General review

The authors present an opinion paper on “eliminating aircraft soot emissions”

According to the abstract the authors aims to give a review of existing technologies to identify strategies to eliminate soot emissions. From my point of view, they have not succeeded to accomplish this aim. There are some important technologies missing from the article, and the actual technologies leading the soot emission reduction are only mentioned briefly and for some reason in the description seems that these technologies  are somehow still on a development phase.

Overall the structure is correct but, in some points there are some repetitions or text that seems to be out of place. For example, in introduction, the last paragraph seems quite out of place.

Detalied review:

Introduction

Lines 22-25, you state that aviation has a significant impact on health and climate due mainly to soot, but then you give two citations that deal with soot in general. Indeed, the review from Niranjan and Thakur mainly compile works on synthetic soot like printex and some on diesel soot, but not aeronautic soot. There is not too many works dealing with health effects of aircraft emissions, but still there are some that are much more relevant for the context than the review paper cited  (for example: Bendtsen et al. 2019 https://doi.org/10.1186/s12989-019-0305-5, Delaval et al. 2022 https://doi.org/10.1016/j.envpol.2022.119521)

Lines 27-29 you are mistaking jet aircraft smoke visibility and smoke number. Both can be linked (Slusher 1971 FAA report FAA-RD-71-23) but the smoke number is defined as the loss of reflectance of a filter used to trap smoke particles from a prescribed mass of exhaust per unit area of filter (SAE ARP 1179).

Lines 64-65, why do you give a citation about light-duty diesel engine to illustrate soot NOx trade off in aircraft engines?

Lines 67-69, I would like to see a reference about the uncertainties of soot contribution to climate in global models being due to the simplification of soot morphology

Lines 92-99, you may want to have a look to this paper in ACPD https://doi.org/10.5194/egusphere-2023-2441

Lines 107-110 the definition of nvPM should have ben given at the beginning of the introduction

Line 111, the discussion about >26.7 KN engines could be moved to line 32

Lines 116-118, what Avgas have to do with soot? Lead is a contaminant strictly linked to the fuel, I do not see the link with soot reduction or combustor technologies.

Section 3.1

Frist of all, be careful with the term alternative fuel, not all alternative fuels are SAF. I am surprised by not seeing any references to CORSIA criteria when speaking about SAF.

Lines 232-233 No, alternative fuels can not be used as drop-in fuels. They can only be used after blending with jet fuel up to the mixing ratios defined by ASTM for each fuel.  There are two alternative fuels which have started the procedure to be certified for their use as 100% drop-in fuel, CHJ and FT-SPK-A but they are not certified yet.

238-239 No, LCAF are defined as: A fossil-based aviation fuel that meets the CORSIA  Sustainability Criteria under this Volume (ICAO Annex 16 Vol IV), so indeed LCAF are fossil fuels that manage to reduce their lifecycle emissions by 10% compared to the baseline of 89 gCO2e/MJ

Line 258 that is the actual blend limit for HEFA not for SAF, other SAF are certified for much lower blend ratio, for example SIP is certified just up to 10% blend ratio.

Line 290, I am missing references to on-going project like ECLIF, EcoDemonstrator or VOLCAN, where in flight measurements has been done even with 100% SAF

Lines 304-307, Fit for 55 and ReFuelEU aviation target for a share of SAF in the annual fuel used, not in the blends. Indeed, how things work actually is just as the author describes, the blends used in airports are normally blends of around 30% SAF. It also has to be noted that even if the EU fix a minimum amount of SAF share by year, industry have proposed higher SAF shares, for example Airbus have set as an objective to use 10% SAF share for 2023 for internal movements and 5% for external movements, and again this is done by using blends between 30 and 49% of SAF for some flights, and not blends 1% of SAF for all flights. So last sentence of the paragraph (306-307) are incorrect and must be removed.

After reading SAF section I have the feeling that the authors are not familiar with aeronautic SAF. There are too many wrong or misleading information.

Section 3.2

Overall all this section is too naïf to be a review of existing technology, only speak of general combustor concepts, but there is no mention of different technologies, just as an example, there are different RQL implementations (LEC, TALON …) and each of this have their own specificities. I am missing also TAPS. Regarding technologies in development, they only mention LEAF, while there are other technologies that are much more advance in development, for example WET engines that will be tested on flight in the near future. Also developments like open-rotor engines might be included, thought it is not directly linked to combustor optimization, but again this technologies will be tested on flight soon while LEAF is at laboratory level demonstration.

Lines 327-329 results shown in Kelesidis et al. 2023 has been obtained with a laboratory combustor, that has nothing to do with a real aircraft combustor, despite they might obtain soot with similar properties that soot obtained in some engine regimes, results about oxygen injection cannot be transferred to a real engine.

Line 408 Emission index is defined as emissions by kg of burned fuel.

Lines 412-414, I wondering how the authors have assigned different engines to different technologies. I do not see where they have obtained LDI engine data, or LPP, are they assuming that TAPS is equivalent to LPP? according with line 416 you include in your graph 27 LPP engines, in the data base there is data for 26 TAPS and TAPS II engines, so seems so (though I don’t know what is the extra engine you consider LPP to sum up the 27 points you use),   Furthermore, what is the point of comparing old SAC engines with new SAC-TI engines? The authors claim that RQL combustors have a large variation, and that this is most likely due there is more data entries for RQL… no, this is due to the fact that you are comparing technologies based on RQL concept but that are differently implemented and also that you are probably including out of production engines (ICAO data base contain data for 215 engines, 43 of those are out of production, your graph includes 208 points).

