From the journal Environmental Science: Atmospheres Peer review history

16-Dec-2020

Dear Dr Donahue:

Manuscript ID: EA-ART-11-2020-000017
TITLE: Peroxy radical kinetics and new particle formation

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Reviewer 1

This is a very interesting manuscript, which extents a previously published conceptual modelling framework for formation of HOM peroxy radicals and their chemical fate under increasing HO2/RO2 ratio. The authors are correct that simulation experiments in the laboratory are (too) often run under very low HO2/RO2 ratios which leads – in the context of HOM – to an overestimation of RO2-RO2 pathways including dimer formation. Overall, the paper is well written and figures, tables are suited to illustrate the arguments. In some instance, I find it difficult to follow directly the arguments, maybe because the text is too compact and some more explicit explanation would be helpful. Although I don’t mind conceptual papers at all, I wish that the authors would somehow compare at least the trends of the findings to data. Otherwise, the manuscript should be published in Environmental Science: Atmospheres after addressing/considering the follow minor comments.

General comments:

a) On one hand the authors refer to “newer” experimental data when they adjust the “coarse” performance on the model, on the other hand the do not show if the trends found by their model are compatible with observations. The boundary conditions are chosen to simulate a CLOUD like chamber, I guess there must be some reason behind it. Aren’t there any data to compare with? I don’t mean simulate the experiments, but showing that the trends and the conceptual thinking are going in the right direction or that deviations can be understood from the limits of the model?
In the same direction, I am wondering, why the authors do not refer to the findings by McFiggans et al. (Nature, 2019). In principle, the laboratory observations here support the findings of the role of small RO2 (isoprene, CH4) and HO2(CO) regarding the suppression of dimers.

b) I am not quite sure about the following. The split into the classes seems to be inconsistent. Does it mean that all others than non-HOM products contain autoxidation steps, ROOH as well as dimers or do they also contain non HOM components? ROOH for example, seem to contain also the non-HOM ROOH, which increase strongly with HO2. HOM-ROOH seem not to count as HOM monomers, correct? If so, two cases are needed to be entangled, the fact that with increasing HO2 more hydroperoxides are formed, and the fact that at the same time more HOM-ROOH are formed, that should be accounted to HOM monomers. For example, I would expect that with increasing HO2 other HOM monomers from the molecular channel and dimers decrease to form HOM-ROOH. But this seems not to be the case?
Again, under the clause “if so”: these HOM ROOH (ROOH) contribute to the log(C) = 1 and 2 bins so possibly increase the "SOA formation by condensation" at atmospheric loads of 1-100ug/m3 total organics. Which would be in contrast of the finding of McFiggans et al. (2019) that the presence of (large amounts) of small RO2 by CH4 and HO2 by CO lowers SOA yields and SOA mass. I suggest that the authors should comment or clarify that in the manuscript.
If not so, the class definitions should be clarified in the text. In any case it would be interesting and helpful to indicate the degree of autoxidation within the HOM classes (log C bins) in the according figures.

Minor comments:
page 2, column 1, last §: Isn’t that number closer to 1E-11 cm3 s-1 for HO2+RO2 reactions, so 1 in 10 collisions. At least in the previous ACP paper by the same authors it was categorized like that.

page 2, column 2, last §: I have difficulties to follow what is meant here. Specifically, the sentences beginning with “We form our parameterization assuming that…” and “There is still significant uncertainty…”.

section 2.1. and table 1: What will happen if small but substantial groups of RO2 and OxnRO2 have specifically high rate coefficients? Could that establish something like a preferred flow into ULVOC dimers? Wouldn’t that have a tendency to survive at high HO2/RO2 and provide still channels into dimers?

page 7, column 2, last line and page 8, column 1, last §: I cannot follow the statements beginning with: “…we see in the fast case, the oxidized dimers are suppressed far less at low CO mixing ratios when their cross reactions are fast.” and “The concentration of these dimers … is comparable to the fast case when no CO is present.”.

page 9, colum2, last §: I don’t understand what is meant by: “Out modeled ULVOC collision frequencies at low CO are consistent with ULVOC nucleation efficiency for all RO2 reactivity cases considered.” Please explain in more detail.

