Review of "Impacts of present and future aircraft NOx and aerosol emissions on atmospheric concentrations and associated radiative forcing of climate" by Terrenoire et al.

This paper studies the impact of present-day and a range of projected aviation NOx and aerosol emissions on atmospheric composition and climate. Aviation NOx has been the focus of numerous studies over the past decades, but here the authors include less well studied contributions from nitrate and sulfate and applies an updated and more refined, including the addition of interactive stratospheric chemistry, version of the LMDZ-INCA global model than previously used.

The results reconfirm previous findings and are in overall agreement with older comparable studies when it comes to features of the NOx-induced perturbations to ozone and methane concentrations. A key new finding is that accounting for indirect effects on nitrate and sulfate aerosols, results in a switch from net positive to net negative aviation NOx forcing. The sign of the net NOx forcing is also found to be highly sensitive to background conditions, with implications for future scenarios. Given that the most recent comprehensive assessment of aviation climate impact placed NOx as the third largest warming contribution, these are important findings. The sensitivity of the net NOx climate effect to these factors, combined with the large uncertainties surrounding atmospheric aerosol in general, shows that further efforts are required from the scientific community.

The paper is very well written and organized, albeit a bit long and detailed at times. The future scenarios used are old and may not capture more recent formulations of the ICAO goals and the Paris Agreement, which is an issue for the relevance of the results in the broader context – however, this is also understandable given the lack of up-to-date aviation emission scenarios and fine within the more process-focused scope of this paper.  

Overall, I consider the paper to be an important contribution to the literature and have mostly minor comments for clarity and readability that should be considered before publication.

General comment:

Both the abstract and Summary and conclusion sections are quite long and detailed, which makes it a bit challenging to extract the key findings and implications. I would encourage the authors to look for possibilities to reduce the level of detail in the abstract, which reads almost like a full summary on its own, as well as eliminate some of repetition that seems to be between the summary and previous sections (or alternatively remove from results section for a more condensed summary of course).

The study includes a large number of experiments; while described in the methods section, I think it could be helpful for the reader to have a table that summarizes them.

Minor comments:

Line 130: Lund et al. 2017 https://doi.org/10.5194/esd-8-547-2017 also included nitrate aerosols

Line 147: missing an “us”?

Line 215: Because the paper repeats the importance of the emission altitude and to be able to better study the zonal concentration changes, it would be useful with the figure (in the SI) showing the zonal mean, vertical emission distribution.

Lines 228-235: could be worth a quick mention of the evolution of the sector in the past 10-12 years (as assessed by Lee et al. 2021) since these emission inventories are older – for context. Also how, do the CO2 emissions for 2006 compare with those in Lee et al.? I seem to recall that CEDS emissions are a bit lower than that study in 2018, also the case for 2006?

Lines 294: the study by Ocko et al. https://acp.copernicus.org/articles/19/14949/2019/ presented some scenarios that where aligned with ICOA goals and policies – worth mentioning comparing here, or in the discussion?

Line 380: given that one of the main novel contributions in this work is the inclusion of aerosols, I suggest the authors also add a brief summary of what has been found regarding model performance for sulfate/nitrate, rather than just referring to other studies.

Line 390: typo

Line 390: can you please specify the latitude bands, unclear what high latitudes are

Line 405: mix of southern and northern hemisphere stations as I understand it – are these three stations the only ones located in the southern hemisphere high lats?

Line 470: maybe mention findings about non-linearities in the scaled small perturbation vs 100% removal approach?

Line 487: what about the approach to derive methane forcing? Describe here?

Line 603: to get a better feel for the magnitude of the changes, it would be useful to express some of this in terms of percentage of the baseline concentration…

Line 620-629: while the sulfate change is pronounced towards the high latitudes, the nitrate change is focused much further south – can the authors provide a brief explanation? Meteorological factors?

Lin 764-782: what would be the potential implications of allowing climate to change, instead of fixing meteorological conditions? Of significance?

Line 897-898: I feel that there is a bit of a mismatch between one of the novelties of this paper and the only two-line mention of this here. For instance, in light of the large uncertainties surrounding aerosols RF, how confident are you in the net response?

