New maximum UV irradiance levels observed in Central Europe

Abstract

Simultaneous measurements of UV irradiance have been made from two adjacent sites (47.5°N, 11.1°E) in Garmisch-Partenkirchen (730 m a.s.1.) and the Zugspitze (2964 m a.s.1.). New maximum erythemally weighted irradiance levels were observed in Garmisch-Partenkirchen, Germany during March 1996 and June 1995. The March episode was associated with a polar stratospheric cloud while the June episode was associated with an unusually low total ozone column (for June) and broken clouds. The June maximum may be especially significant because it was preceded by a minimum of UV irradiance caused by heavy cloud cover, thus the absolute change in the UV irradiance was very extreme. Even though the maximum irradiance levels increased, these measurements show that the monthly mean spectral UV irradiation over Germany in 1995 was not significantly different compared to previous years. This is important because an increase in the number of extreme fluctuations from low UV irradiation followed by very high UV irradiation may be much more dangerous for the biosphere than a small gradual increase in average UV dose, because natural adaptation mechanisms may not be able to cope with these extreme fluctuations. The monthly erythemally weighted irradiation is between 25 and 90% higher on the Zugspitze than on the lower Garmisch-Partenkirchen site. The difference between the two sites cannot be characterized by a single number because the average monthly ratio 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-Partenkirchen) can be used in the assessment of biological effects in the immediate neighbourhood, data from a single site are not sufficient to extrapolate even to different altitudes near the measurement site and certainly should not be used to extrapolate conclusions to the atmospheric chemistry of the global troposphere.