The authors report and discuss peroxy radical measurements performed during flights with the aircraft HALO across Europe. Because there are only few flight measurements of radicals over Europe, these measurements are valuable. However, it is not very clear, what the improvement in the understanding of tropospheric fast photochemistry really is from the manuscript. The author mainly compare measurements with different approaches of steady state calculations. Results are mainly descriptive, but there is little discussion about the meaning for the understanding of photochemistry. The presentation quality needs to be improved. It is partly unclear, how equations for steady state calculations are derived and what the meaning is. This manuscript needs major improvements to be suitable for publication in ACP.

Abstract: The definition of RO2* is unclear. In the first sentence it sounds as if this is the sum of RO2+HO2, but later it looks as if also OH is included. Please clarify and be precise and accurate with definitions.

Abstract L22: How can a production rate agree with a concentration?

Abstract L23: RO2 is not directly produced from the photolysis of ozone and HONO, but OH is that then further reacts to produce RO2* species. Please be accurate how you phrase this.

Abstract L25: For an abstract the statement about the PSS is vague and not well-defined. Please expand here, which processes are considered in the PSS and what quantity is calculated.

Abstract L30: Really RO+NOx ? If RO2* is the sum of RO2+HO2+OH, it is not clear to me, why this statement is about radical interconversion, because radical interconversion reactions cancel out. Please rephrase and clarify.

L90: Reaction R25 should be mentioned as well.

L91: The first half of the sentence is not clear. What do you mean with insolation? Do you mean PSS? This would not be required to ensure rapid photochemical processes. Please rephrase and clarify.

L102: Specifically since the manuscript is about airborne measurements, the temperature and if necessary also the pressure should be given, if values for reaction rate constants are mentioned.

L103: The typical reader may not know, what exactly is meant with “weighted average rate coefficient” and why this is required. Please clarify and rephrase.

L126: It is not obvious, why the measurements of trace gases in Reactions R1 to R26 other than required in Equation 1 minimizes the number of assumptions for calculating RO2*. My expectation would have been that this would allow to perform also full model-calculations of RO2* concentrations, which could be compare PSS calculations. Please explain in more detail.

L135: Please avoid to define and use abbreviations like IOP and MPC and others that are not common. The typical reader will forget them, while reading the manuscript. It only makes it difficult to follow the line of arguments.

L143: What do you mean with “stable flight layers”?

L168: Please add also the pressure, for which you calculated the concentrations.

L172: Why do you only refer to CH3O2 as RO2? Earlier you mention “weighted average rate coefficient” implying that you not only have CH3O2.

L180: I would recommend to give a number how large the humidity effect was for measurements in this work.

L183 ff: The short description of miniDOAS data / data evaluation is hard to understand for the non-expert. Please rephrase. It is also not clear at this point, why this instrument is explain in more detail, whereas other instruments more obvious useful to determine the PSS are not explained.

L186: Please explain RT modelling.

L187: Please explain the abbreviation HAIDI.

L191: Please explain what you mean with “common and related species”.

L202: I would avoid a conclusion about the reason for high RO2 in specific regions before doing the analysis. Your arguments are plausible but there are also other plausible explanations giving the contrary conclusion.

L210: I do not understand the argument “comparable”. What is exactly compared here? Calculating RO2* from PSS can always been done as long as the time required to reach PSS is short enough that concentrations of species do not significantly change. Please explain and rephrase.

Figure 3: Wouldn’t make more sense to show percentiles instead of standard deviations to be independent from outliers?

L224: I cannot follow the argument that differences between mean and median values indicate more or less variability. Median and mean values could be exactly the same, if the distribution of values is symmetric independent on how big the range of values is. It is also not obvious, if you want to say that there is a change how similar median and mean values are. I do not see that the similarity depends on the height.

Line 235: “becomes” instead of “become”

L235: Please clarify what you mean with “low NOx conditions” and why this impacts the significance of H2O2 photolysis.

L238: Please define OVOC before using it in Eq 2

L237. This statements needs explanation. Why can you assume that photolysis of OVOCs is more important compared to reaction with OH? This is not obvious. Which were the most important OVOCs and VOCs and can you quantitatively show that your assumption is valid? Can you also show this for ozonolysis reactions? If you want to calculate the RO2* production rate you may not need to consider OH reaction, because this is a radical conversion reaction and not a primary production, which you may want to calculate. This should be clarified, if you talk about production. Please explain and extend your description.

