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.
Highlights
Release of vapor-phase chemicals from localized sources into the atmosphere results in gaseous plumes propagating away from the source; examples may include industrial emissions, unintended chemical leaks or spills, burning of biomass materials, explosive detonations, or a chemical warfare attack
Given that atmospheric turbulence typically occurs on time scales of ∼10 ms [2], it is expected that plume dimensions and chemical concentrations may vary on similar time scales, especially near the source release point [3]
For all species except N2O, we demonstrate measurement sensitivities expressed as noise-equivalent column densities (NECD) of 1-2 ppm*m with 2.5 ms time resolution
Summary
Release of vapor-phase chemicals from localized sources into the atmosphere results in gaseous plumes propagating away from the source; examples may include industrial emissions, unintended chemical leaks or spills, burning of biomass materials, explosive detonations, or a chemical warfare attack. While propagating, these chemical plumes experience dilution and mixing with the atmosphere before eventually reaching equilibrium levels, adsorbing on surfaces, undergoing chemical reactions, or otherwise becoming undetectable. Measurement of a chemical plume near the release point provides valuable information on source concentrations and emission rates, before the plume is subject to atmospheric dilution. Passive hyperspectral imaging is useful for monitoring plume propagation over large distances and over longer times, and offers imaging context [4], but high speed and quantitative concentration measurements can be challenging to achieve
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