The temporal variability of global ultraviolet solar spectral irradiance measured regularly at Thessaloniki, Greece during the last 15 years is presented. The measurements were conducted by a single- and a double-monochromator Brewer spectroradiometers which operate at the Laboratory of Atmospheric Physics since 1989 and 1993, respectively. Recently the entire series of measurements was re-evaluated and quality controlled, by revising the calibration history of the two instruments and by comparing these measurements with those obtained by a collocated erythemal radiometer and a pyranometer. In addition, the spectral measurements were corrected for the angular response error of each instrument and for the effect of temperature variations.

The longest of the re-evaluated series, which was obtained by the single monochromator, was statistically analyzed to derive estimates of the long-term changes and variability of UV irradiance radiation. Daily integrals were derived with the aid of broadband measurements which were used to simulate the diurnal variation of the spectral irradiance at one minute increments. The effect of clouds and solar zenith angle on the log term variability of UV irradiance are also investigated. Finally, signals of inter-annual natural variations and oscillations on this data set are explored and removed in an attempt to attribute the observed variability to different factors or mechanisms and investigate their effects on the long term changes of UV irradiance at the ground. All long term changes that were calculated have positive signs and vary according to wavelength solar zenith angle and the period of data. Monthly erythemal irradiance increases in the 1990s by about 6%, possibly as a result of reduction of clouds and aerosols.