The International Intercomparison Exercise of Underway fCO2 Systems During the R/V Meteor Cruise 36/1 in the North Atlantic Ocean
Körtzinger, Arne, Ludger Mintrop, and Jan C. Duinker, University of Kiel, Kiel, Germany; Kenneth M. Johnson, and Craig Neill, Brookhaven National Laboratory, Upton, New York; Douglas W. R. Wallace, University of Kiel, Kiel, Germany; Bronte Tilbrook, and Philip Towler, Commonwealth Scientific and Industrial Research Organisation, Holbart, Tasmania, Australia; Hisayuki Inoue, and Masao Ishii, Meteorological Research Institute, Tsukuba, Japan; Gary Shaffer, University of Copenhagen,
Copenhagen, Denmark; Rodrigo Torres, Göteborg University and Chalmers University of Technology, Göteborg, Sweden; Eiji Ohtaki, and Eiji Yamashita, Okayama University, Okayama, Japan; Alain Poisson, Christian Brunet, and Bernard Schauer, Université Pieere et Marie Cure, Laboratoire de Physique et Chimie Marines, Paris, France; Catherine Goyet, and Greg Eischeid, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
Prepared by Alexander Kozyr, CDIAC
NDP-067 (1999) (http://cdiac.ornl.gov/oceans/ndp_067/ndp067.html)
Measurements of the fugacity of carbon dioxide (fCO2) in sur-face seawater are an important part of studies of the global carbon cycle and its anthropogenic pertur-bation. 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. These measures were taken in order to reduce the
largest possible amount of controllable sources of error.
The results demonstrated in this report show that three of the seven underway systems agreed to within 2 ľatm throughout the cruise. This was not only the case for seawater fCO2 measure-ments but also for measurements of the atmospheric mole fraction of CO2 (CO2). It was found that significant offsets of up to 10 ľatm occurred in underway fCO2 measurements for three systems under typical and identical field work conditions. Finally, the discrete fCO2 system measurements agreed within its nominal accuracy of 1% with the three most consistent underway fCO2 systems data sets.
General conclusions and recommendations for underway fCO2 work are based on a detailed comparison and evaluation of this large intercomparison data set and 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 interlaboratory comparability of fCO2 data. WDC-A database
