GENERAL COMMENTS

In this study, Mengze et al. present an analysis of observed (airborne) vs modeled ethane, propane and methane trends for the period 2006-2016. They estimate a global emission of ethane of 19.28 Tg/yr. Their results show trends for upper tropospheric and stratospheric ethane, propane and methane.  

The paper is generally well written and contributes to the scientific understanding of ethane, propane and methane trends globally. I recommend this paper for publication after major revisions.

My biggest concern is that authors jump into conclusions too fast in some sections. There is also a lack of information regarding the model simulation setup. There are multiple places where it is hard to distinguish if the authors are referring to modeled results or observations, making the reading a bit confusing. Also, almost all figure captions (in the manuscript and the supplemental material) should be improved by adding information regarding the legends, and the type of information shown (see specific comments below). Lastly, even though this study shows results for propane and methane, these compounds are barely discussed in the text and their results are not even mentioned in the abstract. I wonder if somehow the title should be changed to clarify the readers that ethane is the main compound discussed in the paper and just a few propane and methane results will be shown.

SPECIFIC COMMENTS

Section 3.2

1. There is no comment on the most significant feature of Figure 1, which is the much stronger and different seasonality of ethane for EUR compared to the rest of the regions. Why are concentrations so high for this region and why the peak shows up 1-2 months prior compared to NAM and whole NH?
2. How does stratospheric ethane lack of general seasonality from this study compares to others?
3. Why does NAM show such seasonality? Any insights of the reasons behind it?

 Lines 219-221.

1. Figure S3 does not indicate the region that the uncertainties correspond to (e.g. NAM, EUR). If it is for the whole globe, it needs to be stated. Again, captions and descriptions in the text should be improved.
2. “.. and model optimizations (section 2.2)”. I couldn’t find the description of the model optimizations in section 2.2. Check or add this information clearly. Use the same terms/words, so that is clear and easy for the reader to find this information. This is extremely important to better understand Figure 2.

Line 222-224. Map with location and time frames of sample collection is needed.

Lines 224-225. How were these observations selected? Which are those observations selected?

Figure 2 – Panel a) I am surprised by seen that the FEF opt contribution is almost similar to RES opt and how little BIB opt contributes from 2006-2014. Also, more information of model setup is needed. Specifically a table with emission inventories used, global and regional (NAM, EUR, etc) totals per sector will be helpful.

Line 267.  Add the word “modeled” in “… the modeled NH upper tropospheric ethane…”.

Line 269. State that SWD and TRO are modeled results.

Lines 269. How can contributions of SWD and TRO be so high in NAM? Can these contributions be explained? Do these modeled results make sense? Also, how can SWD and TRO have such high sectoral contributions, but low contributions when considering trends from the optimized models (Figure 2).

Line 271. Add the word “modeled” in “Figure 3 shows the modeled sectoral…”

Figure 3. Even though this figure looks really nice, it does not provide clear information on the contribution of each sector and region. The size of Whole NH, NAM, EUR, and ASI on the right side is the same for each region. On the left side, the sectors have different sizes, but there is no axis or value assigned to each contribution and also, for each sector the slice corresponding to each region is always the same. My suggestion is to change this figure for another one that shows the contributions by region as stated in lines 271-280. The way it is right now, those contributions are not clear.

Lines 283-284:

1. “Five geographical sectors” ?? Do you mean geographical regions? Explain what do you mean by geographical sectors because mixing regions with emission sources is confusing.
2. Why can’t BIB and BIO emissions be separated by regions? Are the emissions not gridded in the model? Clarification is needed.

Figure 4: Is Model Opt the sum of all the optimized geographical regions? The authors need to clarify this.

Lines 285-286.

1. “… 30%~55%, 35%~50%, 50%~65%, and 30%~40%...”, the symbol “~” must be changed for “-“.
2. Are these percentages average values from 2006-2016? Specify.
3. “Ethane emissions from ASI dominates the trends…” What are the model estimated ethane emissions (in Tg/yr) for ASI compared to the rest of the regions? A table with regional emissions used in the model is needed.

