High-frequency spectrophotometric measurements of total dissolved inorganic carbon in seawater

A new spectrophotometric method was developed to achieve continuous measurements of total dissolved inorganic carbon (DIC) in seawater. It uses a countercurrent flow design and a highly CO2-permeable membrane (Teflon AF 2400) to achieve flow-through CO2 equilibration between an acidified sample and indicator solution with a fast response time of ~22s. This method improves the spatiotemporal resolution by more than one order of magnitude compared to the existing spectrophotometric method. The flow-through equilibration allows for continuous (~1 Hz) detection and real-time data smoothing. The method had a short-term precision of ±2.0 µmol kg-1 for a given flow-through sample. It achieved a field precision of ±3.6 µmol kg-1 and successfully captured high DIC variability down to minute scales. Measurements by the new method over the typical range of oceanic DIC showed good agreement with measurements made by an established method (mean differences -1.6–0.3 µmol kg-1 with 1σ ±6.0–6.7 µmol kg-1). This level of precision and accuracy is comparable to that of the existing spectrophotometric method. The characteristics of the new method make it particularly suitable for high-frequency, submerged measurements required for mobile observing platforms in the ocean. It can also be adapted for high-frequency, spectrophotometric measurements of seawater CO2 fugacity.

Wang Z. A., Chu S. N. & Hoering K. A., 2013. High-frequency spectrophotometric measurements of total dissolved inorganic carbon in seawater. Environmental Science & Technology. Article (subscription required).


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