Measurement report: Atmospheric CH<sub>4</sub> at regional stations of the Korea Meteorological Administration–Global Atmosphere Watch Programme: measurement, characteristics, and long-term changes  of its drivers

Lee, Haeyoung; Seo, Wonick; Li, Shanlan; Lee, Soojeong; Kenea, Samuel Takele; Joo, Sangwon

doi:https://doi.org/10.5194/acp-23-7141-2023

Articles | Volume 23, issue 12

https://doi.org/10.5194/acp-23-7141-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-23-7141-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 23, issue 12

Measurement report

|

28 Jun 2023

Measurement report |

| 28 Jun 2023

Measurement report: Atmospheric CH₄ at regional stations of the Korea Meteorological Administration–Global Atmosphere Watch Programme: measurement, characteristics, and long-term changes of its drivers

Haeyoung Lee, Wonick Seo, Shanlan Li, Soojeong Lee, Samuel Takele Kenea, and Sangwon Joo

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RC1:
'Comment on acp-2022-600', Anonymous Referee #1, 07 Nov 2022
Review's comments

Manuscript Number: acp-2022-600

Title: Measurement report: atmospheric CH₄ at regional stations of the Korea Meteorological Administration/ Global Atmosphere Watch Programme: measurement, characteristics and long-term changes of its drivers

Authors: Lee H., Seo W.-I., Li S., Lee S, Kenea S, and Joo S.

The authors of this study present high-quality record of the atmospheric CH₄ mole fractions measured at three WMO/GAW stations in Korea: Anmyeondo (AMY), Jeju Gosan Suwolbong (JGS), and Ulleungdo (ULD). To confirm the data quality in detail, the authors evaluate the total measurement uncertainties at the three sites, which are less than GAW’s compatibility goal for atmospheric CH₄ measurement (±2 ppb). Then, they investigate the characteristic features of the CH₄ variations by precisely analyzing the excess CH₄ mole fractions from the background variations, the diurnal cycles, the seasonal cycles, and the growth rates. From PSS analysis and the ¹³CH₄ isotopic data obtained at nearby site (TAP), they also discuss the temporal changes in the source region and the dominant source categories from a 5-year period of 2000-2006 to that of 2016-2020. Although I think the discussion based on the PSS and isotopic analyses have some problems, I believe that the data presented in this report are quite reliable and contribute to the global modeling studies especially for the CH₄ emission analysis from the east Asia.

I found that the paper contains material that should be published in Atmospheric Physics and Chemistry. However, I think that the authors should make more efforts to improve the manuscript as is suggested in followings before publication.

General comments:

I’m not sure why the authors discuss in depth the influence of water vapor on the CRDS analyzer in Section 2.3 and 3.1. The CRDS analyzer provides dry CH₄ mole fraction by measuring the water vapor simultaneously. The study of Rella et al. (2013) revealed that the preliminary reduction of water vapor of sample air less than 1% allow CRDS analyzer to measure the CH₄ mole fraction within the level of GAW’s compatibility goal (±2 ppb). The authors describe that the air samples are dried to −50â. I believe that the laboratory standard gases and working standard gases uses in this study are sufficiently dried (I think that the absolute dew points should be clarified in the text). In addition, if the air samples are dried by a cold trap of −50â, the dilution effect is comparable to the reproducibility of the CRDS analyzer. Therefore, I think it would be better to simplify the description about H₂O influence on the CRDS measurement.

I’m very curious about the diurnal cycles of the L3 hourly data. If the data selection method described in Section 2.4.2 effectively remove the local effect, there is no statistically significant diurnal cycles in the L3 data. Such information would be useful to make the L3 data more reliable.

I don’t agree with the discussion based on the PSS and the δ¹³CH₄ analyses in Section 3.5.As for the PSS analysis, I’m not sure how to interpret the plots shown in Figs 9a and 9b. Although the authors seem to consider that the plots reflect the CH₄ flux distributions, the flux distribution is considerably different from the previously reported flux distributions (e.g., Ito et al., 2019). Additionally, I cannot believe that such large temporal change in the flux distribution occurred between the two periods. So, I think that the authors should clarify what the plots based on the PSS analysis mean. The authors only show the plots based on the observation at AMY. But I’m curious about the similar plots based on the data at JGS and ULD. If the distributions based on JGS and ULD are similar to Fig. 9b, such result would convince us to some extent that the PSS analysis is reliable. I’m not sure what the isotopic analysis means. The authors would like to suggest that influences of the biogenic sources increased from 2006-2010 to 2016-2020 because the δ¹³C_(CH4) values for the latter period shows faster decreasing rates than those for the former period. However, the background δ¹³C_(CH4) data also show the secularly decreasing trend (for example, you can see the recent δ¹³C_(CH4) change in NIWA’s home page: https://niwa.co.nz/atmosphere/our-data/trace-gas-plots/methane). So, how do the authors distinguish the regional influence on the δ¹³C_(CH4) values from the background change? Additonally, why don’t the authors use Miller-Tans plot to evaluate of the δ¹³C values for the CH₄ regional sources? On the other hand, the values of the intercept of the Keeling plot for CS and KL increased from 2006-2010 to 2016-2020, suggesting the influence of the biogenic sources relatively decreased. This result is inconsistent with the above result. I’m also curious about the δ¹³C_(CH4) data during the period from 2010 to 2016. Why don’t the authors discuss the period from 2010 to 2016?

