In situ Carbon 13 and Oxygen 18 Ratios of Atmospheric CO2 from Cape Grim, Tasmania, Australia: 1982-1993 CONTRIBUTORS: R. J. Francey C. E. Allison CSIRO Division of Atmospheric Research Private Bag No. 1 Mordialloc, Victoria AUSTRALIA 3195 BRIEF INTRODUCTION Since 1982, a continuous program of sampling atmospheric CO2 to determine stable isotope ratios has been maintained at the Australian Baseline Air Pollution Station, Cape Grim, Tasmania (40 40'56" S, 144 41'18" E). The process of in situ extraction of CO2 from air, the preponderance of samples collected in conditions of strong wind from the marine boundary layer of the Southern Ocean, and the determination of all isotope ratios relative to a common, high-purity CO2 reference gas with isotopic d13C close to atmospheric values, are a unique combination of factors with respect to obtaining a globally representative signal from a surface site. The d13C data help partition the uptake of fossil fuel emissions between ocean and terrestrial reservoirs. The d18O data predominantly reflect the terrestrial hydrological cycle. Using the data provided here, Francey et al. (1995b) observed a gradual decrease in d13C from 1982 to 1993, but with a pronounced flattening from 1988 to 1990; a trend that appears to involve the terrestrial carbon cycle. More details about the CSIRO isotopic measurement program at Cape Grim may be found in Allison et al. (1994) and Francey et al. (1995a). METHODS 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 degrees, condensation nucleus concentration below 600 per cubic cm, 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 intake is dried with a trap immersed in an alcohol bath at a temperature of about -80C. 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 d13C and d18O 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 d13C in atmospheric CO2 requires the simultaneous measurement 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 17O 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 (d) 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 d values are calculated as SA45-ST45 d45 = ----------- ST45 SA46-ST46 d46 = ----------- ST46 When multiplied by 1000, the value d is expressed in units of per mil (o/oo). The d13C and d18O 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 d13C and d18O 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 evolves CO2 with accepted d13C of +1.95 o/oo and d18O 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). DATA FILE FORMAT The file "isotope.cgo" contains in situ carbon 13 and oxygen 18 ratios of atmospheric CO2 from Cape Grim for 1982 through 1993. Each data record contains a decimal year value (e.g., 93.960), d13C and d18O values, and an indication of the air intake height (i.e., either the 10 m or 70 m intake). Only samples measured on the VG602D mass spectrometer have values reported for d18O. This is pending completion of a study of varying differences observed in samples measured on both mass spectrometers during the 18 month sample overlap period (no similar differences influenced the d13C comparison). The decimal year does not carry diurnal information; it is the decimal equivalent of the date only. The d13C and d18O values are expressed in per mil and are calculated as described in the preceding paragraph. REFERENCES 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 d13C 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. ------------------------------------------------------------------------- This descriptive file was created by Tom Boden (Carbon Dioxide Information Analysis Center, Environmental Sciences Division, Oak Ridge National Laboratory) and reviewed by Roger Francey. This file is Environmental Sciences Division publication number 4522. Oak Ridge National Laboratory is managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy under contract number DE-AC05-96OR22464.