An FTIR method for accurate CO2 mole fraction measurements with correction for differences in isotopic composition of gases
The precision of optically based techniques for long term CO2 monitoring in the atmosphere is approaching the ±0.02 µmol mol−1 level, however biases of potentially up to fifteen times greater than this can arise due to differences in isotopic composition of sample and calibration gases used. We hav...
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| Published in: | Metrologia 2019-08, Vol.56 (4), p.1 |
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| Main Authors: | , , , , |
| Format: | Article |
| Language: | eng |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | The precision of optically based techniques for long term CO2 monitoring in the atmosphere is approaching the ±0.02 µmol mol−1 level, however biases of potentially up to fifteen times greater than this can arise due to differences in isotopic composition of sample and calibration gases used. We have developed and describe a Fourier transform infrared spectroscopy (FTIR) method for CO2 mole fraction measurements, using an instrument configured with a liquid N2-cooled mid-infrared indium antimonide (InSb) detector and a 10.01 m multipass Whitetype gas cell (0.75 l) and fitting of the 12C16O2 isotopologue absorption in the wavenumber region (3500-3800) cm−1 where the measurement bias resulting from isotopic variation between sample and calibration gases can be corrected for. The method was completed with a comprehensive uncertainty budget from which considerations on the importance of spectroscopic specific biases could be deduced. The method was validated using 29 standards produced by 15 National Metrology Institutes/Designated Institutes, including the Central and a Regional Calibration Laboratory (McClenny et al 2002 J. Air Waste Manag. Assoc. 52 542-62) from the World Meteorological Organization (2011 WMO/TD-No. 1560, Ed.: Hohenpeissenberg Metrological Observatory), with δ13C values in CO2 ranging from −0.4‰ to −67‰ versus VPDB. The standards were all produced with CO2 mole fractions typical of background atmosphere calibration gases (either 380 µmol mol−1 or 480 µmol mol−1), and reference values for each of them were established during a comparison performed with a GC-FID analyzer, insensitive to their isotopic composition. Measurements have confirmed that a Δδ13C of 1‰ between sample and calibration gases can bias the CO2 mole fraction measurements made by FTIR by 4 nmol mol−1 and a Δδ18O of 1‰ by 2 nmol mol−1. These biases can be corrected with the method described here, allowing FTIR based measurements of the CO2 mole fraction with standard uncertainties reaching 0.05 µmol mol−1 to be achieved, even if sample and calibration gases isotopic ratios differ. These findings have significant implications for the use of CO2 calibration mixtures in particular for the atmospheric community, when high accuracy measurements are required. |
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| ISSN: | 0026-1394 1681-7575 |