Investigation Of The Use Of The Resonance Raman Effect As An Environmental Monitor

An essential element in any strategy designed to cope effectively with atmospheric pollution is the development of more sensitive or more convenient methods for determining the concentrations of pollutant gases in ambient air and in emissions from localized sources. One general class of methods for measuring pollutant concentrations makes use of lasers to probe the atmosphere remotely and obtain spectroscopic information about the molecules present in the segment of atmosphere under consideration. In particular, the use of a laser to measure the Raman spectra of molecular constituents remotely seems to offer some promise of being an effective monitor of gaseous pollutants. This scheme, called Raman Lidar, is implemented in a manner analogous to the use of a radar system. A laser transmitter directs a pulsed beam into the atmosphere. By measuring the time between the initial laser pulse and the returning signal, one determines the range of the atmospheric region being monitored. The light coming from this region is spectroscopically analyzed. The Raman spectrum, which is due to transitions between vibrational states of a molecule, is a unique signature for a particular molecular species. Therefore, a measurement of its intensity allows one to determine the concentration of that molecule in the region of interest. Raman Lidar allows one, in principle, to make a three-dimensional map of molecular concentrations in real time with good spatial resolution out to a distance of several kilometers from a fixed transmitter.