the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Results of a Long-Term International Comparison of Greenhouse Gas and Isotope Measurements at the Global Atmosphere Watch (GAW) Observatory in Alert, Nunavut, Canada
Douglas E. J. Worthy
Michele K. Rauh
Lin Huang
Felix R. Vogel
Alina Chivulescu
Kenneth A. Masarie
Ray L. Langenfelds
Paul B. Krummel
Colin E. Allison
Andrew M. Crotwell
Monica Madronich
Gabrielle Pétron
Ingeborg Levin
Samuel Hammer
Sylvia Michel
Michel Ramonet
Martina Schmidt
Armin Jordan
Heiko Moossen
Michael Rothe
Ralph Keeling
Eric J. Morgan
Abstract. Since 1999, Environment and Climate Change Canada (ECCC) has been coordinating a multi-laboratory comparison of measurements of long-lived greenhouse gases in whole air samples collected at the Global Atmosphere Watch (GAW) Alert Observatory located in the Canadian high Arctic (82°28' N, 62°30' W). In this paper, we evaluate the measurement agreement of atmospheric CO2, CH4, N2O, SF6, and stable isotopes of CO2 (δ13C, δ18O) between leading laboratories from 7 independent international institutions. The measure of success is linked to target goals for network compatibility outlined by the World Meteorological Organization’s (WMO) GAW greenhouse gas measurement community. Overall, based on ~8000 discrete flask samples, we find that the co-located atmospheric CO2 and CH4 measurement records from Alert by CSIRO, MPI-BGC, SIO, UHEI-IUP, ECCC, and NOAA are generally consistent with the WMO compatibility goals of ±0.1 ppm CO2 and ±2 ppb CH4 over the 17-year period (1999 – 2016), although there are periods where differences exceed target levels and persist as systematic bias for months or years. Consistency with the WMO goals for N2O, SF6, and stable isotopes of CO2 (δ13C, δ18O) has not been demonstrated. Additional analysis of co-located comparison measurements between CSIRO, SIO, and NOAA at other geographical sites suggests that the findings at Alert for CO2, CH4, N2O and δ13C-CO2 could be extended across the CSIRO, SIO, and NOAA observing networks. Two approaches are carried out to determine the level of agreement as a collective for the 7 individual laboratories (1) pooling the differences of individual laboratories over the entire sampling records from a designated reference laboratory and determining the 95th percentile range of these data points and (2) averaging the 2 standard deviations (2-sigma) of the means for all flask samples taken in each individual sampling episode over the entire sampling record. For CO2, from 5691 samples, we derive a measurement agreement level of -0.51 to +0.53 ppm using the 95th percentile range of the differences from NOAA measurements. Similarly, we derive a corresponding value of ± 0.37 ppm using the mean of 2-sigma values from 923 individual weekly sampling episodes. For CO2 isotopes using INSTAAR measurements as a reference, we derive measurement agreement values of -0.09 to +0.07 and ± 0.06 ‰ for δ13C and -0.50 to +0.58 and ± 0.31 ‰ for δ18O, for the 95th percentile ranges and the mean of the 2-sigma values, respectively. For other gases, the corresponding values for both approaches are 4.86 to +6.16 and ± 3.62 ppb for CH4, -0.75 to +1.20 and ± 0.64 ppb for N2O, and -0.14 to +0.09 and ± 0.09 ppt for SF6. These upper and lower limits represent our best estimate of the measurement agreement at the 95 % confidence level for these individual laboratories, providing more confidence for using these datasets in various scientific applications (e.g., long-term trend analysis).
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Douglas E. J. Worthy et al.
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CC1: 'Comment on amt-2023-99', Shinji Morimoto, 07 Jul 2023
“Results of a Long-Term International Comparison of Greenhouse Gas and Isotope Measurements at the Global Atmosphere Watch (GAW) Observatory in Alert, Nunavut, Canada” by Worthy et al.
This paper summarizes intercomparison observations of greenhouse gases and CO2 isotope ratios at Alert, Canada, one of the Global Atmospheric Watch (GAW) stations, by seven institutes. While a lot of research institutes carry out observations of greenhouse gases and CO2 isotope ratios, integrating their data requires confirmation of biases between observation results from each institute and, if necessary, bias correction. To achieve this, it is highly effective that each institute analyze air samples collected at the same location nearly simultaneously and examine the results. The authors have organized and conducted such intercomparison observations for over a decade since 1999, verifying the biases between the institutes and their temporal changes for CO2, CH4, N2O, SF6, which are important greenhouse gases, as well as CO2 isotope ratios. This paper is well-organized, and the comparison methodology employed is appropriate. It will be suitable for publication after authors’ consideration of the minor comments listed below.
