We describe a new method for the rapid and quantitative determination of the composition, size and density of atmospheric aerosols from broadband high resolution IR spectra recorded by remote sensing instruments. This method uses a novel technique for the separation of the aerosol spectrum from that of the gas phase. It is fast enough to permit the analysis of high resolution IR satellite measurements in real time, yielding aerosol composition, size distribution and volume density. The method is applied to spectra measured by the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument during flights in March 1992, shortly after the eruption of Mount Pinatubo, and again in November 1994. We report the chemical and physical characteristics of the stratospheric aerosols as a function of the tangent height.