L238: How large were the concentrations of these OVOCs? What do you mean concretely, if you take this as “surrogate”? Equation 3 only considers 4 OVOC species, which rather indicates that you neglect others.

L244: I assume that measurements allowed a calculation of the air concentration density rather than an estimate.

L245ff: Avoid explaining details of a figure that is explained in the legend and / or caption of the figure.

L291: The section header referring to PSS. From what is written earlier, one would expect calculations using Equation 1, but then you start with calculations using Eq 5. Also later in this Section Eq 5 is stated as PSS calculation instead of Equation 1 and not used at all in the end. This is confusing. Please be consistent. It is not clear, why Equation 1 is introduced earlier at all.

L297: It is a bit contradictory to state “interconversion reactions occur without losses”, because interconversion implies that the radical nature is not lost.

L298 ff: Please justify that you can calculate the loss of RO2* -RO2* reaction by an weighted average rate coefficient? What do you use as weights? Without knowing the distribution between HO2 and RO2 it is hard to imagine how this loss rate can be accurately calculated. It is not obvious how this is mathematically done, if you expand the right side of Eq. 5 using [HO2] and [RO2] concentrations. If you assumed e.g. [HO2] = [RO2] = 0.5 [RO2*], this should be clearly said and written down what this means for the equation. The assumption of [HO2]=[RO2] would be expected if the loss of [HO2] and [RO2] is dominated by reaction with NO. Please expand, if this is the case for measurements in your work. In this case, it would be also essential to show and discuss NO measurements and peroxy radical loss rates with NO. What about the loss of RO2* due to the reaction of NO2+OH? Could this have been significantly contributed to the RO2* loss? Your analysis between differences, if you divide data sets between North and South may hint that this loss process was relevant.

L318: I do not understand the statement about the validity of results. Please explain and rephrase.

L330: It would be good, if names of e.g. photolysis frequencies in Equation 5 and 6 were consistent. It should be emphasized that the point of assuming that RO2 consist only of CH3O2 is only, in order to have one RO2 species and therefore not considering differences in RO2+RO2 and RO2+HO2 reaction rate constants. In general, I would recommend to start with Equation 6 and then you easily derive Equation 5. By doing this, you also will be able to explain what you mean with average weighted reaction rate constant in Equation 5.

L338: It is rather confusing that the negative solution is mentioned at this point, but not when you discuss Eq. 5, where the form of the quadratic equation is identical.

L342: The effect that RO2* measurements can be affected by differences in the detection sensitivity of RO2 and HO2 should have been discussed for the results with delta=0.5 (Equation 5).

L344: Please make rather quantitative than qualitative statements about the level of agreement. What effect do you expect from differences in reaction rate constants among RO2, if you do not assume that all RO2 is CH3O2?

L356: It is not clear, which processes you are referring to, if you mention VOC oxidation processes. OH + VOCs would be a radical interconversion process and ozonolysis reactions and Cl chemistry may be not of importance for conditions of the campaign.

L358: Again it is confusing, if you talk about radical conversion reactions, but in fact you mean radical termination reactions. Please rephrase and be clear with the definition throughout the manuscript.

Equation 8 / 9: Similar it is confusing that you name reaction rate constants referring to radical conversion reactions and move the loss into a loss factor. What is the value of the loss factor? It would be easier, if you added more explanation, which loss reactions (products) you include. I read the first loss term as non-radical products from HO2 + NO and HO2 +O3, it is not clear to me, what for example the product of HO2+O3 would be. The factor rho associated with this term is explained as OH loss during the OH-RO2* interconversion, which does not fit, what I read from the equation. There is more explanation needed, what is meant with this term. It is also not clear to me, if the second loss term (organic nitrate formation k25 and k22) is correct and why this is connected to RO2+RO2 reactions (k16b). This needs to be explained in more detail. It would be much easier to understand, if you introduced yields of products produced from radical termination reactions.

L367 ff: It sounds as if you state that the reaction of OH+ HCHO and OH+ CH3CHO are the dominant radical precursor reactions, though so far you only discuss photolysis of them. OH reactions would also not be primary sources, but radical conversion reactions. In this context and for the same reason, it is also not clear, what you mean with RO2* production from CHOCHO and CH3OH oxidation. Please clarify and rephrase.

L373: The context of the statement about the importance of HO2+NO and HO2+O3 is not clear and seems displaced at this point.

