Monitoring the isotopic composition of atmospheric CO2: Measurements from the NOAA Global Air Sampling Network

The stable isotopic composition of atmospheric CO2 is being monitored via measurements made at the University of Colorado-Institute of Arctic and Alpine Research, using air samples collected weekly by the Global Air Sampling Network of the NOAA Climate Monitoring and Diagnostics Laboratory. These measurements, in concert with the monitoring of atmospheric CO2 mixing ratios, offer the potential to characterize quantitatively the mechanisms operating in the global carbon cycle, by recording the isotopic signatures imparted to CO2 as it moves among the atmosphere, biosphere, and oceans. This data set increases the number of measurements of atmospheric CO2 isotopes by nearly an order of magnitude over those previously available. We describe the analytical techniques used to obtain and calibrate these data and report measurements from 25 land-based sites, and two ships in the Pacific Ocean, from samples collected during 1990-1993. The typical precision of our mass spectrometric technique is 0.03 parts per thousand for delta(13)C and 0.05 parts per thousand for delta(13)O. Collecting the flask samples without drying leads to loss of delta(18)O information at many sites. The seasonal cycle in delta(13)C at sites in the northern hemisphere is highly correlated with that of the CO2 mixing ratio, with amplitudes approaching 1 parts per thousand at high latitudes. The seasonal cycle in delta(13)O is of similar amplitude, though variable from year to year and lags the other species by 2-4 months. Interhemispheric differences of the 1992 and 1993 means of the isotopic tracers are in strong contrast: the north pole-south pole difference for delta(13)C is -0.20 parts per thousand, which though highly quantitatively significant is dwarfed by the -2 parts per thousand difference for delta(18)O. In contrast to the record of atmospheric delta(13)C during the 1980s we observe no significant temporal trend in annual mean delta(13)C during 1990-1993.