This manuscript presents original and valuable earth system model results based on sensitivity experiments to investigate the tropospheric VOC chemistry effect on climate. It is well structured, written and presented. I suggest acceptance of the manuscript for publication but I have a few minor comments to be considered.

Comments

Something that I missed in the structure of the manuscript was a discussion of the induced radiative forcing due to the perturbation with explicit VOC chemistry (WACtl) versus the case with a simplified SOAG scheme and no explicit VOC chemistry (MACtl). Would you think that an estimate of effective radiative forcing from these two experiments would be feasible following the regression method of Gregory et al. (2004). For this method, it is the time development before the steady state is reached which is of interest and in your simulations the first 39 years of output (which were discarded because they showed significant trends in global mean surface temperature, indicating that the model had not fully equilibrated) may offer this opportunity.

line 130:  I would suggest to add a few references for SOAG scheme implemented?

lines 198-199: The fact that the two cases were not initialized from the same ocean and sea ice states is a limitation. Although you mention that in the future, you plan to examine possible sensitivity to different initialization approaches, could you speculate based on other studies on the degree that this could influence your results?

lines 297-299: The authors mention that " This makes sense because, if low cloud cover is fixed (which is approximately true around Antarctica) and SIC decreases (increases), then the radiative effect of low clouds gets stronger (weaker), implying a negative (positive) CRE change." Maybe the authors can clarify that this is related to albedo changes and not cloud fractions changes. 

Figure 5: I am puzzling by the fact that the statistical significant near surface warming over the North polar regions seen in Figure 1 is not shown in Figure 5. Do you have any explanation for this inconsistency among the two figures?

line 325: There is a weakening of the NH stratospheric polar jet but it is not clear the equatorward shift of the tropospheric mid-latitude jet.

line 328: This is confusing as in the next sentence you clarify that the responses are opposite to the poleward shift due to global warming. Are you referring  to stratosphere warming (as shown in Figure 5) or the thermal forcings that qualitatively mimic three key aspects of anthropogenic climate change: (warming in the tropical troposphere, cooling in the polar stratosphere, and warming at the polar surface) discussed in Butler et al (2010) referenced here?

lines 330-332: The authors mention that "based on such past work, we would expect that, aside from any regional temperature changes, widespread tropospheric cooling would also shift the midlatitude jets equatorward". Do you refer to past work related to the impact of aerosol cooling? Please prove some references.

line 342: The use of arrows on Fig.6b could help the reader to identify the counterclockwise anomaly extending poleward of the SH HC edge and a clockwise anomaly extending poleward of the NH HC edge. This is only a suggestion.

lines 360-361: Pacific. The statement that "in addition to these HC changes, Fig. 6b also shows weakening of the Ferrel cells in both hemispheres and weakening of the polar cell in SH" needs more elaboration as it is not clear.

line 383: Please define the acronym SPS.

Line 405: The authors mention here that the warming in the Antarctica is likely explained by shortwave heating, which does increase. A link to Figure 8 that shows clearly this would be helpful for the reader.

line 448 : Are PANs included in NOy as part of organic nitrates?

Line 547: The authors mention that in the midlatitudes of both hemispheres, the downward shifts of the clouds result in negative CRE as expected. This is clear for the SH but it is not clearly evident in the case for NH midlatitudes according to Figure 13.

Stanton and Tandon present numerical model calculations, which aim to attribute the role of Volatile Organic Compounds (VOC) on climate. This is a worthwhile question and progress on the topic is needed inlight of efforts like the WCRPs Lighthouse Activity on "Explaining and Predicting Earth System Change". However, this study falls significantly far from being able to answer the question. I think the authors should be comended for their attempt with the modelling but significant flaws in the model design mean that they are unable to answer the problem they have set out to. I would suggest a significant revision, including a change of the title of the manuscript if the present results are to be used as the underpinning data for a revision. 



