Abundances of isotopologues and calibration of CO₂ greenhouse gas measurements

Abstract. We have developed a method to calculate the fractional distribution of CO₂ across all of its component isotopologues based on measured δ¹³C and δ¹⁸O 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 (¹⁶O¹²C¹⁶O, ¹⁶O¹³C¹⁶O, and ¹⁶O¹²C¹⁸O) individually. The three measured values are then combined into total CO₂ (accounting for the rare unmeasured isotopologues), δ¹³C, and δ¹⁸O 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 µmol mol⁻¹ CO₂, ±0.2 ‰ δ¹³C, and ±0.2 ‰ δ¹⁸O, all at 68 % confidence interval (CI).