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Standoff chemical plume detection in turbulent atmospheric conditions with a swept-wavelength external cavity quantum cascade laser

Mark C. Phillips, Bruce E. Bernacki, Sivanandan S. Harilal, Jeremy Yeak, and R. Jason Jones

Open Access Open Access

Abstract

Rapid and sensitive standoff measurement techniques are needed for detection of trace chemicals in outdoor plume releases, for example from industrial emissions, unintended chemical leaks or spills, burning of biomass materials, or chemical warfare attacks. Here, we present results from 235 m standoff detection of transient plumes for 5 gas-phase chemicals: Freon 152a (1,1-difluoroethane), Freon 134a (1,1,1,2-tetrafluoroethane), methanol (CH3OH), nitrous oxide (N2O), and ammonia (NH3). A swept-wavelength external cavity quantum cascade laser (ECQCL) measures infrared absorption spectra over the range 955-1195 cm−1 (8.37- 10.47 µm), from which chemical concentrations are determined via spectral fits. The fast 400 Hz scan rate of the swept-ECQCL enables measurement above the turbulence time-scales, reducing noise and allowing plume fluctuations to be measured. For high-speed plume detection, noise-equivalent column densities of 1-2 ppm*m are demonstrated with 2.5 ms time resolution, improving to 100-400 ppb*m with 100 ms averaging.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Figures (10)
Fig. 1.
Fig. 1. (a) Schematic of standoff detection optics on 300×600 mm breadboard. (b) Vertical configuration of beam steering optics for alignment of ECQCL beam to a remote retro-reflector target and focusing of return light onto DET2. A visible alignment camera was inserted into the beam path for coarse alignment and removed for measurements with the ECQCL. (c) Image of remote retro-reflector obtained using the alignment camera. (d) Photograph of system in measurement position. (e) Google Earth image of measurement configuration with 235 m standoff distance. Abbreviations: WW – BaF2 wedged window, L1- Lens, OAP – off-axis parabola, DET1/2 – infrared photodetectors.

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Fig. 2.
Fig. 2. Effects of atmospheric turbulence on signals recorded by reference detector (DET1, blue) and standoff detector (DET2, red), with the ECQCL at fixed wavenumber of 965 cm−1. (a) Detector signals as function of time. (b)-(d) Absorbance as function of time for decreasing time windows T of (b) 1000 ms, (c) 100 ms, (d) 10 ms, and (e) 1 ms. (f) Absorbance noise (standard deviation of absorbance) over time window T as function of equivalent frequency 1/T.

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Fig. 3.
Fig. 3. Measured scan intensity as a function of time for the return signal from retro-reflector, in the absence of chemical plumes. Scans are separated by 2.5 ms.

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Fig. 4.
Fig. 4. (a) Principal component analysis (PCA) of background absorbance spectra. (b) First 10 PC vectors (absorbance spectra) from PCA analysis, plotted from bottom to top with decreasing variance, and offset for clarity. (c) Series of 10 absorbance spectra from background data set, after fitting and subtraction of baseline using PC background vectors.

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Fig. 5.
Fig. 5. Absorbance spectra and fits measured during standoff plume detections. In (a)-(f), the top curve is the experimentally measured absorbance spectrum (100 ms average), the middle curve is the best fit spectrum, and the lower curve is the fit residual. (a) F152a, (b) F134a, (c) N2O, (d) MeOH, and (e) NH3. Panel (f) shows library absorption spectra for a column density of 106 ppm*m H2O and CO2.

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Fig. 6.
Fig. 6. Detected column density CL (units of ppm*m) versus time for (a) F152a, (b) F134a, (c) N2O, (d) MeOH, and (e) NH3 in standoff plume detection experiment.

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Fig. 7.
Fig. 7. Measured (a) N2O, and (b) MeOH signals versus time with and without inclusion of the PC background vectors in the analysis library to fit the absorbance baseline.

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Fig. 8.
Fig. 8. Effect of measurement rate on retrieved column density of NH3. The solid line shows retrieved values for analysis of scan acquired at a 400 Hz rate. The circles show retrieved values when scans are pre-averaged to a 1 Hz rate before analysis. The inset shows a zoom of a 500 ms region between the dashed vertical lines.

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Fig. 9.
Fig. 9. Allan deviation analysis of background data set with 100,000 measurements (a) with and (b) without inclusion of PC background vectors in analysis library. Results are shown for N2O (green diamonds), MeOH (blue triangles), F152a (red squares), F134a (black open circles), and NH3 (orange inverted triangles).

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Fig. 10.
Fig. 10. Effects of averaging on plume detection for (a) peak column density and (b) signal-to-noise ratio (SNR). Results are shown for N2O (green diamonds), MeOH (blue triangles), F152a (red squares), F134a (black open circles), and NH3 (orange inverted triangles).

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Tables (1)

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Table 1. Noise-equivalent column density (NECD) and noise-equivalent concentration (NEC) for 235 m standoff detection using swept-ECQCL. Plume NECD is in units of column density (ppm*m) for a plume of unknown size. Full path NEC is in units of ppb, assuming a uniform species distribution along entire measurement path length of 470 m.

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