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4. Conclusions

• We proposed an algorithm for retrieval of biologically active irradiance according to the ozone and cloudiness effects that can be applied in reconstruction of EW irradiance variations in the pre-instrumental period. This algorithm was validated on the base of broadband and spectral UV data in different geographical regions. We also have revealed a good agreement between variability of UV cloud transmittance retrieved from ground based observations and cloud transmittance obtained from satellite (TOMS) observations.
• We revealed a distinct upward trend over Ushuaia in EW irradiance and especially in Setlow dose according to NSF ground spectral measurements and satellite data, which is explained by ozone loss and to some extent by concurrent variations in cloudiness. The analysis at some other sites with positive EW trends has shown the increase in EW irradiance to be the result of combination of cloud and ozone variability effects.
• The historical cloud data available for us since the middle of 30s shows different tendencies in CQ_g variations over Eurasia. There was a significant CQ_g growth (about 10%) at the end of 30s over the Northern and Central part of Eastern Europe, a strong CQ_g decrease over continental region of Western Siberia at the end of 40s, and in the 50s - over the Monsoon Far East area. In the latter region the CQ_g growth (higher than 10%) can be noticed in the middle of 80s. A strong inverse correlation between the cyclonic processes and the CQ_g values is observed over Russian Plane. On the whole, according to ground observations in May-September period, the EW variations due to clouds comprise 7-16%, that is in coincidence with TOMS V_cl which mainly vary within 5-20%. EW variations due to ozone are slightly less and lie in the range of 3-11%. During May-September period EW changes due to ozone variations prevail in Southern Hemisphere. It was shown that most of the surface of the Earth is characterized by comparable effects of cloud and ozone. It is important that the area with prevailing ozone influence covers several highly populated regions located in low latitudes with high absolute EW levels (for example, the Mediterranean).