Stratospheric Chemical Lifetime of Aviation Fuel Incomplete Combustion Products

Round 1

Reviewer 1 Report

The authors here suggest that there are up to over 20 compounds emitted by aircraft engines which could have a significant lifetime in the stratosphere and impact stratospheric aerosol with implications for the impact of these aerosol on climate.

The paper is publishable, but not without addressing some major issues.

• No attempt was made to consider whether the compounds with the longest lifetimes and abundances considered here could be, or have been measured, in the stratosphere. In the late 1990s and early 2000s there were a number of field programs to follow commercial aircraft with instrumented aircraft to measure the chemistry of the commercial aircraft exhaust. The European Commission funded several major studies on this topic and, as I recall, most of them did not conclude that the a/c exhaust was of a level to cause a global impact. The authors need to review some of this literature and summarize it. The references used here related to this topic, near the end of the paper, all preceded most of this later work, leaving this paper not conversant with the recent literature on the measurements of these compounds. Here are just a couple of such references:

Jurkat et al., 2011, GRL, 38, L10807, doi:10.1029/2011GL046884.

Schlager, H., P. Konopka, P. Schulte, U. Schumann, H. Ziereis, F. Arnold, M. Klemm, D. E. Hagen, P. D. Whitefield, and J. Ovarlez (1997), In situ observations of air traffic emission signatures in the North Atlantic flight corridor, J. Geophys. Res., 102(D9), 10,739–10,750.

• The approach to describing the altitude, pressure, temperature, acid concentration space to be investigated is convoluted. First lidar measurements from presumably one location, which is not mentioned, is used to provide the stratospheric temperature/pressure profile at 5 altitudes from 11 to 48 km, giving the pressure in log10(mBar), and claiming the pressure at 48 km is zero. First there are literally thousands of temperature measurements made every year throughout the world, along with numerous climatologies or reanalysis data which could be used if it was important to characterize the temperature/pressure environment carefully, providing detail much beyond lidar measurements from a single location. In addition lidars struggle in the lower stratosphere and often use radiosonde data to correct their measurements there. It appears that the authors did a brief search for a temperature profile and this one popped up. Second in the end these measurements are not used, as is evident around line 145 when the authors list the temperature and acid concentration profiles they will use. And these are perfectly reasonable, but leaves the reader wondering what Table 1, the lidar temperature reference, and surrounding text were all about.

• Table 2. What is the pressure at which the boiling points are listed? If at the surface what is the relevance for this study? Or more generally why is the boiling point included?

• The authors calculations producing Figure 1 lead to acid concentrations > 100%, which cannot be real. Instead of addressing this deficiency the authors argue that there is little aerosol above 30 km and so that region of the atmosphere can be ignored for the purposes here. This is reasonable, but why not set that up initially and not extend the calculations to 50 km?

The following notes, made while reading the paper, highlight other areas that need to be fixed, or support the comments above. The numbers are line numbers.

Table 1 and the source of the temperature structure. Why use log10 pressure in mBar? It is easy enough to list pressures in more normal units of hPa, which is an SI unit, while mBar is not. How universal is the temperature structure used. Wouldn’t it be better to use some global zonal mean covering the normal flight regions the aircraft exhaust products are expected to impact. To the extent that temperature is important, as it is for sulfuric acid aerosol, the scheme proposed is a bit simplistic with an isothermal layer from 11 – 20 km. Later the authors abandon this table altogether.

128 – Where is section XX?

132 Why is Turco et al. called out separate from the reference style, and is then not in the references?

132 It should be stated that the numbers 1e5/1e7 are molecules/cm3, and it would be nice to include them also as mixing ratios.

142-145 – It makes sense to not consider these reactions above 30 km, but then why not state this sooner and not extend Table 1 and Figure 1 to above 40 km?

Figure 1.  What does it mean to have an acid percent above 100?

146-149 – Again this seems reasonable, but then why are Table 1 and the reference to the lidar temperature profiles included. There are many ways to measure temperature in the range of interest, and lidar is not the preferred one. Really more for high altitude temperatures.

