The international at-sea intercomparison of fCO(2) systems during the R/V Meteor Cruise 36/1 in the North Atlantic Ocean

The 'International Intercomparison Exercise of fCO(2) Systems' was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canar ia/Spain. Nine groups from six countries (Australia, Denmark, France, Germa ny, Japan, USA) participated in this exercise, bringing together 15 partici pants with seven underway fugacity of carbon dioxide (fCO(2)) systems, one discrete fCO(2) system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic car bon. Here, we compare surface seawater fCO(2) measured synchronously by all participating instruments. A common infrastructure (seawater and calibrati on gas supply), different quality checks (performance of calibration proced ures for CO2, temperature measurements) and a common procedure for calculat ion of final fCO(2) were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO(2) in surface seawater and overlying air c an be made to a high degree of agreement(+/-1 mu atm) with a variety of pos sible equilibrator and system designs. Also, discrete fCO(2) measurements c an be made in good agreement (+/-3 mu atm) with underway fCO(2) data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 m u atm. Based on our results, no "best choice" for the type of the equilibra tor nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO(2) system. Measurem ents of equilibrator temperature do not seem to be made with the required a ccuracy resulting in significant errors in fCO(2) results. Calculation of f CO(2) from high-quality total dissolved inorganic carbon (C-T) and total al kalinity (A(T)) measurements does not yield results comparable in accuracy and precision to fCO(2) measurements. (C) 2000 Elsevier Science B.V. All ri ghts reserved.