Good points: 

Overall the manuscript is well written. The climate analyses are excellent and the team are clearly experts in this area. 



Bad points: 

The experiment design is very much flawed in being able to answer the question of VOCs impacts on climate. We have known for a long time that the effect of VOCs is non-linear on key climate forcers (e.g., O3, CH4, aerosols). By removing the VOCs one is significanlty perturbing the system into a state which does not allow the role of VOCs to be teased out. The study answers the question: "What are the impacts of removing VOC chemistry on pre-industrial climate simulated by CESM2". This is a useful question but far from the grander question raised by the title. I think this point is a critical one that can not be addressed without addressing the core of the study/revising the aims and scope. 



As someone who has looked at aspects of the title problem I am very surprised by the large changes in surface temperature that the authors show in Figure 1. The small difference in Global Mean Surface Temperature (GMST) suggests that the surface response is really not that signficiant.  How does that compare to the spread of PI GMST simulated by CMIP6 models or even ensemble members of CESM2? Are these changes really significantly different from the uncertainty in the literature? The model we use in the group, UKESM-1-LL, shows significant variance across ensemble members (even under PI conditions). As it currently stands, I don't think the results suggest that there is a significant imapct of VOC chemistry on climate; BUT as I said I am not sure that the experiment design allows one to answer this question. 



What was also lacking for me was more of a focus on the causal links between tropospheric composition changes and their impacts on radiative forcers. There is clearly a very large response in OH. Why? I would guess the removal of isoprene, which has been addressed before, many times (see e.g., Bates and Jacob, 2019; Squire et al., 2015; von Kuhlmann et al., 2004). I was surprised not to see mention of isoprene at all in any analyses. Moving from OH one can then identify the impacts of changes in oxidising capacity on aerosols and aerosol precursors. What happens to SO2? Why does Sulfate change the way it does? Some of this can only be understood by constructing budgets of the variables (production and loss) and analysing them. Like I said the climate analysis is good but the attribution to composition changes leaves a lot to be desired and can be thoroughly improved to provide insight into causality (but not attribution of it!). 



In additio to these minor comments I have more minor comments:

L26: The abstract is missing a conclusive statement at the end. 

L38: I don't think oxidization is a word. Change to oxidation. 

L41: Not clear how RO2  makes NO3. Add a reaction or reference. Need to define HO2 and RO2. 

L45: Not only "typically". Actually. That's the nature of a CTM.  See Young et al (2018) for an overview of the types of models and feedbacks/couplings and adopt that nomenclature. 

L56: They didn’t have to prescribe SSTs. They chose to. there are still elements of climate response with fixed SSTs and the use of fixed SSTs is the defacto method for calculating key climate metrics like ERF. 

L61: Define NOx. 

L73: ...limits the model’s ability to produce a "full" climate response. Add "full". 

L84-85: Absolute temperature variations across climate models is way larger than this. Anyway, what is key to climate change is the difference between simulations under different forcings within a model. This statement can be misconstrued and so needs to be toned down, alot. I still don't see the surface temperature response as being at all significant so would like to be convinced more on this point. 

L87: Typically RONO2 are formed from RO2+NO too. Should be made clear if BVOC+NO3 is the only route to RONO2 or if there are other routes, too. And what Organic Nitartes are comprised of in the model. PAN? 

Table 1: It’s unclear how key reactions, like the CH4 oxidation, varies between experiments. Neither is it clear what the emissions of NOx and VOCs are. 

L143: So there is no methane? No CO? Why? NB I am not saying CO is a VOC. 

L175: I agree HONO is important but many things are missing that are important for ozone (see for example my review paper on tropospheric ozone, Archibald et al. 2020). The focus on HONO seems quite parochial. 

L198: I think this is a major weakness of the study. I would welcome comments on the limitiations that this imposes and whether 39 years is really long enough for a fully coupled model to "spin-up".

