Standoff detection of carbon monoxide from the underground high explosive detonation in Physics Experiment 1-A using swept-wavelength ECQCL spectroscopy

Results are presented from a custom-built standoff infrared spectroscopy instrument used to detect above-ground CO generated in the Physics Experiment 1-A test, in which 13 580 kg of high explosives were detonated in an enclosed underground chamber. The instrument used a swept-wavelength external cavity quantum cascade laser (swept-ECQCL) scanned over a wavenumber (wavelength) region of 1965–2290 cm−1 (4.37–5.09 μm) at a 100 Hz rate. The system was configured for standoff open-path measurements with a 223 m distance between the swept-ECQCL instrument and a retro-reflecting mirror, with the beam passing directly above a vent connected to the underground tunnel complex containing the detonation. Spectral fitting provided column densities of CO, CO2, H2O, and N2O, and a spectroscopic temperature, at 1 s intervals over a continuous 27-day period after detonation. The results showed that CO detected above ambient levels throughout most of this period, with a 5 orders-of-magnitude measurement dynamic range. Statistical analysis of the time-resolved CO signal shows a large fraction of low-magnitude and short-duration CO plume fluctuations. Measurement precision/sensitivity for CO detection is characterized to be 0.20 ppm × m at 1 s averaging, improving to 0.05 ppm × m when averaged to 100 s, corresponding to average CO concentrations of 450 ppt at 1 s and 110 ppt at 100 s. The combination of continuous high-speed measurements at high-precision and with high dynamic range provided by the swept-ECQCL sensor provides a valuable capability for detection and characterization of transient plume sources released into the atmosphere.