Optical and Quantum Electronics 7 (1975) 147-177 Review Remote air pollution measurement R. L. BYER Department of Applied Physics, Stanford University, Stanford, Cafifornia 94305 Received 21 November 1974 The availability of high energy tunable laser sources has extended the possibilities for remote air pollution measurement. This paper presents a discussion and compari- son of the Raman method, the resonance and fluorescence backscatter method, long path absorption methods and the differential absorption method. A comparison of the above remote detection methods shows that the absorption methods offer the most sensitivity at the least required transmitted energy. Topographical absorption provides the advantage of a single ended measurement, and differential absorption offers the additional advantage of a fully depth resolved absorption measurement. Recent experimental results confirming the range and sensitivity of the methods are presented. Future widespread use of remote pollution monitoring will probably be limited to the infra-red to interact with molecular spectral bands and to meet eye safety requirements. 1. Introduction A number of review articles on air pollution measurements have already been published [1-6]. However, recent advances in tunable laser technology coupled with experimental verification of remote sensing methods bring remote pollution measurements very close to a practical state. This paper reviews the field of remote pollution measurement and indicates the requirements for tunable laser transmitters. The adverse effects of polluted air on human health is welt established and an estimation of the cost savings for human health alone in the United States is over one billion dollars for a 50~ air pollutant reduction [7]. The Environmental Protection Agency has established maximum pollutant levels and goals for future reduction in pollutant levels. To meet these standards, accurate sensitive measurement methods must be established. Major pollutants of interest include ozone, nitrogen oxides, carbon monoxide, sulphur dioxide and hydrocarbon compounds. These pollutant molecules are present in urban atmospheres in concentrations from 0.01 to over t0 ppm [8]. Remote pollutant measurement methods must have measure- ment sensitivities in this range if they are to be useful for atmospheric monitoring. In addition, the detection methods must be capable of monitoring molecular vapours. Both Raman scat- tering and infra-red absorption on molecular vibration bands satisfy this last requirement. The detection sensitivity inherent in absorption and the regard for eye safety requirements probably limit future remote pollution detection methods to the infra-red spectral region. 1.1. Historical overview Historically, lasers were first used in radar type remote sensing applications. The acronym LIDAR for Light Detection and Ranging was applied to laser ranging systems. In 1963, 9 1975 Chapman and Hall Ltd. 147