HIGHER UV-RADIATION INFERRED FROM LOW OZONE LEVELS AT NORTHERN MIDLATITUDES IN 1992 AND 1993

Atmospheric ozone, cloudiness and aerosol optical depth determine the amount of solar UV radiation that reaches the lower atmosphere at a gi ven location. Ozone measurements during the years 1992 and 1993 showed extremely low values in Central Europe. Monthly mean ozone in January 1992 and February 1993 at Potsdam (52 degrees N, 13 degrees E) were 2 0-25% below the long-term averages. Ah monthly mean values of ozone in both years at Potsdam were lower than their long-term averages, and t he annual mean values of ozone, which were 10% below the long-term ave rage, have been the two lowest since ozone observations started in 196 4. The effect of ozone and cloudiness on the biologically effective UV irradiation in 1992 and 1993 as compared to average conditions was es timated by a combination of irradiance measurements and model calculat ions. The low ozone values should have produced higher than normal UV irradiation for clear sky conditions. If cloudiness is also taken into consideration, the effect of low ozone values on UV irradiance was ei ther further enhanced or reduced depending on the amount of clouds. Fo r example, the lower than normal cloud amount in some months such as i n May and June 1999 further enhanced the biologically effective solar UV irradiance, while the higher than normal cloud amount from January to March 1992 reduced the enhancement of UV irradiation. We estimate t hat the biggest enhancements in UV irradiation at Potsdam should have occurred in the time period from January to April 1993. As the contrib ution of effective UV irradiation in the winter months to the annual U V irradiation dose is shown to be less than 1% at mid-latitudes and mu ch less than 1% at high latitudes, the damage to the biosphere is not expected to be dramatic. However, the enhancements in effective UV irr adation in the summer months of 1992 and 1993 can have led to adverse biological effects. For example, the erythemal irradiation was 20-30% higher and the plant damaging irradiation was between 30 and 40% highe r than normal in the summer of 1992 and from spring to early summer of 1993.