The international at-sea intercomparison of fCO2 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 Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants 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 carbon. Here, we compare surface seawater fCO(2) measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO2, temperature measurements) and a common procedure for calculation 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 can be made to a high degree of agreement(+/-1 mu atm) with a variety of possible equilibrator and system designs. Also, discrete fCO(2) measurements can 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 mu atm. Based on our results, no best choice for the type of the equilibrator 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. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO(2) results. Calculation of fCO(2) from high-quality total dissolved inorganic carbon (C-T) and total alkalinity (A(T)) measurements does not yield results comparable in accuracy and precision to fCO(2) measurements. (C) 2000 Elsevier Science B.V. All rights reserved.