In this paper we discuss some measurement results of spectral optical thickness of the atmosphere conducted in 1991 in southern part of lake Baikal. The observation sites were located near the points of aerosol sampling. The size spectrum of aerosol particles is well approximated by a sum of three lognormal size distributions. The set of experimental data collected and their theoretical analysis show that the optical depth of the atmosphere over southern part of lake Baikal is mainly determined by the properties of aerosols corresponding to the second mode of size distribution. This result well agrees with the data obtained in other geographical regions.