This paper provides a detailed analysis of ozone chemical regimes across the tropics, with a focus on the role of short-lived VOCs.  The paper is well written and the analysis is thorough and it provides new insight, but my main concern is that it is missing the big-picture by focusing too much on the details of VOCs.  Methane plays a huge role in global (and tropical) ozone production, especially in remote regions dominated by lightning NOx.  Methane is increasing rapidly and it is expected to increase over the next few decades, and therefore it will continue to drive ozone production across the tropics, even if NOx emissions were to remain constant.  Methane cannot be ignored and this paper needs to consider the role of methane as the background driver of tropical ozone production.  I provide more details below, and I touch on a few other items that need to be addressed.  

Methane is a major ozone precursor especially in the remote atmosphere and it will play a major role in future ozone increases across the tropics (Young et al., 2013), yet methane is not even discussed in this paper.  Methane cannot be ignored and needs to be addressed. Please consider the following: 

Zhang et al. (2016) ran the CAM-Chem model for 1980-2010 and estimated an increase of the global tropospheric ozone burden of 28.12 Tg (8.9%), due to the increase of anthropogenic emissions and the partial shift of the emissions from mid-latitudes towards the equator.  The increase of methane (15% over 30 years) accounted for one quarter of the ozone burden increase. The increase of methane has continued to the present, with rapid increases over the past 5-10 years (https://gml.noaa.gov/ccgg/trends_ch4/).  Under a future scenario of high anthropogenic emissions and continuously increasing methane concentrations (Griffiths et al., 2021), the global ozone burden is expected to increase for the remainder of the 21st century (see the ssp370 scenario in Figure 6.4 of Szopa et al., 2021), with increases of approximately 10% from 2014 to 2050.  

In terms of ozone production from lightning, methane is a key precursor, first explored above the tropical South Atlantic by Moxim and Levy, 2000.  They found that the upper tropospheric ozone maximum above this region was dominated by ozone production from lightning NOx in conjunction with CO/CH4 chemistry. They suggested an ozone maximum would have existed in this region in preindustrial times.  Similarly, Cooper et al. (2006) calculated that 69–84% (11–13 ppbv) of the observed upper-tropospheric ozone enhancement above North America during summer 2004 was due to in situ ozone production from lightning NOx and background mixing ratios of CO and CH4.  In situ observations from the NASA DC8 showed very low values of reactive hydrocarbons in the UT, indicating a limited impact from fresh surface emissions.  Basically, given plenty of lightning NOx, background methane concenrations and sunny conditions in the tropics, short-lived VOCs aren't required to produce large amounts of ozone.

The current authors conducted sensitivity tests to understand the impact of lightning NOx, and it would be very informative if they can estimate future changes in ozone with increasing methane.  Given that methane has increased almost continuously since the 1980s, and given the high likelihood that it will continue to increase, testing the sensitivity of the model to increased methane should be a priority.  

Line 408

“…model run excluding aircraft NOx does not show significant differences compared to the baseline scenario”.  This result contradicts the recent findings of Wang et al. (2022) who concluded that increasing aircraft emissions are playing a major role in increasing the global tropospheric ozone burden.  Please address this discrepancy.

Line 26

“In the upper troposphere (UT), ozone is the second most important greenhouse gas after water vapor and changes in ozone exert (and will continue to exert) a particularly large impact on the earth’s radiative forcing – especially in the tropopause region and the tropical UT (Lacis et al., 1990; Mohnen et al., 1993; Wuebbles, 1995; Lelieveld and van Dorland, 1995; van Dorland et al., 1997; Staehelin et al., 2001; Iglesias-Suarez et al., 2018).”

On the global scale, ozone is the third most important greenhouse gas after CO2 and methane (not including water vapor) and so I’m surprised by this statement that ozone has a greater impact than CO2 and methane on the radiative balance of the UT.  I looked at the list of references and none of them seem to make a strong case for this claim. Perhaps this was true decades ago when CO2 and methane concentrations were much lower, but they have increased greatly since the study by Lacis et al. 1990.  