Lines 288-289. Clearly state to which atmospheric layer correspond these trends. For example: “…contribution to ethane trends in the upper troposphere and/or stratosphere”.

Lines 295-301. How does these results compare with other studies?

Section 3.3.3 has “methane” in its title, but it is barely discussed in the text.

Section 3.3.4

1. This section should be named differently because it discusses the comparison of the model to ONLY two observations. It’s current name gives the false idea that there was a model simulation using ground station data. I suggest calling it: “Model results compared to two ground stations”.
2. The conclusion from the last sentence should be erased. It cannot be concluded just by comparing 2 stations that the model provides realistic values for ethane surface level. A thorough analysis of multiple surface stations must be done before jumping to that conclusion.
3. I suggest completely getting rid of this section and mention that the use of this model results for surface-level ethane should be studied in the future.

Section 3.3.5

1. Erase the term “budget” in the title and text because this implies sources and sinks of a certain compound and here, only global emission totals are presented.
2. Thus, modify the title of this section.

Lines 334-335. How does the authors conclude that the stratosphere has a minor contribution for observed trends? Authors need to explain clearly why location biased trends can be discounted, even if it is for the same reasons that were discounted for the upper troposphere.

Lines 335-336. Add “over the Whole NH” before “shows a general…” to specify which panel the authors are referring to.

Lines 336-339. Suggest the following change: “The peak in 2010 is not seen at regional levels (NAM, ASI, EUR, Figure 6 (b)(c)(d)), which suggest global upward transport of the upper tropospheric ethane emissions (peaking in 2010-2011) into the stratosphere.” No analysis was made to clearly indicate that there was an upward transport from the upper troposphere, therefore, it cannot be stated as such.

Line 339. Suggest the following change: “The second peak in 2013 can be due to the regional…”. Same reason as explained in the previous comment.

Section 3.4.2 Add (if any) references and comparisons to other studies.

Lines 396-397. Specify which “current inventory” the authors are referring to.

TECHNICAL CORRECTIONS

Figures 1, 2, 3, 4, 6, and 7: Explanation of legends and terms used in the figures are missing in the caption.

There are two figures with number 5 (Lines 640 and 645).

Caption Figure 3. Clearly state sectoral contributions are model results.

Caption of Figure 4 should be revised because not all results correspond to “Optimized geographical sector contribution” as stated in the first sentence (Observations are not included in the caption). Also, clearly state that the optimized geographical contributions are model results.

Suggest the use of a different color scheme for the EUR, NAM, etc regions in Figures 4 and 7 to avoid confusion model optimizations in figure 2 and figure 6.

Sections 3.3.3 and 3.4.2 have exactly the same name. Indications to “troposphere” or “stratosphere” should be added to avoid confusion. The same goes for their corresponding figures in the supplemental material.

This manuscript reports on measurement data for methane, ethane, and propane from the IAGOS-CARIBIC commercial aircraft sampling program. Data from the upper troposphere/lower stratosphere are compared with outputs from the EMAC model. The conclusion from the disagreement in this comparison is that there are discrepancies between inventory and actual emissions of ethane.

This paper follows up on a series of other recent studies that have investigated the burden and atmospheric ethane, for instance [Aydin et al., 2011; Simpson et al., 2012; Franco et al., 2015; Franco et al., 2016; Hausmann et al., 2016; Helmig et al., 2016; Dalsoren et al., 2017; Tzompa-Sosa et al., 2017; Dalsoren et al., 2018; Monks et al., 2018; Tzompa‐Sosa et al., 2019; Angot et al., 2021].

I see a plethora of issues with this manuscript that dampen my enthusiasm for this work.

Overall, the manuscript does not seem all that carefully proofread, having an unusual high number of punctuation and formal errors. The concept of deriving a linear trend determination for the 2006-2016 window seems ill directed, given that it has been demonstrated that there was a decline in atmospheric ethane to approximately 2009, and then an uptick of ethane after that. Ignoring this trend reversal and then averaging trends over these two contrasting periods of atmospheric ethane changes does not seem to make much sense.

