PNNL

Abstract

Chemical plume detection and modeling in complex terrain presents numerous challenges. We present experimental results from outdoor releases of two chemical tracers (sulfur hexafluoride and Freon-152a) from different locations in mountainous terrain. Chemical plumes were detected using two standoff instruments, both located at a distance of 1.5 km from the plume releases. A passive long-wave infrared hyperspectral imaging system was used to show time- and space-resolved plume propagation in regions near the source. An active infrared swept-wavelength external cavity quantum cascade laser system was used in a standoff configuration to measure quantitative chemical column densities with high time resolution and high sensitivity along a single measurement path. Both instruments provided chemical-specific detection of the plumes and provided complementary information over different time and spatial scales. The results show highly variable plume propagation dynamics near the release points, strongly dependent on the local topography and winds. Effects of plume stagnation, plume splitting, and plume mixing were all observed and are explained based on local topographic and wind conditions. Measured plume column densities at distances ~ 100 m from the release point show temporal fluctuations over ~1 s time scales and spatial variations over ~1 m length scales, highlighting the need for high-speed and spatially resolved measurement techniques. The results also provide validation data at the relevant spatial and temporal scales required for high-fidelity terrain-aware microscale plume propagation models.