Measurements of solar ultraviolet radiation reaching the earth's surface became very important during the last few years as both observations and predictions suggest a tendency for increasing UV levels, following the observed ozone decreases (Kerr and McElroy, 1993; Seckmeyer et al., 1994; Zerefos et al., 1995a). Of major public and scientific concern is the spatial and temporal variability of the lower part of the spectrum (UV-B) due to its strong biological activity. Overexposure in UV-B solar radiation may lead to damages in the DNA and in incidences of skin cancer in humans. In addition to its biological effects, solar UV radiation plays also a major role in photochemistry both in the troposphere and the stratosphere. The variability of UV-B radiation penetrating the earth's atmosphere is controlled mainly by the diurnal and seasonal change of solar zenith angle, changes of cloudiness and of course by the variability of ozone, aerosols, and other UV absorbing atmospheric constituents. It is well known that ozone absorbs a substantial part of ultraviolet radiation mainly at wavelengths lower than about 320 nm. The recent observed decreases of total ozone in various regions over the globe have imposed the need for continuous and reliable monitoring of solar UV irradiances at the earth's surface. Attenuation of UV from clouds is a very complicated process, which becomes more complex by the rapid variability of clouds in time and space. Thus, due to the strong spatial variability of UV, monitoring networks are necessary to achieve representative coverage. Such networks can provide valuable information about the actual levels of UV at the ground, but also about its variability in time and space. The data produced are useful for both scientific studies but also for public information and awareness. Many countries have developed, during the last decade, their regional UV monitoring networks, which are based mostly on erythemal broadband detectors (e.g. Scotto et al., 1988; Zerefos et al. 1995b). The principal measured parameter is the global erythemal solar irradiance, although the direct and diffuse components, as well as the short-wave solar irradiance (300-2500 nm) are also monitored in some stations. These additional measurements can be used to extract information on aerosol and cloud effects, but also to demonstrate the effectiveness of diffuse UV radiation in producing biological effects in the presence of clouds. Monitoring of solar ultraviolet radiation and maintaining high quality standards has been one of the most important scientific fields during the last decade. Solar UV-B radiation represents only a small fraction (less than 1%) of the radiant power emitted by the sun, which makes its monitoring extremely difficult, requiring instruments of high precision. The increasing international interest on solar ultraviolet radiation during the last decade stimulated a large activity about the methods and the instrumentation used for its measurements. UV radiation can be measured either spectrally with the use of spectroradiometers, or in narrow- or broadband integrals with the use of filter instruments. Spectroradiometers can provide detailed information about the solar spectrum in their operational spectral region, while filter instruments respond to specific spectral regions and their measurements represent weighted integrals of solar UV radiation.
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