Figures 3,7, 8: I think it would be helpful to show the two CO lines in all Figures. Especially, since the distributions refer to steady states under these conditions.

pag5, column 1, second §: typo UVLOC.

Figure 5: legend contains still the non HOM products.

Reviewer 2

The paper highlights a critical issue directly related to our understanding of the formation of very low volatility molecules within the atmosphere -- molecules which are believed to be responsible, in significant part, for the formation of new particles in the lower atmosphere. By conducting model simulations of alpha-pinene + O3 steady state chamber experiments using an explicit kinetic chemical mechanism, the authors infer that in the absence of reactants (HO2 precursors), the steady state RO2/HO2 ratio is very high, unlike the real atmosphere, where daytime RO2/HO2 is generally believed to be <= 1. In addition the authors show that addition of OVOC to the reaction mix, specifically CO, can help to better mimic the real atmosphere (which is rich in OVOC), by elevating HO2 levels. Particles can be nucleated from collisions of very low volatility ‘dimer’ ROOR products believed to arise from RO2 + RO2 (R~C10) accretion reactions. Alternative fates for RO2 (e.g. reaction with HO2) can thus reduce ROOR formation, and in turn nucleation rate. The authors use the simulations with varying amount of CO (e.g., changing RO2/HO2 ratio) to show that calculated nucleation rates are quite sensitive to RO2/HO2 ratio, and RO2+RO2 kinetics, and caution that extrapolation of nucleation rates/growth from chamber experiments to the atmosphere may be complicated, as chamber experiments are simulated to have much higher RO2/HO2 ratios than the real atmosphere, though, this can be partially mitigated through careful control of experimental conditions (e.g. addition of OVOC such as CO).

Overall, I think this paper makes a very important and timely point, in an elegant fashion. More than a few times, in reviewing papers discussing SOA yields, when authors were queried as to the peroxy radical fate/reaction conditions, unsatisfactory/hand-wavy answers are returned. This is not acceptable, and at a minimum, reaction conditions need to be clearly and accurately reported for the results are to be properly interpreted, and placed into the larger context of atmospheric chemistry! I think this paper should be published after the following items are addressed:

[Note to journal editor]: Addition of line numbers to review manuscripts would greatly help with the conveyance of reviewer remarks.

General considerations:

Given my understanding of the journal target audience (as quite broad and general) I think the abstract and the introduction could use more specific language to describe the chemistry under consideration will likely enhance understanding and reduce confusion. E.G., RO2 derived from rather large biogenic species are the focus of the discussion. In addition, a sentence or two stating importance of particle nucleation to our understanding of the atmosphere will help motivate the study.

Acronyms/Terminology: The general reader is likely to get lost in the wide array of terms/acronyms and how they relate to each other. [SuperDuperLVOC?, [joking]], ULVOC, ELVOC, HOM, dimer. I think the introduction can be improved to better define and relate these terms to one another.

Broaden the introduction: While the nuts and bolts of the paper describe the issue of RO2/HO2 ratio mismatch between chamber experiments and atmosphere, this is likely not the only important issue. Broadening the discussion of potential issues in the introduction may be prudent. For instance, when considering RO2 autoxidation pathways, it is not only the ratio of RO2/HO2 that matters, but also the absolute concentration of partner radicals [NO, HO2, and RO2], as the RO2 lifetime will determine the fraction of a particular RO2 which will autoxidize. Furthermore, much of the real atmosphere likely resides in a chemical regime where NO, HO2, RO2 and autoxidation processes all contribute to the RO2 fate. NO, RO2, autoxidation and even some HO2 reactions with RO2 have significant radical channels products of which can then participate in further reactions and produce products which are different from any single type of RO2 process (ie mixed regime). In addition, for particular RO2 such as acyl peroxy radicals (thought to be important RO2 products from terpene chemistry) reaction with NO2 to form PAN species can be a significant sink in the real atmosphere, sequestering these from participation in ‘dimer’ formation.

Model discussion: Again given the target audience, a bit more specific description of the model maybe appropriate; it is quite central to this paper, and the casual reader may not be likely to go investigate the earlier paper.

