Detection and Measurements of Air Pollutants and Constituents by Millimeter-Wave Microwave Spectroscopy

The properties of millimeter-wave spectrometers as highly specific and sensitive experimental research tools for monitoring gaseous pollutants of interest in pollution research are presented. A spectrometer system operating at frequencies in the vicinity of 70 GHz and employing a semi-confocal Fabry-Perot resonator as the sample cell is described. The resonator is voltage tuned by means of a piezoelectric transducer which enables it to track the microwave source frequency. The operation and performance of the spectrometer are discussed and illustrated by measurements of diluted samples of sulfur dioxide. The noise from the microwave source, the absorption cell and the receiver, which ultimately limits the achievable sensitivity are discussed along with considerations for future sensitivity improvements.