I am skeptical of the quality of the observational data and that the authors did not address any of the very obvious discrepancy of their findings with the prior published literature (see listing above). Year-to-year variability and trends in the ethane mole fraction are ways higher than reports in any of these prior publications. This points towards technical inconsistencies in the sampling program, possible analyte losses in the analytical system, or calibration issues.

Unfortunately, this work does not provide sufficient insight into the analytical protocols and quality controls to evaluate this question. Has the sampling apparatus and analytical lab ever been audited by the World Calibration Centre for VOCs [WCC-VOC, 2021], which most labs with global monitoring programs have been subjected to during the past ten years?  If so, then the results from this audit should be presented and critically evaluated.

The reason for the difference in surface versus upper tropospheric trends needs more discussion/justification. Obviously, this defies the findings from column FTIR observations, e.g. [Helmig et al., 2016] who showed that, while lower in magnitude, lower and upper tropospheric trends were approximately in sink.

Specific comments:

Abstract: The source/program of the measurement data should be mentioned. The model that was used should be mentioned.

Line 14-16:  I am surprised that the oil and gas sector is not clearly listed as an emissions category given its significant contribution to ethane emissions (see the above listed literature that overwhelmingly focused on the oil and gas emissions sector)?

19: Given the overall uncertainty in emissions, four significant figures for the global ethane emissions do not seem justified.

23: Instead of using the term ‘discrepancy’ it would be more meaningful to use a term that clearly identifies if inventory emissions are over- or under-estimated.

30, 32, 41, etc:  There are many, many cases throughout the manuscript where a comma should be placed within a sentence. 

56: Please explain what the number in parenthesis stands for.

103: Are methane and CO2 indeed measured with an ECD?

105-109: Details on the calibration scale, source of calibration standard should be provided, and how and when the scale was verified against the World Meteorological Organization Global Atmospheric Watch program that is used by the international community for global observations of ethane.

139: Replace ‘of’ with ‘by’.

156:  Insert ‘the’ before NOAA.

174:  …related to emissions from the ….

188:  What is the reference year for the % trend determination?

193/194:  No need to put parentheses around the time intervals.

204: Please provide more explanation on the criteria that were used for differentiating upper tropospheric air from stratospheric air and associated uncertainties.

208, Figure 1 caption:  Ethane is presented in mole fraction units (ppb) and not as a concentration. Use of the term ‘concentration’ is therefore technically not correct.

219:  Wording is not correct. Uncertainties are presented in Figure S3.

246 - 262: A more convincing approach would be to first evaluate and optimize the modeled trends using available and much more abundant (and likely better quality) surface data (possibly high elevation mountaintop data, e.g. Jungfraujoch, Mauna Loa, Summit) or FTIR column records, and then move on towards evaluating the upper troposphere trends.

249:  It shouldn’t really be a surprise that the model performs better for methane, given the much lower variability of methane in time and geographically, and given its 40 times longer lifetime?  Please avoid subjective terms such as ‘better’, and instead give a concrete quantitative evaluation.

259: The oil spillage doesn’t really matter for ethane. It would make more sense to provide emissions/leakage/spill estimates for natural gas, including ethane.

262: Insert ‘the’ after although.

283: Please explain/spell out all abbreviations when they are used for the first time.

285: Doesn’t this result contradict the findings from other prior literature, e.g.[Franco et al., 2015; Franco et al., 2016; Hausmann et al., 2016; Helmig et al., 2016] who claim that North America is the main contributing region for the rise in global ethane post 2009? 

295-301: As mentioned previously, I question that trend results over this time window are meaningful, given that this spans over a point in time when there was a reversal of the ethane trend.

314-318:  Again, ‘well’ is a subjective qualifier.  Please provide a concrete quantitative evaluation.

321: Unreasonable number of significant figures.

334:  …is again a minor ….

338:  What do you mean by ‘upper tropospheric ethane emissions’?  I am not aware of ethane emission sources in the upper troposphere?

353: Figure 6 shows time series data, not trends.

378: Shouldn’t the NOAA data source be included here as well?

396: Please name the inventory.

402: ‘Trend’ is not a suitable term here.

411: ..revised …

605:  The listed ethane sources do not add up to the total that is listed at the bottom?

