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More accurate CO2 measurements thanks to isotopic analysis

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.

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.

  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.
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