Contrail formation for aircraft with hydrogen combustion &ndash; Part 1: A systematic microphysical investigation

Zink, Josef; Unterstrasser, Simon; Burkhardt, Ulrike

doi:10.5194/egusphere-2025-3704

Preprints

https://doi.org/10.5194/egusphere-2025-3704

Preprints

03 Sep 2025

| 03 Sep 2025

Contrail formation for aircraft with hydrogen combustion – Part 1: A systematic microphysical investigation

Josef Zink, Simon Unterstrasser, and Ulrike Burkhardt

Abstract. The number of ice crystals formed during the contrail’s jet phase has a long-lasting impact on the life cycle and radiative forcing of contrail cirrus clouds. Contrail formation for conventional kerosene combustion is well studied, and suitable parametrizations for the early ice crystal number have been used to estimate the climate impact of contrail cirrus with a general circulation model. However, a parametrization for the number of ice crystals formed is lacking for hydrogen combustion. To develop such a parametrization, we present a comprehensive set of contrail formation simulations using the particle-based Lagrangian Cloud Module in a box model approach. Unlike kerosene combustion, no soot particles are emitted in the hydrogen combustion case. Thus, ice crystal formation is assumed to occur on ambient aerosols entrained into the exhaust plume. The results show that coarse mode particles have negligible influence on ice crystal number due to their low abundance. Furthermore, ice crystal formation involving multiple co-existing aerosol populations with different properties (number, size, solubility) can be reconstructed from simulations involving single aerosol populations. We also identify atmospheric conditions where homogeneous droplet nucleation can be safely neglected as potential ice formation pathway. Based on more than 20,000 simulations covering a broad range of atmospheric conditions and aerosol properties, we identify a regime where ice crystal formation becomes nearly independent of ambient relative humidity, aerosol size, and solubility. Our results provide a basis for a data-driven parametrization of ice crystal number in contrails from hydrogen combustion, to be presented in a companion paper.

Received: 30 Jul 2025 – Discussion started: 03 Sep 2025

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

Download & links

Preprint (PDF, 807 KB)

Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
Preprint (807 KB)

Download & links

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Journal article(s) based on this preprint

02 Mar 2026

Contrail formation for aircraft with hydrogen combustion – Part 1: A systematic microphysical investigation

Josef Zink, Simon Unterstrasser, and Ulrike Burkhardt

Atmos. Chem. Phys., 26, 3125–3143, https://doi.org/10.5194/acp-26-3125-2026,https://doi.org/10.5194/acp-26-3125-2026, 2026

Short summary

Josef Zink, Simon Unterstrasser, and Ulrike Burkhardt

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-3704', Anonymous Referee #1, 14 Oct 2025

