We have developed a method to calculate the fractional distribution of CO2 across all of its component isotopologues based on measured delta C-13 and delta O-18 values. The fractional distribution can be used with known total CO2 to calculate the amount of substance fraction (mole fraction) of each component isotopologue in air individually. The technique is applicable to any molecule where isotopologue-specific values are desired. We used it with a new CO2 calibration system to account for isotopic differences among the primary CO2 standards that define the WMO X2007 CO2-in-air calibration scale and between the primary standards and standards in subsequent levels of the calibration hierarchy. The new calibration system uses multiple laser spectroscopic techniques to measure mole fractions of the three major CO2 isotopologues ((OCO)-O-16-C-12-O-16, (OCO)-O-16-C-13-O-16, and (OCO)-O-16-C-12-O-18) individually. The three measured values are then combined into total CO2 (accounting for the rare unmeasured isotopologues), delta C-13, and delta O-18 values. The new calibration system significantly improves our ability to transfer the WMO CO2 calibration scale with low uncertainty through our role as the World Meteorological Organization Global Atmosphere Watch Central Calibration Laboratory for CO2. Our current estimates for reproducibility of the new calibration system are +/- 0.01 mu mol mol(-1) CO2, +/- 0.2% delta C-13, and +/- 0.2% delta O-18, all at 68% confidence interval (CI).

http://dx.doi.org/10.5194/amt-10-2669-2017

Journal Article

Office of Oceanic and Atmospheric Research (OAR)