NEW MAXIMUM UV IRRADIANCE LEVELS OBSERVED IN CENTRAL-EUROPE

Simultaneous measurements of UV irradiance have been made from two adj acent sites (47.5 degrees N, 11.1 degrees E) in Garmisch-Partenkirchen (730m a.s.l.) and the Zugspitze (2964m a.s.l.). New maximum erythemal ly weighted irradiance levels were observed in Garmisch-Partenkirchen, Germany during March 1996 and June 1995. The March episode was associ ated with a polar stratospheric cloud while the June episode was assoc iated with an unusually low total ozone column (for June) and broken c louds. The June maximum may be especially significant because it was p receded by a minimum of UV irradiance caused by heavy cloud cover, thu s the absolute change in the UV irradiance was very extreme. Even thou gh the maximum irradiance levels increased, these measurements show th at the monthly mean spectral UV irradiation over Germany in 1995 was n ot significantly different compared to previous years. This is importa nt because an increase in the number of extreme fluctuations from low UV irradiation followed by very high UV irradiation may be much more d angerous for the biosphere than a small gradual increase in average UV dose, because natural adaptation mechanisms may not be able to cope w ith these extreme fluctuations. The monthly erythemally weighted irrad iation is between 25 and 90% higher on the Zugspitze than on the lower Garmisch-Partenkirchen site. The difference between the two sites can not be characterized by a single number because the average monthly ra tio of the irradiances was very variable with respect to both time and wavelength. The variability in the differences between the two sites indicates that the differences are caused by a combination of several factors including Rayleigh scattering, cloud effects, air pollutants ( e.g. tropospheric ozone), aerosols and albedo. These results show that although UV irradiation measurements from one site (e.g. Garmisch-Par tenkirchen) can be used in the assessment of biological effects in the immediate neighbourhood, data from a single site are not sufficient t o extrapolate even to different altitudes near the measurement site an d certainly should not be used to extrapolate conclusions to the atmos pheric chemistry of the global troposphere. (C) 1997 Elsevier Science Ltd.