Line 440-441 As this is written I interpret that you say that engine manufacturers manipulate the data included in ICAO data base, what is unacceptable , this is just a plain offense to ICAO, SAE-E31 and EASA.

Line 446-448 again authors are forgetting a large number of on-going projects linked to these engines. The end of this sentence “ … if they will be adopted in the future”  just illustrate the lack of understanding of the authors in the aeronautic field. LEAP and GEnx engine has been in the market for several years, actually there are almost 10k on service, and if you check the orders of engines only for LEAP engines, this indicates an annual production for the next year of over 2000 engine per year.

Line 450-452 Again an other sentence that indicates that the authors are not familiar with the engine development. CFD combustion models are used in the development of engines, but indeed they represent only a small part on the development, there are several test of different injector configurations, combustion chamber geometries etc, so I do not see the point of the authors here.

Conclusion

I have the feeling that authors are neglecting the potential of engines based on TAPS combustor to reduce both NOx and soot emissions. Indeed, above 30% of engine thrust soot emissions of these engines are close to be under the detection limit of the instruments. The main conclusion seem to be that the solution for eliminate soot from aircraft emissions is the development of CFD models able to predict soot production. This will be of course useful, but I do not think is something realistic. Further more actual models used in engine development are able to give at least order of magnitude values for soot production, and in any case all engine development goes through an experimental optimization of injectors and combustion chamber.

I do not see that this article bring something new to the field, even as a review there are many things missing. For me the question right now in the field  is not so much to reduce further soot emission from actual engines ( at least for thrust > 30%) but, what could be other sources of particulate matter once the soot emissions are reduced at ambient background levels and how contrail formation can be affected. Other interesting topic can be if is worthy to produce Lean burn engines or SAF + rich burn engines can achive  similar reductions on nvPM emissions.

General:

This opinion paper addresses the issue of soot emissions from aircraft engines, which is a significant contributor to air pollution and climate change. The authors review current strategies and technologies for reducing soot emissions, including combustor designs, the use of alternative fuels (SAF), and discussing trade-offs with NOx. They also highlight the challenges in accurately modelling and measuring soot emissions, which complicates efforts to design effective emission reduction strategies.

The paper is well-written with good discussions on soot formation and aircraft combustors. It is of scientific interest and makes some interesting points particularly regarding the targeted approach for SAF usage. I have a few minor comments and suggestions for the authors to consider:

Main comments:

• While the perceived benefits from SAF and of a targeted approach for its implementation are discussed in detail, I’d like to point out that because SAF can currently only be blended at up to 50%, the soot emissions from the SAF blend are ultimately driven by the jet-A it’s being blended with. By that, I mean that the soot emissions from a relatively clean jet-A (fuel hydrogen content > 14.2% for example) might be lower than that of a dirty jet-A (fuel hydrogen content <13.2% for example) blended with SAF because ultimately, it’s the fuel hydrogen content that correlates with the soot emissions. I thought this could be something you could discuss in your paper.
• While the main topic of your paper is soot emissions, there is no mention of other PM emissions from aircraft engines such as lubrication oil, volatile PM, secondary organic aerosol (SOA) which could also significantly contribute to air pollution and climate change by either coating soot or forming new particles in the plume. Some of these emissions, such as sulphates particles, are dependent on fuel properties and therefore could be reduced with SAF. I suggest discussing this as well in your manuscript.
• While you introduced and discussed EC/OC from aircraft soot, I didn’t understand from your manuscript how this parameter was relevant to health and the environment. Why is EC/OC important in this context (e.g., tracks combustion efficiency, EC warming effect, OC contribute to SOA)?

Minor comments:

• Line 3: You define “Nitrogen oxides (NOx)” but not “CO2” (carbon dioxide) – I suggest being consistent.
• Line 8: the term “mobility” is not introduced or discussed in the manuscript. You actually refer here to “electrical mobility” (as measured by SMPSs, DMS500, EEPS, etc) which is an equivalent diameter and I believe this should be discussed, particularly in regards with modelling, which maybe considers a different particle equivalent diameter (e.g., aerodynamic diameter) which could contribute to uncertainty.
• Line 9: what do you mean by “quite high” – please quantify or use the word “relatively” instead.
• Line 72: typo (return to line).
• Line 81 (figure title): “broken line”: Do you mean red dashed line? Also, primary particles are solid black lines, not red.
• Line 128: Can you discuss in more details how OC/TC influences the environment (see comment above).
• d_p: To my knowledge this abbreviation typically refers to particle diameter and not primary particle size, I suggest replacing it by d_pps or d_pp throughout the manuscript to prevent confusion.
• L194: replace “poor” by “poorer”.
• L255, 279: Other more recent scientific literature has correlated soot emissions with fuel hydrogen content (e.g., https://doi.org/10.1016/j.fuel.2020.119637, https://doi.org/10.1016/j.fuel.2021.123045, https://doi.org/10.1016/j.fuel.2019.115903), I suggest adding them where relevant.
• L466: Explain why EC/OC matters.
• L474/475: this sentence says that soot emissions are reduced with lower H/C content when its reality it’s the other way around (higher H/C ratios result in lower soot emissions). Please rephrase.