Principle of Measurement of the Fugacity of CO₂

The principle of the measurement of the fugacity of CO₂ (fCO₂) in seawater is based on the determination of the CO₂ mixing ratio in a gas phase that is in equilibrium with a seawater sample at known temperature and pressure. The CO₂ mixing ratio can either be measured with a nondispersive infrared analyzer (NDIR) or with a gas chromatograph (GC) with flame ionization detector after catalytic conversion of the CO₂ into methane. Whereas the GC approach has a few advantages (e.g., small sample volume and the ability to measure additional trace gases), the more rugged infrared technique has shown better suitability for use at sea and allows measurements in a truly continuous fashion. Depending on the sampling strategy (discrete or continuous), two different families of analytical systems have been developed. For the determination of the fCO₂ in air that is in equilibrium with a discrete sample, a known amount of seawater is isolated in a closed system containing a small known volume of air with a known initial CO₂ mixing ratio. For the determination of the fCO₂ in air that is in equilibrium with a continuous flow of seawater, a fixed volume of air is equilibrated with seawater that flows continuously through an equilibrator.

Continuous (or underway) fCO₂ systems are more widely used in marine CO₂ research. They provide important information about the saturation state of seawater at the air-sea interface when operated on board research vessels with a continuous flow of seawater usually obtained by means of a shipborne pumping system.