The current WMO
CO2 Mole Fraction Scale consists of a set of fifteen CO2 –in-air
primary standard calibration gases ranging in CO2 mole fraction from
250 to 520 micromol/mol. Since the WMO
CO2 Expert Group transferred responsibility for maintaining the WMO Scale from the Scripps Institute of
Oceanography (SIO) to the Climate Monitoring and Diagnostics Laboratory (CMDL) in 1995, the fifteen WMO
primary standards have been calibrated at regular interval, between one and two
years, by the CMDL manometric
system. From mid-1996 to 2001, the assigned CO2 values of the WMO Primaries have been jointly based on the SIO
and CMDL manometric measurements,
and completely on the CMDL
manometric measurements alone from 2001 to present. The uncertainty of the 15
primary standards is estimated to be 0.07 micromol/mol in the one-sigma
absolute scale. Manometric calibration results indicated 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, CMDL
has always transferred the WMO
Scale to the Secondaries via NDIR analyzers. The uncertainties arising from the
analyzer random error and the propagation error due to the uncertainty of the
reference gas concentration are discussed. Precision of NDIR transfer
calibrations is about 0.01 micromol/mol from 1979 to present. Propagation of
the uncertainty is calculated theoretically. In the case of interpolation, the
propagation error is estimated to be between 0.05 and 0.07 micromol/mol when
the Primaries are used as the reference gases via NDIR transfer calibrations.
Author: C. Zhao, and P. Tans (Conglong dot Zhao at noaa dot gov)
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