MONITORING THE ISOTOPIC COMPOSITION OF ATMOSPHERIC CO2 - MEASUREMENTSFROM 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 Arcti c and Alpine Research, using air samples collected weekly by the Globa l Air Sampling Network of the NOAA Climate Monitoring and Diagnostics Laboratory. These measurements, in concert with the monitoring of atmo spheric CO2 mixing ratios, offer the potential to characterize quantit atively the mechanisms operating in the global carbon cycle, by record ing the isotopic signatures imparted to CO2 as it moves among the atmo sphere, biosphere, and oceans. This data set increases the number of m easurements of atmospheric CO2 isotopes by nearly an order of magnitud e over those previously available. We describe the analytical techniqu es used to obtain and calibrate these data and report measurements fro m 25 land-based sites, and two ships in the Pacific Ocean, from sample s collected during 1990-1993. The typical precision of our mass spectr ometric technique is 0.03 parts per thousand for delta(13)C and 0.05 p arts per thousand for delta(13)O. Collecting the flask samples without drying leads to loss of delta(18)O information at many sites. The sea sonal cycle in delta(13)C at sites in the northern hemisphere is highl y correlated with that of the CO2 mixing ratio, with amplitudes approa ching 1 parts per thousand at high latitudes. The seasonal cycle in de lta(13)O is of similar amplitude, though variable from year to year an d lags the other species by 2-4 months. Interhemispheric differences o f the 1992 and 1993 means of the isotopic tracers are in strong contra st: the north pole-south pole difference for delta(13)C is -0.20 parts per thousand, which though highly quantitatively significant is dwarf ed 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 n o significant temporal trend in annual mean delta(13)C during 1990-199 3.