Lidar observation of midlatitude stratospheric aerosols in long-term volcanically quiet period

We analyze the data of measurements of the optical characteristics of stratospheric aerosol layer (SAL), obtained at the Siberian Lidar Station of lAO SB RAS in Tomsk (56°N, 85°E). The data are considered in comparison with results of previous years of observations in Tomsk and at other midlatitude stations of the Northern Hemisphere. The results characterize SAL state in extended volcanically quiet period. The last explosive volcanic eruption was observed in June 1991 (volcano Pinatubo). For the first time such a SAL state is studied using modem ground-based and satellite measurement facilities. The results of observations show that the background levels of the natural aerosols in the stratosphere may be lower than was considered earlier. The anthropogenic growth of background stratospheric aerosol content is not observed. The presence of seasonality and quasi-biennial oscillation (QBO), with a characteristic increase of aerosol content at midlatitudes in periods of intensification of winter-spring meridional transport from tropical belt to midlatitudes and in westerly QBO phases, is an evidence in favor of hypothesis about the presence of tropical reservoir of not only volcanic but also background stratospheric aerosol.