An open-path nitrous oxide laser analyzer for eddy covariance flux and mobile monitoring applications

Nitrous oxide (N₂O) is a major agricultural greenhouse gas and a reactive nitrogen species that drives climate forcing and environmental degradation. Its low ambient concentration and episodic bursts following fertilization or rainfall make detection difficult, requiring instruments capable of high sensitivity and temporal resolution to capture rapid flux dynamics.

This work introduces an open-path N₂O laser analyzer (Model: HT8500, HealthyPhoton Co., Ltd.) designed for future applications in N₂O monitoring and EC flux measurements. The HT8500 utilizes an quantum cascade laser (QCL) to probe the mid-infrared transition of N₂O at 4.54 μm. Laboratory experiments revealed that the HT8500 has a noise level of 0.3 ppbv at a 10-Hz sampling rate with a typical power consumption < 25 Watts.

A long-term field experiment based on the HT8500 over a bare agricultural field in Shandong, China was conducted to test “zero-flux” measurements and computations under different meteorological conditions. The resulting minimum detectable flux (~26.549 μg N m⁻² h⁻¹) indicates performance comparable to commercially available chamber-based N₂O flux measurement scenarios.

In the experiments conducted in Northeast China, the fertilization period was concluded. As temperatures decrease, the diurnal variation in N₂O fluxes dropped significantly, indicating the influence of temperature on eddies when emission sources remain stable. Continued evaluation will clarify how climate conditions and agricultural practices shape flux variability.

Such analyzer was also deployed on a EV based plume sensing platform (Farizon SV), along with open-path NH₃, CH₄, H₂O laser analyzers (model HT8700, HT8600P, respectively). We conducted mobile monitoring campaigns at wastewater treatment plants in Jinan, Beijing and Shanghai, all employed the Anaerobic–Anoxic–Oxic (AAO) process. Synchronized plume signals of GHGs above background were detected, with CH₄:N₂O concentration ratios ranging from 4.06:1 to 5.93:1, indicative of anaerobic contributions and process-dependent emission signatures.