High-frequency measurement of partial pressure and total concentration of carbon dioxide in seawater using microporous hydrophobic membrane contactors

To investigate CO2 chemistry in ocean water with greater time-space resolutions, we developed measurement systems, which have state-of-the-art precision but an order of magnitude or better increase in the frequency of analysis, for carbon dioxide partial pressures (P-CO2) and total carbon dioxide concentrations (T-CO2) in seawater. The P-CO2 system was based on equilibration of a CO2-free carrier gas stream with aqueous carbon dioxide in a flowing seawater sample stream using a commercially available membrane contactor unit normally employed in industrial applications followed by nondispersive infrared (NDIR) absorbance detection of the CO2 in the exit carrier gas. The T-CO2 system was based on injection of a small-volume seawater sample loop (similar to1 mL) into an acid (0.1 N HCl) liquid carrier stream to convert all carbonate and bicarbonate ions to aqueous carbon dioxide; this acidified sample was then passed through a custom-made small-volume membrane contactor unit where the sample's P-CO2 was determined by equilibration of a CO2-free carrier gas followed by NDIR detection. Results from lab tests and a field experiment in the Ross Sea polynya, Antarctica, are presented. The P-CO2 system was determined to have a response time of about 3 s and precision of better than 1 muatm. The T-CO2 system had a maximum analysis rate of one sample per 36 s, and reproducibility was determined to be better than 0.2% for a period of hours.