RO2/HO2 ratio at zero CO: I am quite surprised at results showing RO2/HO2 ratios approaching 1000 in the simulations with no CO. Has this been validated with any experimental observations of specific indicators (e.g. H2O2, ROOH, etc). While I do not have much experience with the RO2 + RO2 chemistry from terpenes, I am quite familiar with RO2+RO2 chemistry derived from smaller (C2-C6) alkenes and dialkenes. Under conditions with RO2 primary production >> HO2 primary production, significant ROOH yields (~30%) are generally observed, independent of PRO2/PHO2. These ROOH are understood to arise from secondary HO2 formation from the subsequent chemistry of the alkoxy radicals generated from initial RO2 + RO2 reactions, and hence cannot be separated from RO2+RO2. Do RO2 + RO2 reactions in your simulations generate RO? HO2? Should they?

Why does the simulation appear to asymptote in RO2/HO2 ratio at about 2 with increasing CO? Is there a way to move this to more relevant atmospheric ratio like 0.1, or is this precluded due the limitation of only using O3 as radical precursor? Is there a better radical precursor system which might give more control over this ratio?


Specific comments:

• Significance statement: In addition to precisely knowing the chamber conditions, I would argue one must actually also use mechanistic information if one is to be successful in extrapolating to the atmosphere, which invariably will have different conditions. This is the primary limitation to the ‘engineers method’ of solving such problems. The derived parameterization will work wonderfully for conditions within those used to derive the parameterization, but will quickly fail when extrapolated outside the test conditions.
• Pg2P4: most may suggest room temp k(RO2+HO2) ~ 1e-11 increasing with size of R to 2e-11,as opposed to 1e-12 (also for RO2 + NO) (1-2 out of ~30 collisions?);
• Pg2P4: might be worthwhile to reference related note from Wennberg (https://igacproject.org/sites/default/files/2016-07/Issue_50_Jul_2013.pdf)
• Pg3P2S1: Is this really true. Suspicious.
• Pg3P3LastSent: Again, I think conditions could be found where each RO2 reacts with HO2, for a general VOC chamber experiment. If this statement only applies to an O3 + AP experiment, with no other oxidants this should be clarified.
• Pg5P2: ‘UVLOC’ --> ‘ULVOC’
• Pg9P3: ‘Out’ --> ‘Our’

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

Response to reviewer comments:

We thank both reviewers for their insightful and constructive comments. We have revised our manuscript in line with their suggestions and provided detailed responses below (in addition we provide a difference document with our revised submission).

REVIEWER REPORT(S):
Referee: 1

Comments to the Author
This is a very interesting manuscript, which extents a previously published conceptual modelling framework for formation of HOM peroxy radicals and their chemical fate under increasing HO2/RO2 ratio. The authors are correct that simulation experiments in the laboratory are (too) often run under very low HO2/RO2 ratios which leads – in the context of HOM – to an overestimation of RO2-RO2 pathways including dimer formation. Overall, the paper is well written and figures, tables are suited to illustrate the arguments. In some instance, I find it difficult to follow directly the arguments, maybe because the text is too compact and some more explicit explanation would be helpful. Although I don’t mind conceptual papers at all, I wish that the authors would somehow compare at least the trends of the findings to data. Otherwise, the manuscript should be published in Environmental Science: Atmospheres after addressing/considering the follow minor comments.

General comments:

a) On one hand the authors refer to “newer” experimental data when they adjust the “coarse” performance on the model, on the other hand the do not show if the trends found by their model are compatible with observations. The boundary conditions are chosen to simulate a CLOUD like chamber, I guess there must be some reason behind it. Aren’t there any data to compare with? I don’t mean simulate the experiments, but showing that the trends and the conceptual thinking are going in the right direction or that deviations can be understood from the limits of the model?
In the same direction, I am wondering, why the authors do not refer to the findings by McFiggans et al. (Nature, 2019). In principle, the laboratory observations here support the findings of the role of small RO2 (isoprene, CH4) and HO2(CO) regarding the suppression of dimers.