Specific comments:

Page 2, line 28: “Kim et al., 2014” should be “Kim et al., 2015”.

Page 2, line 30-32: “Li et al., 2020” and “Li et al., 2022” are not listed in References.

Page 2, line 36: “Turnbull et al., 2015” is not listed in References.

Page 3, line3: “Watanabe et al. 2000” is not listed in References.

Page 4, line 17: What’s the “−90â” mean? If it’s not the trap temperature, it should be removed to avoid confusion.

Page 5, line 6: I believe that the four laboratory standards are kept in high-pressure cylinders. I think that it would be better to clarify what kind of cylinders are used for the laboratory standards (volume and material).

Page 5, line 13-20: I’m a little bit confused by this part. I think that the CRDS analyzer (Picarro) give dry mol fractions of the atmospheric CH₄. So, why does the authors discuss the dilution effect of water vapor?

Page 5, line 17 (Eq. 1): It should be better to clarify what the C_dilution and C_dry mean. And the definition of H_act, “H₂O difference between standard gases and samples”, is rather confusing. Is it “H₂O difference between laboratory standard gases and working standard gases”?

Page 7, line 10-11: It should be better to clarify the detail of the fitting function (order of polynomial and order of harmonics) and the cut-off frequency for the digital filtering based on Thoning et al. (1989).

Page 7, line 12: It should be better to clarify how to compute the monthly data, long-term trend, and seasonal amplitude in detail.

Page 7, Table 2: There are no values of MS for AMY and ULD. Does it mean that MS criteria were not applied to the data for AMY and ULD?

Page 7, line 25-26: The geological locations of WLG and RYO are already given in page 3, line 1-3.

Page 8, line 6-8: It should be better to add the reference for the CH₄ isotopic measurement at INSTAR.

Page 8, line 5 and 9: It says here that the distance between TAP and AMY is 24 km. But other parts in this manuscript, the distance is 28 km. Which value is correct?

Page 8, line 9-10: It should be better to add a figure showing synoptic scale variations of CH₄ observed at AMY and TAP.

Page 8, line 35-page 9, line 4 (2.7 PSS analysis): Since I’m not sure PSS analysis, I tried to read the cited papers, Remann et al., 2004, 2008; Li et al., 2017. But these papers are not listed in References. If the single line for each backward trajectory is used for the calculation of C_(i,j), the sensitivity of the potential source strength increase with the distance from the start point in comparison with the reality. This effect would result in the overestimation of the potential source strength with increasing the distance from the station, wouldn’t it?

Page 12, line 8-9: Are the characteristics of the bivariate polar plots for the three stations (Figs. 3-5) same with those for the other years?

Page 18, line 10-11: The seasonal cycle of the atmospheric CH₄ is also influenced by the atmospheric transport.

Page 18, line 19: The distance between TAP and AMY is repeatedly described in this manuscript.

Page 18, line 20: “during” what?

Page 19, Figure 7b: Why don’t the authors plot the negative growth rate in the figure?

Page 19, line 12: Was the annual growth rate calculated from the annual means based on the monthly means, listed in Table 4? If so, it would be helpful for the readers to describe that in the manuscript.

Page 19, line 15: CH₄ at AMY, JGS, JLD, and RYO showed significant increases from 2016 to 2017, but small (or negative) increases from 2017 to 2018.

Page 20, line 1: There is relatively large differences in the CH₄ growth rate between WLG and the WMO global mean in 2017 and 2020.

Page 20, line 5-6: There is no description about the δ¹³CH₄ data at AMY. Please add the explanations for the data. In addition, since the plot shown in Fig. S3 is not Keeling plot but Miller-Tans plot, the slopes of the plots represent the δ¹³C of the CH₄ sources.

Page 21, line 1: “Sources affecting CS and KL were paddy fields and …” ?