Minor Comments:
- Because the abstract is very long, it would be better to keep it a little simpler. L66-71 should be integrated into method, and L71-80 should be a little more organized and shortened.
- For the atmospheric observations by flask sampling, concentrations of CO2 and N2O, and d18O-CO2 may change during sample storage in flasks. Did participating institutes correct for the changes in concentration and isotope ratio for the sample storage?
L198 Consider referencing Table 3 here (and re-numbering tables) to indicate "approximately the same time."
L243 Concentrations could change during cylinder depressurization.
L340, L384, L439 Whether "stopcock" and "valve" have the same meaning or they are used differently?
L440 "aspirated intake" may be difficult to imagine for those unfamiliar with O2/N2 measurements. Consider removing it if unnecessary.
L465 The meaning of "before being re-united with its mate" is unclear.
L505 An explanation for "JRAS-06 realization" or a reference regarding JRAS-06 are necessary.
L617 Figures 1-5 should be specified here.
L677-681 The concentration scale change from WMO-X2007 to X2019 does not affect the conclusions of this paper. However, does the scale change from SIO-X08A to X12A affect the conclusion regarding the comparison with SIO?
L724 It seems unnatural that the difference in the CO2 concentration between SIO and NOAA shows seasonal variations only at ALT. Is there any possible causes?
L1109 Why is only N2O biased by the sample collection procedure?
L1191, L1194, L1248, L1360, L1409 Formats of a part of reference are different from the AMT standards.
For Figs. 1-6, (a) marker size is too large to make all research institutions' data visible. Also, consider adding legends to (c).
Since there are a lot of tables, could you consider re-organizing and moving some of them to the “supplementary materials”.
Citation: https://doi.org/10.5194/amt-2023-99-CC1 - AC1: 'Reply to CC1', Lin Huang, 09 Sep 2023
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RC1: 'Comment on amt-2023-99', Anonymous Referee #1, 08 Jul 2023
In "Results of a Long-Term International Comparison of Greenhouse Gas and Isotope Measurements at the Global Atmosphere Watch (GAW) Observatory in Alert, Nunavut, Canada" Douglas Worthy and co-authors present the results of long time series of atmospheric flask samples analysed for greenhouse gases to check the whether different laboratories succeed in obtaining the WMO GAW recommended compatibility goals for greenhouse gases, as recommended in WMO (2020), and specifically whether the observations at Alert observatory analysed by the Canadian ECCC are compatible with 5 other global labs.
This important paper is clear and well written, but unfortunately way too long. I appreciate the level of detail and accuracy invested to document all the important details that are useful and needed to understand the conclusions, but in its current shape it is hard for the reader to see the tress for the forest. Also it is good to have all data now available in one place for future analyses. So I recommend to not remove but move many details (e.g. those in section 2.3 and 3.x inlcuding most of the figures) to a supplement and replace those sections with more general and short descriptions of the commonalities.
I also agree with the first reviewer that the abstract also need to be shortened (by about 50%).
I would also appreciate an explanation why the documentation and analysis took so long (first results are from 1999 and latest results analysed are from 2017) and depending on the reasons would like to see recommendations on how this could be improved in the future, as timely information on scale differences between labs is crucial to detect and improve compatibility issues. And what would this mean for the global GAW network, should all associated labs cooperate in these regular comparisons and with which frequence and minimal delay time should the data be analysed? Would a central database like now set up by NOAA be helpful in coordinating the results?
WMO (2020): GAW report #255, 20th WMO/IAEA Meeting on Carbon Dioxide, Other Greenhouse Gases and Related Measurement Techniques (GGMT-2019). Available at: https://library.wmo.int/index.php?lvl=notice_display&id=21758
As I assume a major rewite is needed I only have a few smaller textual comments and suggestions:
L94: please quote the most recent WMO GAW GHG Bulletin (2022) available at https://library.wmo.int/index.php?lvl=notice_display&id=22149
L104: remove being
L107 : world data centers -> the world data center
L115: Start new line with: However,
L124 In this regard, -> Fore these reasons,
L245: explain co-sponsored (what support is provided?)
Citation: https://doi.org/10.5194/amt-2023-99-RC1 - AC2: 'Reply to RC1', Lin Huang, 09 Sep 2023
- AC3: 'Reply to RC1', Lin Huang, 09 Sep 2023
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RC2: 'Comment on amt-2023-99', Martin Steinbacher, 14 Jul 2023
review of manuscript amt-2023-99
Title: Results of a Long-Term International Comparison of Greenhouse Gas and Isotope Measurements at the Global Atmosphere Watch (GAW) Observatory in Alert, Nunavut, Canada by Worthy et al.