For this and the following comment it is an excellent point that we should more thoroughly discuss McFiggans et al., and we have done so. We have added a discussion of McFiggans et al. at several points and in detail in the Discussion. We cannot respond specifically to the questions about “coarse” performance and “newer” data because we cannot find those terms in our manuscript. However, we believe this is referring to section 2.2.1 where we describe differences in the RO2 kinetics from our earlier radical-VBS model. Here we cited results from the CLOUD experiment and now also cite Pye et al. (PNAS, 2019). As the reviewer notes, however, we are in general trying to avoid too close a focus on matching experimental data because we are not yet confident that we have the appropriate degrees of freedom in our representation (both number and tied to the correct processes).

b) I am not quite sure about the following. The split into the classes seems to be inconsistent. Does it mean that all others than non-HOM products contain autoxidation steps, ROOH as well as dimers or do they also contain non HOM components? ROOH for example, seem to contain also the non-HOM ROOH, which increase strongly with HO2. HOM-ROOH seem not to count as HOM monomers, correct? If so, two cases are needed to be entangled, the fact that with increasing HO2 more hydroperoxides are formed, and the fact that at the same time more HOM-ROOH are formed, that should be accounted to HOM monomers. For example, I would expect that with increasing HO2 other HOM monomers from the molecular channel and dimers decrease to form HOM-ROOH. But this seems not to be the case?
Again, under the clause “if so”: these HOM ROOH (ROOH) contribute to the log(C) = 1 and 2 bins so possibly increase the "SOA formation by condensation" at atmospheric loads of 1-100ug/m3 total organics. Which would be in contrast of the finding of McFiggans et al. (2019) that the presence of (large amounts) of small RO2 by CH4 and HO2 by CO lowers SOA yields and SOA mass. I suggest that the authors should comment or clarify that in the manuscript.
If not so, the class definitions should be clarified in the text. In any case it would be interesting and helpful to indicate the degree of autoxidation within the HOM classes (log C bins) in the according figures.

The dimers and ROOH products may or may not follow an autoxidation step and are classified as such (dimers and ROOH rather than as HOMs or non-HOMs) because they specific species were important to track in this work (and differentiating both HOM and non-HOM dimers and ROOH on a plot produced quite an eyesore). However, the reviewer is absolutely correct that some of these ROOH are also HOM monomers, although in every run the non-HOM ROOH concentration is at least an order of magnitude higher than the total concentration of all the HOM ROOH as shown in the plot below, where the solid lines are the non-HOM ROOH and the dashed lines are HOM ROOH. As HO2 is increased there are more non-HOM ROOH formed, and subsequently fewer non-ROOH HOM monomers are formed.

The confusion may arise from our volatility assignment of these ROOH products. This volatility assignment is consistent with the kernel from our original model, and ultimately the optimal oxidation kernels may require refinement in light of the data from McFiggans et al. and other experiments. However, a direct comparison is difficult for several reasons. The most notable is that the experiments described in McFiggans et al. include an OH source from the photolysis of ozone, which increases the total HOx and the amount of alpha-pinene oxidized by OH, whereas the simulation in our work is dark ozonolysis. Note in our Figure 4 that the increased HO2 and thus ROOH production with increasing CO causes significant changes to the volatility distribution for c* >= 10 µg m-3, which is just above the mass loading probed in the McFiggans experiments. However, we agree that discussion of these issues is important, so we have added that in the Discussion.

In addition, language was added in the introduction to clarify the distinction between HOMs and the volatility classes themselves.
Minor comments:
page 2, column 1, last §: Isn’t that number closer to 1E-11 cm3 s-1 for HO2+RO2 reactions, so 1 in 10 collisions. At least in the previous ACP paper by the same authors it was categorized like that.

The RO2+HO2 rate coefficient used in this work was 10-11 cm3 s-1. This was just typo / temporary brain freeze by the senior author.

page 2, column 2, last §: I have difficulties to follow what is meant here. Specifically, the sentences beginning with “We form our parameterization assuming that…” and “There is still significant uncertainty…”.

