We describe a new method for the quantitative characterization of condensed phases in the atmosphere. It uses broad band IR extinction spectra to obtain the density, size distribution, phase and the approximate composition of aerosols within a single retrieval process. The method is based on a linear least squares fitting procedure with physically-based constraints. In this report, the method is applied to the analysis of spectra measured by the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument. The volume density, size distribution and composition of the stratospheric sulfate aerosols observed in several ATMOS missions are reported. The values of these properties for aerosols observed shortly after the eruption of Mount Pinatubo in 1992 are compared with those of aerosols present at much lower levels in 1993 and 1994.