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Greenhouse gases

    Standards to underpin atmospheric monitoring

    Long-term records of essential climate variables, including carbon dioxide (CO2), are essential in tracking the effect of anthropogenic activities on atmospheric composition and modelling atmospheric radiative forcing and the contribution to climate change. The developments in global greenhouse gas emissions and their quantification and verification, has led to a demand for GHG measurement systems, the consistency and accuracy of which need to be assured. This in turn drives the demand for high accuracy calibration standards for GHGs worldwide and the requirement for their consistency.

    As the most important greenhouse gases (GHG) are stable at atmospheric concentrations, calibration standards can be prepared by gravimetry, adding these trace gases in a matrix of dry air. The BIPM has developed central facilities to run international comparisons of such standards prepared by National Metrology Institutes (NMIs) and Designated Institutes (DIs), for CO2, methane (CH4) and nitrous oxide (N2O). For each GHG, two different analytical techniques are used for comparison, to ensure any method bias is detected, and methods developed to reduce their uncertainty to a minimum.

    In addition to the monitoring of CO2 concentrations in the atmosphere, the analysis of CO2 isotopic composition provides valuable information on the sources and sinks of the gas. Recent developments in instrumentation for the measurement of CO2 isotopes in air has resulted in promising results from spectroscopic techniques, which require calibration with novel standards of CO2 in air value-assigned for their isotope ratio. The BIPM recently developed a calibration strategy for FTIR and other Isotope Ratio Infrared Spectroscopy (IRIS) instruments for such measurements[1], and is working with the IAEA in preparing an international comparison of standards for CO2 isotopes.



    1. Flores E., Viallon J., Moussay P., Griffith D.W.T., Wielgosz R.I., Calibration strategies for FTIR and other IRIS instruments for accurate δ13C and δ18O measurements of CO2 in air, Anal. Chem., 2017, 89, 3648-3655

    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).

    CO2 produced from the burning of fossil fuels or forests has quite a different isotopic composition from CO2 in the atmosphere. Measuring isotope ratios allows sources and sinks of carbon to be differentiated, including separating ocean–atmosphere exchange of CO2 from terrestrial exchange, as well as from fossil fuel input. Activities undertaken at the BIPM, describing standards and methods to calibrate instruments that measure isotope ratios of carbon dioxide in air and operating in the infra-red have been published in Analytical Chemistry[1].

    Over recent years the introduction of Isotope Ratio Infrared Spectroscopy (IRIS), based on various spectroscopic techniques, has advanced stable isotope analysis in the atmosphere, allowing in situ field measurements of the isotope ratio of CO2 in air, performed in real time directly on the air sample without separation of CO2 from air.

    The use of standards of CO2 in air, which have been value-assigned for CO2 mole fraction and isotopic ratio (δ13C and δ18O), to calibrate FTIR and other IRIS instruments is described in the joint paper by the BIPM and the University of Wollongong, Australia. It has been demonstrated that calibration with two standards containing CO2 with the same isotopic composition but different mole fraction values, results in δ13C measurements with standard uncertainties of less than 0.1 per mille.

    The calibration strategy is also being applied in the CCQM-K120.a and CCQM-K120.b key comparisons coordinated at the BIPM, which will evaluate the consistency of CO2 in air concentration standards over the range (380-800) µmol/mol. Forty-five standards from sixteen countries are being compared, using a number of measurement techniques, including FTIR, the response of which is corrected for the isotope ratio of CO2 found in the standard.



    1. Flores E., Viallon J., Moussay P., Griffith D.W.T., Wielgosz R.I., Calibration strategies for FTIR and other IRIS instruments for accurate δ13C and δ18O measurements of CO2 in air, Anal. Chem., 2017, 89, 3648-3655

    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

    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.