161 “above the stratopause” ??? … This must be above the tropopause. Why the divergence in reference style with an explicit call out to Murphy. At least this time the reference is available.

194-195 The importance of sulfuric acid to the chemistry of stratospheric aerosols has been known for a long time. Do the authors mean organic stratospheric aerosol?

Author Response

Reviewer #1

The authors here suggest that there are up to over 20 compounds emitted by aircraft engines which could have a significant lifetime in the stratosphere and impact stratospheric aerosol with implications for the impact of these aerosol on climate.

The paper is publishable, but not without addressing some major issues.

• No attempt was made to consider whether the compounds with the longest lifetimes and abundances considered here could be, or have been measured, in the stratosphere. In the late 1990s and early 2000s there were a number of field programs to follow commercial aircraft with instrumented aircraft to measure the chemistry of the commercial aircraft exhaust. The European Commission funded several major studies on this topic and, as I recall, most of them did not conclude that the a/c exhaust was of a level to cause a global impact. The authors need to review some of this literature and summarize it. The references used here related to this topic, near the end of the paper, all preceded most of this later work, leaving this paper not conversant with the recent literature on the measurements of these compounds. Here are just a couple of such references:

Jurkat et al., 2011, GRL, 38, L10807, doi:10.1029/2011GL046884.

Schlager, H., P. Konopka, P. Schulte, U. Schumann, H. Ziereis, F. Arnold, M. Klemm, D. E. Hagen, P. D. Whitefield, and J. Ovarlez (1997), In situ observations of air traffic emission signatures in the North Atlantic flight corridor, J. Geophys. Res., 102(D9), 10,739–10,750.

[Authors]. We are grateful for these references, and have read them and a number that refer to them. Both refer to the measurement of major gas phase products of combustion, such as SO2, NOx, rather than heavier molecular weight organics. The papers conclude that such combustion products have little impact on the stratosphere; however these low molecular weight products are expected to be relatively short-lived, to add only a small amount of oxidation end-product (H2SO4, HNO3 etc.) to natural sources, or both. Specifically, the ITCIMS instrument described by Jurkat et al was set up to analyse HONO, SO2, HCl and HNO3. Schlager et al report CCNs on the basis of particle counts, but not what the CCNs are made of. A number of other papers refer to soot, volatile and non-volatile aerosols and CCNs, but no analyses of these are reported to our knowledge. We have added a new section on this prior literature.

• The approach to describing the altitude, pressure, temperature, acid concentration space to be investigated is convoluted. First lidar measurements from presumably one location, which is not mentioned, is used to provide the stratospheric temperature/pressure profile at 5 altitudes from 11 to 48 km, giving the pressure in log10(mBar), and claiming the pressure at 48 km is zero. First there are literally thousands of temperature measurements made every year throughout the world, along with numerous climatologies or reanalysis data which could be used if it was important to characterize the temperature/pressure environment carefully, providing detail much beyond lidar measurements from a single location. In addition lidars struggle in the lower stratosphere and often use radiosonde data to correct their measurements there. It appears that the authors did a brief search for a temperature profile and this one popped up. Second in the end these measurements are not used, as is evident around line 145 when the authors list the temperature and acid concentration profiles they will use. And these are perfectly reasonable, but leaves the reader wondering what Table 1, the lidar temperature reference, and surrounding text were all about.

[Authors] We understand the referee’s concerns here. Our original work did short review the literature and then chose one representative profile. We have addressed this concern in three ways. Firstly, we have replaced our single representative altitude, temperature and pressure measurements with the complete data set of monthly average data from the Integrated Global Radiosonde Archive from NOAA. This has allowed us to select data by month and by latitude, which captures more realistic ranges. Secondly, we have then used these values directly, as we now describe. Lastly, the referee comments that the method used is convoluted. We have added more detail on the actual method of calculation in an Appendix. We hope the new explanations provide a clearer explanation and a more convincing dataset.

• Table 2. What is the pressure at which the boiling points are listed? If at the surface what is the relevance for this study? Or more generally why is the boiling point included?

[Authors] We included the boiling points (which are estimated for 1 bar pressure) simply as an indication of the relative volatility of the compounds. We have made this more explicit in the paper.

