The study presents valuable measurements of OH and HO₂ radicals using the CIMS technique at a subtropical rural site in southern China during November and December 2022. The data suggest generally lower concentrations of OH and HO₂ compared to previous studies. By categorizing the data into three distinct cases based on backward trajectory analysis, the study highlights good agreement between observations and model predictions under cold, clean conditions but significant overestimation under warm, polluted conditions. This overestimation, which affects secondary pollutant production, underscores the need for further investigation into HOₓ sources and sinks to resolve model-observation discrepancies. While this study provides valuable insights into HOₓ radical behavior in a subtropical rural environment, several aspects require clarification and deeper discussion. Addressing these issues will strengthen the study’s conclusions and enhance its contribution to understanding radical chemistry and secondary pollutant formation in rural and polluted environments. Below are detailed comments:

1. The introduction section needs more comprehensive referencing. Important studies in the field of HOx radical chemistry, particularly those relevant to subtropical and rural environments, should be cited to provide better context and demonstrate the study’s relevance.
2. The calibration procedures for OH and HO₂ require further clarification. Given the critical role of the calibration factor in determining OH concentrations, a detailed explanation of the calibration methodology is essential.
3. The manuscript should specify the scavenge efficiency of OH during the measurement process. How was it ensured that OH radicals were entirely removed? This information is crucial to validate the reported OH concentrations.
4. The efficiency of HO₂ conversion via its reaction with NO should be discussed in detail. Is the conversion complete, or is it assumed to operate at a constant efficiency? This factor significantly affects the accuracy of HO₂ measurements.
5. The manuscript should elaborate on the HO₂ titration process, specifically how HO₂ is converted to OH and subsequently reacts with C₃F₆. The scavenge efficiency for this step may differ from that of OH, which could influence the accuracy of HO₂ measurements.
6. The unit of the calibration factor should not be expressed as 'cm⁻³.' This error needs correction for consistency and clarity.
7. The manuscript claims a measurement accuracy of 44% for HO₂ and 46% for OH. However, this discrepancy is counterintuitive and should be explained, as HO₂ measurements are typically less accurate than OH measurements.
8. Previous studies in rural areas generally report underestimation of OH and HO₂, yet this study finds significant overestimation under certain conditions. This discrepancy requires further discussion, especially concerning the chemical mechanisms and environmental factors leading to such outcomes.
9. The number of significant figures reported for parameters should align with the detection limits of the instrument. Retaining two decimal places for all parameters, irrespective of their precision, is inconsistent.
10. The units in Figure 5 should be corrected from 'ppb/s' to 'ppb/h' for consistency and to align with the typical units used in radical budget analysis.
11. In the legend of Figure 5a, the reaction "OH + NO₂" should be correctly identified as forming HNO₃ rather than HO₂.
12. The manuscript inconsistently classifies the measurement site. Although it is described as a rural site, Figure S5 attributes it to a forest environment. Furthermore, the observed BVOC concentrations are much lower than AVOC concentrations, indicating a rural rather than a forested environment. This ambiguity should be resolved for clarity.
13. The manuscript lacks an assessment of the model's performance in simulating key species such as ozone and OVOCs. Given the uncommon degree of HOₓ overestimation in a similar environment, evaluating the model’s performance against observed concentrations of these species is necessary to validate the findings.

This manuscript details measurements of OH and HO₂ radicals made using a CIMS instrument at a subtropical rural site near the Pearl River Delta in China. These measurements are presented along with radical concentrations from a box model featuring the Master Chemical Mechanism v3.3.1. In general, when constrained to a suite of measured trace gases and meteorological parameters, the model overpredicted the measured radical concentrations, especially during warmer, more polluted periods. The authors also use an additional model, further constrained to the measured radical concentrations, to illustrate that the overprediction of HO_x species results in a significant overestimate of the production rates of secondary pollutants such as ozone and nitric acid.

