In situ Carbon 13 and Oxygen 18 Ratios of Atmospheric CO2 from Cape Grim, Tasmania, Australia: 1982-1993
Francey R. J. and C. E. Allison
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 δ13C close to atmospheric values, are a unique combination of factors with respect to obtaining a globally representative signal from a surface site. 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 per cm-3, and steady continuous CO2 concentrations (variation ± 0.2 ppmv per hour). 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 about -80°C. Mass spectrometer analyses for δ13C and δ18O are carried out by CSIRO's Division of Atmospheric Research in Aspendale, usually one to three weeks following collection.
This record is possibly the most accurate representation of global atmospheric 13C behavior over the last decade and may be used to partition the uptake of fossil-fuel carbon emissions between ocean and terrestrial plant reservoirs. Using these data, Francey et al. (1995) observed a gradual decrease in δ13C from 1982 to 1993, but with a pronounced flattening from 1988 to 1990; a trend that appears to involve the terrestrial carbon cycle.
The database consists of two files, a descriptive "readme" file (153 lines) and a data file ("isotope.cgo", 440 lines) that contains the 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), δ13C and δ18O values, and an indication of the air intake height (i.e., either the 10 m or 70 m intakes). Only samples measured on the older of the two mass spectrometers used have values reported for δ18O. This is pending completion of a study of varying differences observed on samples measured on both mass spectrometers during the eighteen month sample overlap period (no similar differences influenced the δ13C comparison). The decimal year does not carry diurnal information, it is the decimal equivalent of the date only. The δ13C and δ18O values are expressed in per mil (o/oo).