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

Abstract

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 (PCO2) and total carbon dioxide concentrations (TCO2) in seawater. The PCO2 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 TCO2 system was based on injection of a small‐volume seawater sample loop (~1 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 PCO2 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 PCO2 system was determined to have a response time of about 3 s and precision of better than 1 µatm. The TCO2 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.