Quasi-biennial and longer-term changes in clear sky UV-B solar irradiance

Using the longest available clear sky time series of spectral UV-B data (from November 1990 to present) at Thessaloniki, Greece (40 degrees N), estimates are provided on the variability of UV-B on annual and longer time scales. Solar irradiance at two distinct wavelengths are used: the strongly ozone dependent 305nm and the weakly dependent on ozone 325nm. The results are based on Brewer measurements of spectral and erythemally weighted irradiances at 63 degrees and 50 degrees solar zenith angles. The longterm change in UV-B solar irradiance, which can be attributed solely to the observed ozone change of 4.5% per decade, is the order of 10% per decade at 305 nm, 63 degrees SZA.. These long-term change is checked against model calculations and changes imposed by the variability in air quality at Thessaloniki and is statistically significant. The peak-to-peak amplitude of the QBO in total ozone is 8% and its found to be associated with a 21% peak-to-peak amplitude relative to the mean for the solar irradiance at 305nm. Similar results are obtained with the erythemally weighted irradiance and are in agreement with results obtained from model calculations. The observed QBO in solar UV-B has many implications not only because of the QBO biorhythm imposed to many species but also because of its importance in atmospheric chemistry.