A recent publication from the BIPM demonstrates how the isotopic composition of CO₂ in air standards can be used to correct precise spectroscopic measurements to achieve the accurate assignment of concentration. The method was applied to the international comparison of 46 standards from sixteen NMIs and designated institutes, demonstrating reference values with standard uncertainties of 1 part in 10⁴, providing the most accurate comparison of greenhouse gas standards for background and urban concentrations to date.

The paper ^[1] has confirmed that spectroscopic (FTIR, but also applicable to laser-based systems) based measurements of CO₂ mole fraction can be employed to achieve standard uncertainties reaching 0.05 µmol mol⁻¹, even if sample and calibration gases isotopic ratios differ. The measurement of, and correction for, isotope ratios of CO₂, for amount of substance fraction comparisons was recently demonstrated in the CCQM-K120 ^[2] (2019) and CCQM-P188 ^[3] (2019) comparisons of CO₂ in air standards at nominally (380, 480 and 800) µmol/mol. These compared 46 standards from National Metrology Institutes, the WMO's Central Calibration Laboratory and the Integrated Carbon Observation System‑Central Analytical Laboratories (ICOS-CAL) laboratory, as well as the reference systems at the BIPM. The comparisons, carried out with comparative measurements at the BIPM, including bias corrections from differences in isotope ratios in standards, allowed reference values with standard uncertainties of 0.05 µmol/mol to be determined: a four-fold reduction compared to comparisons performed 10 years previously. The pilot study comparison also demonstrated the performance of the manometric reference facility for CO₂ in air standards, which is being developed at the BIPM, as a unique common reference to be used in the on-demand comparison (BIPM.QM-K2) of CO₂ in air standards across the range (380-800) µmol/mol.

Figure 1: Results of key comparisons of CO2 in air standards (CCQM-K120/CCQM-P188 and the previous round CCQM-K52 (in 2006)) at nominally 380 µmol/mol. A four-fold improvement in the uncertainty of the reference value was achieved in the most recent comparisons. (y-axis: Degree of Equivalence/ µmol/mol; x-axis: Participating laboratory).

The comparisons on CO₂, form part of series of comparisons on greenhouse gas standards, also including methane and nitrous oxide, coordinated by the BIPM on behalf of the CCQM Working Group on Gas Analysis and described in two recent publications on 'SI traceability and scales for underpinning atmospheric monitoring of greenhouse gases' ^[4] and 'Advances in reference materials and measurement techniques for greenhouse gas atmospheric observations' ^[5]. Futures comparisons are planned for 2019 (N₂O), 2022 (CO₂) and 2023 (CH₄). A comparison of isotope ratio measurements in CO₂ gas (CCQM-P204) is being coordinated by the BIPM in collaboration with the IAEA, with measurements planned in 2020.

Figure 2: The BIPM CO2-PVT Facility. An on-going comparison of CO2 in air standards (BIPM.QM-K2), across the range (380 to 800) µmol/mol is based on a manometric reference system maintained at the BIPM. The advantages of using a primary manometric method for the comparisons, include: on-demand measurement; minimal gas use; short measurement time; low operating uncertainty (< 0.1 µmol/mol). The reference system developed at the BIPM system is in Silconert-treated stainless-steel, providing a much increased mechanical stability over glass systems, whilst reducing adsorption/desorption of carbon dioxide on surfaces as much as possible.

1. Flores E., Viallon J., Moussay P., Idrees F. and Wielgosz R. I. An FTIR method for accurate CO₂ mole fraction measurements with correction for differences in isotopic composition of gases, Metrologia, 2019, 56(4), 044005.


2. Flores E. et al. CCQM-K120 (Carbon dioxide at background and urban level), Metrologia, 2019, 56, Tech. Suppl., 08001.


3. Flores E., Viallon J., Choteau T., Moussay P., Idrees F., Wielgosz R.I., Meyer C., Rzesanke D. Report of the pilot study CCQM-P188: Carbon dioxide in air (380 to 800) µmol/mol., Metrologia, 2019, 56, Tech. Suppl., 08012.


4. Brewer P.J., Brown R.J.C., Tarasova O.A., Hall B., Rhoderick G.C., Wielgosz R.I. SI traceability and scales for underpinning atmospheric monitoring of greenhouse gases, Metrologia, 2018, 55(5), S174-S181.


5. Brewer P. J., Flores E., Viallon J. et al. Advances in reference materials and measurement techniques for greenhouse gas atmospheric observations, Metrologia, 2019, 56(3), 034006.