• The authors calculations producing Figure 1 lead to acid concentrations > 100%, which cannot be real. Instead of addressing this deficiency the authors argue that there is little aerosol above 30 km and so that region of the atmosphere can be ignored for the purposes here. This is reasonable, but why not set that up initially and not extend the calculations to 50 km?

[Authors] acid concentrations of >100% are ‘real’ in the field, in the sense that they represent oleum, or solutions of SO₃ in H₂SO₄. However we have now substantially improved the algorithm used here (as described in the new Appendix) and no longer get values of >100%. We have also, as the referee suggests, limited the calculations to the lower stratosphere and higher troposphere where commercial aircraft fly.

The following notes, made while reading the paper, highlight other areas that need to be fixed, or support the comments above. The numbers are line numbers.

Table 1 and the source of the temperature structure. Why use log10 pressure in mBar? It is easy enough to list pressures in more normal units of hPa, which is an SI unit, while mBar is not. How universal is the temperature structure used. Wouldn’t it be better to use some global zonal mean covering the normal flight regions the aircraft exhaust products are expected to impact. To the extent that temperature is important, as it is for sulfuric acid aerosol, the scheme proposed is a bit simplistic with an isothermal layer from 11 – 20 km. Later the authors abandon this table altogether.

[Authors] we have removed this table, and replaced it with much more detailed information on the parameters used, in Figure 1 and Table 2. We have selected data from the latitudinal zones where commercial aircraft fly.

128 – Where is section XX?

[Authors] This was an error, we apologise. This has been corrected.

132 Why is Turco et al. called out separate from the reference style, and is then not in the references?

[Authors] This was a formatting error in the references, which we have corrected

132 It should be stated that the numbers 1e5/1e7 are molecules/cm3, and it would be nice to include them also as mixing ratios.

[Authors] We have done this.

142-145 – It makes sense to not consider these reactions above 30 km, but then why not state this sooner and not extend Table 1 and Figure 1 to above 40 km?

[Authors] We have now restricted the analysis to altitudes that are plausibly relevant to commercial jet flight.

Figure 1.  What does it mean to have an acid percent above 100?

[Authors] A percentage above 100% implies ‘fuming’ sulfuric acid or oleum – a solution of SO3 in H2SO4. However the current improved methods do not predict such high concentrations.

146-149 – Again this seems reasonable, but then why are Table 1 and the reference to the lidar temperature profiles included. There are many ways to measure temperature in the range of interest, and lidar is not the preferred one. Really more for high altitude temperatures.

[Authors] We have now removed lidar discussions, and rely on direct balloon measurements.

161 “above the stratopause” ??? … This must be above the tropopause. Why the divergence in reference style with an explicit call out to Murphy. At least this time the reference is available.

[Authors] Yes, this should have been tropopause! Sorry.

194-195 The importance of sulfuric acid to the chemistry of stratospheric aerosols has been known for a long time. Do the authors mean organic stratospheric aerosol?

[Authors] Yes, that is what was intended, we have changed to text to make this clear.

Reviewer 2 Report

This study proposes to assess the chemical reactivity and lifetime of typical products from incomplete combustion products emitted by commercial aircrafts in stratospheric conditions in terms of temperature and sulfuric acid composition using a kinetic model approach. Without proving at all that the organic aerosols have a chemical and radiative impact in the stratosphere (which is not the goal of the study judging by the fact that the model used here is not adapted for comprehensive chemical or climate impact estimations), the authors show their reaction with the ubiquitous stratospheric sulphuric acid tends to increase the lifetime of these combustion products. I think the point of view of this work is interesting since low O/C ratios in the stratosphere are usually believed to stem from photochemical processes.

I find the paper concise and well-written. The manuscript can be published in Atmosphere after addressing the following comments.

Introduction

Line 29: the ubiquitous aerosol layer in the stratosphere is largely composed of spherical droplets. Ice particles (polar stratospheric clouds) are only encountered in winter time polar latitudes when temperatures decrease below a given threshold. So I would not write “Some of these” but ‘Most of these”.

