Open-path cavity ring-down spectroscopy for trace gas measurements in ambient air

Area of Science:

• Atmospheric science
• Spectroscopy
• Environmental monitoring

Background:

• Cavity ring-down spectroscopy (CRDS) is a sensitive technique for gas concentration measurements.
• Open-path CRDS offers advantages for in-situ atmospheric monitoring but faces challenges with performance and limitations.
• Aerosol interference can affect the accuracy of CRDS measurements in ambient air.

Purpose of the Study:

• To evaluate the performance and limitations of near-infrared methane open-path CRDS for atmospheric measurements.
• To develop methods for improving the stability and sensitivity of open-path CRDS systems.
• To validate the performance of the developed open-path CRDS sensor against established methods.

Main Methods:

• Utilized a near-infrared methane cavity ring-down spectroscopy (CRDS) sensor.
• Developed a purge-enclosure to maintain high mirror reflectivity (>0.99996) for over 100 hours.
• Characterized aerosol effects on ring-down decay signals, identifying super-micron particles as a major source of fluctuation.
• Implemented software filtering techniques to mitigate aerosol-induced signal fluctuations.
• Validated sensor measurements against known methane concentrations in closed-path and open-path configurations.

Main Results:

• Achieved stable operation of the open-path CRDS sensor with high mirror reflectivity.
• Identified and quantified the impact of aerosols, particularly super-micron particles, on CRDS signal stability.
• Developed effective software filtering strategies to reduce noise and improve signal quality.
• Demonstrated a noise-equivalent sensitivity of approximately 6x10^-10 cm^-1Hz^-1/2, comparable to closed-path systems.
• Validated open-path measurements through side-by-side comparison with a commercial closed-path system.

Conclusions:

• Open-path CRDS is a viable technique for atmospheric methane measurements with appropriate system design and data processing.
• The developed purge-enclosure and software filtering effectively address key limitations, particularly aerosol interference.
• The sensor achieved high sensitivity and accuracy, comparable to traditional closed-path systems, enabling reliable environmental monitoring.