Characterizing Tracer Flux Ratio Methods for Methane Emission Quantification Using Small Unmanned Aerial System

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This is a well-executed and timely manuscript that addresses a significant issue in emissions monitoring, namely the accurate quantification of methane at the site level. The authors present a robust study comparing various sUAS flight strategies combined with the tracer gas method. The experimental design is rigorous and thoroughly described. The text is well written, the data are clearly presented, and the results are lucid and discussed in detail. The manuscript provides highly valuable information. I recommend the publication of this article, subject to some minor revisions.

 

Lines 24–25: The sentence could be more clearly formulated, given that TD and BU represent the only two possible methodological approaches. For instance, a clearer phrasing might be: “CH₄ emissions can be measured using top-down (TD) or bottom-up (BU) approaches.”

Lines 25–27: Top-down approaches are not limited to fixed ground-based sensors (such as eddy covariance towers); mobile systems are also used, including vehicle-based platforms employed in tracer gas methods, as well as DIAL and DOAS techniques. Moreover, the role of drones is not mentioned, despite being central to the focus of the paper. It is recommended that the authors better articulate the full range of TD approaches—encompassing satellite, aircraft, drone, and ground-based (fixed and mobile) platforms—to more clearly delineate their respective scopes and applications.
Furthermore, mass balance is not the only methodological option. The sentence could be revised to something like: “TD approaches employ satellites, aircraft, drones, or ground-based instruments (fixed or mobile) to measure CH₄ enhancements over a given site, estimating emissions using various methods such as mass balance, micrometeorological techniques, inverse dispersion modelling, or tracer gas approaches.”

Line 32: There is a typographical error (“?”) in the references that should be corrected.

Lines 40–48: Although the manuscript focuses on drone-based applications, it presents a thorough overview of existing sensor technologies without referencing their integration with drones. Yet, there is a growing body of literature on the use of such sensors on drone platforms, which should be cited to better contextualise the discussion in relation to sUAS. For instance, the following studies could be considered:

• Yong, H.; Allen, G.; Mcquilkin, J.; Ricketts, H.; Shaw, J.T., 2024. Lessons learned from a UAV survey and methane emissions calculation at a UK landfill. Waste Management, Volume 180, Pages 47-54. ISSN 0956-053X, https://doi.org/10.1016/j.wasman.2024.03.025.
• Vinković K., Andersen T., de Vries M., Bert Kers B., van Heuven S., Peters W., Hensen A., van den Bulk P., Chen H., 2022. Evaluating the use of an Unmanned Aerial Vehicle (UAV)-based active AirCore system to quantify methane emissions from dairy cows. Science of The Total Environment, Volume 831, 154898, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2022.154898.
• Morales, R., Ravelid, J., Vinkovic, K., Korbeń, P., Tuzson, B., Emmenegger, L., Chen, H., Schmidt, M., Humbel, S., and Brunner, D., 2022. Controlled-release experiment to investigate uncertainties in UAV-based emission quantification for methane point sources. Meas. Tech., 15, 2177–2198, https://doi.org/10.5194/amt-15-2177-2022.

Furthermore, it is noted that open-path technologies are not mentioned. These represent an innovative line of research that complements the overview of sensors discussed. There are also some existing studies in this area aimed at quantifying emissions at the plant scale, with reference to controlled releases:

• Yang, S., Talbot, R.W., Frish, M.B., Golston, L.M., Aubut, N.F., Zondlo, M.A., Gretencord, C., Mc Spiritt, J., 2018. Natural Gas Fugitive Leak Detection Using an Unmanned Aerial Vehicle: Measurement System Description and Mass Balance Approach. Atmosphere, Vol. 9, 383. https://doi.org/10.3390/atmos9100383.
• Golston, L.M.; Aubut, N.F.; Frish, M.B.; Yang, S.; Talbot, R.W.; Gretencord, C.; McSpiritt, J.; Zondlo, M.A., 2018. Natural Gas Fugitive Leak Detection Using an Unmanned Aerial Vehicle: Localisation and Quantification of Emission Rates. Atmosphere 9, 333. https://doi.org/10.3390/atmos9090333.