Material and methods

Mention somewhere that the temperature range used in this study corresponds only to mid-latitude conditions (i.e. extra-polar-vortex and extra-tropical air). Temperatures at the tropical tropopause and in the polar vortex can reach -80°C. This means that the thermodynamic situation in the core of the study does not reflect all the conditions encountered in the whole stratosphere. This should be clearly specified.

Line 76: the authors tend to provide only references. The reader would have appreciated to have a short description of the data sources and of the relevance of the chemical products presented in Table 2, even briefly. Are these products fully representative of the population of compounds emitted from combustion from aircrafts?

Line 97: does this mean that the rates of reactions provided in this study are a lower limit

Section 2.4: I find the model not sufficiently described. How is it initialized? The limitations of the chemical approach and overall of the model itself should be more discussed. Are there other chemical pathways or processes leading to oxidized carbon molecules? The authors should specify here that no photochemistry is included which is a basic process in the stratosphere (although I know the model used here is not relevant for such investigation).

Results

Line 128: please specify the correct section.

Line 133: The authors could add more recent observations of H2SO4 profiles in the stratosphere. Arnold F., J. Curtius, S. Spreng, T. Deshler, Stratospheric aerosol sulfuric acid: first direct in situ measurements using a novel balloon-based mass spectrometer apparatus, Journal of Atmospheric Chemistry, 30, 3-10-1998.

Figure 1: I recommend to compare with % acid profiles from the work of Yue et al. Stratospheric aerosol acidity, density, and refractive index deduced from SAGE II and NMC temperature data, JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. D2, PAGES 3727-3738, 1994, Paper number 93JD02989.

I suggest to swap x- and y- axis since altitude is generally represented on the y-axis in papers related to stratospheric processes.

Within figure 1, please correct: H2SO4: 10^7 molecules/cm3

Line 147: again, the temperature ranges are for extra-vortex and extra-tropical conditions.

Line 160: I would suggest to add the following and more recent reference which describes the various sources of organic aerosols in the stratosphere. Renard, Jean-Baptiste, Gwenaël Berthet, Anny-Chantal Levasseur-Regourd, Sergey Beresnev, Alain Miffre, Patrick Rairoux, Damien Vignelles, and Fabrice Jégou, The complex origin and spatial distribution of non-pure sulfate particles (NSPs) in the stratosphere, Atmosphere, 11, 1031, doi:10.3390/atmos11101031, 2020.

Line 161: Murphy et al.’s work does not deal with atmospheric composition at the stratopause since their observations were limited to the UT/LS region (maximum 19 km). Do the authors mean the tropopause here?

Line 163: “ratios” instead of “rations”. Please correct.

Figure 2: it is not a good quality in term of resolution (especially on the axis). A bracket is missing after “Time (seconds”

Discussion

Line 244: This maybe especially true above Asia during summer monsoon where an aerosol layer composed of ammonium nitrate has been observed. You can add the following reference for that purpose: Höpfner Michael et al., Ammonium nitrate particles formed in upper troposphere from ground ammonia sources during Asian monsoons, https://0-doi-org.brum.beds.ac.uk/10.1038/s41561-019-0385-8,

However, the fact that impure sulphuric acid droplets are largely present in the stratosphere (with dependence on latitude) has been highlighted in particular by Murphy et al. (2007) should be discussed here. What would be for instance the effects of metals in the droplet on the oxidation capacity?

Conclusion

The part dedicated to the estimation of the deposition of incomplete combustion products is not very clear and not sufficiently argued and developed. How do the authors deduce the 10⁹ kg value frm the 9.18x10¹¹ kg of CO2 emitted? What is the link with CO2?

