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The results of long-term (1980-2011) photosynthetically active radiation (PAR) monitoring, which have been held at the Meteorological Observatory of Moscow State University (MSU MO) are presented. The PAR measurements are carried out within wavelength ranges 380-710 nm and 400-700 nm using modified thermoelectrically pyranometer (1980-2001) and LI-COR quantum sensor (from 1999 to the present) with the accuracy 7-10%. Main attention was paid to the variability of the magnitude of the relationship between the broadband total solar radiation (Q) and PAR (QPAR). The dependence of this ratio (qPAR=QPAR/Q) on various meteorological factors is investigated. The simple empirically derived model for PAR estimation is developed. Based on these methods the long-term variations of PAR during the XX Century in the Moscow region was obtained. [ABSTRACT FROM AUTHOR]