The paper presents the results of investigations on chemical composition of soluble fraction of aerosols (anions and cations) and trace gases (SO2 and NO2) over the water area of Lake Baykal in summer. The results demonstrated that, on the whole, the mass concentrations of ions in aerosol and trace gases in the atmosphere over the major part of Lake Baykal are low and comparable with the background values of these components in other uncontaminated regions of the world. However, in the south part of the lake, near large populated areas, an essential excess of concentrations against the background values has been detected. We have also studied the dependence of ion composition of aerosols on the particle size and determined the salt composition of the aerosol over Lake Baykal.