylene) throughout the paper.

Figure 1: I think one panel will show the fact that your results are linear adequately. I would recommend putting most of this figure in the SI. 

L339: A range of (2.0±5.7) ×10-12 cm3 molecule-1 s-1 does not make physical sense. The way the error is written includes negative rate constants. 

L344: You mention a theoretical structural analysis but have no basis sets or computational details in the methods for this. 

Section 4: There are more recent references for the 24-hr average concentration of OH, Cl, and peak Cl atom concentrations during CalNex that probably should be used:

Young et al. ACP 10.5194/acp-14-3427-2014

Wang et al.  ACP 10.5194/acp-19-3981-2019

Lelieveld et al. ACP 10.5194/acp-16-12477-2016

L385: How does the timescale for aerosol partitioning compare to oxidation by OH?

L400-401: There should not be any new information in the conclusion - potential future work should not be included.  

Technical Comments:

The L in 480L or 1080L should be consistently formatted through the paper (i.e. capitalization, italicization, space after the numbers, etc.)

Numbering of equations is inconsistent.

There are subject/verb disagreements, grammar and punctuation errors, and extra spaces and carriage returns that should be checked for. 

Review of:  Kinetic Study of the Atmospheric Oxidation of a Series of Epoxy Compounds by OH Radicals 

This paper summarizes several experiments in a pair of relative rate reactors to determine the rate constants of the OH reactions with a series of epoxides. The kinetics of epoxide reactions with OH have only been studied with a limited number of epoxides to this point. The literature values of those rate constants are compared, and are in good agreement with the values obtained in this study. This paper fills a gap in the literature as the kinetics of these reactions are currently understudied. The authors do a good job in their introduction outlining the importance of this class of compounds and how they are becoming more important through their role in carbon capture technologies.  There are a few technical issues that need to be resolved in the specific comments below. My two large issues are as follows. There is a large section of the results/discussion dedicated to hybridization and ring strain. While interesting from a purely physical chemistry point of view I am not sure how much it actually adds to the paper and might be a bit too physical chemistry for the atmospheric chemistry community at large. Were this a J. Phys Chem. submission I think it would be fine, but I'm not sure about ACP. I think it would be more useful if the authors spent a little more time suggesting improvements/modifications to the Structure Activity Relationship (SAR) instead. This paper clearly demonstrates a scenario where SAR is lacking. It would be more informative to say something beyond “we need more data to properly capture the behaviour of the epoxide reactions with OH.” Now it is possible that improvements to the five suggested methods are not possible, the authors could explain why this is the case. Overall, the paper fits within the scope of ACP and I recommend publication once the Results/Discussion section has been reworked and the technical issues below are addressed.

Specific Comments:

P4 L122: Were there any losses of reactants on their introduction into the chamber? If so how was this determined?

P5 L147: Perhaps I merely missed this in the description of the chambers, but why did the concentrations have to be 8 times higher in the smaller chamber? Is this a lamp intensity issue with the different chamber materials?

Table 1: There are formatting issues here. The column headers do not really describe what is in the columns. Column 1 is the species list not the initial mixing ratio. This needs to be fixed. What are the uncertainties on the initial conditions? This does not seem to stated anywhere and should be included in this table.

P6 L175: Was the linearity only for some compounds or all compounds? This sentence makes it seem like the relationship is not linear for all of the epoxides. This should be reworded, or the lack of linearity needs to be discussed.

Table 2: Why were different reference compounds used for the different epoxides? Was it initially merely a spot check on the kinetics of known reactions or something else? This should be mentioned/discussed.

P7 L192 should probably read “such as for cyclic ethers”.

P8 L215: The ordering of the SAR methods needs to be changed. The authors jump from (c) to (e) and then come back to (d). This leaves the reader confused and believing they missed something.

Figure 1: It is unclear to me which chamber these results came from. It should be indicated in the figure caption and/or panels.  

P10 L243: This is where it would be helpful if the authors made some suggestions or at least discussed a better way to use SAR. Since all of the methods return varying degrees of accuracy for different compounds, it shows that it is not helpful without more data to constrain the SAR prediction. This of course probably is not shocking as with most models if you put either not enough information in (or garbage) you tend to get less than satisfactory results out.

Table 3: Column 2 needs to have the appropriate letter next to the SAR predicted rate constant. Currently it is up the reader to guess/assume.

P12 L285: The sentence should not start with brackets. I am guessing this is an Endnote formatting issue that the authors did not catch.

P13 L315: Perhaps it is my own ignorance but I would not have guessed that an epoxide would be less reactive to OH than its analogous alkane. This is a comment more on me than the authors.

P14 L339: This uncertainty is wrong. How can you potentially have a negative rate constant?

 P14 L357: Are the chambers used in these experiments capable of temperature dependant measurements? How stable is the temperature in the reactor over the course of an experiment as I am guessing the lamps do add some heat.

P15 L372: Should be Cl atoms or radicals?

P15 L382: What are the aerosol uptake rates and how do they compare to oxidation by OH and Cl? A reference to this should be included.

P16 L400: The presentation of future work should either be put in a “Future Work” section or tacked on to the results discussion not the Conclusions section.