In Situ Sensor Technology for Simultaneous Spectrophotometric Measurements of Seawater Total Dissolved Inorganic Carbon and pH

Abstract

A new, in situ sensing system, Channelized Optical System (CHANOS), was recently developed to make high-resolution, simultaneous measurements of total dissolved inorganic carbon (DIC) and pH in seawater. Measurements made by this single, compact sensor can fully characterize the marine carbonate system. The system has a modular design to accommodate two independent, but similar measurement channels for DIC and pH. Both are based on spectrophotometric detection of hydrogen ion concentrations. The pH channel uses a flow-through, sample-indicator mixing design to achieve near instantaneous measurements. The DIC channel adapts a recently developed spectrophotometric method to achieve flow-through CO₂ equilibration between an acidified sample and an indicator solution with a response time of only ∼90 s. During laboratory and in situ testing, CHANOS achieved a precision of ±0.0010 and ±2.5 μmol kg^–1 for pH and DIC, respectively. In situ comparison tests indicated that the accuracies of the pH and DIC channels over a three-week time-series deployment were ±0.0024 and ±4.1 μmol kg^–1, respectively. This study demonstrates that CHANOS can make in situ, climatology-quality measurements by measuring two desirable CO₂ parameters, and is capable of resolving the CO₂ system in dynamic marine environments