A portable, high-precision optical analyzer based on hybrid laser absorption cell for simultaneous measurements of N2O, CH4, and CO2 fluxes from soils

Laser spectrometers have shown good capability in measuring mixing ratios for atmospheric greenhouse gases (GHGs). Given the fact that spectral bands of CO₂, CH₄, and N₂O covering from near to mid-infrared (NIR to MIR) wavelengths, and the limitations in the wavelength coverage of the laser and photodetector, it is very challenging to analyze CO₂, CH₄, and N₂O simultaneously for most cavity-enhanced analyzers using NIR lasers. Alternative solutions, such as combining multiple quantum cascade laser (QCL) beams and analyzing all GHGs in the MIR wavelengths, would significantly increase the instrumental cost.

Here, we present a recently developed analyzer utilizing the advantages of detecting CO₂/H₂O in the NIR spectral region and N₂O/CH₄ in the MIR region, respectively. Through a unique optical design, two independent optical paths are formed in a single Herriott gas cell so that low temperature-related drift and mechanical robustness are achieved. The mixing ratios of CO₂ and H₂O are analyzed by a NIR laser and a photodetector at ~4995cm^-1, while CH₄ and N₂O are analyzed by a QCL and an MCT photodetector at ~1275cm^-1. The analyzer facilitates high-sensitivity, field-deployable measurements of CO₂, CH₄, N₂O and H₂O altogether in a compact, portable instrumental design. In addition, this analyzer can be completely powered by rechargeable battery, facilitating all-day in situ measurements without grid power supply.

In the laboratory, side-to-side comparisons were performed between our newly developed analyzer and another commercial gas analyzer based on cavity ring down spectroscopy (CRDS). The results showed high consistency in GHG mixing-ratio measurements with the two spectrometers. Attached to soil chambers, we also found comparable performance of two analyzers in determining GHG fluxes. In particular, we found that the presented analyzer could precisely capture transient changes in gas mixing ratios from the soil chamber. Recently, field deployment in different soil conditions, including upland forest soils and riparian soils, was carried out for simultaneous N₂O, CH₄, CO₂ soil flux measurements. The overall results suggest that our analyzer is suitable for continuous GHG flux monitoring under variable field conditions, and shows potentials in simultaneous measurements of multiple GHG fluxes from natural ecosystems.