And particularly under real emission conditions:

• Fosco, D., Molfetta, M.D., Renzulli, P., Notarnicola, B., Carella, C., & Fedele, G., 2025. Innovative drone-based methodology for quantifying methane emissions from landfills. Waste Management, Vol. 195, pp. 79–91. https://doi.org/10.1016/j.wasman.2025.01.033.

Line 46: Citation missing [?]

Lines 83-86: It is suggested that the authors remove this sentence, as it refers to well-known advantages of sUAS that are already widely recognised and discussed in the literature. The subsequent sentence conveys these advantages more clearly and in a manner better aligned with the specific context and objectives of the study.

Lines 98-100: I would rephrase this sentence as it contains inaccuracies. The tracer gas method was developed precisely to avoid the need to consider the coefficient α (which, being used to describe turbulent flow, is complex to define without modelling simplifications), rather than to relate increases downwind of the source.

Lines 196-197: “Rate” should be plural.

Line 535: In my opinion, it would be better to write “enhancing the accuracy of emission quantification.”

Line 877: References [4] and [33] appear to refer to the same publication.

 

Additional comments

1. Please ensure consistency in the nomenclature throughout the text and figures. For example, nitrous oxide is sometimes referred to by its chemical formula N₂O and at other times by its full name. Although the meaning is clear, standardisation would improve readability.
2. Was a specific criterion used to ensure that the distances considered were sufficient for full plume mixing, or were literature-based or empirical references used instead?
3. I did not find any references in the section regarding the heights at which transects were performed. This information would be useful to understand the sampling density.
4. Nitrous oxide and ethane have a higher molecular weight than methane (2.74 and 1.87, respectively). This difference could increase the uncertainty of equation (2). Presumably, at the scale of your tests, you observed that it had no significant effect?

 

Author Response

Thank you very much for taking the time to review this manuscript. Please find our detailed responses below; the corresponding revisions are highlighted in blue in the re-submitted files.

Comments 1: [Lines 24–25: The sentence could be more clearly formulated, given that TD and BU represent the only two possible methodological approaches. For instance, a clearer phrasing might be: “CH₄ emissions can be measured using top-down (TD) or bottom-up (BU) approaches.”]
Response 1: [Thank you for bringing this to our attention. We fully agree with your observation and have thoughtfully incorporated your suggestion into the revised manuscript.]

Comments 2: [Lines 25–27: Top-down approaches are not limited to fixed ground-based sensors (such as eddy covariance towers); mobile systems are also used, including vehicle-based platforms employed in tracer gas methods, as well as DIAL and DOAS techniques. ]
Response 2: [Thank you for bringing this to our attention. We fully agree with your observation and have thoughtfully incorporated your suggestion into the revised manuscript.]

Comments 3: [Line 32: There is a typographical error (“?”) in the references that should be corrected.]
Response 3: [Thank you for bringing this to our attention. While we did not find a reference with a "?" typographical error as noted, your comment helped us identify and correct a reference that contained incomplete information.]

Comments 4: [Lines 40–48: Although the manuscript focuses on drone-based applications, it presents a thorough overview of existing sensor technologies without referencing their integration with drones. Yet, there is a growing body of literature on the use of such sensors on drone platforms, which should be cited to better contextualise the discussion in relation to sUAS. For instance, the following studies could be considered]
Response 4: [Thank you for bringing this to our attention. To maintain focus in the introduction, we initially kept the discussion on sensors brief. However, we have now incorporated the literature you suggested to enrich our coverage of sUAS and enhance the depth of the introduction.]

Comments 5: [Line 46: Citation missing [?]]
Response 5: [We reviewed line 46 and confirmed that the statement is properly referenced.]

Comments 6: [Lines 83-86: It is suggested that the authors remove this sentence, as it refers to well-known advantages of sUAS that are already widely recognised and discussed in the literature. The subsequent sentence conveys these advantages more clearly and in a manner better aligned with the specific context and objectives of the study.]
Response 6: [Thank you for your thoughtful suggestion. While we acknowledge that the advantages of sUAS are well established in the literature, we have chosen to retain this sentence because this is the first publication arising from this body of work. The sentence serves to establish a foundational context for referencing this study in future publications related to the same experimental campaign. We believe this brief mention supports continuity and clarity across related manuscripts.]