General comments:
In this manuscript, the authors use box-model simulations to estimate ice crystal number concentrations at the end of the jet regime. These are performed for hydrogen combustion and therefore consider only the entrainment of ambient particles (no exhaust emissions). The authors derive conditions under which homogeneous droplet nucleation may compete with heterogeneous droplet formation (on (entrained) particles). They also conclude that the ambient particle size distribution can be treated using a weighted mean approach. Finally, they perform several sensitivity analyses.
Overall, I find this manuscript provides a clear overview of the key considerations for simulating contrail formation from hydrogen exhausts. It is particularly useful to see an assessment of homogeneous versus heterogeneous droplet formation. However, I find that the manuscript would benefit from being restructured, to highlight the main results and differentiate these from observations about the model. Other specific comments are listed below. Therefore, I would recommend publication after the below comments have been suitably addressed.
Specific comments (in form: location-comment):
Overall structure: the conclusions drawn in Sect. 5 of the manuscript are useful and well-defined. I find that there are two main results: (a) the derivation of a conservative boundary for the importance of homogeneous nucleation of water droplets and (b) ambient aerosol properties and/or meteorological conditions that result in asymptotic ice crystal number concentrations.
In addition, the authors show that (c) omission of the coarse-mode population of ambient particles has a negligible impact on model outputs and (d) they can reduce model compute by applying a weighted mean as in Eq. (3). Although the latter results are undeniably useful for reducing the model complexity and associated simulation times, the manuscript does not clearly motivate whether these are as useful to the wider community as (a) and (b). Therefore, I would recommend that either (i) the authors move (c) and (d) to a methodology/model development section and present (a) and (b) as final results or (ii) provide more justification for the wider applicability and limitations of (c) and (d) and retain these in a results section.
Moreover, I suggest that after restructuring/modifying as above, the main results are presented under a “results” heading rather than “microphysical insights”. Currently, the model development is difficult to disentangle from the results. Accordingly, adopting these changes would more effectively highlight the most applicable results of the study.
Homogeneous droplet nucleation: using information in Figure. 6 and Table. 3, it appears that some simulations may be performed under conditions where homogeneous droplet nucleation competes with heterogeneous droplet formation on entrained ambient particles (i.e., p_a = 400 hPa and T_a = 210 K). However, it is not clear whether homogeneous droplet nucleation is included in the simulations in Sect. 4. Therefore, I would suggest that the authors provide more information on this.
Data availability: I would recommend that the authors consider putting figure data and/or underlying code in a public repository (e.g., Zenodo) rather than only being obtainable from the corresponding author.
Various:
Line 33: this statement would benefit from additional references in support of hydrogen use in aviation. Currently, I find that the reference from Airbus is insufficient to motivate the potential for hydrogen uptake in this sector.
Lines 97 and 100: while the simulations performed by Yu et al., (2024) show the importance of fuel sulfur, the mass of volatile particles is a sum over the masses of all condensable gaseous species, which also include other organic material (Kärcher et al., 2000).
Line 254: this is an interesting observation and would benefit from a fuller explanation. Specifically, why is this the case provided aerosol is entrained and not emitted?
Lines 255-262: It is unclear what the total particle size distribution is being used in these simulations. Are the three modes described using a common total hygroscopicity parameter? If so, presumably it would be possible to represent this trimodal distribution as the linear combination of three monomodal particle size distributions.
Line 359: as some readers will not be familiar with the Koop parameterization, I would suggest that you explain the link between water-supersaturated conditions and the formation of liquid clouds more explicitly.
Line 381: although the temperature dependence is limited for soot-rich exhausts, more recent work (incorporating volatile particles) has shown that ambient temperature regulates contrail ice crystal number concentrations in the soot-poor regime (Yu et al., 2024).
Line 405: this discussion would benefit from additional (supplementary) information on the choice of freezing parameterization adopted in this work.
Figure 8: I would advise clarifying (in both the figure and the main text) that the different colours are being used to represent different co-parameters and that respective ranges are taken from Table. 3.
Minor comments:
Line 13: using the results in Figure. 9, I would advise providing some quantitative information in the abstract to bound this regime (e.g., the information in line 409).
Line 31: I would advise using the term “flight distance” rather than “range”.
Line 73: I would suggest revising the phrasing: “ready to be implemented”.
Lines 77-78 (and Figure 1 caption): I would suggest revising the order of “plume partial water vapor pressure” to “partial pressure of water vapor in the plume”.
Line 80: I would suggest expanding on the significance of “lower calorific value” in this context or removing altogether.
Line 88: I would suggest clarifying that this is only true if the ambient conditions are the same in both cases.
Lines 91-92: I think it is more important to mention that ambient upper-tropospheric temperatures are usually below the Schmidt-Appleman temperature for hydrogen combustion. The comparison with the homogeneous freezing threshold is of secondary importance.
Lines 93-94: I would suggest expanding on why the maximum plume supersaturation is important for the process of contrail formation.
Line 143: I would suggest expanding on the meaning of the term “simple enough”. For instance, does this refer to total computation time?
Line 148: I would suggest revising this sentence for clarity.
Line 175: I would advise defining the meaning of “phase” in this context.
Line 221: I would suggest replacing “spectrum” with “particle size distribution”.
Figure 2: please could you expand (in the text) on why N_{ice, f} shows a slight decrease for large x-values rather than asymptotic behaviour.
Line 274: it would be useful to clarify the units of J_aer(t) in Equation (4).
Line 374: I would suggest introducing a reference for the cutoff at 235 K. Alternatively, this could be enveloped in a supplemental section that outlines your homogeneous ice nucleation parameterization (as described earlier).
Lines 350-354: I would suggest explaining this caveat in more detail as the implication (of the final sentence of this paragraph) is unclear to me.
Technical suggestions (in form: location-comment):
Line 2: “well studied” should be hyphenated.
Line 60: I would suggest replacing “give meteorological condition” with “given set of meteorological conditions”.
Line 63: “globally distributed” should be hyphenated.
Line 76: there is a missing “the” in “(called the plume)”.
Line 77: “Micro-physical” should not be hyphenated.
Lines 77-78 (and Figure 1 caption): I would suggest revising the definition “plume partial water vapor pressure” as “partial pressure of water vapor in the plume”.
Line 121 (and line 210): I would suggest replacing “well” with “highly” and removing the hyphen.
Line 155: I would suggest revising the word order, particularly the use of “used”.
Line 176: “analytically prescribed” should be hyphenated.
Line 178: there is a typographic error in this sentence between “calculated” and “time-resolved”.
Line 205: I would suggest removing “already”.
Line 265: I would suggest rephrasing the “curse of dimensionality”.
References:
Kärcher, B., Turco, R. P., Yu, F., Danilin, M. Y., Weisenstein, D. K., Miake‐Lye, R. C., and Busen, R.: A unified model for ultrafine aircraft particle emissions, J. Geophys. Res., 105, 29379–29386, https://doi.org/10.1029/2000JD900531, 2000.
Yu, F., Kärcher, B., and Anderson, B. E.: Revisiting Contrail Ice Formation: Impact of Primary Soot Particle Sizes and Contribution of Volatile Particles, Environ. Sci. Technol., 58, 17650–17660, https://doi.org/10.1021/acs.est.4c04340, 2024

