Data of many-year sun photometer measurements in the Asian part of Russia are used to analyze the issues of modeling the annual and spectral behaviors of the atmospheric aerosol optical depth (AOD) based on separation of the contributions of fine and coarse aerosol. The specific features of the spatial distribution and seasonal variations in AOD characteristics in different regions are discussed; a three-parameter model of annual behavior of spectral AOD in the range 0.34-4 μm for the Western Siberia (Tomsk) is suggested.