The NOAA Annual Greenhouse Gas Index (https://gml.noaa.gov/aggi/aggi.html) has increased by 49% since 1990, driven by increases in methane and CO2 and corresponding to a radiative forcing increase from 2.3 W m-2 to 3.4 W m-2. The index does not include ozone, but the increase in radiative forcing due to ozone from 1990 to 2014 is only about 0.1 W m-2.  Figure 1 in the supplement to Skeie et al. (2020) has the most recent update on the height/latitude distribution of ozone’s radiative forcing. Do you have any similar plots to compare it to, which would indicate if ozone has a stronger radiative effect in the tropical UT than CO2 or methane?

References:

Cooper, O. R., A. Stohl, M. Trainer, A. Thompson, J. C. Witte, S. J. Oltmans, G. Morris, K. E. Pickering, J. H. Crawford, G. Chen, R. C. Cohen, T. H. Bertram, P. Wooldridge, A. Perring, W. H. Brune, J. Merrill, J. L. Moody, D. Tarasick, P. Nédélec, G. Forbes, M. J. Newchurch, F. J. Schmidlin, B. J. Johnson, S. Turquety,  S. L. Baughcum, X. Ren, F. C. Fehsenfeld, J. F. Meagher, N. Spichtinger, C. C. Brown, S. A. McKeen, I. S. McDermid and T. Leblanc (2006), Large upper tropospheric ozone enhancements above mid-latitude North America during summer: In situ evidence from the IONS and MOZAIC ozone monitoring network, J. Geophys. Res., 111, D24S05, doi:10.1029/2006JD007306.

Griffiths, P. T., Murray, L. T., Zeng, G., Shin, Y. M., Abraham, N. L., Archibald, A. T., Deushi, M., Emmons, L. K., Galbally, I. E., Hassler, B., Horowitz, L. W., Keeble, J., Liu, J., Moeini, O., Naik, V., O'Connor, F. M., Oshima, N., Tarasick, D., Tilmes, S., Turnock, S. T., Wild, O., Young, P. J., and Zanis, P.: Tropospheric ozone in CMIP6 simulations, Atmos. Chem. Phys., 21, 4187–4218, https://doi.org/10.5194/acp-21-4187-2021, 2021. 

Moxim, W. J., and H. Levy II (2000), A model analysis of the tropical South Atlantic Ocean tropospheric ozone maximum: The interaction of transport and chemistry, J. Geophys. Res., 105, 17,393– 17,415

Skeie, R.B., Myhre, G., Hodnebrog, Ø., Cameron-Smith, P.J., Deushi, M., Hegglin, M.I., Horowitz, L.W., Kramer, R.J., Michou, M., Mills, M.J. and Olivié, D.J., 2020. Historical total ozone radiative forcing derived from CMIP6 simulations. Npj Climate and Atmospheric Science, 3(1), p.32.

Szopa, S., V. Naik, B. Adhikary, P. Artaxo, T. Berntsen, W.D. Collins, S. Fuzzi, L. Gallardo, A. Kiendler-Scharr, Z. Klimont, H. Liao, N. Unger, and P. Zanis, 2021: Short-Lived Climate Forcers. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 817–922, doi:10.1017/9781009157896.008

Wang, H., et al. (2022), Global tropospheric ozone trends, attributions, and radiative impacts in 1995–2017: an integrated analysis using aircraft (IAGOS) observations, ozonesonde, and multi-decadal chemical model simulations, Atmos. Chem. Phys., 22, 13753–13782, https://doi.org/10.5194/acp-22-13753-2022

Young, P. J., Archibald, A. T., Bowman, K. W., Lamarque, J.-F., Naik, V., Stevenson, D. S., Tilmes, S., Voulgarakis, A., Wild, O., Bergmann, D., Cameron-Smith, P., Cionni, I., Collins,W. J., Dalsøren, S. B., Doherty, R. M., Eyring, V., Faluvegi, G., Horowitz, L. W., Josse, B., Lee, Y. H., MacKenzie, I. A., Nagashima, T., Plummer, D. A., Righi, M., Rumbold, S. T., Skeie, R. B., Shindell, D. T., Strode, S. A., Sudo, K., Szopa, S., and Zeng, G.: Preindustrial to end 21st century projections of tropospheric ozone from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), Atmos. Chem. Phys., 13, 2063–2090, https://doi.org/10.5194/acp-13-2063-2013, 2013.

Zhang, Y., et al. (2016), Tropospheric ozone change from 1980 to 2010 dominated by equatorward redistribution of emissions, Nature Geoscience, 9(12), p.875, doi: 10.1038/NGEO2827.

