Abstract
Atomic mercury is an atmospheric pollutant that is generated from chlorine-alkali plants, coal-fired power plants, and refuse-incineration plants. In addition, mercury is an interesting geophysical tracer gas associated with certain ore deposits and geothermal and seismic activities. Mercury in the environment is discussed in Ref. 1. Typical background concentrations of this gas are a few ng/m3. Such exceedingly low-number densities can still be measured by the differential absorption lidar (DIAL) technique only since the oscillator strength of the electronic transition at 254 nm is concentrated in an atomic line rather than spread out over the large number of rotational–vibrational molecular transitions normally encountered. Early attempts to measure mercury with the DIAL technique are reported in Ref. 2. We have also described path-averaged measurements of mercury with the differential optical absorption spectroscopy (DOAS) method.3 However, 3-D mapping of Hg reported here had to await the availability of high-power narrowband laser pulses. The measurements were performed with our new mobile lidar system,4 for this measurement equipped with a Quantel Datachrome system with a dual-wavelength option and a linewidth of 0.001 nm in the UV. Using a beta barium borate crystal, pulse energies up to 5 mJ could be generated by direct frequency doubling. Measurement examples are given from a Swedish chlorine-alkali plant. In Fig. 1 the absorption line, as measured through the lantenine mercury outlet at the 500-m distant plant, is illustrated. On- and off-resonance lidar curves and the resulting ratio (DIAL) curve from measurement of the plume downwind are given in Fig. 2. An example of crosswind plume mapping, ~150 m from the outlet, is shown in Fig. 3.
© 1988 Optical Society of America
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