Current Scale: WMO X2007

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.

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

References
Standards that define the WMO X2007 CO2 scale
Cylinder Assigned value
(μmole/mole)
AL47-110 246.650
AL47-102 304.353
AL47-111 323.989
AL47-130 337.307
AL47-121 349.387
AL47-103 353.238
AL47-139 360.893
AL47-105 369.398
AL47-136 381.355
AL47-146 389.550
AL47-101 396.322
AL47-106 412.105
AL47-123 423.066
AL47-107 453.054
AL47-132 521.419