Comments 7: [Lines 98-100: I would rephrase this sentence as it contains inaccuracies. The tracer gas method was developed precisely to avoid the need to consider the coefficient α (which, being used to describe turbulent flow, is complex to define without modelling simplifications), rather than to relate increases downwind of the source.]
Response 7: [Thank you for pointing this out. We have reviewed and corrected the statement accordingly.]

Comments 8: [Lines 196-197: “Rate” should be plural.]
Response 8: [We have corrected this]

Comments 9: [Line 535: In my opinion, it would be better to write “enhancing the accuracy of emission quantification.”]
Response 9: [Thank you for bringing this to our attention. We fully agree with your observation and have thoughtfully incorporated your suggestion into the revised manuscript.]

Comments 10: [Line 877: References [4] and [33] appear to refer to the same publication.]
Response 10: [Thank you for your observation. However, References [4] and [33] refer to distinct papers. Reference [4] focuses on the measurement methods, while Reference [33] presents the measurement results. In fact, there are at least two separate publications that specifically address the measurement results from this study.]

Additional Comments

Comments 1: [Please ensure consistency in the nomenclature throughout the text and figures. For example, nitrous oxide is sometimes referred to by its chemical formula N₂O and at other times by its full name. Although the meaning is clear, standardisation would improve readability.]
Response 1: [Thank you for the suggestion. We have addressed this by standardizing the nomenclature throughout the text and figures. While we use the full name (e.g., nitrous oxide) when first introducing a term—particularly at the beginning of bullet points—we consistently use the chemical formula thereafter to maintain clarity and readability.]

Comments 2: [Was a specific criterion used to ensure that the distances considered were sufficient for full plume mixing, or were literature-based or empirical references used instead?]
Response 2: [Thank you for your question. We did not apply a specific criterion to ensure full plume mixing at the distances considered. One of the objectives of this study was to evaluate how different ratio estimation methods perform under conditions where plumes are not fully mixed. This approach reflects real-world challenges—particularly in midstream operations—where it is often difficult to take measurements at locations with fully mixed plumes.]

Comments 3: [I did not find any references in the section regarding the heights at which transects were performed. This information would be useful to understand the sampling density.]
Response 3: [Thank you for your comment. We did not include a reference in this section because the sampling heights were determined dynamically based on the observed concentration boundary layer during each flight. The drone was flown to span the vertical extent of the plume, and the upper limit of the transect was defined as the height at which the CH₄ concentration enhancement dropped below 0.25 ppm.]

Comments 4: [Nitrous oxide and ethane have a higher molecular weight than methane (2.74 and 1.87, respectively). This difference could increase the uncertainty of equation (2). Presumably, at the scale of your tests, you observed that it had no significant effect?]
Response 4: [Thank you for your insightful comment. In our study, we assumed that molecular weight and density differences between methane and the tracer gases (nitrous oxide and ethane) had negligible impact at the scale of our experiments. However, we recognize the potential influence of these factors on uncertainty. Ongoing tests are being conducted to determine the scale at which molecular weight corrections become significant, and we plan to present those findings in a future publication]

Reviewer 2 Report

Comments and Suggestions for Authors

This paper introduced an optimized technology for measuring and quantifying methane emissions from natural gas facilities, using small unmanned aerial system and tracer flux method. Methods were clearly described, and data and results were very well presented. Advantages and limitations of the optimized flight were adequately discussed. Overall this is a high quality work and could be accepted in current form. 

Author Response

Dear Reviewer,

Thank you very much for taking the time to review our manuscript. We truly appreciate your positive assessment and your recommendation to publish the paper in its current form. Your support and confidence in the quality of our work mean a great deal to us.

Reviewer 3 Report

Comments and Suggestions for Authors

Accurately measuring the facility-level emission rate of greenhouse gases is crucial for mitigating global climate change. In this manuscript, the authors designed several experiments to assess the effectiveness of methane (CH4) emission measurement using unmanned aerial vehicles (UAVs). Particular emphasis was placed on the selection of sampling trajectories and methods for estimating methane-to-tracer concentration ratios. The results offer valuable references for future CH4 emission measurements using UAVs, thus indicating that this manuscript is worthy of publication. Nevertheless, several issues need to be addressed: 

1. The Results section is hard to understand. It is recommended to cite relevant tables or figures to support key statements and conclusions. 

