Title: Sensitivity of atmospheric composition and climate to altitude and latitude hypersonic aircraft emissions.

Authors: Johannes Pletzer and Volker Grewe

General Comments:

This paper addresses the potential impacts of a future fleet (~2050) of hypersonic aircraft emissions (H₂O, H₂, CH₄, and NOx) on middle atmosphere composition (i.e., H₂O, NOy, HOy, etc…) and the resultant chemical impacts on ozone. The stratospheric-adjusted radiative forcing from hypersonic aircraft perturbations is examined in “exhausting” detail. This paper will be a benchmark study for how perturbations from aircraft or any other vehicle emitting gases in the middle atmosphere affect climate forcing.

Overall, I find this work suitable (as is) for publication in ACP. I have a few comments below that the authors may find useful.

Specific Comments:

Line 13: “… can have a large

Line 20: “… simulated years by one-third and thus cost and climate impact.” Do you mean by running simulations in a shorter period, you are not contributing as much to global warming? This may be correct, but is that really necessary to point out (at least at the abstract level)?

Line 26: “…strong noise”? maybe “…“loud noise”?

References seem to be duplicated, e.g., Jockel, et al., 2016a and Jockel et al. 2016b are the same paper. There are many examples of this type of duplication. Please double check all your references.

Page 2. It would be nice for the “uninformed” reader what the primary difference is between a proposed supersonic and hypersonic aircraft are (i.e., relative to the speed of sound).

Line 41: Grammar, please reword. “One of the newest analyses the climate impact and the growth potential using projections of different technological development scenarios (Grew et al., 2021).

Line 51: Not sure what you mean by “…climate change is manifold that of subsonic aircraft…”?

Line 68: (EMAC)j?  Is “j” a typo?

Line 72: “In section three, we present the EMAC model…” You actually present this in sections 2.1. I believe you need to rename all the section numbers here.

Line 88: My next statement is a personal preference… The details of the EMAC model setup could easily be in an appendix. I’m not sure why the reader needs to muddle through all the sub-model process names. Maybe just have a more top-level model description in the main paper. One could probably reference a few EMAC publications in this section and only highlight what was important for this study.

Section 2.3. I would highly recommend a table that discusses the model configurations. You do state in words what simulations are performed in this work – however, I had to read sections 2.1 through 2.3 several times to figure out exactly what you did.  A table would help the reader.

Specified Dynamics. Based on the discussion in section 2, it seems you are using “observed” specified dynamics met fields (present-day ERA-Interm) for the 2050 aircraft scenarios (line 129). You should probably say in a few words why this is justifiable, instead, using model specified dynamics fields from an interactive climate run for the year 2050.

Section 2.4. Enhancing the efficient use of computer time. Very interesting approach!! This could be adopted by many research studies that run a model to a steady state condition.

Section 3.2. Can you put the emissions (H₂O, H₂, NOx) in context, i.e., how many aircraft are considered in this scenario? You mention the LAPCAT-PREPHA aircraft design; but are the number of planes significant to other published SST, HSCT scenarios. This is important when you discuss later the impact of emissions on Ozone and CH₄ lifetime.

Line 260. “For each emitted trace gas (H₂O, NOx, H₂) we have a total of eight simulations, which sum up to 24 simulations in total. The annual magnitude of emitted trace gases is 21.24 teragram of H₂O, 0.031 teragram NO₂ of NOxand 0.236 teragram of H₂.” Question: the scenarios are designed to emit one emission species per simulation. I understand why you chose to emit on species per scenario. However, you might want to mention why did you not combine all the emissions of H₂O, NO₂, and H₂ in one simulation to examine how the chemistry responded (e.g., ozone chemistry)? I do note that later in the paper you did discuss this topic in section 7.3 and 7.4. Possibly point to this section for later discussion.

Figure 3. The color bar goes from -6.0 to >4.8. Question. Why are showing negative colors in your color bar? Seems to me that when you inject H₂O you will always have a positive perturbation?

Figure 4; section 4.1.2.1 Ozone. Why is ozone increasing in region near 10hPa? I.e., is this HOx chemistry interacting with NOx chemistry?

