FREQUENCY-DEPENDENT OPTICAL-CONSTANTS OF WATER ICE OBTAINED DIRECTLY FROM AEROSOL EXTINCTION SPECTRA

We report here a new approach for determining frequency-dependent real and imaginary components of the refractive index of crystalline solids directly from the observation of the infrared spectra of the corresponding aerosols. The interplay between scattering and absorption observed in large (>0.5-mu m-radius) particles allows us to properly scale the imaginary component, determined from the absorption spectrum of small (around 0.3-mu m-radius) aerosols, using calculations based on the Mie scattering theory. Once the imaginary indices are Properly scaled, the corresponding real indices are determined through a Kramers-Kronig analysis. The method is applied to the study of water ice aerosols, and comparisons with previous measurements confirm that the method is sound and accurate. Reported here is a detailed study of the temperature dependence of the refractive index of ice. Experiments are reported over the temperature range from 130 to 210 K, which includes the region of interest for the study of polar stratospheric clouds (PSC's).