This paper uses a 20-year simulation of the EMAC model to demonstrate the reliability of different metrics for defining NO_x and VOC-sensitive regimes for ozone production in the tropical upper troposphere. They find that these chemical regimes are well defined using the formaldehyde to NO₂ ratio (HCHO/NO₂) and the production of HCHO from methyl peroxyradicals (a(CH₃O₂)), parametrized as a function of NO mixing ratios. The sensitivity of the chemical regimes to changes in lighting NO_x production is presented as a case study. Overall, the paper is well written and within the scope of ACP. Understanding the drivers of ozone production in the upper troposphere is important for projecting the role of ozone in climate change.

However, discussion should be added explaining how the results of the present study fit in with earlier publications. Additionally, given the role of lightning NO_x and aircraft emissions in the paper, how these emissions are implemented in EMAC should be described in Section 2.3. Please also explain how the 200 hPa pressure level was chosen for this analysis and if it might have an impact on the results, such as the relative importance of lightning NOx and aircraft emissions. In general, the paper spends a lot of time listing details that are summarized in the figures at the expense of communicating the meaning of the results. Examples are given under the specific comments where more concise language would be beneficial.

Specific Comments:

• In the introduction, please write out chemical reactions that are described and used again in the paper or are important for understanding the regime metrics.

• Line 44: In terms of surface O₃, knowing if a region is NO_x or VOC sensitive is important for determining regulatory policies for controlling air quality. Please state why knowing the regime is useful for the upper troposphere and connect these concepts to the first two paragraphs of the introduction. This is mentioned later, but motivation should be given upfront.

• Line 47, “…most of the indicators for either regime are no longer valid.”: Please provide a brief explanation why. This sentence might make more sense if it was moved to the description of indicators that are not suitable for the upper troposphere. 
• Line 67, “Another indicator is the ratio…”: I recommend starting a new paragraph to distinguish between metrics that are used in the upper troposphere from ones that are air quality specific. Overall, this is a long paragraph that lists different metrics, and reorganizing the discussion would be helpful to communicate what’s relevant for the upper troposphere and why.
• Line 95, “Khodayari et al…”: what region did this modeling study focus on?

• Paragraph beginning line 88: This is a list of past studies that explored ozone production sensitivity regimes in the upper troposphere. In the results or conclusions, please discuss your findings with respect to these studies.
• Figure 1: Is this from the EMAC modeling output, described in section 2.3? If so, state in the figure caption and description of the figure.
• Supplemental Figure S1: Say in the figure caption what the black line and grey shading are. Are these mixing ratios all sampled at local noon?
• Lines 194 – 204: This text is hard to follow. If the authors find these individual mixing ratios are useful, perhaps they should be given as a table or separate figure. If not, could they be summarized to support the support the discussion of the patterns in NO that are due to seasonal and continental vs marine features? Throughout the paper there is text where the authors list values for multiple locations. If there are common features of these locations (i.e., Southern vs Northern Hemisphere) that are driving the patterns seen in the mixing ratio or sensitivity regimes, please point this out instead.
• Line 209, “During DJF…”: This should be stated earlier in the discussion, with citation.

• Line 240: What specifically about your modeled NOPR values is in line with the findings by Apel et al. (2015)?

• Line 273: What do you mean by “mostly absolute values” here; is this columns of HCHO and NO₂?
• Lines 289 – 318: Please consider more concise wording for the description of Figure 6. What are the main results you want the reader to understand from this Figure?
• Figure 7: Add the season (MAM) to the figure caption.
• Line 345: Is the 38 ppbv^-1 value for the slope?
• Figure 9: If the average backgrounds (or another metric) from the base and other sensitivity studies are shown here, the readers can more easily compare these results to those shown in Figure 6 and S16-S17. Additionally, it might be helpful to move Figure S18 to the main text since the results of the aircraft emission sensitivity study is not currently shown in the main text figures.
• Line 378: What do you mean by “the absolute values” here? Is this referring to the ratio for individual regions?
• Lines 406 – 412: How do these results fit in with past studies?
• Line 417: Was an additional run without aircraft emissions conducted?
• Lines 437- 441: Citations should be provided for these statements.
• Conclusion section: The discussion of the role of lightning in the conclusions is terse and doesn’t explain how the study reached these results or how it fits in with the current literature.