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, Wanninkhof & Thoning, 1993). Another overview on different systems is given by Pierrot et al. (2009).
A great variety of pCO2 systems and equilibrators have been described in the literature. Essentially three different design principles can be distinguished (Körtzinger et al., 1996):
- The “shower type” equilibrator.
- The “bubble type” equilibrator.
- The “laminary flow type” equilibrator. A design described by Copin-Montegut (1988) combines aspects of the shower and bubble type.
- A gas-liquid equilibrator that relies on a continuous falling film of water over a spherical surface to drive gas exchange. This equilibrator uses free flowing, falling water to produce a surface for gas exchange, shown to be very resistant to clogging and freezing, and therefore well suited to long term deployment in highly productive waters like estuaries where CO2 concentrations fluctuate hourly, daily, and seasonally (Miller et al., 2019).
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”).
- Feely, R.A., Wanninkhof, R., Milburn, H.B., Cosca, C.E., Stapp, M., Murphy, P. 1998. A new automated underway system for making high precision pCO2 measurements onboard research ships. Analytica Chimica Acta 377: 185–191
- Wanninkhof, R. and Thoning, K., 1993. Measurement of fugacity of CO2 in surface water using continuous and discrete sampling methods. Marine Chemistry 44: 189–204
- Pierrot, D., Neill, C., Sullivan, K., Castle, R., Wanninkhof, R., Lu, H., Johannessen, T., Olsen, A., Feely, R.A. and Cosca, C. 2009. Recommendations for autonomous underway pCO2 measuring systems and data-reduction routines. Deep-Sea Research II 56: 512–522
- Kortzinger, A., Thomas, H., Schneider, B., Gronau, N., Mintrop, L. and Duinker, J.C. 1996. At-sea intercomparison of two newly designed underway pCO2 system—encouraging results. Mar Chem. 1996; 52: 133–145
- Copin-Montegut, C., 1988. A new formula for the effect of temperature on the partial pressure of CO2 in seawater. Marine Chemistry 25: 29–37
- Miller, A.W., Reynolds, A.C. and Minton, M.S. 2019. A spherical falling film gas-liquid equilibrator for rapid and continuous measurements of CO2 and other trace gases. PLoS ONE 14(9): e0222303
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