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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-408
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-408
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  11 Nov 2020

11 Nov 2020

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This preprint is currently under review for the journal AMT.

Revision of the WMO/GAW CO2 Calibration Scale

Bradley D. Hall1, Andrew M. Crotwell1,2, Duane R. Kitzis1,2, Thomas Mefford1,2, Benjamin R. Millera, Michael F. Schibig3,4, and Pieter P. Tans1 Bradley D. Hall et al.
  • 1National Oceanic and Atmospheric Administration, Global Monitoring Laboratory, Boulder, CO, USA
  • 2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 3Climate and Environmental Physics, University of Bern, Bern, Switzerland
  • 4Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland
  • apreviously at: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA

Abstract. The NOAA Global Monitoring Laboratory serves as the WMO/GAW Central Calibration Laboratory (CCL) for CO2 and is responsible for maintaining the WMO/GAW mole fraction scale used as a reference within the WMO/GAW program. The current WMO-CO2-X2007 scale is embodied by 15 aluminum cylinders containing natural air, with CO2 mole fractions determined using the NOAA manometer from 1995 to 2006. We have made two minor corrections to historical manometric records: fixing an error in the applied second virial coefficient of CO2, and accounting for loss of a small amount of CO2 to materials in the manometer during the measurement process. By incorporating these corrections, extending the measurement records of the original 15 primary standards through 2015, and adding four new primary standards to the suite, we define a new scale, identified as WMO-CO2-X2019. The new scale is 0.18 μmol mol−1 (ppm) greater than the previous scale at 400 ppm CO2. While this difference is small in relative terms (0.045 %), it is significant in terms of atmospheric monitoring. All measurements of tertiary-level standards will be reprocessed to WMO-CO2-X2019. The new scale is more internally consistent than WMO-CO2-X2007 owing to revisions in propagation, and should result in an overall improvement in atmospheric data records traceable to the CCL.

Bradley D. Hall et al.

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Latest update: 29 Nov 2020
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Short summary
We have recently revised the carbon dioxide calibration scale used by numerous laboratories that measure atmospheric CO2. The revision follows from an improved understanding of the manometric method used to determine the absolute amount of CO2 in an atmospheric air sample. The new scale is 0.18 μmol mol−1 (ppm) greater than the previous scale at 400 ppm CO2. While this difference is small in relative terms (0.045 %), it is significant in terms of atmospheric monitoring.
We have recently revised the carbon dioxide calibration scale used by numerous laboratories that...
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