the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Carbon reduction requires attention to the contribution of natural gas use: Combustion and leakage
Abstract. Natural gas will continue to replace coal in the process of global energy structure reform, but its leakage potential can delay the realization of global carbon neutrality. To quantify its impact, we established a carbon dioxide (CO2) and methane (CH4) emission flux detection platform on the 220-m platform of the Institute of Atmospheric Physics, Chinese Academy of Sciences, located in northwestern Beijing. The observation results indicated that the daily mean CO2 and CH4 fluxes were 12.21±1.75 µmol·m−2·s−1 and 95.54±18.92 nmol·m-2·s-1, respectively. The daily variations in the emissions of these two gases were highly consistent, and their fluxes were significantly correlated with natural gas consumption, indicating that natural gas has become a common source of CH4 and CO2. Vehicle-based identification demonstrated that methane can escape at the storage and use stages of natural gas. Based on natural gas consumption data, the upper limit of the calculated natural gas leakage rate in Beijing reached 1.12±0.22 %, indicating that the contribution of CH4 to climate change could reach 23 % of that of CO2 on a 20-year scale. Natural gas leakage was estimated to delay the time for China to achieve carbon neutrality by almost three years.
- Preprint
(1084 KB) - Metadata XML
-
Supplement
(2205 KB) - BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2024-3931', Anonymous Referee #1, 18 May 2025
-
AC1: 'Reply on RC1', Guiqian Tang, 23 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3931/egusphere-2024-3931-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Guiqian Tang, 23 Jul 2025
-
RC2: 'Comment on egusphere-2024-3931', Anonymous Referee #2, 15 Jun 2025
General:
The authors report a combination of local scale measurements made with an eddy covariance tower and a mobile unit in the city of Beijing, and inventorial data to assess the impact of methane usage (e.g combustion) and leakage once scaled to China GHG budgets, especially considering the trajectory of replacing coal with natural gas.
The objective of the paper is very important and very ambitious. The authors report leakages are not included in China emission inventory and attempt to provide estimates.
However they base the analysis on local and sporadic measurements, and scale them to the expected carbon neutrality trajectory for entire China. This is an interesting exercise but arbitrary, without any uncertainty analysis, assuming leaks computed (partially) on a local scale are valid elsewhere.
Temporal representativity:
L109-112 (on eddy data): "This measurement lasted from June 11 to September 7, 2022, during which the nitrogen cylinder was replaced, and the instrument was debugged on June 18 and 19. From July 12 to 26, the experiment was stopped due to failure of the tower power supply.
L152-153 (on mobile measurements): “Vehicle-based experiments were conducted in the urban area of Beijing in the winter of 2023 and the summer of 2024”.
It appears that the eddy covariance measurements were made only on summer 2022, with also a data gap; while mobile measurements were made sporadically in the winter and summer of two different years.
This setup poses a serious concern on emission measurements and the fact that those data are scaled to derive country based estimates.
Seasonality is very important on any GHG flux including urban natural gas that is used for heating. This short campaign does not allow any trend or seasonal analysis, while authors claim CO2 and CH4 emission are increasing due to increased natural gas usage. This is not clear.
L175-185 Results: authors report flux data on multiple years and multiple tower elevations, but it’s not clear if there are other towers or where these data originate from. Methods report one tower for only few weeks of measurements.
Spatial representativity:
Footprint analysis of eddy covariance appears lacking and incorrect:
L134-136: “In addition, the flux source area was evaluated via the method of Kljun et al. (Text. S1), and the flux source area covers most of the urban area of Beijing and reflects the average emission characteristics at the regional scale.” This is a totally erroneous definition of a footprint, that does not cover the urban area of Beijing nor a regional scale but instead a limited area around the tower. No footprint analysis is reported.
No attempt to assess the spatial representativeness of the eddy covariance footprint, nor that of the mobile measurements, is made, challenging any spatial upscale from these data.
Overall, the measurements appear insufficient and not properly used to infer larger scale estimates. Annual budgets and country scale budgets for the entire China couldn’t be made with such few data in any case.
Minor comments:
Title appears misleading: "Carbon reduction"
in Fig 4 different units are used for the past and future trends
units missing at line 335
L339-341: "we assume that the leakage rate does not have significant seasonal variability because of the positive correlation between methane flux and natural gas consumption". This is not clear, and seasonality in leak fluxes has been reported in several studies.
ref 40 (likely inventory emission trajectories) does not point to any docuement that can be found online.
