δ¹³C in CO₂ at Estevan Point, Canada

Graphics    Digital Data

Investigators

C. E. Allison, R. J. Francey, and P. B. Krummel
Commonwealth Scientific and Industrial Research Organization (CSIRO),
Atmospheric Research,
Aspendale, Victoria, Australia, 3195

Period of Record

July 1993 - December 2001

Methods

Measurements have been made on air collected in flasks at Estevan Point, Canada, through the CSIRO GASLAB worldwide network. Flasks are filled with air at Estevan Point and returned to the CSIRO GASLAB for analysis; typical sample storage times for flasks collected at Estevan Point range from a few weeks to a few months. No significant effect on the stable carbon isotopic composition, δ¹³C, has been detected as a consequence of the sample storage time.

For stable isotope analysis, CO₂ was extracted from the air in the flasks using an automated cryogenic trapping system. Prior to stable isotopic analysis, the concentrations of CO₂, and N₂O were measured in all samples using gas chromatography (Francey et al., 1996). The CO₂ and N₂O concentrations were used to perform ion corrections on the stable isotope measurements required as a consequence of extracting the isobaric N₂O from air with the CO₂.

The stable isotopic composition of the extracted CO₂ was measured by dual inlet stable isotope ratio mass spectrometry (ThermoFinnigan MAT252). The normal mass spectrometric measurement of δ¹³C in atmospheric CO₂ requires the simultaneous measurements of ion currents due to three mass/charge (m/z) ratios: 44, 45, and 46. Information contained in the m/z 46 ratio is used to quantify and remove the ¹²C¹⁶O¹⁷O isobaric interference to the measurement of ¹³C¹⁶O₂. After isobaric correction (¹⁷O and N₂O), the δ¹³C was adjusted onto the VPDB-CO₂ scale using bracketing measurements of air with known CO₂ stable-isotope composition, maintained in high-pressure cylinders. All air standards used in GASLAB since 1990 remain in use to monitor long-term calibration and to ensure continuity of calibration through new air standards.

The link to VPDB-CO₂ has been monitored by comparisons between a suite of sub-samples of a single high-purity gas calibrated against NBS-19 carbonate (Allison and Francey, 1995) and a number of other high-purity CO₂ gas standards (GS-19, GS-20, OZTECH-3, OZTECH-30, OZTECH-40, NIST RMs 8562, 8563 and 8564). Further checks on the calibration of CO₂ measurements extracted from air samples onto the VPDB-CO₂ scale are maintained through regular analysis of high-pressure air standards, such as the IAEA CLASSIC suite, (Allison et al., 2002, 2003), and flask-air sharing comparisons (e.g., Masarie et al., 2001) maintained between CSIRO and various international laboratories. More detailed calibration information is available in Langenfelds et al. (2001).

The data presented here are monthly means, calculated as the mean of the daily values obtained from a smooth curve fit to the flask data using techniques of Thoning et al. (1989)

Estevan Point, Canada
49°23' N, 126°32' W,
39 m above MSL

Trends

The average annual decrease of about 0.03 per mil per year over the last 10 years reflects the growing influence of fossil-fuel carbon in the atmosphere. Additional influences of temporal variations in oceanic and terrestrial components of the global carbon cycle are also evident (see, for example, Francey et al., 1995; Allison et al., 2001; Francey et al., 2001).

References

• Allison, C. E., and R.J. Francey, 1995. High precision stable isotope measurements of atmospheric trace gases, In Reference and intercomparison materials for stable isotopes of light elements: proceedings of a consultants meeting, Vienna (IAEA-TECDOC-825). Vienna, Austria: International Atomic Energy Agency. p. 131-153.
• Allison, C.E., R.J. Francey, J.W.C. White, B. Vaughn, M. Wahlen, A. Bollenbacher, and T. Nakazawa, 2001. What have we learnt about stable isotope measurements from the IAEA CLASSIC? 11th WMO CO₂ Measurement Experts Meeting: Tokyo, 25-28 September 2001.
• Allison, C.E., R.J. Francey, and L.P. Steele, 2002. The International Atomic Energy Agency circulation of laboratory air standards for stable isotope comparisons: Aims, preparation and preliminary results. In Isotope aided studies of atmospheric carbon dioxide and other greenhouse gases Phase II (IAEA-TECDOC-1269). Vienna, Austria: International Atomic Energy Agency, p. 5-23.
• Allison, C.E., R.J. Francey, J.W.C. White, B. Vaughn, M. Wahlen, A. Bollenbacher, and T. Nakazawa, 2003. What have we learnt about stable isotope measurements from the IAEA CLASSIC? In Report of the Eleventh WMO/IAEA Meeting of Experts on Carbon Dioxide Concentration and Related Tracer Measurement Techniques, Tokyo, Japan, 25-28 Sep. 2001, WMO/GAW Report No. 148, p. 17-30, Geneva.
• Francey, R.J., P.P. Tans, C.E. Allison, I.G. Enting, J.W.C. White, and M. Trolier. Changes in oceanic and terrestrial carbon uptake since 1982. Nature 273(26) 326-330.
• Francey, R.J., L.P. Steele, R.L. Langenfelds, M.P. Lucarelli, C.E. Allison, D.J. Beardsmore, S.A. Coram, N. Derek, F.R. de Silva, D.M. Etheridge, P.J. Fraser, R.J. Henry, B. Turner, E.D. Welch, D.A. Spencer, and L.N. Cooper, 1996. In Global Atmospheric Sampling Laboratory (GASLAB): supporting and extending the Cape Grim trace gas programs. Baseline Atmospheric Program (Australia) 1993, edited by R.J. Francey, A.L. Dick and N. Derek, pp 8 - 29, Bureau of Meteorology and CSIRO Division of Atmospheric Research, Melbourne, Australia.
• Francey, R.J., C.E. Allison, C.M. Trudinger, P.J. Rayner, I.G. Enting, and L.P. Steele, 2001. The interannual variation of global atmospheric δ¹³C and its link to net terrestrial exchange, pp. 43-46 In Sixth International Carbon Dioxide Conference: extended abstracts, Sendai, Japan
• Langenfelds, R.L., L.P. Steele, C.E. Allison, and R.J. Francey, 2001. CSIRO GASLAB Calibration, 2001. Internal Report, CSIRO Atmospheric Research, Aspendale, Australia.
• Masarie, K.A., R.L. Langenfelds, C.E. Allison, T.J. Conway, E.J. Dlugokencky, R.J. Francey, P.C. Novelli, L.P. Steele, P.P. Tans, B. Vaughn, and J.W.C. White, 2001. NOAA/CSIRO Flask Air Intercomparison Experiment: A strategy for directly assessing consistency among atmospheric measurements made by independent laboratories, J. Geophys. Res. 106, 20445-20464.
• Thoning, K.W., P.P. Tans, and W.D. Komhyr, 1989. Atmospheric carbon dioxide at Mauna Loa Observatory, 2, Analysis of the NOAA/GMCC data, 1974 - 1985, J. Geophys. Res. 94, 8549-8565.

9/2003