An Open-Path Eddy-Covariance Laser Spectrometer for Simultaneous Monitoring of CO$_2$, CH$_4$, and H$_2$O

TL;DR

The paper introduces E-CAHORS, a compact laser spectrometer capable of simultaneously measuring CO$_2$, CH$_4$, and H$_2$O in the atmosphere, enabling flux calculations with high precision and stability in field conditions.

Contribution

It presents a novel open-path laser spectrometer with dual detection methods, demonstrating high accuracy, stability, and field applicability for multi-gas atmospheric monitoring.

Findings

High linear response for all gases (R^2 ≈ 0.999)

Strong correlation with commercial analyzers (R=0.91 for CO$_2$, H$_2$O)

Lower noise and greater stability of TDLAS mode in outdoor conditions

Abstract

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 $\mu$m and 3.24 $\mu$m 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 (R$^2$ $\approx$ 0.999) and detection precisions at 10 Hz of 311 ppb for CO$_2$, 8.87 ppb for CH$_4$, and 788 ppb for H$_2$O. Field tests conducted at a grassland site near Moscow showed strong correlations (R = 0.91 for CO$_2$ and H$_2$O, R = 0.67 for CH$_4$) 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).