Maintaining consistent traceability in high-precision isotope measurements of CO_2: a way to verify atmospheric trends of δ~(13)C and δ~(18)O

摘要

Maintaining consistent traceability of high-precision measurements of CO_2 isotopes is critical in order to obtain accurate atmospheric trends of δ~(13)C and δ~(18)O (in CO_2). Although a number of laboratories/organizations around the world have been conducting baseline measurements of atmospheric CO_2 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 (δ~(13)C and δ~(18)O) of atmospheric CO_2 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 CO_2 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 δ~(13)C and δ~(18)O, respectively, which are close to the World Meteorological Organization (WMO) targets for data compatibility. The annual rates of change in δ~(13)C and δ~(18)O of the primary anchor (which links the flask measurements back to the VPDB-CO_2 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 δ~(13)C and δ~(18)O in air CO_2 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 CO_2 to the atmosphere, having a trend toward more negative in δ~(13)C, whereas the lack of change in δ~(18)O likely reflects the influence from the global hydrologic cycle. The total change of δ~(13)C and δ~(18)O during this period is ～-0.27‰ and ～0.00‰, respectively. Finally, the challenges and recommendations as strategies to maintain a consistent traceability are described.