Detection Limit of a Low Power Open-Path Nitrous Oxide Laser Analyzer based Eddy Covariance Flux Experiment in Northern China

As a significant greenhouse gas, nitrous oxide (N₂O) is primarily emitted through anthropogenic activities, with agriculture being the predominant source. These emissions often occur across large spatial scales. The Eddy Covariance (EC) technique stands out as one of the most advanced methods for quantifying N₂O fluxes. However, most commercially available N₂O analyzers are designed for close-path measurements. To achieve the high-frequency gas concentration data required for EC applications, these analyzers typically rely on high-flow pumps, which necessitate substantial power consumption. Consequently, the selection of monitoring sites is frequently constrained by the availability of power.

 

This work introduces an open-path N₂O laser analyzer (Model: HT8500, HealthyPhoton Co., Ltd.) designed for future applications in EC N₂O flux measurements. The HT8500 utilizes an quantum cascade laser (QCL) to probe the mid-infrared transition of N₂O at 4.54 μm. The specific absorption peak has the advantage that the density and spectroscopic effects compensate for each other, resulting in low temperature-related corrections in EC flux measurements. Laboratory experiments revealed that the HT8500 has a noise level of 0.4 ppbv at a 10-Hz sampling rate with a typical power consumption ~ 50 Watts.

 

A long-term field experiment based on the HT8500 over a bare agricultural field in Jinan, Shandong was conducted to test “zero-flux” measurements and computations under different meteorological conditions. Averaged hourly fluxes ranged from near zero at night to less than 0.18 mg m^-2·h^-1 at midday in December, which were not significantly different from zero. Compared to the laboratory detection limit of 19.29 ug N m^-2·h^-1, the field experiment demonstrated a detection limit of approximately 49 ug N m^-2·h^-1, indicating that the HT8500 analyzer maintains its low flux detection limit similar with commercial available chamber-based N₂O flux measurement scenarios.