Simultaneous Monitoring of Atmospheric CH₄, N₂O, and H₂O Using a Single Gas Sensor Based on Mid-IR Quartz-Enhanced...

Area of Science:

• Spectroscopy
• Optical Sensing
• Environmental Monitoring

Background:

• Accurate and simultaneous detection of greenhouse gases like methane (CH4) and nitrous oxide (N2O) is crucial for environmental monitoring.
• Water vapor (H2O) presence can influence the performance of photoacoustic spectroscopy (PAS) sensors.

Purpose of the Study:

• To develop and characterize an optical sensor for simultaneous triple-species monitoring of CH4, N2O, and H2O vapor.
• To investigate the impact of molecular relaxation and water vapor on sensor performance using off-beam quartz-enhanced photoacoustic spectroscopy (OB-QEPAS).

Main Methods:

• An external-cavity quantum cascade laser (EC-QCL) was tuned to absorption lines of CH4, N2O, and H2O near 8 μm.
• Off-beam quartz-enhanced photoacoustic spectroscopy (OB-QEPAS) was employed for gas detection.
• A theoretical model was developed, and real-time Kalman filtering was applied to enhance measurement precision.

Main Results:

• The OB-QEPAS sensor achieved simultaneous detection of CH4, N2O, and H2O.
• Humidified samples showed signal enhancements: 3x for CH4 and 20% for N2O.
• Detection limits were 98 ppbv for CH4, 12 ppbv for N2O, and 750 ppmv for H2O.
• Kalman filtering improved measurement precision by approximately 4x.

Conclusions:

• The developed EC-QCL-based OB-QEPAS sensor is effective for simultaneous CH4, N2O, and H2O monitoring.
• Water vapor positively influences QEPAS signals for CH4 and N2O detection.
• The sensor demonstrates high sensitivity and precision, suitable for environmental applications.