Development of a Millimeter-wave Sensor for Environmental Monitofung*

A millimeter-wave (mm-wave) separ in the frequency range of 225-3 15 GHz range is being developed for continuous emission monitoring of airborne effluents from industrial sites with applicability to environmental compliance monitoring and process control. Detection of chemical species is based on measuring the molecular rotational energy transitions at mm-wave frequencies. The mm-wave technique offers better transmission properties compared to optics in harsh industrial environments such as those with smoke, dust, aerosols, and steam as well as in adverse atmospheric conditions. The laboratory measurements indicate that polar molecules can be measured with a sensitivity of tens of parts-per-million-meter using this technology. Proof of principle of the open-path system has been tested by releasing and detecting innocuous chemicals in the open air. It uses a monostatic radar configuration with transmitter and receiver on one side and a corner cube on the other side of the plume to be measured. A wide-band swept frequency mm-wave signal is transmitted through the plume and the return signal from the coqner cube is detected by a hot-electron-bolometer. Absorption spectra of the plume gases are measured by comparing the return signals with and without the plume in the beam path. Using a novel signal processing technique based on deconvolution, high specificity of detection has been shown for resolving individual chemicals from a mixture. This technology is applicable for real-time measurement of a suite of airborne gaseshapors emitted from vents and stacks of proccss industries. A prototype sensor is being 2 developed for wide-area monitoring of industrial sites and in-place monitoring of stack gases.