Citation: https://doi.org/10.5194/egusphere-2024-3931-RC2 -
AC2: 'Reply on RC2', Guiqian Tang, 23 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3931/egusphere-2024-3931-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Guiqian Tang, 23 Jul 2025
Viewed
HTML | XML | Total | Supplement | BibTeX | EndNote | |
---|---|---|---|---|---|---|
679 | 67 | 24 | 770 | 46 | 25 | 46 |
- HTML: 679
- PDF: 67
- XML: 24
- Total: 770
- Supplement: 46
- BibTeX: 25
- EndNote: 46
Viewed (geographical distribution)
Country | # | Views | % |
---|
Total: | 0 |
HTML: | 0 |
PDF: | 0 |
XML: | 0 |
- 1
Review of “Carbon reduction requires attention to the contribution of natural gas use: Combustion and leakage”.
The manuscript presents the results of a 73 day long observational campaign of methane (CH4) and carbon dioxide (CO2) fluxes made on a tall tower in Beijing. In addition, surface mobile measurements over different time periods were also made to address some specific geospatial regions within the domain covered by the flux tower. The methods to prepare and analyze the data are very standard. The findings include that the emissions of CH4 are likely anthropogenic in nature due to their similarity in time and direction to those of CO2. Comparisons were made with other previous campaigns in earlier years. They then draw some conclusions about the changes in CO2 and CH4 over time and relate these to various different policies.
The authors clearly have demonstrated that their basic measurements of flux are reasonable and representative. There should be no doubt about this point, and hence the fundamental data underlying the project looks sound. However, there are many issues. One such issue about the data is that the individual half-hour averaged flux time series over the entire time studied is not available anywhere. However, the details in the figures of the entire-campaign averaged hour-by-hour data clearly demonstrates that the hour-to-hour and day-to-day variability are both important. They also demonstrate that there are issues likely occuring at the half-hour scale, but they cannot be analyzed or discussed based on the current figures and data provided. Therefore, analyzing the data or evaluating analysis done cannot be validates, and the potential strong impacts of these 30-minute scale variations cannot be analyzed or presented. This weakens the paper.
Furthermore, there are many technical issues with the paper, as outlined in detail below. Perhaps it is just writing style or many small and unintentional mistakes. However, the net of all of these small mistakes and mis-communications in tandem lead to serious doubts as to the overall findings of the work.
Another issue is that the subsequent analysis of the data does not present any new perspectives or science. Advances in analysis by the community over the past few years should be followed. The base 30-minute and vehicle-obtained data both need to be processed at higher temporal and spatial frequency, not a mere “summer” comparison. More rigorous techniques than simple linear correlation between concentration enhancements and single variables (the other species concentration enhancement or temperature) also need to be performed. Uncertainty in the models, in the data, and in the assumptions need to be considered. Analysis of variance and of multi-species need to be performed in tandem. Specific details are presented in more detailed comments below.
Additionally, the use of background subtraction may lead to substantial errors. First, there are the issues of the observational uncertainty in the background value. Second, there are more modern papers demonstrating that background subtraction is not needed. Third, in those cases in which long-range transport is present, background subtraction is flawed in connection with the flux tower computational assumptions. This is because the equations underlying the flux calculation assume that the upper air is clean and that the emissions come from the local surface. Recent papers have demonstrated that there is in fact long-range transport into Beijing from upwind industrial sources in central China, and therefore any such events would need to be excluded from the data before analysis is performed. I raise this point since in analysis done both by my group as well as others, the time period studied in this work contains at least one such long-range transport event. Analysis of the 30-minute time series may help identify this event, and possibly others as well. In addition, this paper introduces the use of a 5-minute window to identify background values. However, given the size of the domain, this is not consistent. The observed wind speed will take more than 5 minutes to go from the edge of the domain to the tower location, and hence the length of the averaging period must be at least this long. This will change from day-to-day and hour-to-hour. The time likely needs to be longer, to account for any atmospheric recycling occurring within the domain.
For all of these reasons, I recommend that the work undergo major revisions before it be considered further. However, due to the strong people on the team, I do believe that with a considerable amount of hard work and time, that they can raise the level of the paper to such that it will make a good ultimate contribution to ACP. I am happy to continue to work with any future revisions which are brought forward.
Specific Issues:
Of course, there are newer techniques such as published in ACP in 2025 this year based on a study of CH4 in central China which completely does away with background subtraction and enhancement calculation. You could consider this new approach as well and completely avoid the issues of enhancement and background subtraction. Or you can work hard to justify why your background subtraction is valid and how it contributes to overall uncertainties in the conclusions.