We continue to analyze the results of many-year studies of the atmospheric aerosol optical thickness (AOT) in the spectral region from 0.37-4 μm measured near Tomsk. Based on the information obtained under different atmospheric conditions (seasons, forest fire smokes), the interconnection between the parameters α and β of the Angstrom formula and AOT in the long-wave spectral range (1-4 μm) is discussed. It was found that β slightly differs from the AOT value at λ > 1 μm. Also, close linear relation between them has been revealed, as well as weak correlation between α and β. A possibility of assessing mean AOT level in the long-wave spectral range (1.2-4 μm) from measurements in the traditional range from 0.37 to 1 μm is shown. The procedure of dividing the atmospheric AOT into two components caused by extinction of light by fine and coarse aerosol is described and quantitative characteristics of their seasonal variability are presented.