Citation: https://doi.org/10.5194/egusphere-2025-3704-RC1
- AC1: 'Comment on egusphere-2025-3704', Josef Zink, 19 Dec 2025
  
  Reply to the Editor and the two reviewers in one combined document.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3704-AC1
RC2:
'Comment on egusphere-2025-3704', Anonymous Referee #2, 25 Nov 2025

see the attachment

Citation: https://doi.org/10.5194/egusphere-2025-3704-RC2
- AC1: 'Comment on egusphere-2025-3704', Josef Zink, 19 Dec 2025
  
  Reply to the Editor and the two reviewers in one combined document.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3704-AC1
AC1: 'Comment on egusphere-2025-3704', Josef Zink, 19 Dec 2025

Reply to the Editor and the two reviewers in one combined document.

Citation: https://doi.org/10.5194/egusphere-2025-3704-AC1

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-3704', Anonymous Referee #1, 14 Oct 2025

General comments:
In this manuscript, the authors use box-model simulations to estimate ice crystal number concentrations at the end of the jet regime. These are performed for hydrogen combustion and therefore consider only the entrainment of ambient particles (no exhaust emissions). The authors derive conditions under which homogeneous droplet nucleation may compete with heterogeneous droplet formation (on (entrained) particles). They also conclude that the ambient particle size distribution can be treated using a weighted mean approach. Finally, they perform several sensitivity analyses.
Overall, I find this manuscript provides a clear overview of the key considerations for simulating contrail formation from hydrogen exhausts. It is particularly useful to see an assessment of homogeneous versus heterogeneous droplet formation. However, I find that the manuscript would benefit from being restructured, to highlight the main results and differentiate these from observations about the model. Other specific comments are listed below. Therefore, I would recommend publication after the below comments have been suitably addressed.
Specific comments (in form: location-comment):
Overall structure: the conclusions drawn in Sect. 5 of the manuscript are useful and well-defined. I find that there are two main results: (a) the derivation of a conservative boundary for the importance of homogeneous nucleation of water droplets and (b) ambient aerosol properties and/or meteorological conditions that result in asymptotic ice crystal number concentrations.
In addition, the authors show that (c) omission of the coarse-mode population of ambient particles has a negligible impact on model outputs and (d) they can reduce model compute by applying a weighted mean as in Eq. (3). Although the latter results are undeniably useful for reducing the model complexity and associated simulation times, the manuscript does not clearly motivate whether these are as useful to the wider community as (a) and (b). Therefore, I would recommend that either (i) the authors move (c) and (d) to a methodology/model development section and present (a) and (b) as final results or (ii) provide more justification for the wider applicability and limitations of (c) and (d) and retain these in a results section.
Moreover, I suggest that after restructuring/modifying as above, the main results are presented under a “results” heading rather than “microphysical insights”. Currently, the model development is difficult to disentangle from the results. Accordingly, adopting these changes would more effectively highlight the most applicable results of the study.
Homogeneous droplet nucleation: using information in Figure. 6 and Table. 3, it appears that some simulations may be performed under conditions where homogeneous droplet nucleation competes with heterogeneous droplet formation on entrained ambient particles (i.e., p_a = 400 hPa and T_a = 210 K). However, it is not clear whether homogeneous droplet nucleation is included in the simulations in Sect. 4. Therefore, I would suggest that the authors provide more information on this.
Data availability: I would recommend that the authors consider putting figure data and/or underlying code in a public repository (e.g., Zenodo) rather than only being obtainable from the corresponding author.
Various:
Line 33: this statement would benefit from additional references in support of hydrogen use in aviation. Currently, I find that the reference from Airbus is insufficient to motivate the potential for hydrogen uptake in this sector.
Lines 97 and 100: while the simulations performed by Yu et al., (2024) show the importance of fuel sulfur, the mass of volatile particles is a sum over the masses of all condensable gaseous species, which also include other organic material (Kärcher et al., 2000).
