This study focuses on the comparison between measured OH, HO2 and the sum of HO2 and RO2 (XO2) radicals and results from 2 different chemical mechanisms, one the RACM (lumped) and the other the MCM (semi-explicit). Measurements were conducted in an isoprene dominated forest in Michigan where a lot of ancillary species as well as ISOPOOH were detected.

This study seems to suggest that quite a large loss rate (termination reaction) for both HO2 and XO2 is needed for both chemical mechanisms investigated to agree with the measurements. Several hypotheses are made such as fast deposition of the radicals on surfaces, faster than used RO2+RO2 reactions for isoprene RO2, RO2 radicals reaction with alkenes and/or segregation could play a role. The authors suggest that most probably a combination of all the above could explain the discrepancy although a rather large loss rate of about 60% during daytime is needed.

The paper is well written and structured, and the arguments are presented in a clear manner. The study is interesting but as it is not possible to give a clear conclusion on what is causing the discrepancies, I would recommend adding a bit of analysis to try and see if something more can be understood and after that I would recommend the publication.

My first suggestion would be to try and perform an experimental budget with the available data. I understand that it might not be possible for the HO2 and RO2 but it would be possible to perform it for the OH radical. In this way it should be clear if the very low OH observed in the morning hours is an instrument artefact as it is quite dubious. The OH budget would also help (possibly) to clarify if indeed there doesn’t seem to be the need for additional sources of OH radicals as the comparison with the model seems to show. I have to say that isomerization reactions for isoprene-RO2 are more or less a given now so I am also wondering why there seems to be such a large overestimate of the OH radical when the most up to date mechs are used.

My second suggestion concerns the XRO2. I am wondering if it would not make sense to remove the HO2 fraction from it from the LIF measurement and then have a more or less RO2 measurement. I understand it does not make much of a difference since the measurement is compared with the some of HO2 and RO2 but to e able to compare with previous studies it would make it easier if it was RO2 instead of XO2.

My third suggestion is about the ISOPOOH. I am not aware of many ISOPOOH ambient measurements and although I could imagine a different publication focusing on that, it would be good to extend the discussion about it in this study. One thing that I find a bit odd is that the measured concentration of ISOPOOH is more or less zero (within the uncertainty) for the all time? I can see a bit of an increase but it is rather small. Even the model results after constraining HO2 and RO2 would expect quite a bit more. Could this be an instrument artefact? Is this consistent with previous measurements? Are there other products that show up which would compensate the production rate of RO2 that, as mentioned, was rather high, and the reacted isoprene must go somewhere.

I noticed that on few occasions the subscripts are not correct so I would recommend checking and fixing that.

I also recommend adding the work by J. Medeiros et al. (2022) which is consistent with LIM1.

Reference

1. Medeiros, D., Blitz, M. A., Seakins, P. W., and Whalley, L. K.: Direct Measurements of Isoprene Autoxidation: Pinpointing Atmospheric Oxidation in Tropical Forests, JACS Au, 2, 809-818, doi:10.1021/jacsau.1c00525, 2022.

This paper presented the measurements of OH, HO2 and XO2 at the PROPHET site in July 2016. OH and HO2 were measured by FAGE with dedicated efforts to minimize the interference. XO2, the sum of HO2 and RO2, were measured by an ethane chemical amplification. The measured radical concentrations were consistent with previous field measurement in 2008 and 2009, considering the meteorological difference. The measurements were compared to box model calculations using RACM2-LIM1 and MCM3.3.1 as a standard test with updated information of isoprene oxidation mechanisms, which enhance the OH concentration by 30% and 20%, respectively, compared to those without LIM1 chemistry, namely RACM2 and MCM3.2. However, this standard models overpredicts OH, HO2 and XO2 more than 60%. RO2+RO2 accretion reactions were added to reduce the model-measurement discrepancy. Further radical budget analysis and model sensitivity tests indicated the model overprediction was related the missing radical sinks.

This paper is well-written and structured reasonable. The discussion is justified. I recommend the paper published in ACP after minor revision. Here is a few suggestions that might help to improve the paper.

1. There were several radical measurements conducted at the PROPHET site. It would be very useful to show an overview plot to compare different field measurements. In selection 3.2 and 3.3, the measurements from 1998, 2008, 2009, and 2016 are compared in different position. For example, a bar plot summarizing the diurnal maximum OH, HO2, RO2 concentrations together with temperature, BVOCs concentrations would help the readers to interpretate the consistency/difference between several campaigns.

1. As the authors suggested the radical sink related to surface loss may be relevant to the model overprediction, it would be interesting to have more quantitative analysis. One could calculate the probability of radical loss upon collision on the ground/canopy similar to the aerosol uptake. In this case, the surface area per volume can be derived from the ratio of ground/canopy area to the volume of mixing layer.

Technical comments.

1. Please check the subscripts for HO2, RO2, XO2 are other chemicals throughout the paper.
2. The name of RACM2, RACM2-LIM1, MCM3.2, MCM3.3.1 should be used properly. For example, Line 565, it should be RACM2-LIM1 instead of RACM-LIM1.