Lines 282-284: I do not find the analogy given between the injected amount of organic material from aviation and of volcanic sulphur for stratospheric chemistry pertinent here. One must match the chemical impact of heterogeneous processes at the surface of sulfate droplets and that of organic particles. The latter is particularly unknown in the stratosphere. A perspective (or outreach) of the authors’ findings towards comprehensive simulations of chemical reactions involving organic compounds in the stratosphere (e.g. using a CCM model see for instance ; Yu, P., Rosenlof, K. H., Liu, S., Telg, H., Thornberry, T. D., Rollins, A. W., Portmann, R. W., Bai, Z., Ray, E. A., Duan, Y., Pan, L. L., Toon, O. B., Bian, J., and Gao, R.-S.: Efficient transport of tropospheric aerosol into the stratosphere via the Asian summer monsoon anticyclone, P. Natl. Acad. Sci. USA, 114, 6972–6977, https://0-doi-org.brum.beds.ac.uk/10.1073/pnas.1701170114, 2017. ; Bossolasco, Adriana, Fabrice Jégou, Pasquale Sellitto, Gwenaël Berthet, Corinna Kloss, and Bernard Legras, Global modelling studies of composition and trends of aerosols linked to the Asian Tropopause Aerosol Layer (ATAL) in the monsoon anticyclone region, Atmos. Chem. Phys., 21, 2745–2764, https://0-doi-org.brum.beds.ac.uk/10.5194/acp-21-2745-2021, 2021) should be provided at the end of the conclusion.

Author Response

Reviewer #2

This study proposes to assess the chemical reactivity and lifetime of typical products from incomplete combustion products emitted by commercial aircrafts in stratospheric conditions in terms of temperature and sulfuric acid composition using a kinetic model approach. Without proving at all that the organic aerosols have a chemical and radiative impact in the stratosphere (which is not the goal of the study judging by the fact that the model used here is not adapted for comprehensive chemical or climate impact estimations), the authors show their reaction with the ubiquitous stratospheric sulphuric acid tends to increase the lifetime of these combustion products. I think the point of view of this work is interesting since low O/C ratios in the stratosphere are usually believed to stem from photochemical processes.

I find the paper concise and well-written. The manuscript can be published in Atmosphere after addressing the following comments.

Introduction

Line 29: the ubiquitous aerosol layer in the stratosphere is largely composed of spherical droplets. Ice particles (polar stratospheric clouds) are only encountered in winter time polar latitudes when temperatures decrease below a given threshold. So I would not write “Some of these” but ‘Most of these”.

 [Authors] Thank you for this, we have changed this.

Material and methods

Mention somewhere that the temperature range used in this study corresponds only to mid-latitude conditions (i.e. extra-polar-vortex and extra-tropical air). Temperatures at the tropical tropopause and in the polar vortex can reach -80°C. This means that the thermodynamic situation in the core of the study does not reflect all the conditions encountered in the whole stratosphere. This should be clearly specified.

[Authors] we have been much more explicit in the latitudes, altitudes and seasons for the temperature data. We have included specific comment on the polar vortex.

Line 76: the authors tend to provide only references. The reader would have appreciated to have a short description of the data sources and of the relevance of the chemical products presented in Table 2, even briefly. Are these products fully representative of the population of compounds emitted from combustion from aircrafts?

[Authors] Line 76 refers to the list of incomplete combustion products. We have added some text here, but the purpose of this paper was not to explore how these products are generated but what happens to them afterwards. We have therefore not expanded greatly on this. As to whether these are representative of all aircraft, and of aircraft at cruise altitude rather than on the ground, to the best of our knowledge this is not known. We make this point explicitly.

Line 97: does this mean that the rates of reactions provided in this study are a lower limit

[Authors]. Formally, yes, but in almost all cases the rate of the fastest reaction is several orders of magnitude higher than the rate of the next fastest, so correcting for additive effects would result in little change. We have made this point explicitly in the revised paper.

Section 2.4: I find the model not sufficiently described. How is it initialized? The limitations of the chemical approach and overall of the model itself should be more discussed. Are there other chemical pathways or processes leading to oxidized carbon molecules? The authors should specify here that no photochemistry is included which is a basic process in the stratosphere (although I know the model used here is not relevant for such investigation).

[Authors]. The reviewer is concerned with two issues here; the model that generates the chemical space of compounds (the subject of section 2.4). On initialization of the algorithm we have added some more explicit descriptions in section 2.4 (regarding the generation of the theoretical chemical space) and the Appendix (regarding the modelling of sulfuric acid concentrations).
The second point is a wider one on the chemistry that generates oxidised molecules. In Section 3.2, we have been more explicit that our model excludes photochemistry, and also made more explicit the relationship between likely photochemistry on the chemical space of all compounds and sulfuric acid chemistry on that same space.

