A Task Group for Greenhouse Gas Scale Comparisons has been established within the CCQM Working Group on Gas Analysis (CCQM-GAWG) with the goal of increasing global accessibility to standards for underpinning long-term monitoring and emissions authentication of CO₂ in the atmosphere with internal consistency of 0.01 μmol/mol over the amount fraction range from 400 μmol/mol to 800 μmol/mol.

Setting up and evaluating a robust economic system based on the mitigation of carbon dioxide is expected to require multiple measurement sites around the world, including around urban areas. This is expected to increase world-wide demand for appropriate CO₂ in air gas standards. The CO₂ scale system enables the production and value assignment of standards that have very small uncertainties in relationship to each other. This high degree of consistency is important for atmospheric monitoring applications where small temporal and spatial changes are important. The reported consistency between tertiary standards is of the order of 0.01 μmol/mol. The best-known example of the scales approach is provided by the WMO, with the SI traceability of their primary standards demonstrated in the recent CCQM-K120 comparison. This relationship between standards in the scale approach is maintained in a database, and the value of any standard can be traced back to the primary standard set defining the scale.

The CCQM-GAWG, chaired by P. Brewer (NPL, UK), includes the WMO Central Calibration Laboratory, maintained at NOAA ESRL GML (USA), and National Metrology Institutes that are active in working towards establishing facilities for CO₂ scale standard provision, with a requirement that differences in scales must be close to the internal consistencies which are currently achieved by the WMO scale approach. To minimize any potential scale bias, it is planned to maintain scale equivalence equations by using CIPM MRA on-going key comparisons.

The CCQM-GAWG Task Group on GHG Scale Comparisons (CCQM-GAWG-TG-GHG) is chaired by J. Carney (NIST, USA) and supported by reference and comparison facilities maintained within the BIPM laboratories. This includes the CO₂-PVT manometric reference facility developed at the BIPM with visiting scientists from NIST (USA) and RISE (Sweden) and validated in the CCQM-P188 comparison. The facility will be the basis for future on-going comparisons of CO₂ in air standards (BIPM.QM-K2) ensuring SI traceability of assigned values and comparisons of CO₂ in air scales (BIPM.QM-K5) to ensure their consistency at the 0.01 μmol/mol level.

BIPM Manometric reference facility for the comparison of carbon dioxide in air standards. The facility was developed at the BIPM together with visiting scientists from the NIST (USA) and the RISE (Sweden), and operates through the principal of cryogenic separation of CO₂ from air samples, with measurement of pressure, temperature and volume of both samples. Initially validated in the CCQM-P188 and CCQM-K120 comparisons, the facility will provide reference values for the future on-going BIPM.QM-K2 comparison of CO₂ in air standards, allowing on-demand comparisons of NMI/DI standards anywhere in the range of 380 μmol/mol to 800 μmol/mol, as well as comparison of CO₂ in air scale standards (BIPM.QM-K5).