The attenuation of solar radiation between 1.8- and 15-μm wavelength was measured with the airborne (DC-8) Jet Propulsion Laboratory Mark IV interferometer during the Airborne Antarctic Ozone Expedition in 1987. The measurements not only provide information about the abundance of stratospheric gases, but also about the optical depths of polar stratospheric clouds (PSCs) at wavelengths of negligible gas absorption. The spectral dependence of the PSC optical depth contains information about PSC particle size and particle composition. Thirty-three PSC cases were analyzed and categorized into two types. Type I clouds contain particles with radii of about 0.5 μm and nitric acid concentrations greater than 40%. Type II clouds contain particles composed of water ice with radii of 6 μm and larger. Cloud altitudes were determined from 1.064-μm backscattering observations of the airborne (DC-8) Langley DIAL lidar system. Based on the PSC geometrical thickness, both mass and particle density were estimated. Type I clouds typically had visible wavelength optical depths of about 0.008, mass densities of about 20 ppb, and about 2 particles/cm3.

The observed type II clouds had optical depths of about 0.03, mass densities of about 400 ppb mass, and about 0.03 particles/cm3. The detected PSC type I clouds extended to altitudes of 21 km and were nearly in the ozone-depleted region of the polar stratosphere. Although PSC type II clouds under different temperature regimes may extend to similar high altitudes, the observed type II cases during September were predominantly found at altitudes below 15 km. Some of these low-altitude clouds may have been associated with cirrus clouds that extended well below the DC-8 aircraft. Simultaneous spectroscopic measurements of nitric acid vapor within the polar vortex (Toon et al., this issue) display lower concentrations in the presence of PSC type I clouds, the deficit being close to the amount of solid nitric acid inferred from the optical depths of these PSCs. This further supports the view that the type I PSCs contain frozen nitric acid. ¿ American Geophysical Union 1989