Factors affecting solar ultraviolet irradiance measured since 1990 at Thessaloniki, Greece
ABSTRACT Factors affecting the solar spectral ultraviolet (UV) irradiance at Thessaloniki, Greece were investigated using measurements with single-and double-monochromator Brewer spectroradiometer, which started operating respectively in 1989 and 1993 and continue up to the present. The two data records were quality controlled, homogenized and finally merged into one dataset, which was used in the analysis. Subsets of these data corresponding to different solar zenith angles (SZAs) and to cloud-free skies were used to quantify the long-term changes in surface UV irradiance at different wavelengths, and the importance of the factors responsible for these changes is discussed. It is shown that the calculated UV changes vary with SZA due to the different atmospheric path of the photons and the dependence of the diffuse to direct irradiance ratio on the SZA. The effect of total ozone and aerosols on UV irradiance is examined and the corresponding radiation amplification factors (RAFs) at the various wavelengths are calculated. The observed changes in UV irradiance due to ozone are smaller than those expected for the changes in total ozone, suggesting that the influence of the ozone is masked by other factors. An important finding of this study is that the improvement in air quality at Thessaloniki, during the period under examination, is the main reason for the observed increase in solar UV irradiance.
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ABSTRACT: There have been relatively few studies that have attempted to separate the effects of the different factors affecting the short- and long-term changes of UV radiation. The effect of ozone is fairly easily represented, but that of other factors ( surface albedo, aerosols, clouds) is more complex. In this paper, we present a methodology that can account for the other effects as well. We have used this methodology to study what part of the short- and long-term variability of measured spectral UV data from Sodankyla", Finland (67degreesN), and Thessaloniki, Greece (40degreesN), is explained by each factor. It was found that the effect of ozone on the short- term variability of monthly mean irradiance can be almost as high as 100%, whereas on average it is about 35%. The corresponding impacts by clouds are typically smaller, 40% and 12%, respectively. During May the albedo-related effect is strongest at Sodankyla", being 21% at its maximum and 7% on average at monthly levels. The amplitude of the variability caused by ozone is much stronger at Sodankyla" than at Thessaloniki, so a longer time series at the former place is needed to detect any possible long-term trend. In the Thessaloniki time series there is no significant ozone-related increase. In the summer data from Thessaloniki, however, there is a long-term increase, which is mainly caused by cloudiness. It was found that it is crucial to check the spectral data for any possible wavelength shift, if the long-term variability at a single wavelength is studied. Moreover, it was demonstrated that if the irradiance measurements of a given solar zenith angle (SZA) range are selected, the use of even the rather narrow band of 2degrees can introduce an effect that hampers trend detection. In other words, it is important to remove the effect of SZA and normalize all the measurements to some constant SZA value.Journal of Geophysical Research 01/2003; 108(D17):4549, doi:10.1029/2003JD003447. · 3.17 Impact Factor
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ABSTRACT: This paper compares two methods of cloud flagging that were developed at the Deutscher Wetterdients (DWD) and at the Laboratory of Atmospheric Physics, University of Thessaloniki (LAP). The two methods are applied to the same data set to uncover their similarities and differences. The LAP method aimed at flagging the quality of global UV irradiance spectral measurements with respect to the purity of their spectral characteristics. while the DWD flags describe the sky conditions and their effects on the radiation field in an absolute sense as well as their short-time variability. In this respect, the two methods appear to have distinct differences, and also similarities, in describing when the sun's disk is occluded or not. (C) 2001 Elsevier Science B.V. All rights reserved.Atmospheric Research 01/2001; 57(1):31-42. · 2.20 Impact Factor
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ABSTRACT: Results from an intercomparison campaign of ultraviolet spectroradiometers that was organized at Nea Michaniona, Greece July, 1-13 1997, are presented. Nineteen instrument systems from 15 different countries took part and provided spectra of global solar UV irradiance for two consecutive days from sunrise to sunset every half hour. No data exchange was allowed between participants in order to achieve absolutely independent results among the instruments. The data analysis procedure included the determination of wavelength shifts and the application of suitable corrections to the measured spectra, their standardization to common spectral resolution of 1 nm full width at half maximum and the application of cosine corrections. Reference spectra were calculated for each observational time, derived for a set of instruments which were objectively selected and used as comparison norms for the assessment of the relative agreement among the various instruments. With regard to the absolute irradiance measurements, the range of the deviations from the reference for all spectra was within +/- 20%. About half of the instruments agreed to within +/-5%, while only three fell outside the +/- 10% agreement limit. As for the accuracy of the wavelength registration of the recorded spectra, for most of the spectroradiometers (14) the calculated wavelength shifts were smaller than 0.2 nm. The overall outcome of the campaign was very encouraging, as it was proven that the agreement among the majority of the instruments was good and comparable to the commonly accepted uncertainties of spectral UV measurements. In addition, many of the instruments provided consistent results relative to at least the previous two intercomparison campaigns, held in 1995 in Ispra, Italy and in 1993 in Garmisch-Partenkirchen, Germany. As a result of this series of intercomparison campaigns, several of the currently operating spectroradiometers operating may be regarded as a core group Of instruments, which with the employment of proper operational procedures are capable of providing quality spectral solar UV measurements.Journal of Geophysical Research-Atmospheres. 01/2001; 106(D12):12509-12525.