Dual-wavelength aerosol lidar backscatter measurements at Mauna Loa Observatory are used to monitor and characterize the 15--30 km stratospheric aerosol layer. The decay of aerosol loading following the El Chich¿n, Mexico (17 ¿N) and Pinatubo, Philippine Islands (15 ¿N) volcanic eruptions of 1982 and 1991, respectively, depends on the phase of the quasi-biennial oscillation (QBO) in tropical stratospheric winds. Averaged over a 3-year period, these effects are removed and an exponential decay with a characteristic (e-1) decay time of about 1 year is observed for both eruptions. By the end of 1996, about 5 1/2 years after the Pinatubo eruption, stratospheric aerosol levels at Mauna Loa had decayed to pre-eruption levels, approximately matching the lowest aerosol levels seen here in the past 17 years (about 6¿10-5 sr-1 at 694 nm integrated between 15.8 and 33 km). However, this background stratospheric aerosol level at Mauna Loa may depend on the QBO, being slightly lower during the westerly phase. Analyses of aerosol backscatter, backscatter wavelength dependence, and trajectories provide evidence for a minor injection of aerosol from the Rabaul eruption in Papua, New Guinea (4 ¿S) in September of 1994.¿ 1997 American Geophysical Union