Kang et al. uses high resolution NO₃^- ToF mass spectrometry to investigate the peroxy alkoxy pathway to highly oxygenated organic molecules (HOMs). Since the alkoxy radical cannot be detected using CIMS, they infer alkoxy reactions occurring by the parity of oxygen and hydrogen number, where odd oxygen C₁₀H₁₅O_x peroxy radicals are assumed to have formed via an alkoxy radical intermediate. There are some limitations to this method (e.g. oxygen parity only works through one generation of peroxy-alkoxy isomerization and NO₃^- CIMS is only sensitive to HOM RO₂) and many assumptions about complex RO₂ and RO chemistry. Nevertheless, the story of this paper, summarized in Figure 8, is sound and may fundamentally shift how we think about HOM formation.  I find this study interesting and believe the audience of ACP will too. Before being published I request the following comments to be addressed.

Specific comments:

1. My main concern is that the discussion often neglects the importance of R structure for the peroxy-alkoxy pathway. This includes RO formation and RO isomerization, both of which are broadly parameterized here. While there are instances where structure is addressed (e.g. Figure 1), below are a few places where I would like to see a deeper discussion,

• Figure 1 is a great illustration of how a step of alkoxy chemistry changes the RO₂ hydrogen parity, in this case from an even to odd number. However, mechanism ABE produces a less oxygenated RO₂. This contradicts the paper conclusion that the peroxy alkoxy pathway leads to more oxygenated organic compounds. A discussion on R here would help clarify branching between mechanisms ABCD and ABE.
• It is difficult to interpret changes in the RO₂ and RO branching chemistry when the ratio of OH:O₃ oxidation changes. Peroxy and alkoxy branching is sensitive to R, and so it matters whether the peroxy radical you make is coming from OH addition, OH abstraction, or ozonolysis. Please address how the ozonolysis rate is accounted for in the AP turnover when you adjust O₃, NO_x, CO, or light aperture.
• Line 84: ROO-OOR -> 2RO₂ + O₂ branching ratio seems to be largely parameterized and probably varies much more. Please clarify if using structure activity relationships.
• Line 427: I don’t agree that RO isomerization will be a statistical probability as you make it out here. It will be extremely dependent on R structure. RO isomerization will create new functionalized R backbones that may favor or inhibit future isomerization steps. Please clarify if using structure activity relationships and also their applicability.
• Line 798: Please clarify what you mean by the alkoxy-peroxy step does not rely on specific chemistry of alpha-pinene. I agree these results are applicable (and important!) for large, functionalized VOCs, but would this statement hold true for alkanes, especially those with 5 or fewer carbon atoms?

2. Although it is pointed out that NO₃^- CIMS is selective towards measuring HOMs, there is little discussion as to what it cannot efficiently detect. Including more information on this is important for readers to interpret what's shown in the plots and also what's not shown (e.g. less oxygenated RO₂ that precede HOM RO₂).

3. For the NO_x experiments, your aim is to increase [NO_x] to produce RO from RO₂+NO reactions. There is a good discussion in the main text and SI about how NO_x affects OH and thus the alpha pinene turnover rate. Have you calculated/modeled how additional NO_x affects OH:HO₂? The previous CO experiments show that increased HO₂ shuts down peroxy alkoxy pathways, which could also explain your data in the NO_x experiments.

4. In Figure 5, the x-axis is plotted as the NO_x concentration. Table S1 indicates almost all of NO_x is NO₂. Can this be changed to NO concentration since it is RO₂+NO reactions that produce alkoxy radicals?  

5. Figure 6 is interesting but may be better suited for the SI. The text points out that signal is changing due to [OH] which is not corrected for. The comparison to MCM does not fit in with this specific study since autoxidation is not implemented and therefore it is without a single path to HOM RO2 (with or without considering the peroxy alkoxy pathway).

6. The branching ratios you derive for both RO formation and subsequent isomerization are important and really jumped out to me in the abstract. However, the discussion in the text comes late and was not clear. I would recommend putting more of a focus on this in the main text.

Minor comments:

Line 70: Should alcohol product have a radical dot?

Line 90: Add NO₂ as product

Line 97: Clarify RO₂+NO produces RO or RONO₂ as products

Line 201: You mention using SA calibration factor for HOMs and specify a value. But no concentrations are reported in the paper. Why is that?

Line 258: You specify the same formula, C₁₀H₁₇O_x+1, twice. Please clarify.

Line 264-267: This paragraph is generally confusing to read. Please clarify. You point out 3 products coming from 2 precursors and then say respectively. It is not clear what products are from which precursors.

Line 268: What is sufficient NO? Even low NO will be important to peroxy-alkoxy chemistry (Nie, W., et al, Nature Comm, 2023)

Line 417: “Different from C₁₀H₁₅O₈ and C₁₀H₁₅O₁₀, formation of C₁₀H₁₅O₆ is obviously exclusively initialized by OH oxidation.” I would remove obviously. Could C₁₀H₁₅O₆ not been formed from AP + O₃ ->C₁₀H₁₅O₄ -> C₁₀H₁₅O₆ through one generation of autoxidation?

Line 495: “The CO experiment resulted in a clear suppression in the abundance of HOM-RO₂ radicals” This is not clear from the data presented in Figure 4 which is normalized. Please show unnormalized data to make this point.

