CO2 Calibration Analytical Method Update:

Historically, NOAA GMD transferred the WMO CO2 mole fraction scale by non-dispersive infrared absorption (NDIR) spectroscopy. Recently we developed a new CO2 analysis system based on 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). These methods are collectively referred to as Laser based Spectroscopy. 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 will be included in a forthcoming publication 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 (Zhao et. al., 1997 and 2006) into individual isotopologue mole fractions based on their measured δ13C and δ18O values. Measured δ13C and δ18O values are from analysis by the Stable Isotope Laboratory at the Institute of Artic and Alpine Research, University of Colorado Boulder (INSTAAR SIL). 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. 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 uses groups of 3-4 bracketing primary standards to calibrate secondary standards and the response is not linear. This makes 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 11 months 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 has been running in parallel with the NDIR system since April 2016. The on-going comparison has shown good agreement between the two systems near ambient CO2 but with significant mole fraction dependent offsets between 300 – 360 ppm and 430 – 500 ppm (Figure 1). These offsets can be traced primarily to the effects of calibrating the NDIR system with subsets of the primary standards. Using subsets in this way makes the results from the NDIR system sensitive to errors in the assigned values of the individual primary standards. Additional manometric evaluations of the primary standards made subsequent to the X2007 assignments show that the X2007 assignments do indeed have some errors. These assignment errors will be corrected in an upcoming scale revision (scheduled for mid-2017), and the corrections remove most of the mole fraction-dependent bias between the two analysis systems. After the scale revision all past calibrations of tertiary standards will be revised. Calibrating the new system by fitting all primary standards makes the new system very insensitive to these assignment errors of individual cylinders. Thus results from the new system are more correct than from the NDIR. However, caution should be used when evaluating cylinders for drift when comparing historical results from the NDIR system and new measurements from the new calibration system. NOAA will continue to recalibrate cylinders on both analysis systems until the scale revision is completed and the discrepancies are understood. However, new fillings will only be measured on the new calibration system.

Figure 1
Comparison results (NDIR minus Laser Spectroscopy) from running the two calibration systems in parallel from April 2016 through Oct 2016 as a function of mole fraction. Blue points are cylinders with δ13C < -20‰. Dashed lines are the reproducibility of the NDIR system (±0.025 1 sigma). The differences show a strong mole fraction dependency that can be traced to the NDIR system being susceptible to errors in the assignment of the primary standards. These system offsets should be considered when evaluating cylinders for drift using historical NDIR measurements with measurements from the new calibration system. The 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. Again, this should be taken into account by the user when evaluating drift in 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 (blue data points) shows a larger offset between the two systems for highly depleted cylinders (δ13C < -20‰). This isotopic effect is currently being quantified for the current NDIR analyzer and will be addressed in the upcoming paper describing the analytical techniques. Users are cautioned again 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

  • 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.