Abstract
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).
Highlights
Long-term atmospheric monitoring of greenhouse gases relies on a stable calibration scale to be able to quantify small spatial gradients and temporal trends
All measurements by National Oceanic and Atmospheric Administration (NOAA) and World Meteorological Organization Global Atmosphere Watch (WMO GAW) contributing programs are directly traceable to this single set of primary standards through a strict hierarchy of calibration
We have undertaken to improve our calibration capabilities and to address key uncertainty components of the scale transfer. These key components are the reproducibility of the scale transfer, the potential for mole-fraction-dependent biases, and the potential issues we describe in this paper relating to the isotopic composition of the primary standards and subsequent standards in the calibration hierarchy
Summary
Long-term atmospheric monitoring of greenhouse gases relies on a stable calibration scale to be able to quantify small spatial gradients and temporal trends. All measurements by NOAA and WMO GAW contributing programs are directly traceable to this single set of primary standards through a strict hierarchy of calibration. The relatively large uncertainty of the individual manometrically assigned values would potentially introduce significant biases due to the use of subsets of primary standards. The re-assigned values (average manometer value minus the residual) were assumed to be the best assigned value for the primary standards This in theory should allow the use of subsets of the primary standards when transferring the scale from primary to secondary. Current reproducibility of standards using the NDIR calibration system is 0.03 μmol mol−1 (68 % CI) (Carbon Dioxide WMO Scale, 2017) This is a significant component of the targeted 0.1 μmol mol−1 (or 0.05 μmol mol−1 in the Southern Hemisphere) network compatibility goal (WMO, 2016). These key components are the reproducibility of the scale transfer, the potential for mole-fraction-dependent biases, and the potential issues we describe in this paper relating to the isotopic composition of the primary standards and subsequent standards in the calibration hierarchy
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