Results

Line 128: please specify the correct section.

[Authors] We have corrected this

Line 133: The authors could add more recent observations of H2SO4 profiles in the stratosphere. Arnold F., J. Curtius, S. Spreng, T. Deshler, Stratospheric aerosol sulfuric acid: first direct in situ measurements using a novel balloon-based mass spectrometer apparatus, Journal of Atmospheric Chemistry, 30, 3-10-1998.

 [Authors] We now cite several more recent studies, including the Arnold one cited here, for which we thank the reviewer.

Figure 1: I recommend to compare with % acid profiles from the work of Yue et al. Stratospheric aerosol acidity, density, and refractive index deduced from SAGE II and NMC temperature data, JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. D2, PAGES 3727-3738, 1994, Paper number 93JD02989.

[Authors] We are grateful for this reference, and have discussed it in the text.

I suggest to swap x- and y- axis since altitude is generally represented on the y-axis in papers related to stratospheric processes.

[Authors] we understand the reviewer’s point, but it is more general practice to have the independent variable (here altitude) on the X axis and the dependent variable (here concentration of acid etc) on the y axis. We have stayed with this convention.

Within figure 1, please correct: H2SO4: 10^7 molecules/cm3

 [Authors] We have corrected this.

Line 147: again, the temperature ranges are for extra-vortex and extra-tropical conditions.

[Authors] Agreed, and we have now explicitly discussed this.

Line 160: I would suggest to add the following and more recent reference which describes the various sources of organic aerosols in the stratosphere. Renard, Jean-Baptiste, Gwenaël Berthet, Anny-Chantal Levasseur-Regourd, Sergey Beresnev, Alain Miffre, Patrick Rairoux, Damien Vignelles, and Fabrice Jégou, The complex origin and spatial distribution of non-pure sulfate particles (NSPs) in the stratosphere, Atmosphere, 11, 1031, doi:10.3390/atmos11101031, 2020.

 [Authors] We thank the reviewer for this suggestion, and have added the reference and modified the text accordingly.

Line 161: Murphy et al.’s work does not deal with atmospheric composition at the stratopause since their observations were limited to the UT/LS region (maximum 19 km). Do the authors mean the tropopause here?

[Authors] Yes, our apologies, this was a typing error and should have been ‘tropopause’.

Line 163: “ratios” instead of “rations”. Please correct.

[Authors] Corrected

Figure 2: it is not a good quality in term of resolution (especially on the axis). A bracket is missing after “Time (seconds”

[Authors] We have improved the quality of this figure

 Discussion

Line 244: This maybe especially true above Asia during summer monsoon where an aerosol layer composed of ammonium nitrate has been observed. You can add the following reference for that purpose: Höpfner Michael et al., Ammonium nitrate particles formed in upper troposphere from ground ammonia sources during Asian monsoons, https://0-doi-org.brum.beds.ac.uk/10.1038/s41561-019-0385-8,

[Authors] This is a very interesting idea, which we have mentioned.

However, the fact that impure sulphuric acid droplets are largely present in the stratosphere (with dependence on latitude) has been highlighted in particular by Murphy et al. (2007) should be discussed here. What would be for instance the effects of metals in the droplet on the oxidation capacity?

 [Authors]. This is a very complex issue, and one that we feel is beyond the scope of what we have done here to fully explore. However we understand the reviewer’s comments here, and have added relevant text in the Discussion.

Conclusion

The part dedicated to the estimation of the deposition of incomplete combustion products is not very clear and not sufficiently argued and developed. How do the authors deduce the 10⁹ kg value frm the 9.18x10¹¹ kg of CO2 emitted? What is the link with CO2?

[Authors] This is simply from an estimate of the efficiency of jet fuel combustion. We have made this clearer, and ‘softened’ the statement of conclusions.

