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

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.