2. Add the scale heights of the bars representing N2O and CH4 concentrations in Figure 1(b). 

3. In lines 124-125, check the units ppb/s and ppt/s. 

4. How were the background values of CH4 determined?

5. In section 2.3, it is suggested to present the three scenarios in a table, clearly describing what was emitted and the location of the emission. 

6. Figure 2 was not referenced in the text. 

7. Provide detailed information about all the flights, including the time, trajectories, and meteorological conditions, etc. 

Author Response

Thank you very much for taking the time to review this manuscript. Please find our detailed responses below; the corresponding revisions are highlighted in blue in the re-submitted files.

Comments 1: [The Results section is hard to understand. It is recommended to cite relevant tables or figures to support key statements and conclusions.]
Response 1: [Thank you for your comment. We appreciate your suggestion regarding the clarity of the Results section. Upon review, we found that the relevant figures and tables were already cited to support the key statements and conclusions.]

Comments 2: [Add the scale heights of the bars representing N2O and CH4 concentrations in Figure 1(b).]
Response 2: [Thank you for your suggestion. Figure 1(b) has been updated to include the peak concentration values for both N₂O and CH₄, as well as other relevant information to enhance interpretability. Additionally, the scale heights of the bars representing N₂O and CH₄ concentrations have been added to provide clearer context for the visual comparison.]

Comments 3: [ In lines 124-125, check the units ppb/s and ppt/s.]
Response 3: [Thank you for your observation. Upon review, we confirm that the units “ppb/s” and “ppt/s” used in lines 124–125 are correct as stated. These units were taken directly from the user manual provided by the instrument’s manufacturer.]

Comments 4: [How were the background values of CH4 determined?]
Response 4: [Thank you for your question. The background determination methodology is described in Sections 2.1 (lines 114-115) and 2.2 (lines 157-158). Specifically, each flight began with an upwind transect at the highest planned altitude to characterize background concentrations. These background values were defined as stable measurements taken outside the influence of the plume, with no detectable enhancement in CH₄ levels.]

Comments 5: [In section 2.3, it is suggested to present the three scenarios in a table, clearly describing what was emitted and the location of the emission]
Response 5: [Thank you for the helpful suggestion. We have added the requested summary as Table 1, which outlines the three experimental scenarios and the substances emitted in each case. To conserve space and avoid redundancy, the specific emission locations were not included in the table, as they are already clearly illustrated in Figure 1(b).]

Comments 6: [Figure 2 was not referenced in the text. ]
Response 6: [Thank you for your comment. We acknowledge that Figure 2 was not referenced earlier in the text. To address this, we have now added a clear citation to Figure 2 in lines 220–223 to ensure proper integration and improve the flow of the manuscript.]

Comments 7: [Provide detailed information about all the flights, including the time, trajectories, and meteorological conditions, etc. ]
Response 7: [Thank you for your comment. We appreciate your suggestion to provide comprehensive flight information. The flight times have been included in the manuscript and can be found in lines 352 and 370. Additional relevant details, including flight trajectories and other contextual information, are also presented throughout the manuscript. Regarding meteorological conditions, we have not included this information as it is not directly relevant to our study]

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have responded to the issues I put forward. However, I still maintain that "ppb/s" is not a standard way to express sensitivity. It is recommended to express it as "** ppb within an integration time of 1 s". 

Author Response

Comment 1: [The authors have responded to the issues I put forward. However, I still maintain that "ppb/s" is not a standard way to express sensitivity. It is recommended to express it as "** ppb within an integration time of 1 s"]
Response 1: [We value your feedback and have revised wording to better reflect your recommendation. Our original wording was based off the specific wording from the instrument data sheets provided by the manufacturer of the MIRA Strato analyzer(s) and so we will also plan to keep your recommendation in mind going forward in future correspondence and publications to better reflect this standard]