An Operational Intercalibration Experiment
Between CMDL and CSIRO to Measure Several Atmospheric Trace Species
Atmospheric trace species that are important to the question of global and climate change have been measured by both CMDL and CSIRO for many years through their networks of flask sampling sites. There has been some overlap of these networks, particularly at the locations of the baseline observatories. A desirable, even essential goal is to be able to "seamlessly merge" the trace species data obtained by the two laboratories, creating a larger and self-consistent data set that can be reliably used for modeling and interpretive studies of trace gas budgets, with particular emphasis on the global carbon cycle. The advantages of this approach have already been demonstrated through several studies of the carbon cycle which make use of the isotopic measurements of carbon dioxide [e.g., Ciais et al., 1995a,b,c]. The initial phases of an intercomparison of stable isotope measurements of CO2 by both laboratories has been provided by Francey et al. .
The most commonly adopted method of determining the degree of agreement in
the measurement of atmospheric trace gases by different laboratories has been
the periodic exchange of standard gases, usually contained in high-pressure
cylinders. This method was in use for many years by those laboratories measuring
atmospheric CO2, and is endorsed by the World Meteorological Organization
[Pearman, 1993]. While these types of experiments can yield important
information, they suffer from the disadvantages that the frequency of intercalibration
is too low, perhaps occurring only once every few years, and the analysis methodology
sometimes differs from that employed in the routine observing programs.
In an attempt to improve upon the existing methods of trace species intercalibration between CSIRO and the CMDL Carbon Cycle Group (CCG), we proposed an experiment in 1991 that was agreed upon between the two laboratories. It began in late 1991 and is ongoing. The glass flasks that are filled with air (in pairs) approximately once a week for CCG at the Cape Grim baseline station are routed through CSIRO's GASLAB on their way back to Boulder. One member of every second pair of flasks (i.e., one in every four flasks) is measured in GASLAB for several trace species (CO2, CH4, CO, H2, and 13C and 18O in CO2). The flask sample measured in this way is clearly marked, therefore, it can be identified when the air samples reach Boulder. All flask air samples, including the one measured in GASLAB, are then measured for the same suite of trace species in the CCG laboratories. This type of experiment is possible only because the analytical methods used in GASLAB consume relatively small volumes of air, typically less than 200 mL for the full suite of measurements. The techniques used for the GASLAB measurements are outlined in Francey et al. . A crucial requirement for this experiment to be successful is that the trace composition of the samples measured in GASLAB must not be altered in any way by this process. The design of the experiment should allow any such alteration to be identified.
Exchanges of data from this experiment between the two laboratories were initially
sporadic. Visual inspection indicated that there was reasonable agreement for
CH4, CO, and 13CO2. An offset of about 0.6
ppm between the CO2 records was thought to be due to an offset in
the calibration scales being used by the two laboratories. A thorough analysis
of the full data set is being prepared for publication. This analysis and improvements
in procedures to allow regular and systematic comparison of the results by staff
of both laboratories has been greatly facilitated by support from both laboratories
for an extended visit by one of the authors (KAM) to CSIRO Division of Atmospheric
Research during 1996.
Ciais, P., P.P. Tans, M. Trolier, J.W.C. White, and R.J. Francey, A large northern hemisphere terrestrial CO2 sink indicated by the 13C/12C ratio of atmospheric CO2, Science, 269, 1098-1102, 1995a.
Ciais, P., R.J. Francey, P.P. Tans, J.W.C. White, and M. Trolier, An analytical error estimate for the ocean and land uptake of CO2 using 13C observations in the atmosphere, NOAA Tech. Memo. ERL CMDL-8, NOAA Environmental Research Laboratories, Climate Monitoring and Diagnostics Laboratory, Boulder, Colorado, 13 pp., 1995b.
Ciais, P., P.P. Tans, J.W.C. White, M. Trolier, R.J. Francey, J.A. Berry, D.R. Randall, P.J. Sellers, J.G. Collatz, and D.S. Schimel, Partitioning of ocean and land uptake of CO2 as inferred by 13C measurements from the NOAA Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network, J. Geophys. Res., 100, 5051-5070, 1995c.
Francey, R.J., C.E. Allison, L.P. Steele, R.L. Langenfelds, E.D. Welch, J.W.C. White, M. Trolier, P.P. Tans, and K.A. Masarie, Intercomparison of stable isotope measurements of CO2, in Climate Monitoring and Diagnostics Laboratory No. 22 Summary Report 1993, edited by J.T. Peterson and R.M. Rosson, pp. 106-110, NOAA Environmental Research Laboratories, Boulder, CO, 1994.
Francey, R.J., L.P. Steele, R.L. Langenfelds, M.P. Lucarelli, C.E. Allison, D.J. Beardsmore, S.A. Coram, N. Derek, F.R. de Silva, D.M. Etheridge, P.J. Fraser, R.J. Henry, B. Turner, E.D. Welch, D.A. Spencer and L.N. Cooper, Global Atmospheric Sampling Laboratory (GASLAB): Supporting and extending the Cape Grim trace gas programs, in Baseline Atmospheric Program (Australia) 1993, edited by R.J. Francey, A.L. Dick, and N. Derek, pp. 8-29, Bureau of Meteorology and CSIRO, Division of Atmospheric Research, Melbourne, Australia, 1996.
Pearman, G., Survey of the results of the 1991-1992 round-robin interlaboratory
intercomparison, in Report of the Seventh WMO Meeting of Experts on Carbon
Dioxide Concentration and Isotopic Measurement Techniques, Rome, Italy,
September 7-10, 1993, edited by G.I. Pearman and J.T. Peterson, pp. 104-107,
World Meteorological Organization, Global Atmosphere Watch, Report No. 88, 1993.