L421: How can you exclude that there is no over-estimation of loss processes instead of an under-estimation  of production processes? What is the impact in the uncertainty of the HO2/RO2 ratio in the case, when VOCs concentrations were high?

L422: Why would OH recycling processes increase the calculated RO2*, if radical regeneration terms cancel out in the calculations for the sum measurement of RO2*?

L436: It would be interesting to see a more quantitative analysis of the impact of the uncertainty in HCHO measurements on the results.

L490 ff: The calculation of OH concentrations does not really fit this manuscript and would require a much deeper description that currently done. The statement that the OH calculated from Eq 5 is higher than reported OH concentration means that OH reactivity is underestimated cannot easily be justified. I would recommend to cancel this entire paragraph. It does not add anything to the content of the manuscript and may even be rather misleading as it is now.

Section 4.4.1. / Equation 11: Again the definitions of the effective rate coefficients is not clear. Also the use of NOx makes it hard to see, what exactly is calculated. This makes it very difficult to follow any of the subsequent quantitative statements. The connection to previous Equations is also not clear. What is the difference to Equation 9, which should consider radical loss in NOx reactions? What is used for the production rate for example? The authors should make much clearer what is calculated and what the meaning of the calculation is. As it is written now, it is not clear, what the authors want to discuss in this section.

This manuscript presents some rare airborne RO₂ measurements over much of Europe. These are useful measurements that should be shared, especially the vertical dependence of concentrations, but the analysis is not focused enough and requires significant revisions.

Major comments:

line 101 (in the intro) shows the photostationary state (PSS) equation for [HO₂ + RO₂], and describes previous studies that compared this value to measured HO₂ + RO₂. The introduction ends with “Consequently, this data set provides an excellent opportunity to gain a deeper insight into the source and sink reactions of RO2* and the applicability of the PSS assumption for the different pollution regimes and related weather conditions in the free troposphere”.

I was looking forward to seeing what insights the authors had to provide regarding the applicability of the PSS assumption…. but it does not show up at all later in the main text! The authors make use of the equation P(RO₂*) = L(RO₂*) quite a bit, but not the above NO-NO₂-O3-RO₂ photostationary state assumption.

Line 119: in the abstract RO₂* is defined as RO2 + HO2. On line 119, it’s defined as RO2 + HO2 + OH. Quantitatively there’s little difference since [OH] is much smaller than the other two terms, but conceptually this is very important. Line 24 states “RO2* is primarily produced following the  photolysis of ozone (O3), formaldehyde (HCHO), glyoxal (CHOCHO), and nitrous acid (HONO) in the airmasses investigated”, which is true for (RO₂+HO₂+OH) but not for (RO₂ + HO₂). Please be consistent in terminology.

later in abstract: “The dominant removal processes of RO2* in the airmasses measured up to 2000 m are the loss of OH and RO through the reaction with NOx during the radical interconversion”. This is very confusing – if a reaction is a radical interconversion reaction, then no radicals are lost. Moreover, reactions of RO with NOx are rare and not discussed at all later in the manuscript.

Line 40 – remove comma

Line 90 – should say “R23 and R25 are two of the most…”

 Section 3: the description of perceas is confusing. Nowhere does it even mention that the sampled air is mixed with NO and CO in amplification mode – the basics of PERCA operation. I do recognize that the instrument has been described in the referenced papers, but just a few more details would be helpful.

line 174: the relative sensitivity to CH3O₂ vs. HO₂ (α) – was this just based on their previous study, or was it experimentally determined again in between flights? Similarly, please provide more information on calibrations – how many were done? Were the eCL values stable (and their dependence on humidity) or were different values used for each flight?

That’s great that glyoxal and methyl glyoxal, in addition to the other OVOCs, were measured.

Line 202 “Typically, the highest RO2* mixing ratios were observed below 3000 m over Southern Europe. This is attributed to the higher insolation and temperatures favouring  the rapid production of RO2* from the photochemical oxidations of CO and VOCs”

I question the inclusion of temperature in that sentence. If the authors are simply presenting a *correlation* between highest RO₂ mixing ratios and temperature that is fine, but to *attribute* the high mixing ratios to elevated temperature requires some discussion. Are they inferring that the reaction rate constants are faster at higher temperatures? This is certainly not true for all of the reactions. Or are they referring to the increased emissions of biogenic VOC emissions at higher temperatures, leading to higher bVOC concentrations? This by itself won’t necessarily lead to higher RO₂* mixing ratios.

