Historically, NOAA GML transferred the WMO CO₂ mole fraction scale by non-dispersive infrared absorption (NDIR) spectroscopy. In 2016 we began using a new CO₂ analysis system based on three laser spectroscopic techniques; cavity ring-down spectroscopy (CRDS), off-axis integrated cavity output spectroscopy (off-axis ICOS), and quantum cascade-tunable infrared laser differential absorption spectroscopy (QC-TILDAS). The new calibration system makes use of the ability of these analyzers to measure individual CO₂ isotopologues to fully account for isotopic differences among members of the primary standard set and between the primary standards and subsequent levels of standards in the calibration hierarchy.

A full description of the analytical system and the isotope accounting method is presented in Tans et al. (2017) so only a brief description is given here. The overall calibration strategy is to decompose the total CO₂ mole fraction assigned to the primary standards by manometric measurement (Hall et al., 2020; Zhao et al., 1997; and Zhao et al., 2006) into individual isotopologue mole fractions based on their assigned δ¹³C and δ¹⁸O values and use these to calibrate the analyzer responses for each individual isotopologue. Assigned δ¹³C and δ¹⁸O values are from analysis by the Stable Isotope Laboratory at the Institute of Arctic and Alpine Research, University of Colorado, Boulder (INSTAAR SIL)on the JRAS-06 realization of the VPDB-CO₂ scales for δ¹³C and δ¹⁸O of CO₂. The isotopologue mole fractions of the standards are used to simultaneously calibrate the CRDS instrument for ¹⁶O¹²C¹⁶O and either the off-axis ICOS or the QC-TILDAS for the ¹⁶O¹³C¹⁶O and ¹⁸O¹²C¹⁶O isotopologues. Secondary standards are measured relative to these isotopologue specific calibration curves to determine isotopologue mole fractions of the secondary standards. The three isotopologue mole fractions are then converted back into total CO₂, δ¹³C and δ¹⁸O values accounting for the unmeasured rare isotopologues. A similar method is used when tertiary standards are calibrated against secondary standards. Several publications have described this technique (Wehr et al., 2013; Flores et al., 2017; Tans et al., 2017; and Griffith, 2018) All CO₂ calibration data in the database and reported to the user are total CO₂, not the isotopologue specific mole fractions.

The new calibration system offers a significant advantage over the NDIR because it is calibrated over the whole range of the scale and the response to the major isotopologue (¹⁶O¹²C¹⁶O) is linear within uncertainty. The NDIR system used groups of 3-4 bracketing primary standards to calibrate secondary standards and the response is not linear. This made the NDIR system susceptible to mole fraction dependent biases as discussed below.

The estimated reproducibility of total CO₂ measurements on the new system is ± 0.01 ppm (1 sigma) based on approximately 5 years of target tank calibrations. The δ¹³C and δ¹⁸O measurements agree well with measurements by INSTAAR SIL of flasks filled from cylinders (average NOAA – SIL is 0.0 ± 0.2‰ for δ¹³C and 0.0 ± 0.2‰ for δ¹⁸O (each at 1 sigma)). The isotopic measurements are not to be used as a substitute to having cylinders directly measured by IRMS when isotopic standards are required. They are designed to be used only for making isotopic corrections to measurements of atmospheric CO₂ on instruments that are sensitive to isotopic differences between standards and samples.

The new calibration system ran in parallel with the NDIR system from April 2016 to August 2017. This comparison shows good agreement between the two systems on the current WMO CO₂ X2019 scale. On the now obsolete WMO CO₂ X2007 scale there was a significant mole fraction dependent bias between the two systems (Figure 1, top panel). These offsets can be traced to the effects of calibrating the NDIR system with subsets of the primary standards coupled with mistakes in the implementation of the X2007 scale. Using subsets in this way makes the results from the NDIR system sensitive to incorrectly assigned values of individual primary standards. The mole fraction dependent bias between the laser spectroscopic and NDIR calibrations was resolved with the release of the WMO CO₂ X2019 scale (Figure 1, bottom panel). The X2019 scale revision was retroactively propagated to all past calibrations of tertiary standards.

Comparison results (NDIR minus Laser Spectroscopy) from running the two calibration systems in parallel from April 2016 through Aug 2017 as a function of mole fraction. Top panel: results on the now obsolete WMO CO₂ X2007 scale; bottom panel: results on the current WMO CO₂ X2019 scale. Open symbols are cylinders with δ¹³C < -20‰. Dashed lines are the reproducibility of the NDIR system (±0.03 1 sigma). The differences on X2007 show a strong mole fraction dependency that can be traced to the NDIR sensitivity to mis-assigned primary standards. These biases are resolved with the X2019 scale revision. The isotopically depleted tanks show a slightly larger difference indicating the current NDIR analyzer is biased by the isotopic composition of the standards relative to the samples. This should be taken into account by the user when evaluating drift in isotopically depleted cylinders and comparing historical NDIR measurements with the new calibration system measurements.

The results from the new calibration system also show the NDIR system to be sensitive to differences in the isotopic composition of the tertiary cylinders calibrated. Figure 1 (open symbols on both scales) shows a larger offset between the two systems for iostopically depleted cylinders (δ¹³C < -20‰). Users are cautioned when comparing historical NDIR measurements of isotopically depleted tanks with measurements on the new calibration system. The new calibration system is much better at eliminating biases due to isotopic differences, but drift determinations may be compromised by the sensitivity of the NDIR to the isotopic differences.

References

• Flores, E., J. Viallon, P. Moussay, D. W. T. Griffith, and R. I. Wielgosz (2017), Calibration strategies for FT-IR and other isotope ratio infrared spectrometer instruments for accurate δ¹³C and δ¹⁸O measurements of CO₂ in air. Analytical Chemistry, 89(6), 3648-3655.
• Griffith, D.W.T. (2018), Calibration of isotopologue-specific optical trace gas analysers: A practical guide. Atmos. Meas. Tech., 11, 6189-6201, https://doi.org/10.5194/amt-2018-187.
• Hall, B. D., Crotwell, A. M., Kitzis, D. R., Mefford, T., Miller, B. R., Schibig, M. F., and Tans, P. P.: Revision of the World Meteorological Organization Global Atmosphere Watch (WMO/GAW) CO2 calibration scale, Atmos. Meas. Tech., 14, 3015–3032, https://doi.org/10.5194/amt-14-3015-2021, 2021.
• Tans, P. P., A. M. Crotwell, and K. W. Thoning (2017), Abundances of isotopologues and calibration of CO₂ greenhouse gas measurements. Atmospheric Measurement Techniques, 10(7), 2669–2685, https://doi.org/10.5194/amt-10-2669-2017.
• Wehr, R., J. W. Munger, D. D. Nelson, J. B. McManus, , M. S. Zahniser, S. C. Wofsy, S. R. Saleska (2013), Long-term eddy covariance measurements of the isotopic composition of the ecosystem-atmosphere exchange of CO₂ in a temperate forest. Agricultural and Forest Meteorology. 181: p. 69-84.
• Zhao, C., P.P. Tans, and K.W. Thoning (1997), A high precision manometric system for absolute calibrations of CO₂ in dry air. Journal of Geophysical Research 102 (D5):5885-5894
• Zhao, C. L. and P. P. Tans (2006), Estimating uncertainty of the WMO mole fraction scale for carbon dioxide in air, Journal of Geophysical Research-Atmospheres, 111(D8), 10.1029/2005JD006003.