Line 254: this is an interesting observation and would benefit from a fuller explanation. Specifically, why is this the case provided aerosol is entrained and not emitted?
Lines 255-262: It is unclear what the total particle size distribution is being used in these simulations. Are the three modes described using a common total hygroscopicity parameter? If so, presumably it would be possible to represent this trimodal distribution as the linear combination of three monomodal particle size distributions.
Line 359: as some readers will not be familiar with the Koop parameterization, I would suggest that you explain the link between water-supersaturated conditions and the formation of liquid clouds more explicitly.
Line 381: although the temperature dependence is limited for soot-rich exhausts, more recent work (incorporating volatile particles) has shown that ambient temperature regulates contrail ice crystal number concentrations in the soot-poor regime (Yu et al., 2024).
Line 405: this discussion would benefit from additional (supplementary) information on the choice of freezing parameterization adopted in this work.
Figure 8: I would advise clarifying (in both the figure and the main text) that the different colours are being used to represent different co-parameters and that respective ranges are taken from Table. 3.
Minor comments:
Line 13: using the results in Figure. 9, I would advise providing some quantitative information in the abstract to bound this regime (e.g., the information in line 409).
Line 31: I would advise using the term “flight distance” rather than “range”.
Line 73: I would suggest revising the phrasing: “ready to be implemented”.
Lines 77-78 (and Figure 1 caption): I would suggest revising the order of “plume partial water vapor pressure” to “partial pressure of water vapor in the plume”.
Line 80: I would suggest expanding on the significance of “lower calorific value” in this context or removing altogether.
Line 88: I would suggest clarifying that this is only true if the ambient conditions are the same in both cases.
Lines 91-92: I think it is more important to mention that ambient upper-tropospheric temperatures are usually below the Schmidt-Appleman temperature for hydrogen combustion. The comparison with the homogeneous freezing threshold is of secondary importance.
Lines 93-94: I would suggest expanding on why the maximum plume supersaturation is important for the process of contrail formation.
Line 143: I would suggest expanding on the meaning of the term “simple enough”. For instance, does this refer to total computation time?
Line 148: I would suggest revising this sentence for clarity.
Line 175: I would advise defining the meaning of “phase” in this context.
Line 221: I would suggest replacing “spectrum” with “particle size distribution”.
Figure 2: please could you expand (in the text) on why N_{ice, f} shows a slight decrease for large x-values rather than asymptotic behaviour.
Line 274: it would be useful to clarify the units of J_aer(t) in Equation (4).
Line 374: I would suggest introducing a reference for the cutoff at 235 K. Alternatively, this could be enveloped in a supplemental section that outlines your homogeneous ice nucleation parameterization (as described earlier).
Lines 350-354: I would suggest explaining this caveat in more detail as the implication (of the final sentence of this paragraph) is unclear to me.
Technical suggestions (in form: location-comment):
Line 2: “well studied” should be hyphenated.
Line 60: I would suggest replacing “give meteorological condition” with “given set of meteorological conditions”.
Line 63: “globally distributed” should be hyphenated.
Line 76: there is a missing “the” in “(called the plume)”.
Line 77: “Micro-physical” should not be hyphenated.
Lines 77-78 (and Figure 1 caption): I would suggest revising the definition “plume partial water vapor pressure” as “partial pressure of water vapor in the plume”.
Line 121 (and line 210): I would suggest replacing “well” with “highly” and removing the hyphen.
Line 155: I would suggest revising the word order, particularly the use of “used”.
Line 176: “analytically prescribed” should be hyphenated.
Line 178: there is a typographic error in this sentence between “calculated” and “time-resolved”.
Line 205: I would suggest removing “already”.
Line 265: I would suggest rephrasing the “curse of dimensionality”.
References:
Kärcher, B., Turco, R. P., Yu, F., Danilin, M. Y., Weisenstein, D. K., Miake‐Lye, R. C., and Busen, R.: A unified model for ultrafine aircraft particle emissions, J. Geophys. Res., 105, 29379–29386, https://doi.org/10.1029/2000JD900531, 2000.
Yu, F., Kärcher, B., and Anderson, B. E.: Revisiting Contrail Ice Formation: Impact of Primary Soot Particle Sizes and Contribution of Volatile Particles, Environ. Sci. Technol., 58, 17650–17660, https://doi.org/10.1021/acs.est.4c04340, 2024

