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

We present E-CAHORS-a compact mid-infrared open-path diode-laser spectrometer designed for the simultaneous measurement of carbon dioxide, methane, and water vapor concentrations in the near-surface atmospheric layer. These measurements, combined with simultaneous data from a three-dimensional anemometer, can be used to determine fluxes using the eddy-covariance method. The instrument utilizes two interband cascade lasers operating at 2.78 µm and 3.24 µm within a novel four-pass M-shaped optical cell, which provides high signal power and long-term field operation without requiring active air sampling. Two detection techniques-tunable diode laser absorption spectroscopy (TDLAS) and a simplified wavelength modulation spectroscopy (sWMS)-were implemented and evaluated. Laboratory calibration demonstrated linear responses for all gases (R2 ≈ 0.999) and detection precisions at 10 Hz of 311 ppb for CO2, 8.87 ppb for CH4, and 788 ppb for H2O. Field tests conducted at a grassland site near Moscow showed strong correlations (R = 0.91 for CO2 and H2O, R = 0.74 for CH4) with commercial LI-COR LI-7200 and LI-7700 analyzers. The TDLAS mode demonstrated lower noise and greater stability under outdoor conditions, while sWMS provided baseline-free spectra but was more sensitive to power fluctuations. E-CAHORS combines high precision, multi-species sensing capability with low power consumption (10 W) and a compact design (4.2 kg).