Trends in spectral UV radiation from long-term measurements at Hoher Sonnblick, Austria

High-quality long-term records of spectral UV irradiance from the Network for the Detection of Atmospheric Composition Change-affiliated Bentham spectroradiometer at the high-mountain site Hoher Sonnblick (47.05A degrees N, 12.95A degrees E, 3,106 m above sea level) from the period 1997-2011 have been investigated for the existence of trends. Throughout the year, significant upward trends are found at wavelengths of 315 nm and longer. The magnitudes at 315 nm range from +9.3 A +/- 4.5 %/dec at 45A degrees solar zenith angle (SZA) to +14.2 A +/- 3.7 %/dec at SZA 65A degrees for all-sky conditions. The trend estimates at 305 nm are considerably smaller and less significant, yielding between +5.1 A +/- 6.5 and +7.9 A +/- 7.3 %/dec, depending on SZA. Seasonally, the largest trends are found during winter and spring. Total ozone has significantly increased by year-round +1.9 A +/- 1.3 %/dec since 1997 and therefore cannot explain these significant increases. They are rather attributed to decreases in total cloud cover and aerosol optical depth.