An Open-Path Eddy-Covariance Laser Spectrometer for Simultaneous Monitoring of CO₂, CH₄, and H₂O

Area of Science:

• Atmospheric Science
• Spectroscopy
• Environmental Monitoring

Background:

• Accurate measurement of atmospheric gases like CO2, CH4, and H2O is crucial for understanding climate change and environmental processes.
• Traditional methods for gas flux determination often require complex sampling systems and can be limited in scope.

Purpose of the Study:

• To introduce E-CAHORS, a novel compact open-path diode-laser spectrometer for simultaneous measurement of CO2, CH4, and H2O.
• To evaluate the performance of E-CAHORS in laboratory and field conditions for atmospheric flux studies.

Main Methods:

• Utilized two interband cascade lasers (2.78 µm and 3.24 µm) in a four-pass M-shaped optical cell.
• Implemented and compared tunable diode laser absorption spectroscopy (TDLAS) and simplified wavelength modulation spectroscopy (sWMS) detection techniques.
• Integrated with a three-dimensional anemometer for eddy-covariance flux calculations.

Main Results:

• Achieved high linearity (R² ≈ 0.999) and precisions of 311 ppb (CO2), 8.87 ppb (CH4), and 788 ppb (H2O) at 10 Hz in laboratory calibration.
• Demonstrated strong field correlations (R = 0.91 for CO2/H2O, R = 0.74 for CH4) with commercial analyzers.
• TDLAS mode showed better stability outdoors, while sWMS offered baseline-free spectra but was sensitive to power fluctuations.

Conclusions:

• E-CAHORS offers a precise, multi-species sensing capability for near-surface atmospheric layer measurements.
• The instrument's compact design, low power consumption (10 W), and portability make it suitable for field deployment.
• E-CAHORS facilitates accurate greenhouse gas flux determination via the eddy-covariance method.