Citation: https://doi.org/10.5194/egusphere-2025-3704-RC1
- AC1: 'Comment on egusphere-2025-3704', Josef Zink, 19 Dec 2025
  
  Reply to the Editor and the two reviewers in one combined document.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3704-AC1
RC2:
'Comment on egusphere-2025-3704', Anonymous Referee #2, 25 Nov 2025

see the attachment

Citation: https://doi.org/10.5194/egusphere-2025-3704-RC2
- AC1: 'Comment on egusphere-2025-3704', Josef Zink, 19 Dec 2025
  
  Reply to the Editor and the two reviewers in one combined document.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3704-AC1
AC1: 'Comment on egusphere-2025-3704', Josef Zink, 19 Dec 2025

Reply to the Editor and the two reviewers in one combined document.

Citation: https://doi.org/10.5194/egusphere-2025-3704-AC1

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Josef Zink on behalf of the Authors (19 Dec 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (29 Dec 2025) by Carsten Warneke

RR by Anonymous Referee #1 (15 Jan 2026)

Suggestions for revision or reasons for rejection

General comments:

I would like to thank the authors for taking time to carefully address each of my specific comments in detail. Concerning my general comments, the authors have opted to restructure the manuscript by relocating various parts into a dedicated results section. This now more clearly differentiates the model description from the findings obtained using the model and is an effective change.
Nevertheless, I have one additional (general) suggestion that I believe would further improve the manuscript which concerns the presentation of the results.

To be most useful to the wider scientific community, I find that the most applicable findings could be stated more clearly in the results section. As an example, my understanding is that the key result in Sects. 3.11 and 3.12 is that (when simulating contrails formed by hydrogen combustion) the complexity in the prescribed ambient aerosol particle size distribution can be reduced straightforwardly by: (a) neglecting contributions from coarse mode aerosol and (b) preparing estimated ice crystal numbers using a weighted mean approach. These are very useful observations for future modelling studies, however I find that while these are justified at length in the text, their general applicability is not emphasized in the discussion/results section.

Note that while I have chosen the case of Sects. 3.11 and 3.12, I find that the same is true of the results presented in Sect 3.2 and 3.2.2. Therefore, I would advise that the authors consider: (i) placing a sentence at the end of each results section with the key result and (ii) emphasizing (potentially in the conclusions) that their findings are more generally applicable to all contrail simulations for hydrogen combustion (see the bullet points in Sect. 5), if this is indeed the case.

