Maintaining consistent traceability in high-precision isotope measurements of CO₂: a way to verify atmospheric trends of δ¹³C and δ¹⁸O

Abstract. Maintaining consistent traceability of high-precision measurements of CO₂ isotopes is critical in order to obtain accurate atmospheric trends of δ¹³C and δ¹⁸O (in CO₂). Although a number of laboratories/organizations around the world have been conducting baseline measurements of atmospheric CO₂ isotopes for several decades, reports on the traceability and maintenance are rare. In this paper, a principle and an approach for maintaining consistent traceability in high-precision isotope measurements (δ¹³C and δ¹⁸O) of atmospheric CO₂ are described. The concept of Big Delta is introduced and its role in maintaining traceability of the isotope measurements is described and discussed extensively. The uncertainties of the traceability have been estimated based on annual calibration records over the last 10 yr. The overall uncertainties of CO₂ isotope measurements for individual ambient samples analyzed by the program at Environment Canada have been estimated (excluding these associated with the sampling). The values are 0.02 and 0.05‰ in δ¹³C and δ¹⁸O, respectively, which are close to the World Meteorological Organization (WMO) targets for data compatibility. The annual rates of change in δ¹³C and δ¹⁸O of the primary anchor (which links the flask measurements back to the VPDB-CO₂ scale) are close to zero (−0.0016 ± 0.0012‰, and −0.006 ± 0.003‰ per year, respectively) over a period of 10 yr (2001–2011). The average annual changes of δ¹³C and δ¹⁸O in air CO₂ at Alert GAW station over the period from 1999 to 2010 have been evaluated and confirmed; they are −0.025 ± 0.003‰ and 0.000 ± 0.010‰, respectively. The results are consistent with a continuous contribution of fossil fuel CO₂ to the atmosphere, having a trend toward more negative in δ¹³C, whereas the lack of change in δ¹⁸O likely reflects the influence from the global hydrologic cycle. The total change of δ¹³C and δ¹⁸O during this period is ~0.27‰ and ~0.00‰, respectively. Finally, the challenges and recommendations as strategies to maintain a consistent traceability are described.