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The current WMO CO2 Mole Fraction Scale consists of a set of 15 CO2-in-air primary standard calibration gases ranging in CO2 mole fraction from 250 to 520 μmol mol^-1 CO₂ in dry air. The 15 WMO primary standards have been calibrated at regular intervals, between 1 and 2 years, by the ESRL/GMD manometric system. The uncertainty of the 15 primary standards was estimated to be 0.069 μmol mol^-1 (one-sigma). Manometric calibrations results indicate that there is no evidence of overall drift of the Primaries from 1996 to 2004. In order to lengthen the useful life of the Primary standards, GMD has always transferred the scale via NDIR analyzers to the secondary standards. The mean reproducibility of NDIR transfer calibrations was about 0.03 μmol mol^-1 (two sigma) from 1979 to present.

You can also read
The WMO Mole Fraction Scales for CO₂ and other greenhouse gases, and uncertainty of the atmospheric measurements

See:

Zhao, C. L. and P. P. Tans (2006), Estimating uncertainty of the WMO mole fraction scale for carbon dioxide in air, Journal of Geophysical Research-Atmospheres, 111(D8), 10.1029/2005JD006003.

Zhao, C., P.P. Tans and K.W. Thoning (1997), A high precision manometric system for absolute calibrations of CO₂ in dry air. Journal of Geophysical Research102(D5):5885-5894