| 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. |
| 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 |
| 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. |
