An optoacoustic system for the measurement of low concentrations of atmospheric pollutants

Abstract

The optoacoustic effect, through which absorption of chopped optical energy results in the production of sound at the chopping frequency, has become an important analytical tool during the past decade. High‐power lasers, operated at visible and infrared wavelengths, have made possible the generation of measurable acoustic signals through optical absorption by a variety of gases as well as by aerosols and solid materials. Device optimization has been attempted by a number of researchers. Elements of the problem are discussed here in the context of a system intended for the measurement of concentrations of nitrogen dioxide as low as 1 part in l09. Of concern are the use of an acoustically resonant gas cell, reduction of extraneous sound generated where the chopped laser beam passes through cell windows, and enhancement of signal relative to acoustic and electronic noise.