Abstract

Measurements of the fugacity of carbon dioxide (fCO₂) in surface seawater are an important part of studies of the global carbon cycle and its anthropogenic perturbation. An important step toward the thorough interpretation of the vast amount of available fCO₂ data is the establishment of a database system that would make such measurements more widely available for use in understanding the basin- and global-scale distribution of fCO₂ and its influence on the oceanic uptake of anthropogenic CO₂. Such an effort, however, is based on knowledge of the comparability of data sets from different laboratories. Currently, however, there is not much known about this subject. In the light of the aforementioned situation, an International Intercomparison Exercise of Underway fCO₂ Systems was proposed and carried out by the Institut für Meereskunde Kiel (IfMK) (Institute of Marine Research at the University of Kiel), Kiel, Germany, during the R/V Meteor Cruise 36/1 from Hamilton, Bermuda, to Las Palmas, Gran Canaria, Spain. Nine groups from six countries (Australia, Denmark, Germany, France, Japan, and the United States) participated in this ambitious exercise, bringing together 15 participants with 7 underway fCO₂ systems, 1 discrete fCO₂ system, and 2 underway pH systems, as well as discrete systems for alkalinity and total dissolved inorganic carbon. This report presents only the results of the underway measurements of fCO₂.

The main idea of the exercise was to compare surface seawater fCO₂ synchronously measured by all participating instruments under identical conditions. This synchronicity was accomplished by providing the infrastructure during the exercise, such as a common seawater and calibration gas supply. Another important issue was checks of the performance of the calibration procedures for CO₂ and of all equilibrator temperature sensors. Furthermore a common procedure for the calculation of final fCO₂ was applied to all data sets. All these measures were taken in order to reduce the largest possible amount of controllable sources of error.

In this report we will demonstrate that the results of three of the seven underway systems agreed to within 2 µatm throughout the cruise. This was not only the case for seawater fCO₂ measurements but also for measurements of the atmospheric mole fraction of CO₂ (xCO₂). One system was in good agreement (+/-2 µatm) for most of the time but showed a considerable positive offset of up to 9 µatm for about 40 h. However, it was found that significant offsets of up to 10 µatm occurred in underway fCO₂ measurements for three systems under typical and identical field work conditions. Although at least in one case this may be a consequence of a technical failure, it is an indication of significant systematic differences in other cases. Finally, the discrete fCO₂ system measurements agreed within its nominal accuracy of 1% with the three most consistent underway fCO₂ systems data sets.

On the basis of a detailed comparison and evaluation of this large intercomparison data set we try to come up with general conclusions and recommendations for underway fCO₂ work. These may serve as background information for a successful preparation of a coherent database of surface ocean fCO₂ values. The results of this exercise certainly underline the need to address carefully the important issue of the inter-laboratory comparability of fCO₂ data.

Reference

• Koertzinger, A., L. Mintrop, J.C. Duinker, K. Johnson, C. Neill, D.W.R. Wallace, B. Tilbrook, P. Towler, H. Inoue, M. Ishii, G. Shaffer, R. Torres, E. Ohtaki, E. Yamashita, A. Poisson, C. Brunet, B. Schauer, C. Goyet, G. Eischeid, and A. Kozyr (ed.). 1999. The International Intercomparison Exercise of Underway fCO₂ Systems During the R/V Meteor Cruise 36/1 in the North Atlantic Ocean. ORNL/CDIAC-114, NDP-067. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, U.S.A. doi: 10.3334/CDIAC/otg.ndp067.