A 2-year time series of high resolution lidar backscatter profiles at 0.532 μm, taken at the NOAA Fritz Peak Observatory (39.9 ¿N, 105.3 ¿W) is analyzed to investigate the evolution of the stratospheric aerosol following the eruption of Mount Pinatubo in June, 1991. Aerosol from the eruption first appeared as transient layers just above the tropopause in late summer and early fall of 1991. This was followed by a rapid increase in aerosol centered near 21 km, with an exponential risetime of ~22 days. The maximum in late December 1991 was followed by a slow decline, punctuated by seasonal increases below 18 km and with an exponential decay timescale of ~300 days near 20 km. Aerosol backscatter is converted to mass and a principal component analysis (PCA) is performed to explore the statistical properties of aerosol variability. More than 80% of the variability in aerosol mass is described by only three components, corresponding to variations in the layers 17-22 km (PC1, 44%), below 17 km (PC2, 27%), and above 22 km (PC3, 9%). Since most of the temporal variations occur independently in these three layers, this work provides further insight into the nature of stratospheric transport from the tropics to midlatitudes. ¿ American Geophysical Union 1995