A method to derive long-term time series and trends of UV-B radiation (1968-1997) from observations at Hohenpeissenberg (Bavaria)

The aim of this study is to estimate the solar ultraviolet (UV) radiation reaching the ground during the last 30 years. Solar UV-B irradiance (290-320 nm) measured by the Brewer spectrophotometer 10 at Hohenpeissenberg, Germany (48°N, 11°E), was compared with other ground-based measurements at Hohenpeissenberg. Eight years (1990-1997) of operationally measured UV were used to establish statistical relations between spectral UV and total ozone amount. Data were included in the analysis only if the sunshine recorder reported 10/10. This criterion allows clouds to be present, but only optically thin clouds may cover the Sun. The resulting data set contains approximately one fourth of all available measurements. The atmospheric conditions were classified by four well-defined groups. Turbidity classes were derived from the ratio of global and diffuse radiation, which is well correlated with aerosol optical thickness during sunshine. Measurements of total ozone, global, and diffuse radiation are available for the last 30 years. Thus it is possible to calculate the mean maximum UV based on monthly mean total ozone values during this period from the statistical model. The mean maximum UV is characterized by a value of solar zenith angle (SZA) which roughly represents the monthly mean of the daily SZA at local noon. The decrease in total ozone has led to an increase in UV. The spectral global irradiance at 300 nm at a solar zenith angle of 40° increased in April by about 100±10% and in August by about 25±3% in the 30 year period. At 305 nm we found an increase of about 40±4% in April and of only about 10±2% in August. In general, the increase in UV radiation is stronger early in the year and at short wavelength (300 nm) than in summer and autumn and at longer wavelength.