Figure 2: To the very best of my knowledge, the ethane spike that is reported in 2010 has not been seen in any other data products, including FTIR column observations and surface measurements, including those taken at mountain sites. This makes me quite skeptical of these results. In my opinion, this requires a much more thorough evaluation/verification, including through careful consideration of available other data sources. Can this be explained by interannual differences in the fraction of stratosphere-troposphere exchange that the aircrafts encountered during their sampling? That would then raise the question or representative each year’s aircraft data are?

Figure 3: The size of the four colored emission regions does not seem to agree with the data in Table 1.

Figure 5:  Graph shows time series data, not trend results. The 2011 propane spike is very significant, representing a 50% increase from prior years. This dramatic change is much larger than year-to-year variability seen in other data products. This seems unreasonable and makes me question the quality of the underlying data. This calls for much more documentation on the quality control procedures and a convincing demonstration of the long-term stability of the sampling and analysis program.

Fig 6/7: I would like to see information on how the data were treated, averaged, binned, and how uncertainty ranges were determined. These figures are the pivotal example for my skepticism of the data quality and the knowledge/understanding of the authors of the atmospheric behavior of these NMHC. For the ASI panel, there is a more than 50% drop of ethane in the observations from 2013 to 2015. So, that’s a reduction of ethane to less than half in two years. No other data set that is referred to in the above-cited literature shows year-to-year changes of that magnitude. A concentration drop like this is highly unreasonable, most likely impossible. The best explanation that I can think of is that there were sampling or measurement problems that the authors did not become aware of and investigate. Or that there were strong differences in the fraction of stratospheric versus tropospheric air that were sampled each year and that the differentiation wasn’t accomplished properly?

Figure 8: Similar reservations as already explained for Figure 6/7: I can not believe how propane would drop in the stratosphere by a factor of more than four from 2006 to 2011. This defies every other data set on atmospheric NMHCs that I have seen in the published literature.

Literature cited

Angot, H., C. Davel, C. Wiedinmyer, G. Pétron, J. Chopra, J. Hueber, B. Blanchard, I. Bourgeois, I. Vimont, S. A. Montzka, B. R. Miller, J. W. Elkins, and D. Helmig (2021), Temporary pause in the growth of atmospheric ethane and propane in 2015–2018, Atmos. Chem. Phys. Discus., https://doi.org/10.5194/acp-2021-285.

Aydin, M., K. R. Verhulst, E. S. Saltzman, M. O. Battle, S. A. Montzka, D. R. Blake, Q. Tang, and M. J. Prather (2011), Recent decreases in fossil-fuel emissions of ethane and methane derived from firn air, Nature, 476, 198-201, doi:10.1038/nature10352.

Dalsoren, S. B., G. MYhre, Ø. Hodnebrog, C. L. Myhre, A. Stohl, I. Pisso, S. Schwietzke, L. Hoeglund-Isaksson, D. Helmig, S. Reimann, S. Sauvage, N. Schmidbauer, K. A. Read, L. Carpenter, A. C. Lewis, S. Punjabi, and M. Wallasch (2017), Ethane and propane emissions substantially underestimated in community emission datasets, Nature Geoscience, In press.

Dalsoren, S. B., G. Myhre, O. Hodnebrog, C. L. Myhre, A. Stohl, I. Pisso, S. Schwietzke, L. Hoglund-Isaksson, D. Helmig, S. Reimann, S. Sauvage, N. Schmidbauer, K. A. Read, L. J. Carpenter, A. C. Lewis, S. Punjabi, and M. Wallasch (2018), Discrepancy between simulated and observed ethane and propane levels explained by underestimated fossil emissions, Nature Geoscience, 11, 178-186, doi:10.1038/s41561-018-0073-0.

Franco, B., W. Bader, G. C. Toon, C. Bray, A. Perrin, E. V. Fischer, K. Sudo, C. D. Boone, B. Bovy, B. Lejeune, C. Servais, and E. Mahieu (2015), Retrieval of ethane from ground-based FTIR solar spectra using improved spectroscopy: Recent burden increase above Jungfraujoch, Journal of Quantitative Spectroscopy & Radiative Transfer, 160, 36-49, doi:10.1016/j.jqsrt.2015.03.017.

