Based on many-year (1986-2007) observations at Siberian Lidar Station of Institute of Atmospheric Optics SB RAS in Tomsk (56.5°N; 85.0°E), we identified the periods of background state of the stratospheric aerosol layer and determined the trends of variations the background, non-volcanic component of the stratospheric aerosol (SA). For the period 1999-2006, the weak negative trend is statistically insignificant. The content of the background SA in the long volcanically quiet period 1997-2007 does not suffer significant long-term changes under the impact of the natural or anthropogenic factors. It is possible to observe short-term (few-day) anomalous variations of SA content under impact of the natural factors. The seasonal variations of the content of background SA show pronounced behavior with maximum in the winter period and minimum in summer.