High-accuracy spectroradiometry of solar ultraviolet radiation

The study of ultraviolet effects on the biosphere requires an accurate knowledge of UV-B irradiances reaching the Earth's surface. Here we present the technical specifications and the performance of a spectroradiometer for the highly accurate determination of actual UV levels. The sensitivity, the spectral resolution and the wavelength accuracy of the instrument, as well as results from a series of intercomparisons, justify use of the term "highaccuracy". Spectroradiometric measurements made with our instrument in Germany and New Zealand show that the decreasing stratospheric ozone levels in both hemispheres, and the increasing tropospheric ozone levels in Germany, have led to significant differences in UV-B radiation between Germany and New Zealand. The ratios of the weighted irradiances range from 1,5 to 2, depending on the biological weighting function, in clear-sky summer conditions. A comparison of spectral UV measurements in Germany between 1992 and 1993 showed enhanced levels of UV-B radiation in the late spring and early summer of 1993 caused by reduced total ozone columns. The monthly averaged total ozone columns in 1993 were the lowest since ozone levels were systematically recorded in Germany. The data also show that UV-A radiation was lower in 1993 than in 1992, due to differences in cloudiness between the two years.