Spectra of the real and imaginary parts of the refractive index m = n ′ - in˝ from 2.5 µm to 40 µm of ammonium sulfate, Sahara dust (as collected at Barbados), volcanic dust (pumice), and dust from a coalfired power plant are presented. With a peak absorption index n˝ = 1.0 at 10 µm Sahara dust turns out to be the strongest absorber in the atmospheric window. The dispersion of n′ is also very pronounced near 9 µm and 20 µm. Volcanic and coal-fire dust also absorb in the window region stronger than most of our earlier-investigated more normal aerosol fractions. The data for ammonium sulfate are compared with Remsberg's measurements on an aqueous solution.
Frederic E. Volz, "Infrared Optical Constants of Ammonium Sulfate, Sahara Dust, Volcanic Pumice, and Flyash," Appl. Opt. 12, 564-568 (1973)