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Propagation of visible light from the top of the atmosphere to the ground surface

The characteristics of the solar spectrum at the top of the atmosphere and near the ground surface in different seasons are presented. The solar radiation flux in the photosynthetic active regime (PAR) is considered at the normal incidence of solar rays on the ground surface with respect to the top of the atmosphere. The transparency of the atmosphere at the latitude of 42 degree(s) during 1993-1997 and April until October 1993 is presented. Extinction of the direct PAR under different conditions of aerosol pollution is considered.