We expanded the discussion of this rate coefficient parameterization to make it clearer. Simply put, the RO2 rate coefficients get a lot faster as electron withdrawing functional groups influence the OO moiety.

section 2.1. and table 1: What will happen if small but substantial groups of RO2 and OxnRO2 have specifically high rate coefficients? Could that establish something like a preferred flow into ULVOC dimers? Wouldn’t that have a tendency to survive at high HO2/RO2 and provide still channels into dimers?

This is certainly possible and most closely resembles our “fast” case in this work, where the more autoxidized RO2 form dimers rapidly and the non-autoxidized (and most abundant) RO2 associate much slower. This particular case was chosen as having more electron-withdrawing groups (like -OOH groups) near the peroxy moiety increase the association rate coefficient. Thus the more autoxidation a peroxy radical has undergone the more of those groups will be near the peroxy moiety (or the more likely those groups will be near the peroxy moiety) and they will have faster association rate coefficients. We also note in the discussion that the recent paper from Zhao et al. (PNAS 2020) defined “slow” and “fast” autoxidation RO2 to obtain dimer yields matching both chamber and flowtube experimental data. The bottom line is that there is a very rich phase space, and we are striving to identify the major dimensions within that space to capture and generalize the chemical behavior.

page 7, column 2, last line and page 8, column 1, last §: I cannot follow the statements beginning with: “…we see in the fast case, the oxidized dimers are suppressed far less at low CO mixing ratios when their cross reactions are fast.” and “The concentration of these dimers … is comparable to the fast case when no CO is present.”.

We have tried to make the language less confusing and changed these sentences to the following:

“The concentration of the most oxidized dimers in the slow and middle cases is comparable to the fast case when no CO is present, but they are suppressed more aggressively by HO2 in the slow and middle cases, creating situations where we might measure appreciable dimers under low CO chamber conditions, but not under atmospheric conditions with lower RO2:HO2.”

page 9, colum2, last §: I don’t understand what is meant by: “Out modeled ULVOC collision frequencies at low CO are consistent with ULVOC nucleation efficiency for all RO2 reactivity cases considered.” Please explain in more detail.

We have elaborated that to now read, “Our modeled ULVOC collision frequencies at low CO are consistent with ULVOC nucleation efficiency for all RO2 reactivity cases considered, indicating that any of these association rate scenarios would be consistent with experimental findings at low CO mixing ratios.”

Figures 3,7, 8: I think it would be helpful to show the two CO lines in all Figures. Especially, since the distributions refer to steady states under these conditions.

We added the two CO lines to the plots in Figures 3, 7, and 8 so that they now appear consistently for any plot vs CO.

pag5, column 1, second §: typo UVLOC.

Fixed.

Figure 5: legend contains still the non HOM products.

Fixed.

Referee: 2

Comments to the Author
The paper highlights a critical issue directly related to our understanding of the formation of very low volatility molecules within the atmosphere -- molecules which are believed to be responsible, in significant part, for the formation of new particles in the lower atmosphere. By conducting model simulations of alpha-pinene + O3 steady state chamber experiments using an explicit kinetic chemical mechanism, the authors infer that in the absence of reactants (HO2 precursors), the steady state RO2/HO2 ratio is very high, unlike the real atmosphere, where daytime RO2/HO2 is generally believed to be <= 1. In addition the authors show that addition of OVOC to the reaction mix, specifically CO, can help to better mimic the real atmosphere (which is rich in OVOC), by elevating HO2 levels. Particles can be nucleated from collisions of very low volatility ‘dimer’ ROOR products believed to arise from RO2 + RO2 (R~C10) accretion reactions. Alternative fates for RO2 (e.g. reaction with HO2) can thus reduce ROOR formation, and in turn nucleation rate. The authors use the simulations with varying amount of CO (e.g., changing RO2/HO2 ratio) to show that calculated nucleation rates are quite sensitive to RO2/HO2 ratio, and RO2+RO2 kinetics, and caution that extrapolation of nucleation rates/growth from chamber experiments to the atmosphere may be complicated, as chamber experiments are simulated to have much higher RO2/HO2 ratios than the real atmosphere, though, this can be partially mitigated through careful control of experimental conditions (e.g. addition of OVOC such as CO).