For HONO photolysis, do the calculated numbers reflect the gross OH formation from HONO photolysis or the net amount (subtracting out the reverse reaction OH + NO)?

line 244 and 304: “calculated”, not “estimated”

Figure 4: I would have found it more useful to see a plot of altitude vs. P(RO₂*) rather than [RO₂*] colored by P(RO₂*).

line 266- needs some re-wording. “…the high amount of H2O in the air masses probed results in the O3 photolysis and subsequent reaction of O1D with H2O (R1-R2a) and is the highest RO2* radical production rate”. The H2O itself does not cause O3 photolysis… rather the high H2O leads to the reaction O(1D) + H2O being the most important RO₂* source.

line 305: The sentence “The [RO2*] < 0.5….” is awkward, change to something like “Measurements in which [RO₂*] were less than xyz…”

Figure 7 and text: it’s good that someone has done this analysis! I think the correlations observed in figure 8 are about as good as could be expected, though it’s interesting that the impact of NOx is not so clear.

line 338: “The second solution gives…” I don’t see any solutions…. please clarify.

line 340 – “the measured RO2* (RO2*m) mixing ratio” is confusing – what is RO₂*(RO₂*m)? Is this a product of RO₂* and RO₂*m? Or should it just be RO₂*m?

line 341: “RO2*m and RO2*c are the measured and calculated RO2* respectively for d = 1, i.e. RO2* = HO2 and d = 0.5, i.e. HO2 = RO2.” Confusing. RO₂*m is measured RO₂, and RO₂*c is calculated, but for which case – d = 1 or 0.5? This section should be prefaced with some text along the lines of “because not all peroxy radicals are detected equally by the instrument, the comparison of measured and calculated RO₂* values is complicated. To investigate this, we …”

eq 11: the first term on the right hand side of the equation refers to the RO₂* loss reactions HO₂ + HO₂, RO₂ + RO₂, and HO₂ + RO₂. The 2^nd term should represent the RO₂* loss reactions RO₂ + NO₂ and HO₂ + NO₂ and OH + NO₂, but not RO₂ + NO or HO₂ + NO as those are radical interconversion reactions. I recommend simply writing out the full equation as it is confusing to always deal with “HO₂ + RO2” and “NOx” in these rate equations.

line 524: dependence, not dependency

Review of ‘On the understanding of tropospheric fast photochemistry: airborne observations of peroxy radicals during the EMeRGe-Europe campaign’

The manuscript presents airborne RO2* observations from the EMeRGe-Europe campaign which was designed to study the chemistry in the outflows from major population centres. The concentration and variability of RO2* with altitude, latitude and inside and outside of the urban plumes is of interest to the community.  The authors compare the observations to a couple of steady state calculations for RO2*. Some details on the % breakdown of the primary sources of RO2* are provided, but there is little discussion on the main sinks for RO2*, which I think should be added to the manuscript.

There are some major problems with the manuscript currently: The steady state calculations used are flawed; see my major comments below. Many of the figures are extremely ‘busy’, and I believe some of the axes have been labelled incorrectly, and so it becomes very difficult to follow the discussion related to these plots.

It is difficult for a reader to draw any solid conclusions on the observations and comparison to calculated RO2* because, not only are there unknowns relating to VOCs present, which impact the calculated RO2*. The VOCs present also affect the ambient HO2:RO2 ratio, so the absolute sensitivity of the instrument becomes uncertain. I do think that the results from this study should be published, but major revisions to the analyses performed are needed before publication.

Major comments

Equation 2: The sum of OH+VOC reactions do not constitute a primary source of radicals and should be removed from this equation

Equation 3 & 5: The authors need to be clear that the photolysis rates for HCHO and CHOCHO account only for the radical forming channels.

Figure 4, 5 and 6: I don’t see the value in binning p(RO2*) as a function of production rate. I think total production rate as a function of altitude and observed [RO2*] would be easier to visualise and take information from. I suggest p(RO2*) is broken down into % precursor contribution in figure 4 (so figure 5 wouldn’t be needed). I also find figure 6 extremely difficult to read. From this figure it is impossible to see the HONO concentration profile for example. I suggest showing the altitude profiles of the key RO2* precursor species as shown in figure 2 (there is no need to break these profiles down into p(RO2*) production rate. In figure 6, I would focus on HONO, OVOC and O3 altitude profiles as H2O (v) and j(O1D) profiles are provided in figure 2.

