In Situ δ13CO2 from Cape Grim, Tasmania, Australia: 1982-1993
R.J. Francey and C.E. Allison
CSIRO, Division of Atmospheric Research,
Private Bag No. 1,
Mordialloc, Victoria, Australia 3195
Period of Record
Air samples are collected during baseline condition episodes at a frequency of around one sample per week. Baseline conditions are characterized by wind direction in the sector 190-280°, condensation nucleus concentration below 600 cm3, and steady, continuous CO2 concentrations (variation + 0.2 ppmv per hour). The Cape Grim in situ extraction line is based on 3 high-efficiency glass U-tube traps with internal cooling coils. A vacuum pump draws air from either the 10 m or 70 m intakes, and sampling alternates between the two intakes. The air from the intakes is dried with a trap inmersed in an alcohol bath at a temperature of about -80°C. CO2 is collected in a second trap immersed in liquid nitrogen. A third liquid nitrogen trap guards against oil vapor back-steaming from the vacuum pump. Air flow is maintained for 2 hours, usually during late morning, at 300 mL/min. After this time, the trap holding the CO2 is raised to the alcohol bath temperature, and the CO2 is transferred cryogenically into a 100 mL glass flask for transport to CSIRO's Division of Atmospheric Research in Aspendale.
Mass spectrometer analysis for δ13C and δ18O are carried out in Aspendale usually one to three weeks after collection. Since 1990, a Finnigan-MAT 252 (MAT252) mass spectrometer has been used for the isotopic analyses. Before 1990, a Vacuum Generators Micromass 602D (VG602D) mass spectrometer was used for the analyses. Cape Grim in situ CO2 samples collected during the period August 1990 to October 1991 were analyzed on both the MAT252 and VG602D. The VG602D was decommissioned in late 1991.
The normal mass spectrometric measurement of δ13C in atmospheric CO2 requires the simultaneous measurements of ion currents due to three mass-to-charge ratios (m/e) at 44, 45, and 46. The m/e 46 is primarily measured so that any 170 contribution to the m/e 45 can be calculated and removed to obtain the actual 13C contribution.
Isotopic composition is expressed by using the delta (δ) notation, which requires that the ratios 45/44 and 46/44 be measured for both the sample (SA45 and SA46) and reference gas (ST45 and ST46). The δ values are calculated as
δ45 = (SA45 - ST45) / ST45
δ46 = (SA46 - ST46) / ST46
When multiplied by 1000, the value δ is expressed in units of per mil (o/oo).
The δ13C and δ18O obtained indicate the difference in isotopic composition of the sample CO2 from that of the CSIRO reference CO2, after correction for a nitrous oxide contribution at m/e 44. The N2O correction uses concentration measurements of N2O and CO2 continuously monitored at the station. To allow comparison with data from other laboratories, the δ13C and δ18O are converted to differences from the international V-PDB reference scale. The basis of this scale is PDB-CO2, CO2 derived from PDB CaCO3 from the rostrum of a Cretaceous belemnite (Belemnitella americana) collected in the Peedee formation of South Carolina, USA. The carbonate standard, PDB, is no longer available, and the current international standard is NBS-19, a limestone sample that involves CO2 with accepted δ13C of +1.95 o/oo and δ18O of -2.20 o/oo with respect to V-PDB.
More detail about the sampling methods, instrumentation, and isotopic calculations are provided in Allison et al. (1994) and Francey et al. (1994, 1995a).
Cape Grim, Tasmania, Australia
40°41' S, 144°41' E
94 m above MSL
The Cape Grim in situ record is possibly the most accurate representation of global atmospheric δ13C behavior during the 1980s and 1990s. Changes in δ13C of atmospheric CO2 are useful in elucidating the relative roles of oceanic and terrestrial uptake of CO2 from fossil fuel emissions. Francey et al. (1995) observed a gradual decrease in δ13C for Cape Grim for 1982 to 1993, but with a pronounced flattening from 1988 to 1990. Francey et al. (1995) reported the flattening of the trend from 1988 to 1990 appears to involve the terrestrial carbon cycle.
- Allison, C.E., R.J. Francey, R.L. Langenfelds, and E.D. Welch. 1994. Comparison of high precision Cape Grim CO2 stable isotope measurements using two mass spectrometers. pp. 10-19. In A.L. Dick and J.L. Gras (eds.), Baseline Atmospheric Program Australia 1991. Department of the Environment, Sport and Territories and CSIRO Division of Atmospheric Research, Melbourne, Australia.
- Ciais, P., P.P. Tans, J.W.C. White, M. Trolier, R.J. Francey, J.A. Berry, D.R. Randall, P.J. Sellers, J.G. Collatz, and D.S. Schimel. 1995. Partitioning of ocean and land uptake of CO2 as inferred by δ13C measurements from the NOAA Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network. Journal of Geophysical Research. 100:5051-70.
- Ciais, P., P.P. Tans, M. Trolier, J.W.C. White, and R.J. Francey. 1995. A large northern hemisphere terrestrial CO2 sink indicated by 13C/12C of atmospheric CO2. Science 269(5227):1098-1102.
- Francey, R.J., C.E. Allison, L.P. Steele, R.L. Langenfelds, E.D. Welch, J.W.C. White, M. Trolier, P.P. Tans, and K.A. Masarie. 1994. Intercomparison of stable isotope measurements of CO2. pgs. 106-110. In J.T. Peterson and R.M. Rosson (eds.), Summary Report 1993, Climate Monitoring and Diagnostics Laboratory No. 22, US Department of Commerce, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA.
- Francey, R.J., C.E. Allison, and E.D. Welch. 1995a. The 11-year high precision in situ CO2 stable isotope record from Cape Grim, 1982-1992. pp. 16-25. In C. Dick and P.J. Fraser (eds.), Baseline Atmospheric Program Australia 1992. Department of Administrative Services and CSIRO Division of Atmospheric Research, Melbourne, Australia.
- Francey, R.J., P.P. Tans, C.E. Allison, I.G. Enting, J.W.C. White, and M. Trolier. 1995b. Changes in the oceanic and terrestrial carbon uptake since 1982. Nature 373:326-330.
CITE AS: Francey, R.J., and C.E. Allison. 1998. In situ δ13 CO2 from Cape Grim, Tasmania, Australia: 1982-1993. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A
7/26/98 Prepared by Mónica Martínez and Tom Boden