Overall, I think this paper makes a very important and timely point, in an elegant fashion. More than a few times, in reviewing papers discussing SOA yields, when authors were queried as to the peroxy radical fate/reaction conditions, unsatisfactory/hand-wavy answers are returned. This is not acceptable, and at a minimum, reaction conditions need to be clearly and accurately reported for the results are to be properly interpreted, and placed into the larger context of atmospheric chemistry! I think this paper should be published after the following items are addressed:

[Note to journal editor]: Addition of line numbers to review manuscripts would greatly help with the conveyance of reviewer remarks.

We apologize and have added line numbers in the revision.

General considerations:

Given my understanding of the journal target audience (as quite broad and general) I think the abstract and the introduction could use more specific language to describe the chemistry under consideration will likely enhance understanding and reduce confusion. E.G., RO2 derived from rather large biogenic species are the focus of the discussion. In addition, a sentence or two stating importance of particle nucleation to our understanding of the atmosphere will help motivate the study.

We have expanded the introduction to provide more context for the work including language about the importance of new particle formation as well as discussion of the importance of studying large biogenic species.

Acronyms/Terminology: The general reader is likely to get lost in the wide array of terms/acronyms and how they relate to each other. [SuperDuperLVOC?, [joking]], ULVOC, ELVOC, HOM, dimer. I think the introduction can be improved to better define and relate these terms to one another.

We have provided definitions of these in the introduction. and should we ever need a lower volatility designation, we will consider “SuperDuperLVOC” in due course.

Broaden the introduction: While the nuts and bolts of the paper describe the issue of RO2/HO2 ratio mismatch between chamber experiments and atmosphere, this is likely not the only important issue. Broadening the discussion of potential issues in the introduction may be prudent. For instance, when considering RO2 autoxidation pathways, it is not only the ratio of RO2/HO2 that matters, but also the absolute concentration of partner radicals [NO, HO2, and RO2], as the RO2 lifetime will determine the fraction of a particular RO2 which will autoxidize. Furthermore, much of the real atmosphere likely resides in a chemical regime where NO, HO2, RO2 and autoxidation processes all contribute to the RO2 fate. NO, RO2, autoxidation and even some HO2 reactions with RO2 have significant radical channels products of which can then participate in further reactions and produce products which are different from any single type of RO2 process (ie mixed regime). In addition, for particular RO2 such as acyl peroxy radicals (thought to be important RO2 products from terpene chemistry) reaction with NO2 to form PAN species can be a significant sink in the real atmosphere, sequestering these from participation in ‘dimer’ formation.

We have expanded the introduction as suggested.

Model discussion: Again given the target audience, a bit more specific description of the model maybe appropriate; it is quite central to this paper, and the casual reader may not be likely to go investigate the earlier paper.

We agree that more discussion of the model is appropriate and added a brief summary in Section 2.2.

RO2/HO2 ratio at zero CO: I am quite surprised at results showing RO2/HO2 ratios approaching 1000 in the simulations with no CO. Has this been validated with any experimental observations of specific indicators (e.g. H2O2, ROOH, etc). While I do not have much experience with the RO2 + RO2 chemistry from terpenes, I am quite familiar with RO2+RO2 chemistry derived from smaller (C2-C6) alkenes and dialkenes. Under conditions with RO2 primary production >> HO2 primary production, significant ROOH yields (~30%) are generally observed, independent of PRO2/PHO2. These ROOH are understood to arise from secondary HO2 formation from the subsequent chemistry of the alkoxy radicals generated from initial RO2 + RO2 reactions, and hence cannot be separated from RO2+RO2. Do RO2 + RO2 reactions in your simulations generate RO? HO2? Should they?

The reviewer is correct that the mechanism we employ here does not explicitly represent any alkoxy radical chemistry. We now point this out in the manuscript. This is a design choice, as our goal is to focus on the RO2 chemistry, but in future work we will explore more explicit representation of the organic radical cycling through RO. We are not aware of many experimental constraints on this from the alpha-pinene system, and our experience has been that the organic radical chemistry for these highly oxidized products can show surprising behavior (as with autoxidation and the dimerization reactions). Our strategy is thus to add this complexity systematically; however, obtaining better experimental constraints on this is clearly important.