Section 4.3: This section is difficult to follow and take away any clear conclusions. I don’t think anything is gained from gradually increasing the analytical expression. I suggest just beginning with the most comprehensive expression and discussing the components of the expression that have the biggest impact on [RO2*c]. Although additional terms have been added to equation 8 and 9, a number of these terms represent propagation of one radical species to another and so should not be considered. The P(ROx) and D(ROx) expressions given in Tan et al., (2019) and Whalley et al., (2021) with the additional photolytic sources from acetaldehyde, acetone and glyoxal available from EMERGE would seem to me like the most robust expression to use.  

Section 4.3: This section begins with a series of correlation plots of RO2* measured vs the square root of the primary production of RO2*. I wondered if these figures could be put into context, by drawing on previous research by Ehhalt and Rohrer (JGR-Atmos., 105, 3565–3571, 2000) and Vaughan et al. (ACP, 2149-2172, 2012)? These papers demonstrate the linear dependence of OH on p(OH) (or jO1D) and square root dependence of HO2 on p(OH) (or jO1D) using the following expression: [OH] or [HO2] or [RO2*] = (a x JO1D^b + c). Where ‘a’ represents the influence of all chemical sources and sinks, ‘b’ accounts for the effect of combining all photolytic processes that produce OH, HO2 or RO2 into a single power function of J(O1D) and ‘c’ is the contribution from all light-independent processes. This sort of analyses may be revealing in highlighting differences between different regions that could, for example, be related to differences in VOCs. It may highlight times when, light-independent processes, such as ozonolysis reactions are of significance.

Section 4.3: It would be useful to show the breakdown of the termination pathways of RO2* as a function of altitude in this section; similar to the primary production pathwayss presented earlier.

Figures 10, 11, & 12: The colour coding for the different parameters considered, other than for the sum of VOCs (fig. 12c), don’t show a clear trend and I suggest the majority of these figures are moved to the SI, so as not to detract focus from the main discussion. Looking at fig. 12c, the calculated RO2* over-predicts the measured RO2* under high VOC loading. This, however, directly contradicts what is written on line 420, where the authors state ‘RO2*m is systematically underestimated for ΣVOCs greater than 7 ppbv’ and, therefore, the subsequent discussion surrounding missing VOCs in the calculation becomes moot. Unless this is a plotting error (axes labelled incorrectly), this mistake means that the conclusions drawn on ln 573, 574 are wrong. Figure 14a, which looks at the ratio of RO2*m/RO2*c, does seem to suggest that the axes have been labelled incorrectly in the earlier figures, but the authors need to confirm this is the case and correct these figures.

Ln 375 – 381: What were the range of concentrations of methylglyoxal during EMERGE? It seems a little surprising to me that, including the production of RO2* from methylglyoxal photolysis, leads to RO2*c systematically overestimating the measured RO2*m.

Assuming RO2*m is underestimated by RO2*c in regions of high VOC loading (see comment above on fig 12c), it would be useful to gauge how much additional p(RO2*) is needed in the calculation to bring RO2*c into agreement with the observations.

Specific comments:

Abstract: ‘measurements of the sum of hydroperoxyl (HO2) and organic peroxy (RO2) radicals that react with NO to produce NO2, i.e. RO2 *’ to measurements of the sum of hydroperoxyl (HO2) and organic peroxy (RO2) radicals (i.e. RO2 *) that react with NO to produce NO2’

Ln 103: ‘kNO+(HO2+RO2) is the weighted average rate coefficient assumed for the reactions of peroxy radicals with NO’ the authors should state the rate that has been used.

Ln 202: From figure 3, j(O1D) increases with altitude, so the highest RO2* concentrations (below 3000m) cannot be attributed to higher insolation alone. Rather the net j(O1D)*[H2O] leads to the greatest primary production of OH below 3000 m.

This paper presents aircraft measurements of peroxy radicals during the EMeRGe-Europe campaign. The authors compare their measurements to predictions from several iterations of a photostationary state analysis. The authors find that the predicted peroxy radical concentrations were lower than the measured concentrations and suggest that photolysis and oxidation of OVOCs not included in the steady-state expression were responsible for the discrepancies.