Franco, B., E. Mahieu, L. K. Emmons, Z. A. Tzompa-Sosa, E. V. Fisher, K. Sudo, B. Bovy, S. Conway, D. Griffin, J. Hannigan, K. Strong, and K. A. Walker (2016), Evaluating ethane and methane emissions associated with the development of oil and natural gas extraction in North America, Environmental Research Letters, 11, 1-11.

Hausmann, P., R. Sussmann, and D. Smale (2016), Contribution of oil and natural gas production to renewed increase of atmospheric methane (2007–2014): top-down estimate from ethane and methane column observations, Atmos. Chem. Phys. , 16, 3227-3244.

Helmig, D., S. Rossabi, J. Hueber, P. Tans, S. Montzka, K. Masarie, K. Thoning, C. Plass-Duelmer, A. Claude, L. Carpenter, A. C. Lewis, S. Punjabi, S. Reimann, M. Vollmer, R. Steinbrecher, J. Hannigan, L. Emmons, E. Mahieu, B. Franco, D. Smale, and A. Pozzer (2016), Reversal of global atmospheric ethane and propane trends largely due to US Oil and natural gas production, Nature Geoscience, 9, 490–495, doi:doi:10.1038/ngeo2721.

Monks, S., C. Wilson, L. Emmons, D. Helmig, J. Hannigan, D. Blake, and N. Blake (2018), Using an inverse model to reconcile differences in simulated and observed global ethane concentrations and trends between 2008 and 2014, Journal of Geophysical Research, 11262-11282.

Simpson, I. J., M. P. S. Andersen, S. Meinardi, L. Bruhwiler, N. J. Blake, D. Helmig, F. S. Rowland, and D. R. Blake (2012), Long-term decline of global atmospheric ethane concentrations and implications for methane, Nature, 488, 490-494, doi:10.1038/nature11342.

Tzompa-Sosa, Z. A., E. Mahieu, B. Franco, C. A. Keller, A. J. Turner, D. Helmig, A. Fried, D. Richter, P. Weibring, J. Walega, T. I. Yacovitch, S. C. Herndon, D. R. Blake, F. Hase, J. W. Hannigan, S. Conway, K. Strong, M. Schneider, and E. V. Fischer (2017), Revisiting global fossil fuel and biofuel emissions of ethane, Journal of Geophysical Research-Atmospheres, 122, 2493-2512, doi:10.1002/2016jd025767.

Tzompa‐Sosa, Z. A., B. H. Henderson, C. A. Keller, K. Travis, E. Mahieu, B. Franco, M. Estes, D. Helmig, A. Fried, D. Richter, P. Weibring, J. Walega, D. R. Blake, J. W. Hannigan, I. Ortega, S. Conway, K. Strong, and E. V. Fischer (2019), Atmospheric implications of large C2‐C5 alkane emissions from the U.S. oil and gas Industry, Journal of Geophysical Research, 124, 1148-1169.

WCC-VOC (2021), World Calibration Centre for Volatile Organic Compounds, http://www.imk-ifu.kit.edu/wcc-voc/, Karlsruhe Intitute of Technology.

General comments

Li et al. present long-term atmospheric ethane, propane, and methane data obtained from airborne observation in the UTLS region in the Northern Hemisphere through IAGOC-CARIBIC project for the period 2006–2016. The authors also present simulation outputs from the EMAC model to interpret the contributions of different source regions or different source sectors to the observed ethane trend.

The manuscript is generally well written. The airborne observational ethane, propane, and methane data in the UTLS region is unique and useful because there is only a limited platform that can measure with high precision ­such trace gases regularly in this region - perhaps only IAGOC-CARIBIC (this project) and CONTRAIL project (e.g., Machida et al. 2008; Sawa et al. 2015). The observational data will certainly be reused by many researchers in this field, e.g., validate the emission inventories (as the authors did), interpret the atmospheric transport and chemical processes in the UTLS region, through the comparison with model simulations.

