Development of a cavity ringdown laser absorption spectrometer for detection of trace levels of mercury

A potential new laser-based air pollution measurement technique, capable of measuring ultralow concentrations of urban air toxins in the held and in r eal time, is examined. Cavity ringdown laser absorption spectroscopy (CRLAS ) holds promise as an air pollution monitor because it is a highly sensitiv e species detection technique that uses either pulsed or continuous tunable laser sources. The sensitivity results from an extremely long absorption p ath length and the fact that the quantity measured, the cavity decay time, is unaffected by fluctuations in the laser source. In laboratory experiment s, we reach detection limits for mercury of the order of 0.50 parts per tri llion. We developed a CRLAS system in our laboratory and measured Hg with t he system, investigating issues such as background interference. We report experimental results for mercury detection limits, the dynamic range of the sensor, detection of Hg in an absorbing background of ozone and SO2, and d etection of a mercury-containing compound (HgCl2 in this case). (C) 2000 Op tical Society of America OCIS codes: 010.1120, 120.4640, 120.6200, 230.0230 , 300.1030, 300.6540.