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Standards to differentiate CO2 emission sources in the atmosphere

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. Standards and methods to calibrate instruments that measure isotope ratios of carbon dioxide in air and operating in the infra-red have been described in a recent publication in Analytical Chemistry[1].

To better understand the carbon cycle, clear identification of sources and sinks of CO2 is required as well as the ability to quantify their relative contribution to the atmosphere at a variety of spatial scales. The study of isotopic composition of CO2 in the atmosphere permits the identification of sources and sinks of carbon on the local, regional, and global scale.

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 ‰.

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 FT-IR and Other Isotope Ratio Infrared Spectrometer Instruments for Accurate δ13C and δ18O Measurements of CO2 in Air, Anal. Chem., 15 February 2017, doi:10.1021/acs.analchem.6b05063
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