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Assessment of the stability and relationships between the NIES-09, SIO-X12, and WMO-X2019 CO2 scales
by Mr. Andrew Crotwell, Dr. Ralph Keeling, Dr. Motoki Sasakawa, Dr. Stephen Walker, Dr. Bradley Hall


Long-term monitoring of atmospheric CO2 requires stable reference scales to ensure observed trends and spatial gradients are not influenced by changes in the underlying scale. A critical aspect of determining scale stability is the direct comparison with other independently held scales. The National Institute for Environmental Studies (NIES), the Scripps Institution of Oceanography (SIO), and the National Oceanic and Atmospheric Administration (NOAA) have maintained independent CO2 in air scales for decades for use by the atmospheric monitoring community. The scales are based on two distinct SI traceable methods; manometry at SIO and NOAA, and gravimetry at NIES. A primary objective of all three programs is to ensure the internal consistency of the scales over long time periods. Changes or updates to the scales are applied retroactively where appropriate to preserve the internal consistency. Versions of the scales are differentiated by name to improve transparency. Current versions of the three scales held by NEIS, SIO, and NOAA are NIES-09, SIO-X12, and WMO-X2019 (so named due to the role NOAA plays as the WMO/GAW Central Calibration Laboratory for CO2). These three scales are the most widely used CO2 in air scales in the atmospheric monitoring community and the vast majority of available data are traceable to them. The relationships between these scales are important indicators of the relative stability of the scales and are becoming increasingly important to data users who wish to combine data from the respective programs in global models. Here we present historical and on-going scale comparison results from cylinder exchanges between the three programs on the NIES-09, SIO-X12, and WMO-X2019 scales. We use this data to derive relationships between the three scales and evaluate the relative stabilities of the independent scales. These scale relationships allow data traceable to the three scales to be merged in models and other scientific studies without scale differences influencing interpretation.

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Topic : Theme 2: Accuracy requirements for atmospheric composition measurements across economic sectors, and temporal and spatial scales.
Reference : T2-A13

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