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 1990's by about 6%, possibly as a result of reduction of clouds and aerosols.
The stratospheric ozone depletion during the last two decades, the increase of UV-B irradiance levels at the ground and the possible impact on the biosphere has led scientists to develop and use instruments of high accuracy for UV measurements.
During the last two years, 9 UV stations have been established in different environments in Greece and Cyprus, with the aim to establish a long-term monitoring network. The instruments of the network (NILU-UV multichannel filter radiometers) can provide measurements of irradiance in the UV and the visible part of the solar spectrum.
In this study, first results from the calibration measurements and the quality assurance procedures are presented. The stability of the maximum of spectral response and the full width at half maximum was measured within 0.5 nm. Lamp tests were performed and downward drifts up to 40% in UVA channel sensitivity were observed. Calibration factors derived from lamp measurements could provide measurements of UV dose rate and total ozone with quite good agreement when compared with standard ultraviolet instruments.
The decrease in stratospheric ozone observed during last decades and the possible relationship with the observed increase UV-B radiation in biosphere has been extensively discussed in relevant studies. However, the detection of UV trends is difficult due to lack of long-term measurements of UV radiation at the Earth's surface, and the large variability introduced by changes in cloudiness, aerosols and surface albedo.
Recently, several methods for reconstruction of UV radiation levels for the past at single sites have been proposed. In most of these studies, measurements of total ozone and empirical or model derived relations for the impact of clouds and surface albedo on UV transmittance have been used. In this study, a method for estimating erythemal (CIE) UV doses using measurements of total ozone and total solar radiation is presented for Thessaloniki, Greece (40.5°N, 23°E). Measurements of total solar radiation and UV erythemal dose rates for five years period (2000-2004) were used to estimate the effect of clouds and aerosols as a function of solar zenith angle (SZA). The method is then tested, when compared with measurements from previous years. The mean difference (%) between measured and modeled cloud modification factors for UV dose rates was calculated between -1.3% and 2.4% for all SZA groups.