Calibration of Nondispersive Infrared CO2Analyzers with CO2-in-Air Reference Gases

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A set of eight CO2-in-air secondary standard calibration gases has been established by NOAA/Geophysical Monitoring for Climatic Change (GMCC) for use in its global CO2 monitoring program. Use of these gases obviates the need for pressure broadening corrections that arises when CO2 air sample analyses are performed with nondispersive infrared analyzers using CO2-in-N2 calibration gases. The gases are composed of clean mountain air, with suitably adjusted CO2 content, pumped into 49 1 chrome-molybdenum steel tanks to 150 atm pressure. Five of the tank gases have been calibrated manometrically at the Scripps Institution of Oceanography (SIO) to an accuracy of about ±0.10 parts per million (ppm). Calibration of the remaining gases is traceable to manometric primary standards maintained at SIO. The secondary standards are used periodically in Boulder, Colorado, to calibrate tertiary CO2 standards with which, in turn, field station standards and working reference gases are calibrated. Both the tertiary standards and the field station gases are commercially available CO2-in-synthetic-air gases which contain natural proportions of nitrogen, oxygen and argon. Calibrations by SIO since 1978 suggest that the CO2 of NOAA/GMCC secondary standard gases may be drifting at a rate of 0.05.ppm yr−1. Several CO2-in-air tanks used in Boulder for CO2 flask air sample analyses have exhibited drifts of up to 0.39 ppm yr−1. The CO2-in-synthelic-air reference gases appear to be more stable, generally exhibiting CO2 concentration drift rates of from 0 to 0.10 ppm yr−1.