Page 21, line 13: I’m not sure what “this assumption” means.

Page 21 line 3-14: Are all the flask data collected at TAP plotted in Figs 9? Are the source regions of all the flask data classified into three sectors, CS, CN, and KL? How are they classified?

Page 26, line 21-23: “Shuang-Xi et al., 2013” is not cited in the text.

Page 27, line 3-5: “Winderlich et al., 2010” is not cited in the text.
Citation: https://doi.org/10.5194/acp-2022-600-RC1
- AC1: 'Reply on RC1', Haeyoung Lee, 17 Feb 2023
  
  Thank you for your comments on the paper’s value. We also appreciate your helpful comments to improve our manuscript. According to your specific comments and considerations, we revised our manuscript. Please confirm our attachment here.
  
  Citation: https://doi.org/10.5194/acp-2022-600-AC1
RC2:
'Comment on acp-2022-600', Anonymous Referee #2, 23 Nov 2022

The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-600/acp-2022-600-RC2-supplement.pdf

Citation: https://doi.org/10.5194/acp-2022-600-RC2
- AC2: 'Reply on RC2', Haeyoung Lee, 17 Feb 2023
  
  Thank you for your comments on the paper’s value. We also appreciate your helpful comments to improve our manuscript. According to your specific comments, we revised our manuscript, especially isotope analysis results. Also, we tried to explain PSS analysis more detailed. Please confirm our attachment here.
  
  Citation: https://doi.org/10.5194/acp-2022-600-AC2

Peer review completion

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

AR by Haeyoung Lee on behalf of the Authors (17 Feb 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (27 Feb 2023) by Tanja Schuck

RR by Anonymous Referee #2 (12 Mar 2023)

RR by Anonymous Referee #1 (13 Mar 2023)

Suggestions for revision or reasons for rejection

Review's comments
Manuscript Number: acp-2022-600
Title: Measurement report: atmospheric CH4 at regional stations of the Korea Meteorological Administration/ Global Atmosphere Watch Programme: measurement, characteristics and long-term changes of its drivers
Authors: Lee H., Seo W.-I., Li S., Lee S, Kenea S, and Joo S.

I have gone through the response to my comments and the revised manuscript and found the manuscript has been improved to some extent. However, I think that there remain some problems which should be resolved before acceptance. The authors should respond the following comments before publication.