Specific comments:

Line 44: although the authors have made efforts to justify this sentence by introducing several additional references in lines 32 – 38, I find that the phrasing of line 44 is still too definitive. Specifically, use of the term: “great potential”.

Line 59: I would suggest that the authors clarify whether they are referring to the depth/vertical extent of the contrail when using the term “vertical direction”.

Line 132: Note that theta (G) is used by e.g., Kärcher et al., 2015 and Ponsonby et al., 2025, to represent the temperature at which the mixing line contacts (tangentially) the saturation vapour pressure above water. Theta (RH) is then used by Kärcher et al., 2015, for the ambient temperature at which this occurs. The use of symbols in this manuscript is instead aligned with Bier et al., 2024. Although this is entirely valid, I recommend that the authors introduce a section on notation/variable use to prevent any potential confusion that may arise with this choice of naming convention.

Lines 443 – 446: I appreciate that the authors have introduced this comment to clarify my previous question. However, I am still unclear why the authors choose to present these results if their earlier analysis implies that they cannot confidently rule out homogeneous droplet nucleation. I suggest that the authors consider removing these data points altogether when presenting results in Fig. 9 and in the later discussion. This change would also strengthen the narrative of the article, since the findings in Sect. 3.2 (the boundary outside which homogeneous droplet nucleation is unexpected to contribute) would then directly impact the the choice of parameter space for model initialization (Table. 2).

Lines 511 and 513: I suggest that the authors reference recent in-situ measurements performed behind a hydrogen combustor during the Blue Condor campaign and address any statements concerning “validation”.

References:

Bier, A., Unterstrasser, S., Zink, J., Hillenbrand, D., Jurkat-Witschas, T., and Lottermoser, A.: Contrail formation on ambient aerosol particles for aircraft with hydrogen combustion: a box model trajectory study, Atmos. Chem. Phys., 24, 2319–2344, https://doi.org/10.5194/acp-24-2319-2024, 2024.

Kärcher, B., Burkhardt, U., Bier, A., Bock, L., and Ford, I. J.: The microphysical pathway to contrail formation, JGR Atmospheres, 120, 7893–7927, https://doi.org/10.1002/2015JD023491, 2015.

Ponsonby, J., Teoh, R., Kärcher, B., and Stettler, M. E. J.: An updated microphysical model for particle activation in contrails: the role of volatile plume particles, Atmos. Chem. Phys., 25, 18617–18637, https://doi.org/10.5194/acp-25-18617-2025, 2025.

Hide

RR by Anonymous Referee #3 (05 Feb 2026)

Suggestions for revision or reasons for rejection

The manuscript explores simulated contrail properties relevant for a no-particle-emitting engine that emits a water vapor plume that mixes with ambient aerosol-laden air. The water vapor can then either condense heterogeneously on these ambient particles or homogeneously nucleate liquid water droplets (HDN). Both processes form liquid droplets that then freeze homogeneously to form contrail ice crystals. The simulations are interesting and well thought out. The results of the study show that ambient temperature and aerosol number concentration are the drivers of contrail ice crystal number, which is not particularly surprising. The scientific value of the paper comes from insights of how the plume maximum supersaturation might vary with ambient particle microphysical properties for the limiting case of a no-particle-emitting aircraft engine. Overall, the manuscript is well written and appropriate for ACP. My major concern relates to the assumption that aircraft with hydrogen-powered gas turbines do not emit any aerosols that contribute to contrail formation, and I'd recommend that 2 scenarios be included that account for near-field oil emissions alongside ambient aerosols as potential cloud condensation nuclei for contrail formation.

Specific comments:

1) Hydrogen-powered gas turbines require lubrication oil that has been shown to form considerable numbers of particles (US NASEM, 2025; Decker, 2024; Yu et al., 2010; Yu et al., 2012). The size of these oil particle depends on how they are emitted from the engine. For example, recent testing with low-soot CFM engines that vent oil vapor at same location as the hot combustor core flow indicate that the oil vaporizes and subsequently forms new particles of several nanometers in diameter. Other engines vent the oil vapor in the engine bypass or outside the nacelle, where there is likely to be a mixture of primary-emitted Aitken- or accumulation-size-mode droplets and semi-volatile vapor that can subsequently condense on existing particles or form new particles. While the influence of future engineering design choices makes the particle emissions characteristics of future hydrogen-burning gas turbine engine highly uncertain, it seems unlikely to me that these engines will be truly no-particle-emitting as is explored in this study.

