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     on matters related to measurement science and measurement standards.
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Comparisons and calibrations

    Carbon dioxide (CO2) in air

    Changes in carbon dioxide concentration in the atmosphere are of high environmental and political importance, with increasing levels of emissions and background monitoring. Performance criteria for CO2 monitoring networks are being set, with the most stringent of these being for background ambient levels where data compatibility from different monitoring sites should be at the level of 0.1 µmol/mol for mole fractions at 400 µmol/mol. This in turn requires accurate calibration standards, which in the most part are produced gravimetrically, and the uncertainty of which should be sufficiently small to allow network data compatibility goals to be met.

    The BIPM, in collaboration with the NIST, is organizing an international comparison, CCQM-K120) of primary CO2 in air standards, which contains two parts: part (a) for standards in the range (380-480) µmol/mol and part (b) in the range (480-800) µmol/mol. For this comparison, 16 participants send up to three cylinders each to the BIPM where they are analysed for the CO2 amount fraction by two different analytical techniques, completed by the measurement of the CO2 isotopic composition.

    Prior to the comparison the BIPM used primary standards of the NIST, NOAA and NPL to develop comparison methods based on Gas Chromatography with a Flame Ionization Detector (GC-FID) and on Fourier Transform Infrared spectroscopy (FTIR). Measurement uncertainties as low as 20 nmol mol–1 (0.005%) have been demonstrated with FTIR, requiring knowledge of the CO2 isotopic composition for accurate measurements, which are also measured. GC-FID, which is insensitive to isotope ratios in the CO2, provides measurement uncertainties of 40 nmol mol–1.

    The BIPM is developing an independent system to assign the CO2 amount fractions in gas standards based on pressure, volume and temperature measurements: the CO2-PVT measurement system. This facility will support the CCQM-K120 comparison and planned ongoing comparisons for CO2 in air (BIPM.QM-K2).

    Methane (CH4) in air

    Methane contributes 18.1% of the overall global radiative forcing and is the second most important greenhouse gas after carbon dioxide. Atmospheric methane concentrations are now 254% of the pre-industrial level. Measurements of CH4 in the atmosphere are calibrated with gas standards prepared by gravimetry by the addition of pure CH4 into a matrix of dry air. The most stringent compatibility goals for global methane monitoring are set at plus or minus 2 nmol/mol.

    In 2010 the BIPM in collaboration with NIST developed accurate methods for methane in air standard comparisons based on Gas Chromatography with a Flame Ionization Detector (GC-FID) and Cavity Ring-Down Spectroscopy (CRDS). Standards prepared in whole air and synthetic air were compared, to demonstrate that proper preparation of the latter could avoid well-known biases that could occur with the CRDS measurement technique[1].

    The BIPM facilities were successfully used in 2013 for the coordination of CCQM-K82, for methane in air standards ranging from 1800 nmol/mol to 2200 nmol/mol[2]. The improvement in the compatibility of CH4 in air standards during the period of 2003 to 2013 can be seen by comparing the results of CCQM-K82 with those of CCQM-P41 organized in 2003.

    1. Flores E., Rhoderick G.C., Viallon J., Moussay P., Choteau T., Gameson L, Guenther F.R., Wielgosz R.I., Methane standards made in whole and synthetic air compared by cavity ring down spectroscopy and gas chromatography with flame ionization detection for atmospheric monitoring, Anal. Chem. (2015) 87 3272-3279
    2. Flores E., Viallon J., Choteau T., Moussay P., Wielgosz R.I., Kang N., Kim B.M., Zalewska E., van der Veen A.A., Konopelko L., International comparison CCQM-K82: Methane in air at ambient level (1800-2200) nmol/mol, Metrologia (2015) 52 Tech. Suppl. 08001

    Nitrous oxide (N2O) in synthetic air

    Although nitrous oxide is present at much lower levels than CO2 (about a thousand times) in the atmosphere, it has a Global Warming Potential (GWP) that is 296 times that of CO2. The most important N2O sources are agriculture, combustion processes (including catalytic conversion in cars), and industry.

    As for CO2 and CH4, N2O measurements are calibrated with standards prepared by static gravimetry, with relative uncertainties between 0.2% and 1%. After a first international comparison coordinated by KRISS in 2008 (CCQM-K68), efforts have concentrated on further reducing gravimetric uncertainties, as world-wide measurements of N2O need to be compatible to better than 0.03% to be meaningfully interpreted. The BIPM is currently preparing the repeat comparison CCQM-K68.2019 in collaboration with KRISS.

    KRISS has provided a Gas Chromatography analyser with an Electron Capture Detector (GC-ECD) to the BIPM, together with a set of standards prepared at KRISS to validate the future comparison method. In addition, an instrument based on Infrared Spectroscopy with a Tuneable Diode Laser (TILDA-CS), will also be used to compare the standards.

    Nitrogen monoxide (NO) in N2

    Pilot Study:

    A facility for the comparison of primary nitrogen monoxide gas standards is maintained at the BIPM. The NO facility enables the analysis of primary reference NO/N2 mixtures in the range (30 to 70) µmol/mol, with a measurement uncertainty of about 0.05 % – approximately the same as the uncertainty of gravimetric preparation of such mixtures. In addition, impurities in the standards are quantified with Fourier Transform Infrared spectrometry (FTIR) at the BIPM.

    The system is entirely automated by software developed at the BIPM. In 2005-2006 it was used as the central analytical facility for the international comparison of nitrogen monoxide gas standards CCQM-P73, involving 12 NMIs and coordinated by the BIPM[1]. The results illustrated the reductions in uncertainties that can be achieved through a comparison with measurements performed at a central facility. Since then it has been maintained to undertake the key comparison CCQM-K137 on NO standards at 30 µmol mol–1 and 70 µmol mol–1. At this latter concentration, preparing NO in nitrogen standards is seen as an example of a core capability of NMIs active in gas analysis.

    The facility was also used to label secondary gas standards for the ozone Gas Phase Titration (GPT) facility, thereby providing traceability of the GPT system to the primary NO/N2 standards. By comparison with the more traditional method for measuring ozone by UV absorption, this allowed the calculation of an independent value of the ozone absorption cross-section currently under review by a Task Group of the CCQM's Gas Analysis Working Group.

    1. Wielgosz R.I., Esler M., Viallon J., Moussay P., Oh S.H., Kim B.M., Tshilongo J., Mokgoro I.S., Maruyama M., Mace T., Sutour C., Stovcik V., Musil S., Castorena A.P., Murillo F.R., Kustikov Y.A., Pankratov V.V., Gromova E.V., Thorn W., J., Guenther F.R., Smeulders D., Baptista G., Dias F., Wessel R.M., Nieuwenkamp G. and van der Veen A.M.H., Final report on CCQM-P73: International comparison of nitrogen monoxide in nitrogen gas standards (30-70) µmol/mol, 2008, Metrologia 45 Tech. Suppl. 08002