The manuscript, 'Further Constraining the Role of In-Atmosphere Production on the Global HFC-23 Budget', by Adam and co-authors, presents the findings of an atmospheric modelling study that investigated the potential production of HFC-23 (a highly potent greenhouse gas) from precursor gases within the atmosphere. The study uses a state-of-the-art atmospheric chemistry model updated for HFC and HFO chemistry, making use of updated photolysis data for the critical intermediate trifluoroacetaldehyde. A series of sensitivity simulations were performed to propagate the uncertainties associated with several significant atmospheric processes. The study is well designed and the manuscript is well written, with the results communicated clearly. Furthermore, the results are highly relevant as they rule out significant atmospheric production of HFC-23. This suggests that direct emissions (currently unreported/unknown) must be responsible for the growth of HFC-23 in the atmosphere. I would highly recommend publication of the manuscript once some minor points have been addressed/clarified.

P5, L126: Please distinguish the references for observations and global emission estimates. AGAGE provides observations; Western et al. provides global emission estimates based on these observations. A simple 'and .., respectively.' should do it.

P5, L138: When reading through this and the following paragraph, I stumbled several times over the assumptions made. Here: why GDP scaling, when we know that transitions are different in different world regions. Later on, the assumption is questioned and a link is made to the sensitivity runs that explore the importance of the spatial distribution of emissions. I think this section would be easier to follow if it is made clear from the beginning that HFO emission totals and distributions are not well known and therefore different sensitivity tests were carried out. Already mention why spatial distribution is more important for HFOs than for HFCs (currently stated in L250 and following). This can be followed by the current description of the base case emissions and finally the mention of alternatives emission distributions. Essentially, this suggests putting the last paragraph in the section forward.

Table 2: Are these the lifetimes calculated from the base simulation or literature values? Please clarify.

Section 2.3.1: Since this section takes up quite some space and is the basis for the re-evaluation of reaction pathways, I would suggest to also include Figure S1 in the main text. The overall length of the manuscript is rather short and there seems to be no need to hide the Figure (and also the SI tables) in a supplement. Alternatively, the section and figures could be moved into an appendix.

Equation 2 and Figure S1: Equation 2 for the product yield does not fit to the values displayed in Figure S1b. S1b shows PY=1 for lambda<266, the equation suggests 0.5. Please clarify.

P8 L196: How is the actinic flux calculated in STOCHEM? The wavelength dependency of the actinic flux will obviously impact the relative importance of pathway a vs b. 

P8, L196: How is the actinic flux calculated in STOCHEM? The wavelength dependency of the actinic flux will obviously impact the relative importance of pathway a vs b.

P9, L222: The upper end of deposition rates seems to be rather extreme. Not even very water-soluble compounds like HNO3 have such high deposition velocities, especially not over the ocean or bare soils. I understand that extremes should be explored but at the same time these should be within physical bounds.

P10, L 246: Assuming that the transition to HFOs is progressing fastest in Europe and, hence, the base case already represents an upper-limit emission scenario: how realistic is the evaluated emission range 0.33 - 3 times the base case?

Section 3: When discussing the results, it should be clarified if the ensemble mean/median or the base case are considered to give the most realistic results. A statement on how much the two differ may be helpful as well.  Since some of the sensitivity runs are leaning towards extreme parameter tests, it is not obvious that the ensemble mean is an unbiased, best estimate.

P13, L323: What is the uncertainty estimate on the global HFC emissions from Western et al. (2026)? Assuming linearity this could be propagated to evaluate its influence in comparison to the HFO emission and other sensitivity runs. However, I agree that it will most likely be smaller than for the HFO emission sensitivity, which covered a broad range.

P13/14: For understanding the comparison with the Van Hoomissen et al. (2025) results it would be helpful to repeat the kind of model that was used in their study.

P14, L353: While global total HFC-23 production is similar in these runs with different spatial distribution of HFO emissions: Do the runs lead to considerable regional differences in HFC-23 production and, hence, mole fraction differences? Would these differences be sufficiently large to compare simulated versus observed HFC-23 latitudinal gradients?

Section 3.4: Are the cited literature values based on global model simulations as well or simply on kinetic data and average OH assumptions? Please clarify if the Liang et al. (2022) citation also valid for the HFO-1336mzz(Z) and HCFO-1233zd(E) lifetimes.

The study by Adam et al. investigates the role of in-atmosphere production on the global HFC-23 budget. They use a 3D chemistry-climate model and perform several sensitivity simulations. The main question they want to answer is if in-atmosphere production can explain the gap between reported and measured emissions. They find that the in-atmosphere production only contributes to a minor part and that the reason for this gap are still unclear. Using a box model the authors additionally investigate the global warming potential and find that some HFOs have a larger impact on climate than previously thought.

The study seems to be quite sound, however lacks a clear description of  e.g. the models used and the simulations performed. The topic itself also is not well explained. For example for what are HFCs used and why is the investigation of their budget important? As I remember these are the replacements species for the CFCs. What has been established by the Montreal Protocol and its subsequent amendments? The whole topic/problem of using these species needs to be better explained so that this study is also understandable for non-experts. 

The same holds for sources and sinks of the HFOs. Since the focus of this study is on the budget of HFC-23, in the manuscript a clear description of the sources and sinks should be provided. While the sources are quite extensively discussed, the sinks are hardly mentioned and solely indirectly covered the sensitivity tests.

Generally, my feeling is that there is a misbalance in writing. The manuscript goes into too much detail where it is not needed, however barely scratches on the surface where more details are needed.