However, the current manuscript is lacking the information on methods (e.g., model simulation setup, trend analysis, measurement uncertainty), making the readers a bit confused and difficult to interpret the presented results (and dataset). For example, I couldn’t find how the emission inventories are optimized from prior emission inventories to match the observation data. Under the lack of such important information, it is difficult to judge these model outputs are worth publication or not, despite the uniqueness and potential usefulness of the observational data itself. Also as for the measurements, the authors just cited the previous (~10 years ago) publications for ethane and propane measurements (Baker et al. 2010) and for methane measurements (Schuck et al. 2009), but I would recommend the author to describe information on the data quality control over the whole analysis period (2006–2016). For example, how the long-term stability of the standard gases has been maintained? Such information should be necessary for the readers and data users when using the data and interpreting the long-term trend of these trace gases over the decades, especially for ethane and propane, whose measurement uncertainties are comparable with the discussed observed trends.   

Considering the above, I would recommend this paper for publication after major revisions.

Specific comments

Title: I would recommend that the authors reconsider the title because this study present almost only the ethane trends and the data is obtained in the Upper Troposphere – Lower Stratosphere (UTLS) in the northern hemisphere, not in the global.

L10 “global”: Same as the comment in the title.

L66: Please clarify “the global ethane trend” and “long-term global ethane datasets”. Emissions or atmospheric mole fractions? Also, what is “local influences”?

L84 “global”: Same as the comment in the title.

L105-108: Please describe the calibration scales used in this study. Just “please see the references” is inconvenient for the readers who expect to use the dataset for this study. Also please mention the long-term stability of calibration gasses, as well as long-term reproducibility of the sample measurements.

L124: Cl is important for the modeling of ethane, methane, and propane. The authors need to discuss the issue more in the section 3 (e.g., L345-350 and L374-375).

L129-135: I think this part is very important for readers to know what the model optimization setup is. However, I don’t think it is well written at this stage.

L129-132: Did you increase not the fossil fuel combustion only, but the total anthropogenic emissions? If so, what is the justification of this? Then did you increase the total anthropogenic emissions to match which Tg/yr in Pozzer et al. (2020) (13.2 or 15.3 Tg/yr, as in Section 1)?

L132-135:  How to estimate the “45%”? Without the information of the optimization method, it is difficult to interpret the model results hereafter.

L144 “Prophet”: As long as I see the main figures (e.g. Fig. 2, 4, 5), I wonder how this algorithm can capture the discussed long-term trend over the decade. The results show noisy inter-annual variations, not averaged smoothed trend. Maybe the author also need to show the raw data obtained in this study, as done in Fig. S1.

L150-162 “changepoint_prior_scale”: Please explain the definition of the parameter; otherwise, the readers cannot understand what the uncertainty interval represents.

L155-158: If so, please show the comparison of fitting between NOAA algorithm and Prophet algorithm in the supplement Fig. S1?

L165-205: I wonder why the estimated ethane trend of this study is so different from those of previous studies (same as in L237-245, L364-371). Calculating the trend for another periods would be useful to directly compare the results (e.g., 2009-2014), but I also wonder how the “Prophet” can robustly estimate the trend, due to the large inter-annual variations (as commented in L144). Also, the authors need to explain how they separate the air mass origin between troposphere and stratosphere before presenting the result (e.g., in Method). If the criteria of troposphere and stratosphere is set to be “PV = 2”, as in L204, please describe it earlier.

L207: According to Fig. S2, the sample number obtained in Rest of the world (ROW) is dominant more than half of all data in the stratosphere. Then I wonder where the air samples are collected in the ROW and wonder if the current air mass classification is valid for interpreting the trend?

L213-215: Is the seasonality in ASI not statistically significant? It is interestingly showing the same peak timing with UT in June. Is it potentially indicating the intrusion of tropospheric air masses into stratosphere due to Asia summer monsoon (e.g., Park et al. 2007; Xiong et al. 2009).

L228-234: It seems saying about the discrepancy of the inter-annual variations between observation and model climatology, not that of the whole trend for 2006–2016. Also, the observational trends estimated by Prophet show very large inter-annual variations and thus it seems difficult to compare the long-term trends by this analysis.

L264-270: I would suggest the authors describe the source sub-category information in the method section. And then explain which sub-category emissions are optimized.