Why does the simulation appear to asymptote in RO2/HO2 ratio at about 2 with increasing CO? Is there a way to move this to more relevant atmospheric ratio like 0.1, or is this precluded due the limitation of only using O3 as radical precursor? Is there a better radical precursor system which might give more control over this ratio?

The only source of HOx in these simulations is the OH generation from the alpha-pinene+O3 reaction and since that yield is only 0.8 for every RO2 produced, even if all the RO2 reacted with an HO2, the RO2 to HO2 ratio would still be above one. But, yes, this is specific to the system we have chosen to model: dark alpha-pinene ozonolysis. In addition, alpha-pinene + O3 experiments have high RO2:HO2 because they tend to use high alpha-pinene concentrations as we have modeled here; lowering this in the model does lower that asymptote closer to RO2:HO2 = 1. While this system is common and thus important to understand, there are certainly ways to design an experiment where this ratio could be better controlled. One of our goals is to use this model framework to design those experiments.


Specific comments:

• Significance statement: In addition to precisely knowing the chamber conditions, I would argue one must actually also use mechanistic information if one is to be successful in extrapolating to the atmosphere, which invariably will have different conditions. This is the primary limitation to the ‘engineers method’ of solving such problems. The derived parameterization will work wonderfully for conditions within those used to derive the parameterization, but will quickly fail when extrapolated outside the test conditions.

We have added the following to the significance statement to address the importance of mechanistic information when extrapolating experimental data to the real atmosphere:

“In addition, parameterizations derived from those experiments may have limited applicability if they lack molecular detail; understanding this chemistry at the molecular level allows for a broader extension of experimental results.”

• Pg2P4: most may suggest room temp k(RO2+HO2) ~ 1e-11 increasing with size of R to 2e-11,as opposed to 1e-12 (also for RO2 + NO) (1-2 out of ~30 collisions?);

The RO2+HO2 rate coefficient used in this work was 10-11 cm3 s-1. This was a typo.

• Pg2P4: might be worthwhile to reference related note from Wennberg (https://igacproject.org/sites/default/files/2016-07/Issue_50_Jul_2013.pdf)

We now cite the Wennberg note.

• Pg3P2S1: Is this really true. Suspicious.
This is true in the context of how the model was originally run – with no CO present and thus very low HO2. Under low NOx conditions, ROOH is formed, but never has appreciable yields under the conditions in that paper. Language has been added to clarify that in the text.

• Pg3P3LastSent: Again, I think conditions could be found where each RO2 reacts with HO2, for a general VOC chamber experiment. If this statement only applies to an O3 + AP experiment, with no other oxidants this should be clarified.

We agree and have added “(in the chamber, especially in dark ozonolysis experiments)” as a qualification. We believe that the text is very clearly conditional – we state that high RO2:HO2 may be a problem, not that chamber experiments are necessarily out of balance. While OH oxidation is an available pathway in this model, the following figure, which we have included in the manuscript as well, shows that CO effectively acts as an OH scavenger leading to an increase in HO2 at the expense of OH. This means that the OH oxidation pathway becomes less important as CO increases, likely changing the chemistry in the model as well as experiments where the only oxidant is ozone compared to the actual atmosphere. Most notably, this means that “aging” chemistry will be almost completely shut down in high CO chamber experiments, allowing (or forcing) them to focus exclusively on first-generation chemistry.

14-Jan-2021

Dear Dr Donahue:

Manuscript ID: EA-ART-11-2020-000017.R1
TITLE: Peroxy radical kinetics and new particle formation

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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Jamie Purcell

Dr Jamie Purcell MRSC
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Reviewer 2

I believe the changes to the manuscript will certainly improve its approach-ability by general scientists. I recommend publication. Only a couple minor notes:

1) may be helpful to add the C* or vapor pressure boundaries to your vary nice description of the low volatility categories.

2) I may have missed it, but I did not see the mention in the manuscript of how alkoxy radicals were treated in the current simulations (assumed to have no further chemistry?). I will look forward to future work, where this chemistry is extended, and to comparison with chamber data.