An open path H2O/CO2 gas analyzer for eddy correlation systems: theory and design.

Area of Science:

• Environmental Science
• Atmospheric Chemistry
• Spectroscopy

Background:

• Accurate measurement of atmospheric gases like H2O and CO2 is vital for understanding turbulent environments.
• Traditional methods may lack the speed and precision required for eddy correlation measurements.
• Near-surface turbulent environments present unique challenges for gas analysis due to rapid fluctuations.

Purpose of the Study:

• To present a novel, fast-response, open-path tunable diode laser absorption spectroscopy (TDLAS) gas analyzer.
• To enable high-resolution (ppb levels) measurements of H2O and CO2 concentrations.
• To facilitate accurate eddy correlation flux measurements in near-surface turbulent environments.

Main Methods:

• Utilized a dual laser system with a fiber-connected optical head and a folded optical path.
• Employed Fourier (discrete cosine) transform absorbance ratio analysis methods.
• Incorporated modified Hitran parameters (n, gamma, E'') for enhanced absorbance function synthesis and mole fraction calculation.

Main Results:

• Achieved ppb-level resolution for H2O at 1.3964 µm and CO2 at 2.014 µm.
• Developed and presented a mole fraction equation based on the extended Hitran model.
• Outlined an absolute calibration technique and methods for calibration retention.

Conclusions:

• The developed TDLAS analyzer is suitable for eddy correlation measurements in turbulent environments.
• The instrument's design and analysis methods ensure high accuracy and precision.
• Simulation results verify the analyzer's specifications for mole fraction estimation and error analysis.