1) I’m not sure what the sentence, “However, isotopic signature using δ13CH4 explained that CH4 sources are becoming mixture of not only biogenic but also fossil fuel sources in East Asia from 2006 to 2020”, means. I believe that the CH4 sources in East Asia included fossil fuel-related source also before 2006.
2) The added sentence in the revised manuscript, “… since generic water correction cannot be applied to each instrument in different environments” (page 4, line 21) is a little bit misleading. The previous studies including Rella et al. (2013) and Hazan et al. (2016) indicated that the generic manufacturer’s water correction function does not necessarily provides appropriate values for wet air samples. However, as Rella et al. (2013) indicated, the preliminary reduction of water vapor of sample air less than 1% allow us to measure the CH4 mole fraction within the level of GAW’s compatibility goal (±2 ppb). So, I think it would be better for the authors to simply mention that the sample air is dried to reduce the humidity effect less than the detection limit instead of correcting the humidity effect based on the generic manufacturer’s water correction function or independently determined water correction function.
3) In the revised manuscript, the authors mentioned “AMY started flask sampling for CH4 and δ13C(CH4) in December 2013 with same method like TAP and these data were used only for characterization of CH4 at AMY in section 3.5” (page 8, line 22-23). But there is no discussion based on the δ13C(CH4) data for AMY in this paper.
4) The authors should explain how to prepare the Miller-Tans plots shown in Fig. 9b, Fig. 6S, and Fig. 7s because most readers are not necessarily familiar with the plot. In particular, the basic equation for the Miller-Tans plot should be described in the manuscript. And then, the authors should clarify how to determine the background values of the CH4 concentration and isotope ratio. Additionally, a proper reference should be cited in the manuscript for the Miller-Tans plot.
5) I’m not sure the discussion based on the regional isotopic signatures from the Miller-Tans plots in page 22 line 12-20. The isotopic values can give us the information about the mix of the isotopically different CH4 sources of the regional emissions causing the excess CH4 at TAP. Therefore, the temporal and spatial changes in the isotopic ratio probably reflect the changes in the mix of the sources. But there is no information about emission strength in the isotopic ratio for the regional emissions. Therefore, I think it is difficult to discuss the variation in the CH4 growth rate from the result based on the isotopic analysis in this study. For example, I cannot understand why the authors conclude “This suggested that CH4 growth rate in East Asia was affected not only biogenic but also pyrogenic sources, unlike global.” (page 22, line 12-13). If the authors want to discuss the cause of the trend variation in East Asia, the isotopic ratios for the base line should be discussed. But the authors also mentioned “The long-term trend at AMY was very similar to the global trend” (page 21, line 21-22). So, I think the trend of isotopic ratio at AYM is also similar to the global trend. The similar discussion about the trend in East Asia based on the isotopic analysis is also described in Section 4 (page 24, line14-15).
6) I’m curious about whether there is seasonality in the source regions (CN, CS, and KL) affecting the observation at TAP. If there is seasonality, does it explain to some extent the differences in the isotopic ratios for the source regions shown in this study? For example, if the air masses are transported from CN to TAP during winter, it is expected that the isotopic signature would be relatively higher because the biogenic CH4 emissions are reduced during winter.
7) I found several mistakes in English in the manuscript as some examples are shown in the followings:
Page 1, line 20-21: “… growth rate increased 3.3 ppb yr-1 …” → ” … growth rate increased by 3.3 ppb yr-1 …”
Page 1, line 30: “…compared with that of other long-lived greenhouse gases…” → ” …compared with those of other long-lived greenhouse gases…
Page 7, line 18: “… days moving median of of HA…”
Page 10, line 2: “The BLH can was obtained …”
Page 22, line 13: “… was affected not only biogenic but also pyrogenic…” → ”…was affected by not only biogenic but also pyrogenic …”
Therefore, I think that it should be better to check the language in general.
8) Added sentences for Eq. 1 (page 6, line 7-9) are rather confusing. The Eq. (1) simply expresses the dilution effect caused by the water vapor (Hact). But I cannot understand what the sentences, “Here Hact is the H2O difference between laboratory and working standards. Cdry is the CH4 mole fraction of laboratory standard while Cdilution is the expressed CH4 mole fraction when Hact existed.”, mean. Do the authors consider the water mole fraction in the laboratory standard is zero? If not, the exact relationship among the H2O differences between laboratory and working standards (HW-HL), the CH4 mole fraction of standard with HL (CdilutionL) and HW (CdilutionW) is expressed by the following equations:
CdilutionW = CdilutionL – 0.01(HW-HL) ·Cdry
Or
CdilutionW / CdilutionL = (1 – 0.01·HW) / (1 – 0.01·HL),
where Cdry represents the dry CH4 mole fraction of the standard.
However, I think above discussion is too detailed. In the manuscript, it would be better to simply examine the absolute H2O mole fraction of the standard gases and the dried air samples and discuss the dilution effects according to Eq. (1). I believe that the resultant changes in the CH4 mole fractions caused by the H2O dilution effect are negligible.
9) Page 5, line 28: The URL https://gml.noaa.gov/ccl/service.html should be https://gml.noaa.gov/ccl/services.html.
10) Page 23, Fig. 9(a): It would be better to clarify the boundaries of the northern China (CN), southern China and Korea local (KL) in the map.

Referee Report: PDF

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ED: Publish subject to minor revisions (review by editor) (13 Mar 2023) by Tanja Schuck

AR by Haeyoung Lee on behalf of the Authors (27 Mar 2023) Author's response

EF by Polina Shvedko (29 Mar 2023) Manuscript Author's tracked changes Supplement

ED: Publish subject to minor revisions (review by editor) (05 Apr 2023) by Tanja Schuck

AR by Haeyoung Lee on behalf of the Authors (11 Apr 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (13 Apr 2023) by Tanja Schuck

AR by Haeyoung Lee on behalf of the Authors (14 Apr 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (19 Apr 2023) by Tanja Schuck

AR by Haeyoung Lee on behalf of the Authors (04 May 2023) Manuscript

Short summary

We introduced three Korea Meteorological Administration (KMA) monitoring stations with monitoring systems and measurement uncertainty. We also analyzed the regional characteristics of CH₄ at each KMA station. CH₄ levels measured at KMA stations are compared to those measured at other Asian stations. From the long-term records of CH₄ and δ¹³CH₄ at AMY, we confirmed that the source of CH₄xs changed from the past (2006 to 2010) to recent (2016 to 2020) years in East Asia.

Measurement report: Atmospheric CH4 at regional stations of the Korea Meteorological Administration–Global Atmosphere Watch Programme: measurement, characteristics, and long-term changes of its drivers

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Measurement report: Atmospheric CH₄ at regional stations of the Korea Meteorological Administration–Global Atmosphere Watch Programme: measurement, characteristics, and long-term changes of its drivers