Abstract

To avoid systematic errors in differential absorption lidar experiments, which use the atmospheric aerosol as a distributed reflector, the assumption has to be made that the volume backscattering coefficient is approximately the same at the two wavelengths. A model of the aerosol has been used to show that in the 9–11-μm region this assumption may be invalid since absorption by the aerosol makes the backscattering term a sensitive function of frequency and relative humidity. To illustrate the problem we examine five pairs of laser frequencies recommended in the literature for monitoring ammonia, ozone, ethylene, and Freon-11 and show that peak-to-peak differential backscatter of >10% is common across the whole humidity range. We also show that the sensitivity of backscattering to humidity changes is such that the volume backscattering coefficient at the on-line wavelength can become larger than the off-line value as the relative humidity increases, an effect that may nullify any rise in trace gas concentration that happens to take place.

© 1981 Optical Society of America

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