Japan Meteorological Agency (JMA)
CO2 ROUND-ROBIN REFERENCE GAS INTERCOMPARISON 1995 - 1997

James Peterson, Pieter Tans, and Duane Kitzis (NOAA, USA)

At the WMO-sponsored CO2 Experts Meeting held in Boulder, Colorado, during July 1995, it was decided to proceed with a standard reference gas intercomparison. The purpose of the intercomparison would be NOT to distribute a calibration scale, but rather to determine the precision of the current practice of international calibration. The Experts stated that a goal for inter-Laboratory precision is 0.1 ppm. All international institutions with CO2 monitoring programs were encouraged to participate. Dr. Pieter Tans served as the intercomparison coordinator. Dr. James Peterson served as the referee.
The protocol for the standard reference gas intercomparison was similar to that used for the previous intercomparison held during 1991 - 1992. An important difference, however, would be that all participants should also report values for several gases in addition to CO2, if they have the capability to do so. These included methane, carbon monoxide, and δ13C and δ18O of CO2.
Twenty-four Laboratories agreed to participate (see Table 1). They were divided into three globally-distributed groups; namely, North America and the Southern Hemisphere, Asia, and Europe. The NOAA Climate Monitoring and Diagnostics Laboratory prepared nine cylinders for the intercomparison; three each of approximate CO2 mixing ratios of 345, 360, and 375 ppm. Then, sets of three cylinders were circulated within each geographical group. Each Laboratory was instructed to treat each cylinder as containing gases of unknown mixing ratios and to analyze the three cylinders as if they were "working tanks" within their programs. The results were reported to the referee, who kept the results confidential until all Laboratories had reported. Each set was analyzed first and last at NOAA to determine if there was any change in mixing ratios while the cylinders were circulating.
The last two institutions on the list were initially scheduled to participate in the intercomparison but did not because of delays in circulating the standard tanks and a desire to complete the exercise in time to report preliminary results at the Aspendale Expert Meeting.
The results for carbon dioxide concentrations reported to the referee and presented at the Aspendale meeting are shown below in Table 2. Results for the other species measured for the intercomparison will not be presented here. Interested readers are referred to the full report to be published by WMO.

TABLE 1. List of participating institutions by geographical group.
GROUP ONE
United States - National Oceanic and Atmospheric Administration
New Zealand - National Institute of Water & Atmospheric Research
Australia - Commonwealth Scientific & Industrial Research Organization
United States - Scripps Institution of Oceanography
United States - Harvard University
United States - National Institute of Standards & Technology
Canada - Atmospheric Environment Service
GROUP TWO
United States - National Oceanic and Atmospheric Administration
China - Chinese Meteorological Agency
Japan - Tohoku University
Japan - National Institute for Resources and Environment
Japan - National Institute for Environmental Studies
Japan - Meteorological Research Institute
Japan - Japan Meteorological Agency
Korea - Seoul National University
GROUP THREE
United States - National Oceanic and Atmospheric Administration
France - Centre des Faibles Radioactivites
Italy - Italian Meteorological Service/Monte Cimone
Italy - Energy Nuclear and Energy Alternative (ENEA)
Germany - Umweltbundesamt
Germany - University of Heidelberg
Hungary - Hungarian Meteorological Agency
Sweden - Stockholm University
Spain - National Institute of Meteorology

TABLE 2. Round Robin Results -- CARBON DIOXIDE CONCENTRATIONS
LaboratoryAnalysis DateTank Number
GROUP ONE135711551230
US - NOAAAug-94342.21358.90376.79
NZ - NIWAFeb-96342.20359.01376.98
AU - CSIROMay-96342.11358.89376.76
US - SIOAug-96NO REPORT
US - HARVOct-96342.24359.05376.96
US - NISTMay-97342.16359.01376.90
CA - AESJun-97342.14358.91376.76
US - NOAAAug-97342.16358.92376.77
GROUP TWO198811591223
US - NOAAAug-94342.72359.49371.94
CN - CMADec-95342.66359.48371.91
JP - TOHOKUJun-96342.72359.53371.95
JP - NIREJul-96342.62359.45371.87
JP - NIESAug-96342.71359.58372.05
JP - MRIDec-96342.77359.56372.07
JP - JMAJan-97342.80359.80372.23
KR - SNUMar-97342.97359.62372.22
US - NOAAJul-97342.74359.50371.92
GROUP THREE115711641224
US - NOAAAug-94345.40356.94372.37
FR - CFRJan-96345.50357.00372.53
IT - M. CIM.Mar-96345.44356.98372.44
IT - ENEAApr-96345.11356.86372.56
DE - UMWELT.Jul-96345.17356.73372.23
DE - U HEIDEL.Sep-96345.25356.83372.28
HU - HMADec-96344.18355.88371.57
US - NOAAJul-97345.40356.89372.37

To show the results in a more comprehensive way that allows one to compare the reports of all Laboratories together, we prepared Table 3. Here, the results of Table 2 are combined showing the report of each Laboratory as the difference from NOAA. This is not to imply that the NOAA values are "correct," but rather, it is only a means to intercompare all the Laboratories since the tanks of each Group were measured at NOAA.

TABLE 3. Round Robin Results -- CARBON DIOXIDE Concentration. Differences from
NOAA (LABORATORY minus NOAA, ppm)
LABORATORYANALYSIS DATECONCENTRATION DIFFERENCE
FROM NOAA (ppm)
Tank Concentration RangeLowMediumHigh
NZ - NIWAFeb-960.020.100.20
AU - CSIROMay-96-0.07-0.02-0.02
US - HARVOct-960.060.140.18
US - NISTMay-97-0.020.100.12
CA - AESJun-97-0.040.00-0.02
CN - CMADec-95-0.07-0.01-0.02
JP - TOHOKUJun-96-0.010.040.02
JP - NIREJul-96-0.11-0.04-0.06
JP - NIESAug-96-0.020.090.12
JP - MRIDec-960.040.070.14
JP - JMAJan-970.070.310.30
KR - SNUMar-970.240.130.29
FR - CFRJan-960.100.080.16
IT - MT. CIMONEMar-960.040.060.07
IT - ENEAApr-96-0.29-0.060.19
DE - UMWELT.Jul-960.00-0.020.11
DE - U HEIDEL.Sep-96-0.15-0.09-0.09
HU - HMSDec-96-1.22-1.04-0.80

The results of this intercomparison were generally positive and encouraging. There was significant improvement from the last intercomparison (1991 - 1992), especially for the high concentration range (~375 ppm). Only one Laboratory showed a large disagreement. Specifically, excluding that Laboratory, 16 of 18 Laboratories were within a range of 0.25 ppm for the Low Scale (~343 ppm), 17 of 18 were within a range of 0.25 for the Middle Scale (~358 ppm), and 16 of 18 were within a range of 0.30 for the High Scale (~375 ppm). For reference, note that background concentrations in early 1998 at high northern latitudes exceeded 370 ppm, in the tropics they were about 365 ppm, and at South Pole they averaged about 362 ppm.
While the Experts noted with satisfaction the significant improvement in inter-Laboratory agreement from the last intercomparison, they also agreed that much hard work remains to achieve the stated goal of 0.1 ppm precision for routine monitoring by the international community.
A full report of the intercomparison, including results of the other species, is in preparation. A report "CO2 Round-Robin Reference Gas Intercomparison, 1995 - 1997," by Peterson, Tans, and Kitzis will be printed as a WMO GAW Technical Publication.

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Last Updated: 2002.07.25
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