Maintaining consistent traceability in high-precision isotope measurements of CO 2 : a way to verify atmospheric trends of 13 C

Maintaining consistent traceability of high- precision measurements of CO2 isotopes is critical in or- der to obtain accurate atmospheric trends of 13 C and 18 O (in CO2). Although a number of laboratories/organizations around the world have been conducting baseline measure- ments of atmospheric CO2 isotopes for several decades, re- ports on the traceability and maintenance are rare. In this pa- per, a principle and an approach for maintaining consistent traceability in high-precision isotope measurements ( 13 C and 18 O) of atmospheric CO2 are described. The concept of Big Delta is introduced and its role in maintaining trace- ability of the isotope measurements is described and dis- cussed 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 measure- ments for individual ambient samples analyzed by the pro- gram at Environment Canada have been estimated (exclud- ing these associated with the sampling). The values are 0.02 and 0.05 ‰ in 13 C and 18 O, respectively, which are close to the World Meteorological Organization (WMO) targets for data compatibility. The annual rates of change in 13 C and 18 O of the primary anchor (which links the flask mea- surements back to the VPDB-CO2 scale) are close to zero ( 0.0016± 0.0012 ‰, and 0.006± 0.003 ‰ per year, re- spectively) over a period of 10 yr (2001-2011). The aver- age annual changes of 13 C and 18 O 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 atmo- sphere, having a trend toward more negative in 13 C, whereas the lack of change in 18 O likely reflects the influence from the global hydrologic cycle. The total change of 13 C and 18 O during this period is 0.27 ‰ and 0.00 ‰, re- spectively. Finally, the challenges and recommendations as strategies to maintain a consistent traceability are described.