Figure 1: This is rather puzzling a result. For a start the largest changes are over the oceans.  This is where some fixed SST runs would help to remove any noise caused by changes in ocean circulation which will have a long time signal (and require more than 140 years of run). The main changes in temperature seem associated with sea ice zones. Is that correct? What could cause such large and significant ~ grid-box level changes in surface temperature over land? I can’t think of a mechanism associated with chemistry. 

Figure 1-Figure 6 and results and discussion. How large are these changes compared to for example the spread of CMIP6 models and or the spread of LENS/2 simulations with CESM/2. 

L445: Are RONO2 coupled to the radiation scheme? 

Figure 9-12: % changes would be much more helpful. Please also express species in units that are more widley used in the literature. pptw for example is not used in atmospheric chemistry circles. Instead use mass per unit volume (g/m3 for example). 

L465: What matters more is how SOA are dealt with in the model, not what the literature says. Can you please expand on the coupling of SOA to radiation (through direct and indirect effects). 

L483:  This causality is not possible to determine. One would need to isolate ONLY the VBS to assert this. 

L523: This can be because: 1) you have less OH so less SO2 forms sulfate 2) you have more clouds and rain so more SO4 is wet deposited. Which is it? I think you need to examin the SO4 budget. 

References: 

A. T. Archibald, J. L. Neu, Y. F. Elshorbany, O. R. Cooper, P. J. Young, H. Akiyoshi, R. A. Cox, M. Coyle, R. G. Derwent, M. Deushi, A. Finco, G. J. Frost, I. E. Galbally, G. Gerosa, C. Granier, P. T. Griffiths, R. Hossaini, L. Hu, P. Jöckel, B. Josse, M. Y. Lin, M. Mertens, O. Morgenstern, M. Naja, V. Naik, S. Oltmans, D. A. Plummer, L. E. Revell, A. Saiz-Lopez, P. Saxena, Y. M. Shin, I. Shahid, D. Shallcross, S. Tilmes, T. Trickl, T. J. Wallington, T. Wang, H. M. Worden, G. Zeng; Tropospheric Ozone Assessment Report: A critical review of changes in the tropospheric ozone burden and budget from 1850 to 2100. Elementa: Science of the Anthropocene 2 November 2020; 8 (1): 034. doi: https://doi.org/10.1525/elementa.2020.034



Bates, K. H. and Jacob, D. J.: A new model mechanism for atmospheric oxidation of isoprene: global effects on oxidants, nitrogen oxides, organic products, and secondary organic aerosol, Atmos. Chem. Phys., 19, 9613–9640, https://doi.org/10.5194/acp-19-9613-2019, 2019.

Squire, O. J., Archibald, A. T., Griffiths, P. T., Jenkin, M. E., Smith, D., and Pyle, J. A.: Influence of isoprene chemical mechanism on modelled changes in tropospheric ozone due to climate and land use over the 21st century, Atmos. Chem. Phys., 15, 5123–5143, https://doi.org/10.5194/acp-15-5123-2015, 2015.

von Kuhlmann, R., Lawrence, M. G., Pöschl, U., and Crutzen, P. J.: Sensitivities in global scale modeling of isoprene, Atmos. Chem. Phys., 4, 1–17, https://doi.org/10.5194/acp-4-1-2004, 2004.



P. J. Young, V. Naik, A. M. Fiore, A. Gaudel, J. Guo, M. Y. Lin, J. L. Neu, D. D. Parrish, H. E. Rieder, J. L. Schnell, S. Tilmes, O. Wild, L. Zhang, J. Ziemke, J. Brandt, A. Delcloo, R. M. Doherty, C. Geels, M. I. Hegglin, L. Hu, U. Im, R. Kumar, A. Luhar, L. Murray, D. Plummer, J. Rodriguez, A. Saiz-Lopez, M. G. Schultz, M. T. Woodhouse, G. Zeng; Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends. Elementa: Science of the Anthropocene 1 January 2018; 6 10. doi: https://doi.org/10.1525/elementa.265