General comments:
The manuscript presents a comprehensive analysis of results from a co-located long-term flask sampling comparison in the high Arctic. The flask samples cover a period of 17 years. Seven international institutions, all well renowned in the atmospheric monitoring of greenhouse gases, participated in the comparison. The paper is well structured and written. It is very comprehensive and provides an excellent overview of what can be achieved in terms of precision and accuracy by the leading experts in greenhouse gas observations. The manuscript includes an excessive number of tables. The authors may consider moving Tables 6 to 22 into the supplementary material for the sake of conciseness of the main part.
The paper is definitely within the scope of ‘Atmospheric Measurement Techniques’. I have only a few minor comments and suggestions for improvement, all listed below.
Chapter 2 briefly summarizes the different sampling approaches and the analytical techniques used. I do not suggest to elaborate on many additional details here since relevant references to the existing literature are provided in the bibliography. However, I was wondering if some specific sampling and analytical methods do systematically contribute to the differences described in the results section. Factors could be the pressure in the flasks during filling, drying/no drying/the way of drying (cryo vs. Mg(ClO4)2) of the air prior to sampling, the time between sampling and analysis (which I assume can be rather long considering the remoteness of the site; rough numbers could be given), effects of single-stopcock vs. double-stopcock flasks (i.e. flushing of the flasks), or (changes) in the analytical technique. Relevant specifications should be mentioned accordingly. What are the most striking lessons-learnt after all these years?
For example, the change of the analytical technique from NDIR to CRDS at SIO in 2012 seems to be observable in Fig. 1 b (panel with the purple symbols, less noise in the difference in the more recent data).
Chapter 3 is rather descriptive. There, reference could be made to the different sampling and analysis methods (see my comment just above) if they can explain some of the observed differences.
Due to the characteristics of ALT, almost exclusively clean-air samples were analyzed and compared. Can you make a (qualitative) statement about the to-be-expected agreement under more polluted conditions?
At least for some of the 17 years, continuous in-situ measurements (for some of the species) were performed. How do the continuous data compare with the flask data? Please add at least a short (quantitative) statement, if applicable.
Are the sampling procedures at MLO and CGO identical to the described methods for ALT?
Specific comments:
Line 182ff.: low operating costs are mentioned as a benefit of the flask sampling program. I tend to agree in terms of investment, but the logistics, shipping costs, and manpower requirements for the off-line analysis are significant. Can you somehow quantify the effort over all these years? 8'000 flasks are mentioned in the abstract but I assume that there are also other quite impressive numbers. Numbers of boxes shipped to ALT? Total distance travelled by the flasks? … Any idea about the manpower required for flask preparation, sampling, and analysis? Total costs of the endeavour? No reference is made to any funding agency in the acknowledgements. This makes it even more impressive (that you were able to keep up this comparison exercise over all these years).
Line 281 – 283: "Flask air samples were collected at Alert during persistent southwesterly wind conditions, when wind speeds were greater than 1.5 m s-1 for several hours prior to sample air collection." In other words, there was no sampling when the wind conditions were inappropriate? If so, sampling was retried as soon as the conditioned matched the requirements, or did you skip this sampling and stick to the regular (weekly, bi-weekly) schedule (the following week)?
Lines 344 – 345: "When meteorological conditions are favorable for sampling, the NOAA sampler is taken outside and several meters away from the GAW laboratory to collect the air samples." What happened when conditions were unfavorable? Flasks for the other institutions were filled but no NOAA flasks? How is "favorable" defined?
Lines 367 ff.: Is it correct that SIO flasks are not pressurized at all (only the valve of the evacuated is opened for filling)?
Line 387: "… pressurized, vented and re-pressurized …" To which pressure?
Line 398 "double-stopcock flasks" vs. line 405 "double-valve stopcock flasks". Please harmonize the wording.
Table 4 reads "GC/ECD" etc. while "GC-ECD" etc. is used in the text.
Calibration scales given in Table 4: most scales are WMO/GAW scales hosted by the GAW Central Calibration Laboratory at NOAA (X2007, X2004A, …); SIO uses X08A, which does not seem to be an official WMO/GAW scale. Is there a reference that documents the WMO CO2 X2007 to X08A consistency?
Citation: https://doi.org/10.5194/amt-2023-99-RC2 - AC4: 'Reply on RC2', Lin Huang, 09 Sep 2023
Douglas E. J. Worthy et al.
Douglas E. J. Worthy et al.
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