A 9-year time series of multispectral radiometric observations taken at Rattlesnake Mountain Observatory (46.4 N, 119.6 W) is analyzed to determine the long-term behavior of the El Chichon-induced perturbation to the stratospheric aerosol layer. A technique for determining the volcanic enhancement is described. Time series data for the volcanic enhancement in stratospheric optical depth at wavelengths of 1010, 785, 535, 486, and 428 nm are presented. The main features of the results, slight wavelength dependence and a seasonal oscillation superimposed on the expected exponential decay at all wavelengths, are discussed. ¿ American Geophysical Union 1988