Figure 5: section 4.1.2.3 Methane. I understand why CH₄ is decreased with H₂O emissions – why does it increase? What is the mechanism?

Section 4.4.1. It seems that the perturbations (emissions) are not impacting the CH₄ lifetime in any significant way!

Section 4.4.3.1. This statement is interesting. “The order of magnitude of changes per molecule of emitted species shows that a molecule of H₂ is roughly 50 % as effective in enhancing the mid-atmospheric H₂O concentration as a molecule of emitted H₂O (Tbl. 3).” Can you discuss the chemistry here? I.e., H₂ +OH => H₂O + H. H₂ can also be converted to H and OH (i.e., HOx). The HOx can also be converted back to H₂O.

Section 4.4.3.2. This section and the previous are very informative on the reactivity of emitted H2! Very nice.  “Interestingly, while in absolute values the H₂ emissions are of minor importance to the O₃ depletion, the average effectiveness in destroying O₃ is roughly 5-6 times larger for H₂ than for H₂O (Tbl. 3).”  I.e., H₂ (vs H₂O) has opposite effect on Ozone (compared to H₂O; section 4.4.3.1). I do note that this is partially discussed in section 7.2.

Figure 10 is a very nice summary figure.

This paper presents an assessment of the impacts of H2, H2O and NOx emissions from high-flying hypersonic aircraft. This is based on a series of calculations with a detailed 3D chemistry-climate model which are analysed for the changes in composition and radiative forcing.

This is an interesting topic and well within the scope for ACP. The tools and methodology are entirely appropriate for the problem and there are a number of useful results in this paper. However, in my opinion the paper is not suitable for publication in its current form. The paper is very long and I find the presentation very chaotic. I think the main points could be communicated in much less text and far fewer figures. There are also a number of typos and mistakes in the text which also detract from the overall impression.

Therefore, I think the paper needs major revisions. My main comments are below. Given that my advice is a significant shortening of the paper I have not provided comments and all parts of the text.

Abstract.

I think that the abstract should be more quantitative. It should state that the results are based on 3D CCM simulations. I don’t understand the message in the final sentence.

Introduction.

I cannot see where you define/explain what a hypersonic aircraft is. You need to justify your use of 30 and 38 km for the emissions.

The section numbering in the final paragraph is wrong.

Model Experiments

The model experiments should be given labels, including the new runs for this paper. Sometimes they are referred to as ‘ours’ which is not clear. The text says that all runs use nudged dynamics? Is that the case even for the future composition? The simulation years are also always in the range 2007-2017, or so. This is the case for the future scenarios, yes? Overall there are a number of things to do to make the model simulations clear for the reader.

Satellite Validation

I don’t see how comparing present-day observations with model runs which use future composition adds anything beyond the comparison of the same model with realistic present-day composition. If the only difference in the model is the source gas loadings then one has to say that the present-day model has been evaluated and you have just changed the boundary conditions. If you want to show the impact of the source gas changes in the model then that is a different issue.

SWOOSH may be a long climatology but only 4 years are used (2013-2016). What are the main datasets in that make up SWOOSH in this period? A 40% or so difference in stratospheric water vapour seems very large to me. How does CH4 compare? What about total hydrogen (2CH4 + H2O (+H2?)) Is there an issue with the age of air (too much CH4, too little H2O) at a certain location. I don’t think a 40% error in H2O can be ignored.

Figures

There are too many figures (35!). I know that some are in an Appendix but they are referred to throughout the text as though they are main figures and not supplementary ones. In effect, the reader is reading a paper of 35 figures which is way too long. Some panels in the figures are small with small font size.

Tables

The tables need checking and tidying up. Table 2 is just explaining a legend which appears in later tables without any reference back to Table 2. It would be simpler just to put this code in the heading of e.g. Table 3.  The second ‘Magnitude’ column of Table 1 must also be ‘per year’? Why is there the need for the final three columns of Table 1 – they are all the same.

Results

Please shorten and rationalise the text and figures that are used to communicate the main results. The description of the the modelled changes for different latitudes/altitudes can be covered quite concisely but the relevant mechanisms at work should also be discussed (e.g. HOx, NOx chemistry etc).