In Situ High-Precision Measurement of Deep-Sea Dissolved Methane by Quartz-Enhanced Photoacoustic and Light-Induced...

Area of Science:

• Geochemistry and Oceanography
• Environmental Monitoring
• Sensor Technology

Background:

• In situ analysis of deep-sea dissolved gases is crucial for ecological geology, resource exploration, and climate change research.
• Existing methods for dissolved gas analysis often lack the speed, accuracy, or in situ capabilities required for deep-sea environments.
• Methane (CH4) is a key greenhouse gas and indicator of geological processes, making its precise measurement vital.

Purpose of the Study:

• To develop and validate a novel in situ sensor for accurate and rapid measurement of deep-sea dissolved methane.
• To integrate advanced spectroscopic techniques for enhanced sensitivity and self-correction capabilities.
• To demonstrate the sensor's performance and applicability in a real-world deep-sea deployment.

Main Methods:

• Utilized quartz-enhanced photoacoustic spectroscopy (QEPAS) and light-induced thermoelastic spectroscopy (LITES) with frequency division multiplexing.
• Designed a compact sensor system (φ120 mm × 430 mm, 7.6 W power consumption) with a small photoacoustic cell (1.2 mL).
• Calibrated the sensor's spectral response for CH4 concentrations (0.01–5%) under varying pressure and temperature conditions, and investigated the influence of water vapor.

Main Results:

• Achieved a minimum detection limit of 0.21 ppm for methane based on Allan variance analysis over several hours.
• Demonstrated a rapid time response of 4 minutes for dissolved methane detection due to the compact photoacoustic cell.
• Successfully deployed the sensor system in the South China Sea at 1380 m, collecting three days of continuous dissolved methane data near "HaiMa" cold seeps.

Conclusions:

• The developed QEPAS and LITES based sensor provides a robust and accurate solution for in situ deep-sea dissolved methane monitoring.
• The sensor's compact design, low power consumption, and rapid response time make it suitable for autonomous marine applications.
• The successful field deployment validates the sensor's capability to provide valuable data for understanding deep-sea ecosystems and geological processes.