Compositions and effective radii of polar stratospheric clouds (PSCs) in the Antarctic region from 5 June to 28 August of 2003 are determined using transmittance data from the Improved Limb Atmospheric Spectrometer‐II (ILAS‐II). Dual compositions are derived for 83 Antarctic cases. The primary components are β‐NAT (β form of nitric acid trihydrate), NAW (nitric acid water, or SBS, supercooled binary solution), and ICE. Other minor composition components are LTA (liquid ternary aerosol, or STS, supercooled ternary solution), α‐NAT (α form of NAT), NAD (nitric acid dihydrate), and SAW (sulfuric acid water). Three single compositions are found; that is, β‐NAT particles were observed from 11 June to 12 August, NAW particles were observed from 28 June to 24 July, and ICE particles were observed from 28 July to 25 August. During this period, dual compositions are also found, i.e., NAW + β‐NAT, β‐NAT + ICE, NAW + ICE, β‐NAT + NAD, β‐NAT + LTA, and β‐NAT + SAW. For this observation period, temperatures varied from 195 K to 180 K while measurements were made progressively in time as latitude varied from 65°S to 80°S. This mixture of compositions is assumed to be either two separate patches of PSCs or a mixture of two different types along the line of sight. The feasibility of the coexistence of dual compositions in a PSC and importance of determination of the PSC particle type for the study of heterogeneous chemistry for ozone are briefly discussed.

In this study, we introduce a new ratio method that uses two transmittance profiles to obtain relative aerosol extinction coefficients as a function of wavelength. The current technique can be used for cases when two transmittance profiles have different aerosol extinction signatures relative to each other. The technique is applied to HALOE data, from which we infer aerosol composition and particle size distribution information during the time period from November 1991 to February 1992 when aerosol-loaded air masses are not well mixed. We begin with a brief description of Mie theory, present satellite-based observations of signal profiles, and describe how aerosol properties are derived along with associated error budgets. Aerosol ExtinctionAerosol properties, such as particle size distribution and composition, can be inferred from observed aerosol extinction coefficients (km -•) using the theoretical expression of extinc-