Global CO2-Measurements

FIGURE 2 WMO round-robin intercalibrations of CO2 measurement laboratories (identified by country only). Plotted are measured differences from mixing ratios assigned by NOAA CMDL. Data for (a) a circulation conducted between 1991-93, and (b) between 1995-97. 

Preliminary results of this round robin are given in Fig. 3 (Allison et al, in press). Measured differences are reported with respect to initial measurements conducted at CSIRO in November 1996. CSIRO(2) refers to CSIRO measurements conducted after circulation in July 1998, confirming the stability of the tank standards. Measurements on pure CO2 samples scatter by about 0.02%o, outside the required target. For the analyses of the whole-air standards the situation is much worse, with reported values scattered over a range of almost 0.1 %o, suggesting serious 

The situation is even more serious than indicated by the round-robin comparisons. Since 1992, with the aim of confirming our ability to merge data from two different measuring laboratories, CMDL and CSIRO commenced an operational intercalibration (also referred to as the ICP, Inter-Comparison Program, also the flask-air-sharing comparison). Both CSIRO and CMDL 

FIGURE 3 IAEA round robin intercalibrations t)f 5 C of CO, using both pure CO, (GS20) and whole air in high-pressure cylinders (in which 5 C is related to CO, mixing-ratio difference from ambient values by about —0.03%o ppm ). USA and Japanese measurement laboratories are identified by number only. Plotted are measured differences from clfi C assigned by CSIRO prior to circulation. CSIRO(2) refers to analyses after circulation. 

The reason for this offset in flasks compared to high-pressure cylinders is not yet fully understood. I lowever, development of a low-flow (15 ml min ), high-precision ( 7 ppb), and highly 

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