Detection of sulphur dioxide in stack plume by laser Raman radar

Abstract

Laser Raman radar offers a promising approach to the remote sensing of atmospheric pollutants in stack plume. An analysis revealed that the combination of the second harmonic of a Nd :YAG laser, a synchronous single photoelectron counting (SSPC) scheme, and narrow passband interference filter employed for light source, signal processor, and separation of Raman-scattered light respectively gave the most sensitive system for stack effluent monitoring. Using a laser Raman radar constructed on the basis of the analysis, the concentration of SO~ in the plume emitted from a 150 m high stack of a power plant was measured at a distance of 228 m.

Laser-Raman radar ?Laser-Raman scattering methods for remote detection and analysis of atmospheric pollution

“…The system was installed in a small coach and consisted basically of a frequency-114 doubled, Q-switched Nd :YAG laser (5320 A) and a 50 cm diameter cassegrain telescope, along with a combination of narrow passband interference filters and synchronous single photoelectron counting equipment for the high sensitivity detection. The in-field tests of this first practical system have demonstrated successfully the remote measurement of trace amounts of SO z in a smoke plume from a 150 m high stack of an operating power plant from a slant range of more than 200 m, as reported in the contributed paper by Nakahara et al [20] in this special issue. Fig.…”

“…A problem may arise, for instance, in the case of the detection of trace amounts of CO in the presence of a high density of N~ molecules, as in combustion plumes. Even in the case of SO ~ detection in stack plumes, the false signal due to the O-branch of CO 2 was found actually to interfere with the accurate measurement of SOs concentration, although this difficulty has been overcome by the use of an interference filter with a FWHM passband of 4.4 A and the rejection ratio of over 10 ~ [20]. Precise and complete theoretical estimation of spectral distribution of Raman bands from molecular mixtures simulating polluted air, especially as a function of relative concentration and temperature, is worthwhile in designing and operating the laser-Raman radar, and also in evaluating the limits of this technique.…”

“…10 shows that a well-designed laser-Raman radar system will have a sensitivity capable of detecting the atmospheric pollutants of medium level concentration such as a few ppm at a range of a few hundred meters. Although this figure is not sufficiently sensitive to allow the measurement of dispersed air-pollution levels in the ambient air, 119 it can possess the unique capability to monitor and analyse a variety of chemicals from stationary emission sources such as smoke stacks [15,16,20]. For a more remarkable improvement in detection sensitivity, one should rely upon a great increase in the Raman scattering cross section by the resonance enhancement of the polarizability tensor, i.e.…”

“…Also, in order to realize interference-free or reduced-interference detection of many important air pollutants, the spectral distribution of their Raman bands should be taken into account in designing as well as in operating the practical system [19,20]. This section is devoted to discussing briefly these two physical factors associated with the basic Raman spectroscopy.…”

Techniques for laser remote sensing of the environment

“…The Raman technique has been extended to demonstrate spectroscopic detection of air pollutants such as SO 2> CO 2 , C 2 H 4 , NO, CO, H 2 S, CH 4 in automobile exhaust gases (Kobayashi and Inaba, 1971;Inaba and Kobayashi, 1972), SO 2 and kerosene (Hirschf eld et al, 1973), and SO 2 and N 2 (Nakahara et al, 1972). These laser Raman radar techniques and applications have been reviewed by Inaba (1976).…”