Active standoff mixing-ratio measurements of N ² O from topographic targets using an open-path quantum cascade laser system

Active stand-off detection and hard-target lidars are common methodologies for gas identification, chemical emission tracing, hazardous material sensing, or explosive detection to name a few. By their nature, this type of instrument heavily relies on the reflectivity or backscattering properties of distant targets. While some applications allow the use of retroreflectors, most mobile systems require the use of actual topographic targets, such as the ground, roads, buildings, roofs, or vegetation. In this work, N ² O path-averaged mixing ratios are measured with the 10 Hz frequency using a quantum cascade laser open path system operating at 7.7 μm wavelength. Measurements are performed by detecting the light backscattered from common topographic targets located 5.5 m away from the instrument. For each topographic target, the detection limit and accuracy of the retrieved mixing ratios are presented and discussed showing detection limits between 0.008 and 1.36 ppm depending on the target and mixing ratio relative errors between 4 and 80 %.