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Measurement of CO2
The main principle of pCO2 measurement is based on the equilibration of a carrier gas phase with a seawater sample and subsequent determination of the CO2 in the carrier gas by an infrared analyser. As the pCO2 in seawater strongly varies with temperature a correction is necessary to compensate for the difference between equilibration temperature and the in-situ seawater temperature. The equilibrated surface water values should be accurate within 2 µatm. This will necessitate pressure measurements within 0.2 mBar and water temperature measurements with an accuracy of 0.01 C. Different underway-measuring systems have been developed and applied for regional and global studies (e.g., in Feely et al. 1998, Cooper et al. 1998, Wanninkhof & Thoning 1993. Another overview on different systems was given at the “Underway pCO2 System Workshop” October 2-3 2002 at NOAA/AOML in Miami, Fl, (http://www.aoml.noaa.gov/ocd/pco2/descriptions.html).
A great variety of pCO2 systems and equilibrators have been described in the literature. Essentially three different design principles can be distinguished (from Körtzinger et al 1996):
- The “shower type” equilibrator (e.g., Keeling et al. 1965; Kelley 1970; Weiss1981; Inoue et al. 1987; Robertson et al, 1993; Goyet and Peltzer, 1994,
- The “bubble type” equilibrator (e.g., Takahashi 1961; Goyet et al. 1991; Schneider et al. 1992; Kimoto and Harashima 1993; Ohtaki et al. 1993), and
- The “laminary flow type” equilibrator (Poisson et al., 1993).
A design described by Copin-Montegut (1985) combines aspects of the shower and bubble type.
A typical example for two underway pCO2 systems of the “bubble type”, that were developed independently at the Institute for Marine Sciences, Kiel (IFM-Geomar) and at the Baltic Sea Research Institute, Warnemünde (IOW) are described in: Körtzinger et al. (1996): A continuous flow of seawater passes through an open system equilibration cell, which is vented to the atmosphere. This allows the equilibrium process to take place at ambient pressure at any time. A fixed volume of air is re-circulated continuously through the system so as to be in almost continuous equilibrium with the constantly renewed seawater phase. In a “bubble type” equilibrator this airflow is bubbled through the water phase. After passage through the equilibration cell the air stream is pumped to a non-dispersive infrared gas analyzer, where the mole fraction of CO2 is measured relative to a dry and CO2-free reference gas (absolute mode). Both systems feature a LI-COR” LI-6262 CO2/H2O gas analyser, which is a dual-channel instrument that simultaneously, measures the CO2, and H2O mole fractions. The gas stream needs no drying prior to infrared gas detection as the biasing effect of water vapour on the measurement of CO2 is eliminated based on the H2O measurement.
These systems were successfully applied in the assessment of regional and global carbon budgets and for the detection of biological processes (see 4.9. “Applications of new emerging technologies”).
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