While the measurements likely provide important new information, the paper is difficult to read. In addition to an analysis of the ability of the photostationary state expression to reproduce the measured peroxy radical concentrations, the authors also provide an analysis of the rates and sources of radical production and the estimated rate of ozone production. Unfortunately, the main conclusions of the paper are lost in the extended discussion. There are also problems with their chemical mechanism and the form of the steady-state equations that they are using to estimate the peroxy radical concentrations.

Overall, this paper presents some interesting and valuable measurements of peroxy radical concentrations. The paper may be suitable for publication after correcting their photostationary state expressions and re-analyzing their results. The paper would also benefit from moving much of this analysis and the discussion of the rates of radical and ozone production to a supplement and focus the main discussion on their primary conclusions as outlined in the abstract and the summary.

Major comments

The authors need to correct and clarify their conclusions stated in the abstract and the text regarding loss of RO radicals (lines 120, 358 and 566 for example). I’m surprised that they are considering the RO + NO reaction an important loss mechanism for alkoxy radicals in the troposphere when the traditional understanding of the fate of these reactions in the atmosphere is reaction with O2 or isomerization and/or decomposition.  While the RO + NO termination reaction (reaction 22) may be important in laboratory studies, it is unlikely that this termination reaction for alkoxy radicals larger than methoxy or ethoxy could compete with reaction with O2 or isomerization/decomposition under atmospheric conditions (see Orlando et al., Chem. Rev. 103, 4657−4689, 2003). This would likely become apparent if they had included the rate of isomerization/decomposition of alkoxy radicals in their photostationary state expressions in addition to reaction with NO and O2 in their attempt to calculate the fraction of RO termination vs propagation (equations S12 and others). Instead, termination of peroxy radicals through reactions with NOx leading to the formation organic nitrates such as the RO2 + NO -> RONO2 reaction are likely more important. Unfortunately, it appears that the authors are not including these reactions in their chemical mechanism.

As a result, their steady-state equations that attempt to incorporate the formation of organic nitrates as radical termination reactions are incorrect (equations 8 and 9). The authors should incorporate an average organic nitrate yield from the RO2 + NO reaction instead of incorrectly attempting to account for the formation of RONO relative to reaction with O2 using rate constants for methoxy radical with NO and O2. It is not clear how this correction would impact their calculated peroxy radical concentrations, but their results should be recalculated and reanalyzed in a revision of their manuscript.

Specific comments

The authors seem to confuse radical initiation and termination processes with radical production and loss through propagation in several places in the manuscript. For example, it appears that the authors intended to calculate the rate of OH, HO2, and RO2 radical initiation using equation 2, but the equation incorrectly includes the rate of radical propagation by the OH + VOC reaction. Even though they neglect this term in their analysis, they should remove it from the equation and clarify their use of radical production vs. initiation throughout the manuscript and supplement.

In their revision, the authors should consider only including the results of their overall photostationary state calculations (after correction) in the main text and include the incremental analysis in the supplement (Figures 9-11). This would reduce the length of this discussion and the number of similar plots, making the discussion easier to follow.

In addition to the correlation plots shown in Figure 12, it would be useful to include the calculated RO2* concentrations in the plots of the measured RO2* concentrations as a function of altitude (Figure 3 and perhaps Figure 4), illustrating the agreement/disagreement as a function of height. The data shown in Figure 4 is not consistent with their reported binning as there appears to be a point below 500 m even though there are no reported measurements at this altitude.

Figure 6 is confusing and difficult to read. It is unclear how this figure adds to the discussion compared to Figure 5. It too could be moved to the supplement, perhaps separating some of the different plots to make it easier to read.

Much of the discussion in section 4.3 may change after the authors have corrected their photostationary state equations and recalculated their RO2* concentrations. In any case, the authors should consider dividing this section into more subsections to improve readability, including moving some supporting figures and discussion to the supplement (Figures 13 and 16 for example).

I expect that the main conclusions would also change, as RO2 loss due to organic nitration formation during radical propagation is likely to be much more important than loss of RO radicals. The authors should consider including calculations of the fractional contributions to radical termination as a function of height similar to the fractional contributions to radical initiation illustrated in Figure 5.

The new calculations could also impact the NOx dependence of the calculated RO2* as shown in Figure 17. While the authors comment on the agreement of the measured RO2* with the expected trend, there is no discussion of the agreement with the calculated RO2*. The authors should expand this discussion after correcting their calculated RO2* concentrations.