Line 910: is table S4 the correct reference here or am I misunderstanding the sentences?

Line 1013: typo

Line 1055: given the uncertainties surrounding the ERF/RF conversion factors, why not include a comparison with the RF numbers provided by Lee et all. (as well).

Review of the “Impact of present and future aircraft NOx and aerosol emissions on atmospheric composition and associated direct radiative forcing on climate” by Terrenoire et al.

This study explores and gathers various aspects of the impact of aircraft NOx emissions on atmospheric composition and climate utilizing a series of present and future aviation projections, and an updated global chemistry-aerosol-climate model, LMDZ-INCA. In addition to the well-established effects of aircraft NOx emission, this paper also investigates the impacts that arise from the formation of nitrate and sulfate aerosols that constitute a novelty of the publication as it has been addressed in only a few modeling studies so far. It is also associated with large uncertainties, which might be worth highlighting more in this publication.

The paper is well-written and scientifically sound. The applied methods are valid and clear. The findings of the study are of interest to the community. I recommend this paper for publication, preceded by a few minor comments.

General comments:

Abstract reads more like a summary. It is too long, with too many technicalities, and it is difficult to follow. I would suggest authors consider re-writing it, concentrating on the main findings and their implications.

The text is full of details, which on one side is understandable taking into account the number of experiments that were performed for this study. On the other hand, maybe authors can help to level this complexity of the paper by preparing a kind of ‘takeaway figure’. Figure that would summarize the main results, highlight the novelty of this study, etc. I believe the paper might gain the readability by having such a figure, but I leave this decision with the authors.

Specific comments:

Lines 90-94: some references can be useful here

Line 219: motigtaion ---> mitigation

Line 239: the list (e.g., as a Table) of performed experiments, maybe with their short description, might be helpful

Lines 464: why your “present-day” is 2004 here, while in the previous section is 2006?

Line 490: the description of methane RF calculation and all the methane-induced components are missing? Maybe part of your section 6.2 can be moved to 3.4, or vice versa, for consistency.

Line 528: Figure 5 (and subsequent Figures) why May? Why not July, or JJA? On the other hand, your RF numbers are based on annual averages, so maybe your chemistry analysis can show the annual means too?

Line 657: since you discuss the future scenarios run with interactive chemistry, it would be interesting to know how future climate change might influence your results. What is the sensitivity of aviation forcings to future climate? Is the use of present-day meteorology justified here?

Line 682: wouldn’t it be more suitable to discuss the effects of the mitigation scenarios based on the annual change, not a one-month response?

Line 795: 7.2 ---> 7.3

Line 833: having fixed surface CH4 concentrations means your CH4 and OH interactions are constrained, so it is not obvious how you derived your CH4 feedback factor.  

Line 875 and Table 5: your long-term ozone estimate is much smaller than what can be found in other studies, e.g., 47% (Wild et al., 2001), 58% (Kohler et al., 2008), 42% (Hoor et al., 2009), 51%, 43% (Pitari et al., 2016) of the CH4 RF. Most of the latest aircraft studies include 50% (IPCC AR4, Myhre et al., 2013) in their calculations.

The same applies to your stratospheric water vapor; most studies calculate it to be 15% (Myhre et al., 2007) of the CH4 RF, while yours is around three times smaller. These need some clarification or/and justification.  

Line 906: based the Etminan ---> based on the Etminan

Line 946: and 2004 ---> and 2005

Line 888 and Table S3: the CH4 increase via the Etminan parametrization you report is 15%, and it is smaller than what other aircraft studies calculate (e.g., Grewe et al., 2019), Skowron et al., 2021). Any explanation for that? At the same time, a few paragraphs above (line 822), you mention the 24.5% increase. It is confusing.

Line 1055: considering significant uncertainties associated with ERF/RF factors (especially those related to NOx as based on just one study according to Lee et al., 2021), wouldn’t it be better to compare RF numbers? It would also be consistent with Holmes et al. (2011) comparison. Another aspect is the nature of these studies, Lee et al. (2021) and Holmes et al. (2011) are multi-model ensembles, so maybe presenting also ranges that they report together with their best estimates might help your comparison to be more feasible.