I recommend that two additional scenarios be explored that consider how continuous co-emission of semi-volatile particles, for example:
-- Case 1: A hydrogen gas turbine engine with breather vent location at the engine tail cone (like a CFM engine), where the hot oil vapor nucleates a 10-nm-diameter size mode that mixes with the water vapor plume
-- Case 2: A hydrogen gas turbine engine with breather vent location outside the engine nacelle (like a Rolls-Royce engine), where primary emissions of 100-nm size mode oil droplets mixes with the water vapor plume

I think it will be really interesting to see if the co-emission of even modest numbers of volatile particles are enough to completely dwarf the contribution of background aerosols.

Alternatively, the title of the paper could be changed to remove references to hydrogen combustion and instead focus solely on the idealized case of a theoretical no-particle-emitting engine.

2) Line 10 and Line 486: The word "solubility" is used here, but the model (and elsewhere in the manuscript) focuses on hygroscopicity.

3) Line 138: It's not just that these particle are abundant at cruise altitude, but that they are co-emitted with the water vapor and mix into the early engine emissions plume where the contrail forms.

Lubrication Oil References:
US NASEM, 2025, https://doi.org/10.17226/29073 (pg. 28)
Decker et al., 2024, https://pubs.acs.org/doi/10.1021/acsestair.4c00184
Yu et al., 2012, https://pubs.acs.org/doi/10.1021/es301692t
Yu et al., 2010, https://pubs.acs.org/doi/10.1021/es102145z

Hide

ED: Publish subject to minor revisions (review by editor) (05 Feb 2026) by Carsten Warneke

AR by Josef Zink on behalf of the Authors (12 Feb 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (14 Feb 2026) by Carsten Warneke

AR by Josef Zink on behalf of the Authors (15 Feb 2026)

Journal article(s) based on this preprint

02 Mar 2026

Contrail formation for aircraft with hydrogen combustion – Part 1: A systematic microphysical investigation

Josef Zink, Simon Unterstrasser, and Ulrike Burkhardt

Atmos. Chem. Phys., 26, 3125–3143, https://doi.org/10.5194/acp-26-3125-2026,https://doi.org/10.5194/acp-26-3125-2026, 2026

Short summary

Josef Zink, Simon Unterstrasser, and Ulrike Burkhardt

Viewed

Total article views: 1,665 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	BibTeX	EndNote
1,451	173	41	1,665	32	36

HTML: 1,451
PDF: 173
XML: 41
Total: 1,665
BibTeX: 32
EndNote: 36

Views and downloads (calculated since 03 Sep 2025)

Month	HTML	PDF	XML	Total
Sep 2025	1,220	18	5	1,243
Oct 2025	60	24	8	92
Nov 2025	48	31	10	89
Dec 2025	41	26	8	75
Jan 2026	38	40	4	82
Feb 2026	41	33	6	80
Mar 2026	3	1	0	4

Cumulative views and downloads (calculated since 03 Sep 2025)

Month	HTML	PDF	XML	Total
Sep 2025	1,220	18	5	1,243
Oct 2025	60	24	8	92
Nov 2025	48	31	10	89
Dec 2025	41	26	8	75
Jan 2026	38	40	4	82
Feb 2026	41	33	6	80
Mar 2026	3	1	0	4

Viewed (geographical distribution)

Total article views: 1,809 (including HTML, PDF, and XML) Thereof 1,809 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 03 Mar 2026

Download

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Preprint (807 KB)
Metadata XML

Short summary

The climate impact of aviation-induced contrail cirrus clouds is strongly influenced by the number of ice crystals that form in the wake of an aircraft under certain conditions. In this study, we systematically investigate the key processes that influence the number of ice crystals formed for hydrogen combustion. A large simulation data set provides the basis for integrating our results into large-scale models used to estimate the climate impact of contrail cirrus clouds.


Total:	0
HTML:	0
PDF:	0
XML:	0