SPECTRAL SIGNATURES OF POLAR STRATOSPHERIC CLOUDS AND SULFATE AEROSOL

Multiwavelength observations of Antarctic and midlatitude aerosol by t he Cryogenic Limb Array Etalon Spectrometer (CLAES) experiment on the Upper Atmosphere Research Satellite are used to demonstrate a techniqu e that identifies the location of polar stratospheric clouds. The tech nique discussed uses the normalized area of the triangle formed by the aerosol extinctions at 925, 1257, and 1605 cm-1 (10.8, 8.0, and 6.2 m um) to derive a spectral aerosol measure M of the aerosol spectrum. Mi e calculations for spherical particles and T-matrix calculations for s pheroidal particles are used to generate theoretical spectral extincti on curves for sulfate and polar stratospheric cloud particles. The val ues of the spectral aerosol measure M for the sulfate and polar strato spheric cloud particles are shown to be different. Aerosol extinction data, corresponding to temperatures between 180 and 220 K at a pressur e of 46 hPa (near 21-km altitude) for 18 August 1999, are used to demo nstrate the technique. Thermodynamic calculations, based upon frost-po int calculations and laboratory phase-equilibrium studies of nitric ac id trihydrate, are used to predict the location of nitric acid trihydr ate cloud particles.