Accurate field measurements of OH and HO₂ are extremely challenging, and the results presented in this manuscript are important to characterizing radical chemistry and the overall oxidative capacity of the atmosphere. While it is clear that a significant amount of work went into the field measurements described in this study, a lack of detail regarding the instrumentation and calibration procedures and limited discussion surrounding the model overestimation of the measured HO_x concentrations limits the conclusions that can be drawn from the results. The strength of the manuscript would be greatly improved if the authors expanded sections 3.2 and 3.3 and offered insight into potential explanations for the discrepancies between measured and modeled HO_x concentrations. More specific comments are included below:

• Section 2.2 – There are few details on the calibration procedure for OH and HO₂. Given the importance of the calibration factor for each species, a brief description of the process should be included. The authors do describe that equivalent amounts of OH and HO₂ are produced by the calibration source – is the residence time of radicals in the calibration source sufficient such that wall interactions and radical-radical loss mechanisms must be considered to determine the concentrations of OH and HO₂ that exit the calibrator and enter the sampling inlet?
• Section 2.2  and Table S3 – Similar to above, there are limited details regarding the timing of the measurement sequence and the addition of the scavenger gas. The manuscript should not describe the OH, HO₂, and H₂SO₄ measurements as simultaneous but should instead detail the amount of time spent in each measurement mode. In the main text, the authors also describe that OH and HO₂ concentrations are derived from a simple subtraction of background signals, while Table S3 lists the scavenging efficiency for OH and HO₂. How are these scavenging efficiency values determined and how are they factored into the determination of radical concentrations? Are there any lingering effects of the scavenger gas that must be considered similar to the residual NO that is described in Text S2? These details would provide additional confidence in the radical measurements.
• Page 8, Line 11. After this description of HO₂ uptake on aerosols, this process is not included as a loss mechanism in Figure 5 or discussed in the remainder of the manuscript despite SMPS measurements of particle size and number being shown in Figure 3. Is this uptake negligible compared to other loss mechanisms shown in Figure 5? Is RO₂ loss on aerosols also included in the model?
• Page 16, Lined 7-15: As mentioned above, the significant overprediction of OH and HO₂ during the PRD and CEC should be the main focus but the current manuscript offers very little in the form of discussion. I suggest expanding this section to include the rate of HO₂ loss necessary to account for the difference between modeled and measured concentrations, how this rate compares to other processes shown in Figure 5, and potential explanations for the discrepancies.
• Figure 6: While the more significant discrepancies during the daytime should be the focus of the discussion, the model also underestimates HO₂ at night during CEC and CNC. This should also be mentioned in the manuscript and could be added to the discussion.
• Figure 3 and Section 3.1.2 – I suggest highlighting the different measurement periods in Figure 3 to better communicate which observations are included in the PRD, CEC, and CNC periods. At first glance, it appears that the majority of the highest observed HO_x concentrations occurred during the three cases, and the lowest HO_x concentrations (December 1-6 and 10-19) are omitted from the analysis. Were model runs also performed for these days?
• Figure S6: The standard deviation of AVOC and OVOC measurements increases suddenly during the daytime in the PRD case. Is it possible that the short gap in VOC measurements shown in Figure 3 is included in the average?

Minor comments:

• The instrument to measure HONO is not listed in Table S2
• Figure 5: The y-axis label should be ppb h^-1 not ppb s^-1 and OH + NO₂ should form HNO₃. In general, this figure is not easy to interpret due to the different axis scales and very small colored sections relative to reactions with NO. A total radical (RO_x) budget that does not include propagation channels may better illustrate the relative importance of initiation and termination processes in the model.
• Figure S4: As all data from the campaign is averaged together, this figure is misleading for some species that vary significantly from November to December such as isoprene or HONO. I suggest separating the data into two or three averaging periods or combining this figure with Figure S6 to illustrate how HONO, isoprene, and ozone changed during the transition from PRD to CEC and CNC.
• Tables 1, S2, and S4: Aligning text to the left of each column would improve readability. There is also a problem with the resolution of Table S2.

