Oceanic pCO₂ Measurements with a Multi-Layered, Composite Hollow-Fiber Membrane

Abstract

In order to measure the CO₂ mixing ratios in air equilibrated with seawater (xCO₂^S), we tested a multi-layered composite hollow-fiber membrane (MHF) module aboard the TS Hokuto-maru in the North Pacific in January-February 1996 and July-August 1996. The MHF module is much smaller (300cm³) than the currently used shower-head type equilibrator (110dm³ installation volume). The mean difference in xCO₂^S (ΔxCO₂^S) values between the two systems determined 14 minutes apart was 1.6ppm with a standard deviation of 2.2ppm (n=73) for the southbound cruise in January, and -1.9ppm with a standard deviation of 1.8ppm (n=103) for the northbound cruise in February, 1996.
The difference in average ΔxCO₂^S between the two legs was mainly caused by the effect of water temperature difference between the shower-head type equilibrator and the MHF module. By taking into account the effect on xCO₂^S of water-temperature difference between the two equilibrators, the mean ΔxCO₂^S was as low as 0.1ppm (n=732) with a standard deviation of 3.7ppm for the cruise in July-August 1996. These results indicate that no systematic difference exists between the two equilibrators. The MHF has potential for future use as an equilibrator that is compact and easy to install.