Lines 282-284: I do not find the analogy given between the injected amount of organic material from aviation and of volcanic sulphur for stratospheric chemistry pertinent here. One must match the chemical impact of heterogeneous processes at the surface of sulfate droplets and that of organic particles. The latter is particularly unknown in the stratosphere. A perspective (or outreach) of the authors’ findings towards comprehensive simulations of chemical reactions involving organic compounds in the stratosphere (e.g. using a CCM model see for instance ; Yu, P., Rosenlof, K. H., Liu, S., Telg, H., Thornberry, T. D., Rollins, A. W., Portmann, R. W., Bai, Z., Ray, E. A., Duan, Y., Pan, L. L., Toon, O. B., Bian, J., and Gao, R.-S.: Efficient transport of tropospheric aerosol into the stratosphere via the Asian summer monsoon anticyclone, P. Natl. Acad. Sci. USA, 114, 6972–6977, https://0-doi-org.brum.beds.ac.uk/10.1073/pnas.1701170114, 2017. ; Bossolasco, Adriana, Fabrice Jégou, Pasquale Sellitto, Gwenaël Berthet, Corinna Kloss, and Bernard Legras, Global modelling studies of composition and trends of aerosols linked to the Asian Tropopause Aerosol Layer (ATAL) in the monsoon anticyclone region, Atmos. Chem. Phys., 21, 2745–2764, https://0-doi-org.brum.beds.ac.uk/10.5194/acp-21-2745-2021, 2021) should be provided at the end of the conclusion.

[Authors]. The reviewer makes a good point. Rather than discuss this in detail here, we have removed the reference to volcanic sources. We have included a call for more detailed chemical modelling, as the reviewer suggests, but we feel that providing further perspective is the work of a more detailed review and would be out of place in what is a paper modelling a very specific aspect of potential stratospheric chemistry.

Reviewer 3 Report

please see the attachment.

Comments for author File: Comments.pdf

Author Response

Reviewer #3

This paper uses a predictive model investigated the potential chemical lifetime of commercial aviation fuel in stratosphere. The topic is of great significance as stratospheric aerosols are knowns as important factors that impact global climate and atmospheric environment. However, the organization of the manuscript could be improved and there are still some questions need to be answered before publication. The major points are:

1. The method of this work are somewhet unclear. The reason whty the authors chose the methods, i.e. the advantage, should be clarified./ The accuracy of the methods is not quantified, so the conclusions also need to be verified

[Authors]. We have now provided a much more extensive discussion of methods, including their accuracy, in the Appendix.

2. As noted later in Sec 4.2, the consideration of stratospheric chemical procedded in this work is incomplete. In this case, the difference between the results obtained under simpler assumptions and the real situation is hard to be estimated. The practical value of the results cannot be measured.

[Authors]. We have added specific discussion in section 5 to both state this, and point the way forward to how to make this more practical.

Given the amount of work I think the paper still needs, I will withhold any specific suggestions at present. I would be happy to provide this information once these major points are taken care of.

Round 2

Reviewer 1 Report

The authors approach to: 1) the atmosphere and the environment in which the chemical reactions of interest may proceed, 2) references to previous measurements of aircraft emissions, and 3) the distinction between lower molecular weight compounds, measured in the past, and the higher molecular weight compounds of this study, have all improved considerably, such that the paper is nearly ready for publication.

The authors may want to consider the following. I do not need to see the revised manuscript.

The two somewhat major additions, which the authors should consider including, are: 1) To search the literature on mass spectrometry measurements in the stratosphere (e.g. Murphy et al., and others) for any evidence of signatures from the higher molecular weight, incomplete combustion products, forming the focus of this study. Were the mass specs used capable of reaching the masses expected? If not that could be stated. At present all the evidence for the higher weight molecules is based on surface tests of jet engine emissions. 2) To indicate what would be required to measure the higher weight molecules of interest in this study.

A few other typos, suggestions, comments:

90 – a hanging v?

139 – … to affect …

Figure 4 runs off the page, so the reviewer cannot see the right end of the figure. The time scale could use a more helpful unit, such as years, giving the reader a better sense of the longevity.

291 suggest … of > 4 months …. If the time is presented in “human readable format”, the rest of the sentence is not necessary.