The manuscript reports about CIMS measurements of OH and HO2 in a subtropical rural site and the comparison of them with the results of MCM box model. Although in general simultaneous measurements of OH and HO2 provide very helpful information for understanding of radical budgets and the article reporting such measurements would potentially be of an interest, the present study cannot be published in its present form because it actually does not report HO2 measurements. The method used in this study for detection of peroxy radicals by their conversion to OH in reaction with added NO will result in about the same conversion efficiency for both HO2 and organic peroxy radicals RO2. To distinguish between the HO2 and RO2 radicals using the conversion scheme with NO several groups previously developed CIMS methods based on a modulation of chemical conditions in their instruments to measure either HO2 or RO2, predominantly (Hanke et al., 2002; Edwards et al., 2003; Hornbrook et al., 2011). In brief, HO2 mode requires efficient suppression of RO2 to HO2 conversion in reaction of alkoxy (RO) radicals with O2 in favor of the formation of alkyl nitrites:

RO2 + NO => RO + NO2

RO + O2 => R’O + HO2

RO + NO + M=> RONO + M

Although the authors of the present manuscript make reference to the study of Edwards et al., 2003, they use NO concentration of 1.2 ppm leading to about 90% conversion of RO to HO2 and resulting in similar contribution of HO2 and RO2 to the detected signal, assuming their similar ambient concentrations. Referring in the manuscript supplement to the study of Fuchs, 2014 as an example of using the same NO concentration of 1.2 ppm for HO2 detection the authors do not take into account low pressure in the FAGE conversion stage, hence low O2, making RO+O2 reaction negligible and allowing HO2 measurements with low interference from RO2, although not for all of RO2 (Fuchs et al., 2011). Finally, the authors do mention once “interference” from RO2 by saying that “To access HO2 interference caused by the ambient RO2 conversion, the model underwent a three-days spins-up to simulate the ambient RO2 concentration”. However, it doesn’t seem to be a correct procedure to make correction using the model and after that compare the measurements corrected in this way with the same model.

The present OH and “HO2” measurements may still be of value and present the basis of an important publication. However, for this the measurements should be correctly interpreted and presented with detailed description of a calibration procedure of the peroxy radicals.

References:

Edwards, G. D., Cantrell, C. A., Stephens, S., Hill, B., Goyea, O., Shetter, R. E., Mauldin, R. L., Kosciuch, E., Tanner, D. J., and Eisele, F. L.: Chemical Ionization Mass Spectrometer Instrument for the Measurement of Tropospheric HO2 and RO2, Anal. Chem., 75, 5317–5327, https://doi.org/10.1021/ac034402b, 2003

Fuchs, H., Bohn, B., Hofzumahaus, A., Holland, F., Lu, K. D., Nehr, S., Rohrer, F., and Wahner, A.: Detection of HO2 by laserinduced fluorescence: calibration and interferences from RO2 radicals, Atmos. Meas. Tech., 4, 1209–1225, doi:10.5194/amt4-1209-2011, 2011

Fuchs, H., Acir, I.-H., Bohn, B., Brauers, T., Dorn, H.-P., Häseler, R., Hofzumahaus, A., Holland, F., Kaminski, M., Li, X., Lu, K., Lutz, A., Nehr, S., Rohrer, F., Tillmann, R., Wegener, R., and Wahner, A.: OH regeneration from methacrolein oxidation investigated in the atmosphere simulation chamber SAPHIR, Atmos. Chem. Phys., 14, 35 7895–7908, https://doi.org/10.5194/acp-14-7895-2014, 2014  

Hanke, M., Uecker, J., Reiner, T., and Arnold, F.: Atmospheric peroxy radicals: ROXMAS, a new mass-spectrometric methodology for the speciated measurements of HO2 and RO2 and first results, Int. J. Mass. Spectrom., 213, 91–99, 2002

Hornbrook, R. S., Crawford, J. H., Edwards, G. D., Goyea, O., Mauldin III, R. L., Olson, J. S., and Cantrell, C. A.: Measurements of tropospheric HO2 and RO2 by oxygen dilution modulation and chemical ionization mass spectrometry, Atmos. Meas. Tech., 4, 735–756, doi:10.5194/amt-4-735-2011, 2011.