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International comparison demonstrates accuracy of the global atmospheric CO2 measurement scale

The results of the CCQM comparisons on CO2 in air standards (CCQM-K120 and CCQM-P188) were presented at the 20th WMO/IAEA Meeting on Carbon Dioxide, Other Greenhouse Gases, and Related Measurement Techniques (GGMT-2019), which was held in Jeju (Republic of Korea) from 2-5 September 2019. This coincided with the presentation of an updated carbon dioxide scale (WMO-CO2-X2019), the primary reference for the WMO-GAW monitoring network on which all global background observations of carbon dioxide are based.

The scale will be used by the WMO Global Atmosphere Watch programme in its global monitoring network for tracking trends in the background carbon dioxide amount fraction in the atmosphere. The accuracy of the scale was demonstrated in the key comparison (CCQM-K120), organized by the CCQM Gas Analysis Working Group with comparative measurements performed at the BIPM. The comparison was a substantial undertaking involving the analysis of the composition of 46 gas reference materials from 13 National Metrology Institutes (NMIs) and The National Oceanic and Atmospheric Administration (NOAA), the institute designated by the WMO.

Progress in reducing uncertainties in SI traceable standards has led to a key comparison reference value for carbon dioxide in air concentrations with state-of-the-art uncertainties of parts in 104. This has enabled benchmarking of international comparability and provided support for an update to the global scale. Standards developed by NMIs with gravimetric preparation were a major contributor in delivering reference values for CO2 in air with the smallest uncertainties achieved to date. The comparison also required methods to accurately calibrate optically-based instruments for CO2 isotope ratio (δ13C and δ18O) measurements to be developed at the BIPM as well as a manometric facility for future ongoing comparisons of CO2 in air standards, the performance of which was demonstrated in the CCQM-P188 study.

The developments to the global scale recognize the substantial collaborative progress made between the metrology and atmospheric monitoring communities since the WMO signed the CIPM Mutual Recognition Arrangement (CIPM MRA) in 2010.

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