The Use of Lasers in Pollution Monitoring

Optical techniques have opened up new possibilities in air pollution monitoring because of their remote-sensing capability, very high specificity, and short observation time. Techniques involving the use of lasers include Raman scattering, emission either from resonantly excited or from hot gases, and resonant absorption. Unique advantages in these applications are provided by the recently developed tunable lasers, including organic dye lasers, parametric oscillators, spin-flip Raman lasers, and semiconductor lasers. The absorption technique which promises to have the widest range of application has been tested in the laboratory by using tunable diode lasers. High-resolution absorption spectra have been measured for SF6, NH3, and C2H4 at 10.6 μm, SO2 at 8,6 μm, CO at 4.7 μm, and NO at 5.2 μm, and the C2H4 (ethylene) content of automobile exhausts has been measured by means of a derivative technique. A sensitivity sufficient to detect one ppm of NH3 in air has been achieved in a 10-μm-long gas cell. The 50 to 100-kHz resolution achievable with diode lasers is about four orders of magnitude better than the resolution of conventional spectrometers and more than adequate to resolve Doppler broadened gas absorption lines which are on the order of 100 MHz wide.