Midinfrared Sensor System Based on Tunable Laser Absorption Spectroscopy for Dissolved Carbon Dioxide Analysis in the South China Sea: System-Level Integration and Deployment

Abstract

System-level integration of a midinfrared carbon dioxide (CO₂) sensor system based on tunable laser absorption spectroscopy (TLAS) was realized for the analysis of dissolved CO₂ in seawater, employing an interband cascade laser (ICL) centered at 4319 nm and a multipass cell (MPC) with an optical path length of 29.8 m. At a low measurement pressure of 30 Torr, three absorption lines of ¹²CO₂ were selected to realize different measurement ranges and a ¹³CO₂ absorption line was targeted for simultaneous isotopic abundance analysis of δ¹³CO₂. The sensor system was compactly integrated into a standalone system with automatic operation for underwater field deployment, and the working process was controlled by a specially designed electrical system. A gas-liquid separator system was developed for CO₂ extraction from water, and a pressure-control mechanism with two operation modes (i.e., static and dynamic modes) was proposed to make the sensor system applicable under a deep-sea environment. A series of experiments were carried out in the laboratory for performance assessment of the developed sensor system employed for the analysis of dissolved CO₂ in water. The sensor was deployed for a field test for natural gas hydrates exploration at an underwater depth of 0-2000 m in the South China Sea, with the sensor operating normally during the deployment.