Scientific Background of the Exercise

Currently marine scientists are applying different concepts to quantify the oceanic uptake of CO₂. These efforts are being undertaken in the light of the atmospheric CO₂ perturbation and its possible impact on the earth's climate. One important concept is based on the determination of the partial pressure difference of CO₂ (delta pCO₂) between the surface seawater and the overlying air, which is the thermodynamic driving force for any net exchange of CO₂. By means of a transfer coefficient, a measured (pCO₂ can be converted into a momentary net flux of CO₂ across the air-sea interface. Given the strong spatial and temporal variability of pCO₂ in the ocean, this concept faces the challenge of coming up with representative mean delta pCO₂ values on a global grid. If this concept is to be successful in pinning down the present oceanic uptake of CO₂ reliably, the combined efforts of research groups all over the world are necessary. The Intergovernmental Oceanographic Council (IOC)/Scientific Committee on Oceanic Research (SCOR) Carbon Dioxide Advisory Panel recently established an international inventory of pCO₂ measurements that have been identified so far. One important requirement in this context is a good inter-laboratory comparability of the data sets, which were generated by quite different types of analytical systems. While the analytical precision of the various systems in use is mostly of the order of 1 µatm or better, not much is known presently about the comparability between different laboratories.

As a first important step to assess the current state of this parameter, an international shore-based intercomparison exercise of underway fugacity of CO₂ (fCO₂) systems was carried out by Andrew Dickson in June 1994 at the Scripps Institution of Oceanography, Marine Physical Laboratory, La Jolla, California, U.S.A. (http://www-mpl.ucsd.edu/people/adickson/CO2_QC) on behalf of the Joint IOC/SCOR CO₂ Advisory Panel. However, the general consensus in the scientific community was that a necessary second step would be an at-sea intercomparison under more typical and identical operation conditions. Such an exercise, to be carried out during the R/V Meteor cruise 36/1, was proposed by the Kiel CO₂ group in June 1995 and received very positive feedback within the scientific community. For a number of reasons the proposed cruise leg was perfectly suited for such an exercise. Funding of the exercise came through the German Joint Global Ocean Flux Study (JGOFS) program. More than fifteen research groups, representing a fairly good geographical distribution, were contacted and invited to participate in the exercise, nine of which were finally able to do so (Koertzinger et al. 1996a).