L272-274: Same as the general comment and specific comment (L129-135 and L264-270). I wonder how the author optimized the emission inventories.

L283-284 “AIR+BIB+BIO”: It is said they cannot be separated into regions. Then how are they included into the model simulation?

L295-301 & L364-371: How the authors calculate the long-term trend and uncertainties? Please describe it somewhere.

L304-307: But as presented in L264-270 and Fig. 2b-d and, the pronounced peak in 2010-2011 for NAM, ASI, and EUR are mainly explained by SWD (solid waste and waste water) and TRO (road transportation). Are these results consistent with the sentence here? As seen in Fig.2a, the peaks in FEF and RES are actually seen in 2010-2011. Are these happened in ROW then?

L316-317: Which figure are you referring to?

L321-L333: Again how to estimate the optimised emissions? Just by comparing with the observation with model output with “try and error” method? Needs more explanation. Also I wonder how meaningful to present the emission amount under 2 decimal places. As long as I understand, the presented optimization method is very rough and seems not to be able to estimate within such small amount of emissions.

L329-330: But, please be noted that the atmospheric ethane, methane, and propane in the UTLS region can be largely influenced by atmospheric transport and chemical destruction processes.

L336-337: It is not clear why the peaks in 2010 not seen at regional levels even though it was caused by global upward transport of the upper tropospheric ethane emissions. Maybe the contribution of ROW is important? Also see the comment in L207.

L345-350: Any possible explanation that the Cl in the stratosphere has increased after 2013? As mentioned here, the discrepancy between model and observation since 2013 is mainly seen only in ASI. Does it mean the Cl destruction process is only influential in the stratosphere in ASI?

L373-374: But why it is not seen in methane? Needs more explanation.

L374-375: Any possibility of ethane destruction by Cl increased after 2013? For example, the emissions CFCs from eastern mainland China has increased after around 2013 (Rigby et al. 2019; Montzke et al. 2021), which may have affected the increase of Cl production in the stratosphere and may have caused the larger destruction of ethane in ASI?

L379 Data availability: Please include the PV values, as well as other observed meteorological information, into the dataset for each air sample. These are important for readers to know how the air masses are classified into troposphere or stratosphere. Also please add the measurement information in more detail (e.g., calibration scales, precision, measurement number, standard deviation (if exists), and reference lists of the measurements). For the simplicity and usability, I would even suggest that the tropospheric and stratospheric data sheet can be combined into one sheet with timeseries, just by adding a flag column like “trop” or “strat”.

Figure 2: Gray lines are not plotted in Fig 2b–d

Figure 3: I’m not sure how this figure could be useful for readers. It is said the vertical scale of each source sector represents the source contributions, but the contributions to each geographical region shouldn’t be the same.

Figure 7: ­First I wondered why the authors need to show the “AIR + BIB + BIO_opt” here, with geographical sector contributions. After reading the sentence L283-284, I found it is said “AIR+BIB+BIO” cannot be separated into regions. But, then I wonder how they are included into the model simulation.

Technical corrections

L19: As already commented, I don’t think the two decimal place is meaningful under the current rough model optimization method.

L56: Tg/yr² ?

L394: Same as L19. ­­

Figure 5: There are two Figure 5!

References

Baker, A. K., Slemr, F., & Brenninkmeijer, C. A. M. (2010). Analysis of non-methane hydrocarbons in air samples collected aboard the CARIBIC passenger aircraft. Atmospheric Measurement Techniques, 3(1), 311-321. https://doi.org/10.5194/amt-3-311-2010

Schuck, T. J., Brenninkmeijer, C. A. M., Slemr, F., Xueref-Remy, I., & Zahn, A. (2009). Greenhouse gas analysis of air samples collected onboard the CARIBIC passenger aircraft. Atmospheric Measurement Techniques, 2(2), 449-464. https://doi.org/10.5194/amt-2-449-2009

Machida et al. (2008). Worldwide measurements of atmospheric CO2 and other trace gas species using commercial airlines. Journal of Atmospheric and Oceanic Technology, 25(10), 1744-1754. https://doi.org/10.1175/2008JTECHA1082.1

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