Line 655: “…strong increase in O:C caused by NOx addition is only explainable by impacts of HOM-RO…” I agree that your data supports this conclusion, but I would caution against saying this is the only explanation since there are not measurements for how the less oxygenated RO₂ were affected

Additionally, please reread the manuscript to minor typos (extra word in line 491, missing word in line 638, subscripts in line 726)

With chamber oxidation experiments, Kang et al. show that bimolecular reactions of alpha-pinene + OH derived peroxy radicals with RO2 and NO enhance the formation HOM by alkoxy-peroxy steps instead of inhibiting it as previously thought. RO2 + NO reactions appear to be particularly important in this regard, leading to products with up to 15 oxygen atoms, three oxygens higher than experiments without NO. As they highlight in their conclusion, this would imply that the formation of HOM is more favorable under polluted conditions than clean, opposite to what is widely accepted in the community. This work is important, timely  and highly relevant to the work of others in the field. I recommend its publication. I have the following comments that I request be addressed.

1. One concern I have is the scarce measurement of the C10H17Ox family of peroxy radicals. These first-generation peroxy radicals from OH addition to alpha-pinene are completely unmeasured, except for C10H17O10, while first-generation peroxy radicals from H-abstraction, a minor channel in comparison, are measured.

• It is clear from Xu et al. 2019 and Berndt 2021 that OH addition to alpha-pinene initiates autoxidation and the formation of peroxy radicals at least up to C10H17O7. Further reactions of some of these are likely the source of the C10H17O10 measured here. Insensitivity of the NO3-CIMS method toward these products is unlikely to be the reason as Figure S5 in this paper shows that C10H15O6-9 are measured above the detection limit.
• The authors attribute their low detection of C10H17Ox compounds partly to their measurement conditions. While some secondary oxidation of pinonaldehyde could explain the C10H15Ox compounds, the authors find primary products from OH H-abstraction to also be at least equally important (Figure S6). This necessitates a more detailed discussion regarding why primary OH-addition products, which should dominate a-pinene + OH reactions, are not measured or measured minimally. Perhaps the additional -OH group in the latter leads more efficiently to termination during autoxidation, and these are then candidates for secondary oxidation by OH?
• Figure S5 shows multiple C10H18Ox measured. As the authors state in line 257 of the manuscript, these products can only form from bimolecular reactions of C10H17Ox and C10H17Ox+1 (correct the typo in the manuscript, you have two instances of C10H17Ox+1). So, the C10H17Ox peroxy radical precursors of C10H18Ox clearly form during these experiments.

2. In addition to the reaction classes described by the authors, some of the products could form from RO2 + OH reaction producing trioxides (ROOOH) (Assaf et al.). This reaction could be particularly important under the elevated OH conditions of the experiments carried out here. The importance of this reaction should perhaps be modelled out.

• Related, could the increase in C10H15O2n+1 signals at elevated levels of OH (line 420) be attributed in some part to the RO2 + OH => ROOOH => RO + HO2 (and not just HOM-RO2 + RO2 as currently stated)?
• This mechanism could also explain in part the CO experiments. In the absence of CO, higher OH concentrations can lead to C10H15O2n+1 products via the ROOOH pathway above, which switches in the presence of CO when OH concentrations are lower. The trioxide does get more stable against decomposition with the increase in carbon chain length, so the contribution of the channel is perhaps minimal.

3. About the C10H16O7 signal in page 10 which dominates the C10H16Ox family in their measurements, the authors state that the contribution to this signal from C10H17Ox is low (line 351). Does the majority of the C10H16O7 signal measured then come from reactions of C10H15Ox? Maybe provide additional details regarding the reactions that are likely involved, whether R2 or R3b or something else. If it’s R2, does [HO2] explain the measured intensity?

4. Lines 415-419: C10H15O6 can form from a-pinene ozonolysis. In fact, Meder et al. 2025 cited here measure multiple isomers of this peroxy radical. Also, consider a different word than “unimportant” in line 419. C10H15O6 is crucial to formation of the next peroxy radical in the autoxidation chain, C10H15O8. It can also react bimolecularly to form the closed-shell C10H14O5 species, as reported in Meder et al.

5. Regarding the effect of CO on the formation of HOM RO, the authors cite Jenkin et al. 2019 to say that the branching to alkoxy radicals from RO2 + HO2 reactions should be low (line 464). However, this is highly dependent on the structure of the R. According to Jenkin et al., there is an almost 50:50 branching towards ROOH and RO for beta-oxo peroxy radicals (Table 8 in Jenkin et al 2019). The authors should discuss the importance of the RO2 + HO2 reaction in the context of the structures of the Rs in their system.

6. How does the decrease in [OH] from CO addition affect the RO2 intensities and distribution? In line 494 the authors put the onus of HOM-RO2 suppression completely on HOM-RO2 + HO2 reactions, but how much of the suppression is due to lower [OH]?

7. The increase in C10H15Ox signals with the increase in NOx: is there a possibility that some ozone is forming from NO2 photolysis? If I understand the method section correctly, UV-A lights are on during these experiments, so won’t NO2 photolysis increase O3 concentrations, explaining at least partly, the observed increase in C10H15Ox signals?

Assaf, E., Schoemaecker, C., Vereecken, L. and Fittschen, C., 2018. Experimental and theoretical investigation of the reaction of RO2 radicals with OH radicals: Dependence of the HO2 yield on the size of the alkyl group. International journal of chemical kinetics, 50(9), pp.670-680