An Evaluation of Lasers for Ambient Air Pollution Measurement

Abstract

Laser based air pollution measurement techniques which might possibly be applied to future air pollution monitors are examined. Four specific systems for which prototypes are currently being developed are reviewed from the standpoint of a user. The known problems are discussed. The four systems depend on either resonance absorption or resonance fluorescence. The first system is a long measurement path step tuned CO₂ laser designed to measure ozone, methane, and ethylene. The second system is a solid state tuned diode laser which is designed to measure carbon monoxide and nitric oxide. By continuously modulating the laser frequency a derivative signal is obtained which is ratioed with the transmission signal. The resulting normalized derivative signal is essentially independent of fluctuations in laser power or any broad band attenuation. This technique represents a fundamental improvement over the first system. The third system has been termed the opto-acoustic technique which measures resonance absorption directly by focusing the beam through a small cell. The laser beam is chopped by a variable speed chopper. The laser beam in passing through the gas in the cell heats the gas which produces a pressure variation. The pressure variation is detected by a microphone. Measurements have been made by this technique of concentration below one part per billion. The fourth technique utilizes resonant fluorescence and has been demonstrated to measure NO₂ with a noise equivalent concentration of 4–5 ppb.