The basic idea of the exercise was to operate as many underway fCO2 systems simultaneously for as much time as possible. Combined with in situ salinity and temperature as well as navigational and meteorological data, this combined underway fCO2 data set is the mainstay of the exercise. Whereas shore-based intercomparison exercises allow researchers to devise special experiments that reflect extreme situations, ship-based exercises have to rely fully on the conditions that are provided by the ocean. The chosen cruise track reflects the attempt to include-within the limits of a single and comparatively short cruise-extreme oceanic regimes. Whereas the situation was very stable in the Eastern North Atlantic with not much variability in surface seawater temperatures and salinities and likewise fCO2, the North Atlantic Drift region off Newfoundland provided extreme variability with steep gradients. The overall temperature range during the exercise was from 6.0°C to 25.1°C, while the salinity varied between 32.3 and 37.0. In the western part our cruise track hit warm and cold ring features. Associated with these rings were steep frontal gradients with changes of up to 15°C and more than 3 in salinity over a few nautical miles.
These different regimes provide different information about the performance and comparability of the participating systems. The stable situation during the second half of the exercise allows the detection of systematic offsets between the data sets, thus providing the basic information about the inter-laboratory comparability. In contrast to this, the strong gradient regime mimics to some extent the step experiments of shore-based intercomparison exercises. The fast change between two "batches" of seawater, which are characterized by different fCO2 values, reveals the different time constants of the analytical systems. Fast responding systems are able to follow the signal much more closely than the more slowly responding ones. So, even if there are no systematic differences between two systems, the systems may have quite different response times, which translates into different spatial resolution in underway work.
Right from the beginning, it was regarded as high priority to measure as many parameters [i.e., pH, fCO2, total dissolved organic carbon (CT), and total alkalinity (AT)] of the marine CO2 system as possible rather than restricting the exercise to mere fCO2 measurements. For this purpose, we followed two different sampling strategies (i.e., underway sampling and discrete sampling). As all participating fCO2 systems (CSIRO, IfMK, MRI, NBI, OU, UP&MC, WHOI; see R/V Meteor Cruise 36/1 Information for a list of participating institutions) were operated in an underway mode on the same seawater source, it was highly desirable to back up these fCO2 measurements with additional underway measurements of other CO2 parameters. This was accomplished by underway pH measurements with two different spectrophotometric systems (SIO, WHOI) as well as underway CT measurements (BNL/IfMK) with a newly modified single-operator multiparameter metabolic analyzer (SOMMA) coulometric titration system (Johnson et al. 1998), all of which were hooked up to the seawater pumping system. Discrete sampling was carried out for discrete measurements of fCO2 (BNL), CT (BNL/IfMK), AT (IfMK), and salinity (IfMK) as well as nutrients (IfMK) in samples taken regularly from the same seawater pumping system.
By measuring more than two parameters of the CO2 system in seawater, the system is overdetermined, as all parameters can be calculated from any combination of two measured parameters and knowledge of the thermodynamic relationships involved. This was the case for both sampling strategies. Overdetermination will therefore allow for consistency checks on the data sets. It may also provide additional information in the question of the best set of thermodynamic constants for the CO2 system. The broad CO2 database furthermore serves as valuable background information and will strongly enhance further interpretation of the results. The exercise also included checks on ancillary measurements, such as temperature and barometric pressure, as performed by most of the analytical systems. All temperature sensors were compared against a calibrated Pt-100 reference thermometer. The barometric pressure readings were also referenced against a high-quality digital barometer. In many cases, these checks revealed offsets and miscalibrations, which, if not corrected for, would have led to significant biases of the final fCO2 values. These checks helped to identify the error contribution from these sources. They also allowed us to correct all fCO2 measurements for these effects to reveal any systematic differences that cannot be attributed to the quality of temperature and pressure measurements.
Further checks were carried out with the calibration gases. The suite of calibration gases supplied by the organizer covered a range of CO2 concentrations between 250 and 500 ppmv with nominal values of 250, 300, 350, 400, 450, and 500 ppmv. While every group required one or more of these calibration gases for their calibration procedure, they measured all other concentrations as unknown samples on their systems. The results provide information on the quality and reliability of the calibration procedures over the whole range from 250 to 500 ppmv. As the infrared detectors used by all groups generally show nonlinear response functions, the calibration procedure is a crucial point.
akozyr 03/03/1999