Specific comments:

P1, L1-15: The abstract should be written in a more concise way and omit insignificant details as e.g. the specific uncertainties in the measurements and the HFC-23 source gases that contribute to climate change. On the other hand what has been done in this study, which model has been used could be explained in more detail. The model name and that also sensitivity studies have been performed should be clearly mentioned. 

P1-4: General comment on the Introduction: too long and should be shortened and only focus on what information is really needed for understanding this study. Also the structure should be adjusted. I would suggest to start with what are HFCs, what are they used for, why is it important to understand emissions and the global budget, what are the open questions and what is done in your study and how does it contribute to answering the open questions?

P2, L30-38: Move this paragraph further up. It should be mentioned that HFCs are the replacement for the CFCs. 

P2, L39ff: How are these estimates derived and how are the CFCs measured. What are the uncertainties?

P2, L40ff: The sentence is not clear. Please check if something is missing here and correct accordingly.

P3, L62: This needs to be elaborated more. Which modelling studies on HFC-23 exists and which models were used. Is your statement that so far no CTMs have been used really correct?

P3, L64: What measurements exactly were made when and where? This also should be elaborated a bit more.  

P3, L80: It would be much more helpful if the previous studies would be explicitly discussed. Split up the references and explicitly discuss what e.g. has Van Hoomissen et al. (2025) done or Nielsen et al. (2025) or Perez-Pena et al. (2025) have done in their studies.  

P2-3, until L80: The Introduction should be thoroughly revised. There are too many unnecessary details given and the rather necessary information/details are omitted.

P4, L94: So far only the sources have been discussed, bot not the sinks.

P4, L94: General comment on the introduction would be to shorten, restructure and provide only the necessary information (what are HFCs, what are they used for, sources and sinks, previous studies) and more details on that. 

P4, L97ff: More information on the model (technical details and set-up) should be given. What time period has been modelled and which resolution has been used. Have global simulations or regional been performed? For what kind of studies is the model generally used, what is known about the general quality and reliability of the model simulation?

P4, L113ff: Usually the greek letter tau is used for the lifetime. I would introduce this letter and then add “tau =“ to the lifetimes instead of just writing solely the lifetimes or “tropospheric lifetime =“ 

P5, L123-124: Too many repetitions of mentioning Table 1.

P5, Table 1 and text: What is the meaning of the source gas suffixes mzz, zd etc:? 

P6, L147: Which previous work? The work by Vollmer et al. (2025)? If yes, this should be stated more clearly.

P6, 155: What sensitivity studies have been performed? Is here the reference to the section where these are described missing?

P7, L172: Add a greek lambda before the wavelength, so that it reads lambda = ….. nm. 

P7, L185: Also here a reference to Sect. 2.4 where the sensitivity studies are described is missing.

P8, L210: Add here the numbers for the lowest and highest levels.

P9, L217: More details and the dry and wet deposition processes should be given. On what exactly deposits the HFCs?

P9, L232: Reference to Sect. 2.4 is missing. Better would be if you would provide all details on the sensitivity study in Sect. 2.4 and not spread out throughout the manuscript.

P10, L266: This table is important and should appear in the main text and not in the supplement. As the other referee mentioned the supplementary material would be worth to be added to the main manuscript. I totally agree.

P11, L282: More details on the box model used and simulations done should be added.

P11, L300: Are you considering here the global HFC production? 

P14 and 15, Sect. 3.2.2. and 3.2.3: A reference to the table where the sensitivity runs are listed is missing in these sections.

P16, L428: Add reference? Or are these lifetimes also found in Liang et al. (2022)?

P17, L433: GWP calculations done with the box model? Id yes, clearly mention this. What set-up was used? Provide more details and what exactly is meant with 10-, 100- and 500-year time horizons. Over 10, 100 and 500 times or in 10, 100 and 500 years?

P17, Table 5 caption: What is the difference between direct and indirect global warming potentials? This should be clearly described in the text.

P17, L440: “are discussed in the next section” correct? Obsolete?

P18, L474: What is meant with F-gas?

Technical corrections:

P2, L37: was -> is

P3, L70: parametrise -> parameterise

P3, L76: Publication year of the Van Hoomissen et al. reference is missing. 

P4, L97: Is STOCHEM-CRI an abbreviation? If yes, the meaning of the abbreviation should be introduced.

P6, L150: emissions inventories -> emission inventories or better emission inventory. Aren’t you using only one inventory?

P6, L153: emissions estimates -> emission estimates

P7, L183: Figure S1 -> Fig. S1

P8, L202: add “by” so that it reads “by Burkholder et al. (2019)”.

P8, L205 and 206: Omit “Supplementary” and change “Figure S1” to “Fig.. S1”.

P8, L209: Add “at wavelengths” after “occurs”.

P8, Table 3 caption: pressures -> pressure levels?

P9, L216-217: Repetition of “considered in the model” in the sentence.

P10, L266: Omit “Supplementary”.

P11, L285: Sentence “was a uniform 0.4 ppt” not clear. Please check and correct.

P11, L306: Remove parentheses around the Van Hoomissen et al. reference and incorporate the reference into the text -> by Van Hoomissen et al. (2025)

P12, Figure 1 caption: section 2.4 -> Sect. 2.4

P13, Figure 2 caption: in solid bars -> as solid bars

P13, L325 and 326: emissions scenarios -> emission scenarios 

P13, L330: Figure 2 -> Fig. 2

P14, L385: Figure 1 -> Fig. 1

P14, L360: section 2.2 -> Sect. 2.2

P16, L426: Remove parentheses around reference of Liang et al. and write “in Liang et al. (2022)”.

P16, L429: of the literature -> to the literature

P18, L458: emissions gap -> emission gap