Infrared optical properties of atmospheric aerosols

Abstract

The infrared optical properties of atmospheric aerosols can strongly affect the radiative transfer properties of the atmosphere. One past method for determining the transmission properties of aerosols depended on measurements on potassium bromide pellets (containing the aerosols) on an infrared spectrophotometer. However, for some materials there is a discrepancy between measurements made with a laser source and those made with a spectrophotometer; but for other materials there is good agreement. The reason for the disagreement appears to be in the resolution of the measurement. Typically, a Perkin-Elmer model 283 infrared spectrophotometer, with a 12-min scan and a normal slit program, will have a resolution of 10−1 μm in the 10-μm region of the spectrum. Single-line lasers have line resolutions between 10−4 and 10−6 μm. Thus, when using lasers for transmission measurements through the atmosphere, the absorption is dependent on the optical properties of the aerosols at exactly the laser wavelength. For certain types of dust (e.g., Sahara sands) measured with a sulfur coated integrating sphere and using a tunable 10.6-μm laser as the source, it is found that the measured sample transmission may be several orders of magnitude smaller than when measured with a broadband spectrophotometer. The cause of the sharp features may be a doping or natural organic contaminant in the aerosol.