327-330 – These statements are not justified by the geo-engineering under consideration, which is generally some form of adding sulfuric acid to the stratosphere. The concern then is usually the opposite, the effect will not last that long such that the material will have to be nearly continuously replenished. The half-life of volcanic stratospheric aerosol, which is also sulfuric acid/water aerosol for any significant impact in the stratosphere, is on the order of one year. If the authors are aware of other higher molecular weight materials being considered, that should be stated here rather than these generic statements.

356 – suggest … but it may be … or .. there may be sufficient information to …

Author Response

The authors approach to: 1) the atmosphere and the environment in which the chemical reactions of interest may proceed, 2) references to previous measurements of aircraft emissions, and 3) the distinction between lower molecular weight compounds, measured in the past, and the higher molecular weight compounds of this study, have all improved considerably, such that the paper is nearly ready for publication.

The authors may want to consider the following. I do not need to see the revised manuscript.

The two somewhat major additions, which the authors should consider including, are: 1) To search the literature on mass spectrometry measurements in the stratosphere (e.g. Murphy et al., and others) for any evidence of signatures from the higher molecular weight, incomplete combustion products, forming the focus of this study. Were the mass specs used capable of reaching the masses expected? If not that could be stated. At present all the evidence for the higher weight molecules is based on surface tests of jet engine emissions. 2) To indicate what would be required to measure the higher weight molecules of interest in this study.

[Authors] Again, this reviewer has made a very good point, which we thank them for. As the ‘geoengineering’ section was controversial and the reviewer felt it inappropriate for this paper, we have removed it and replaced it with a new section on mass spectrometry detection, both potentially why current practice has not detected high molecular weight organics in aerosols, and how this might be addressed with software or instrumentation upgrades.

A few other typos, suggestions, comments:

90 – a hanging v?

[Authors] We have corrected this

139 – … to affect …

[Authors] We have corrected this

Figure 4 runs off the page, so the reviewer cannot see the right end of the figure. The time scale could use a more helpful unit, such as years, giving the reader a better sense of the longevity.

291 suggest … of > 4 months …. If the time is presented in “human readable format”, the rest of the sentence is not necessary.

[Authors]. We would rather leave the results in SI units. We appreciate that this is less intuitive, but it makes comparison with many other results (also reported in SI units) easier.

327-330 – These statements are not justified by the geo-engineering under consideration, which is generally some form of adding sulfuric acid to the stratosphere. The concern then is usually the opposite, the effect will not last that long such that the material will have to be nearly continuously replenished. The half-life of volcanic stratospheric aerosol, which is also sulfuric acid/water aerosol for any significant impact in the stratosphere, is on the order of one year. If the authors are aware of other higher molecular weight materials being considered, that should be stated here rather than these generic statements.

[Authors]. As noted above, we have removed the geoengineering paragraph.

356 – suggest … but it may be … or .. there may be sufficient information to …

[Authors] we have adopted this suggestion, thank you.

Reviewer 2 Report

The manuscript has been properly improved and clarified. I particularly like the effort on the discussion with the available literature.

Author Response

The manuscript has been properly improved and clarified. I particularly like the effort on the discussion with the available literature.

[Authors] We thank the reviewer for this

Reviewer 3 Report

Please see the attachment.

Comments for author File: Comments.pdf

Author Response

I have read the revised manuscript. The authors have added some descriptions and discussions,. The manuscript can be published in Atmosphere after  addressing the following comments.

Minor comments:

Line 84: Change “Right” to “Right-“ in order to be consistent with “Centre-“ and “Left-“

{Authors] We have corrected this

Line 90: Delete “v”

 [Authors] We have corrected this

Figure 2: Change “% acid” to “Acid (%)”

[Authors] We have changed this

Line 183: Change “Figure 1” to “Figure 2”

[Authors] We have corrected this

Line 234: Change “Figure 2” to “Figure 3”

[Authors] We have corrected this

Line 244: “This suggests …””this” stands for what??

[Authors] We have been more specific – ‘this’ was the analysis summarised in Figure 3

Line 262: Change “Figure 3” to “Figure 4”

[Authors] We have corrected this

Figure 4: X axis is too compact and not obvious. Please change it.

[Authors] We have changed this.