This work describes experiments aimed at understanding how the chemistry of alpha-pinene oxidation changes under different HO2:RO2 conditions. As described in the paper, this is crucial to understand as high precursor concentrations and low concentrations of small RO2 molecules in chamber experiments lead to lower HO2:RO2 than is generally expected in the real atmosphere. While this is speculated to lead to lower SOA yields, this work shows that as well as evidence for the chemistry shifting when RO2 versus HO2 dominate. Additionally the authors show an interesting method for estimating SOA yields from a CIMS measurement of large gas-phase molecules. This work brings attention to a limitation of interpreting many chamber experiments that show dominant RO2-RO2 chemistry to the real atmosphere. Additionally, this work is thorough in describing their results in the context of known alpha-pinene chemistry and is clear about the limitations in their interpretation. I feel this work fits well within the scope of ACP and I recommend publication once a few issues have been addressed.

General comments

1. I am a little confused about Exp2. Specifically what is the importance of doing these consecutively versus having two separate experiments (one seeded, one unseeded)? What is the difference between an unseeded experiment and a pure gas phase experiment?
2. A little more discussion of the atmospheric relevance of the conditions would be helpful. Where in the atmosphere is an HO2:RO2 of 1 relevant? As both of these values vary across the globe are there areas where a lower or higher ratio would in fact be more representative?
3. A little more discussion of how this impacts how chamber experiments should be run could also be useful. Can you reference any typical HO2:RO2 for chamber experiments (even just ones inferred from modeling specific chamber conditions)? Besides Schervish and Donahue (2021), which was exclusively a modeling paper not aimed at specifically reproducing any experiment, is there evidence for non-atmospherically relevant ratios in chamber experiments? Would a simple adjustment of yields be sufficient to account for low HO2:RO2 in chambers? Would a model such as the box model with MCM that you used allow these experiments to be interpreted at higher HO2:RO2? Or are experiments that intentionally increase this ratio necessary?
4. The model results are referenced a few times, but never shown. These should at least be included in the SI whenever they are mentioned in the text. Additionally a discrepancy between the modeled and measured reduction in C10-HOM-RO2 is mentioned, but is never appropriately explained. Was any other comparison done between specific product or RO2 families and the model results besides just total C10-HOM-RO2? It seems like that could provide more evidence for some of the interpretations made here (for example for the results in Fig 7).

Specific comments:

1. Line 26: A reduction relative to what
2. Line 66: This paragraph seems more appropriate in the methods section.
3. Line 91: This sentence is confusing. Suggests this process as what?
4. Line 92: The autoxidation rate for some RO2 may be in this range while others might be much slower or faster. Does this accurately represent the average?
5. Line 94: A reference should be provided to justify the autoxidation rate coefficient slowing down as more oxygen is added.
6. Line 238: I think you’ve mixed up these rate coefficients.
7. Line 262: By “desired value” do you mean the value it was before CO addition?
8. Line 283: The value of the RO2+RO2 rate coefficient has been shown to vary substantially based on the structure of the RO2’s. Were any sensitivity studies done to see if the model was particularly sensitive to the value chosen?
9. Line 363: The discussion here is confusing to me. You do see a reduction in HOM-RO2 so why is this relevant to discuss?
10. Line 410: What RO2 ratio?
11. Line 472: Here it is stated that HOM-C10H17Ox are less abundant, but earlier (line 449) it is stated that it is expected there are the products of the dominant OH pathway despite being detected as lower in the NO3-CIMS. Is this why it is assumed there is an abundance of less oxidized C10H17Ox peroxy radicals? Are there any measurements to validate this?
12. Line 502-502: Wouldn’t 1 alkoxy step lead to the same parity change? Why is it then suggested in both cases where a difference in the amount of parity change exists?
13. Line 636-637: What would cause there to be a larger particle-phase source of these compounds at high HO2:RO2?
14. Line 650: Should this be “reduction to 72%”?
15. There are a few compiling errors in the main text and SI. I would recommend going through and carefully checking all the equation/figure references are showing up correctly.

Review of the article: “Impact of HO2/1 RO2 ratio on highly oxygenated α-pinene photooxidation products and secondary organic aerosol formation potential” by Baker et al.

Summary and general comment:

Baker et al. present very interesting results obtained from laboratory experiments done at the SAPHIR STAR chamber based in Julich. In these experiments the authors have analysed the HOMs formation using a-pinene as precursors and their impact on SOA. In these experiments they focus on two different main conditions: High and low HO₂/RO₂ ratio. This study is very important because up to now most of the laboratory experiments have been done at relatively low HO₂/RO₂ ratio however, if we want to mimic atmospheric daytime conditions, we need experiments also at high HO₂/RO₂ ratio. In these experiments, they have found that at high ratio they observed a decrease of HOM-accretion products and several other changes. Basically, as the authors mentioned, the study showed that low HO2/RO2 ratios can lead to an overestimation of SOA yields. Because of all these findings, I believe that this article is suitable for publication in ACP. Below I have added few minor comments.

Minor comments:

Line 22-24 Rephrase the sentences:The HO2/RO2 ratio was increased by adding CO, while keeping the OH concentration constant. We determined the HOM’s SOA formation potential, considering their fraction remaining in the gas phase after seeding with (NH4)2SO4 aerosol.”

I think that these sentences lack of details and probably don’t belong in the abstract. I would either explain the details of how the experiments were done or simply mention the high and low ratio and remove them from the abstract.

Introduction

The introduction is very well written, and it includes a nice set of references. My only minor comment here is related to the final sentences at line 79-80. The authors mentioned the used of mass spectrometry with chemical ionization (HR-TOF-CIMS). This is a very general term where these techniques can include a very large set of instruments and different ionization unit. Would be nice if the author can explain better with techniques they used and why.

Method

The method part is also very clear. However, I have one main comment. I might have missed that but how many experiments and how many repetition were done? While reading the paper I had the feeling that there were not many repetitions for each experiment type. I am aware that those experiments requite lots of work and lots of data analysis. I was just wondering how representative they are and would be the impact on their conclusions.

Results and Discussions

The authors include a vast description of their results. One comment here is about figure 4 and its discussion. I understand very well that the accretion products decrease. However, I would expect an increase in the monomers. The authors mentioned that shortly, but I believe more discussions is needed. For example, can it be due to the limited numbers of experiments and relatively low statistics? Is there any other explanation? Maybe is due to the normalization the authors used?

As motioned above the rest of the discussions is very clear and full of details.