C. Meleti

A note on the recent increase of Solar UV‐B radiation over northern middle latitudes

In this work we present the results from the analysis of regular solar UV-B spectral measurements obtained at Thessaloniki (40°N) during a period of three years, from November 1990 through November 1993. The results presented here refer to both clear-skies (cloudiness ≤3/8) as well as to all-skies conditions, and to solar zenith angles of 63°±1°. These results support earlier findings of increased solar UV-B irradiances, under all-skies conditions, associated with observed ozone decline at the same period. The observed acceleration of the ozone decline-of about 5% per year-during the three-year period resulted to increased solar UV irradiance levels at Thessaloniki by 12.3% per year at 305 nm and 2.7% per year at 325 nm. These figures are slightly modified under all-skies conditions, and they are in agreement with findings reported independently by other scientists.

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°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° and 50° solar zenith angles. The long-term 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° 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.

THE RECENT UVB VARIABILITY OVER SOUTHEASTERN EUROPE

Abstract The UVB erythemal irradiance measurements obtained at Thessaloniki, Greece (40°N) are analysed. The variability of the erythemal irradiance is compared with that of the total columns of ozone and SO2. It is shown that these two factors explain about 50% and 26% respectively of the total variability of the erythemal irradiance. The influence of the reduced total ozone levels observed during the last 4 years on the increase in the solar UV irradiance at 305 nm is also presented. Finally, a comparison between stations with different air pollution levels under clear skies shows that air pollution can significantly reduce the UVB levels received at the ground.

Measurements of aerosol optical depth at Ispra: Analysis of the correlation with UV‐B, UV‐A, and total solar irradiance

Aerosol optical depths measured with a Brewer spectrophotometer in the UV region and with a CIMEL Sun photometer in the visible and near infrared, both in operation at Ispra, Italy, are presented and analyzed. The analysis of the data acquired during 1 year includes a comparison of the spectral aerosol optical depth measured with the Brewer spectrophotometer at various wavelengths, as well as comparisons between the aerosol optical depth measured by the Brewer and the simultaneous measurements taken with the CIMEL Sun photometer. The strong correlation between the measurements of aerosol optical depth obtained from the two instruments demonstrates proper behavior of the instruments and of the methodologies used to retrieve the data. The variability of global UV-B and UV-A and total solar irradiance is investigated in relation to simultaneous spectral aerosol optical depth measured by the Brewer spectrophotometer and the CIMEL Sun photometer. The effect of the total ozone column on the correlation between the global UV-B data and the aerosols is analyzed.

Solar dimming and brightening over Thessaloniki, Greece, and Beijing, China

This work presents evidence that ultraviolet (UV)-A solar irradiances show increasing trends at Thessaloniki, Greece, where air quality has been improving because of air pollution abatement strategies. In contrast, over Beijing, China, where air quality measures were taken later, solar brightening was delayed. It is shown that until the early 1990s, UV-A irradiances over Thessaloniki show a downward trend of –0.5% yr−1, which reverses sign and becomes positive in the last decade (+0.8% yr−1). This brightening is related to a decreasing trend in local aerosol amounts. Both the negative rate of change (dimming) and the positive rate of change (brightening) are amplified in the UV-A solar irradiances, compared with the total solar irradiance, by a factor of 2.6. Satellite derived short-wave radiation over Beijing showed negative changes of –0.4% (1984–1991) and –0.1% yr−1 during 1994–2006. The negative trend in solar radiation continued even during 2000–2006. Satellite-derived aerosol optical depth (AOD) increased by +1.0% yr−1 during 2000–2006, in agreement with in situ measurements of increasing AOD. Therefore, a statistically significant change from dimming to brightening in Beijing could not be seen in the last decade, but it is expected to occur in the near future.

Case study of UV-B modification during episodes of urban air pollution

Examination of the record of solar ultraviolet measurements made during the MEDCAPHOT study of air pollution in the Athens basin revealed a substantial reduction of UV-B on days with high levels of pollution. The magnitude of this relation is illustrated by a comparison of the UV on two days of a pollution episode with that on two days of low pollution.

Changes in surface solar UV irradiances and total ozone during the solar eclipse of August 11, 1999

During the solar eclipse of August 11, 1999, intensive measurements of UV solar irradiance and total ozone were performed at a number of observatories located near the path of the Moons shadow. At the Laboratory of Atmospheric Physics (LAP) of the Aristotle University of Thessaloniki, Greece, global and direct spectra of UV solar irradiances (285–365 nm) were recorded with a double monochromator, and erythemal irradiances were measured with broadband pyranometers. In addition, higher-frequency measurements of global and direct irradiances at six UV wavelengths were performed with a single Brewer spectrophotometer. Total ozone measurements were also performed with Dobson and Brewer spectrophotometers at Hradec Kralove (Czech Republic), Ispra (Italy), Sestola (Italy), Hohenpeissenberg (Germany), Bucharest (Romania), Arosa (Switzerland), and Thessaloniki (Greece). From the spectral UV measurements the limb darkening effect of the solar disk was tentatively quantified from differences of measured solar spectral irradiances at the peak of the eclipse (near to limb conditions) and before the eclipse. Two blackbody curves were fit to the preeclipse and peak eclipse spectra, which have shown a difference in effective temperatures of about 165°K between the limb and the whole of the solar disk. The limb darkening effect is larger at the shorter UV wavelengths. The ratio of the diffuse to direct solar irradiances during the eclipse shows that the diffuse component is reduced much less compared to the decline of the direct solar irradiance at the shorter wavelengths. Moreover, a 20-min oscillation of erythemal UV-B solar irradiance was observed before and after the time of the eclipse maximum under clear skies, indicating a possible 20-min fluctuation in total ozone, presumably caused by the eclipse-induced gravity waves. This work also shows that routine total ozone measurements with a Brewer or a Dobson spectrophotometer should be used with caution during a solar eclipse. This is because the diffuse light increases by more than 30% with respect to the direct solar radiation, increasing more at the shorter wavelength side of the UV spectrum. This plausible mechanism introduces an artificial decrease in total ozone during solar eclipse of more than 30 Dobson units (DU), which is confirmed by all Brewer and Dobson measurements. Changes in total ozone cited earlier in the refereed literature have not been confirmed in the present study.

Changes in surface UV solar irradiance and ozone over the balkans during the eclipse of August 11, 1999

Abstract Intensive measurements of UV solar irradiance, total ozone and surface ozone were carried out during the solar eclipse of 11 August 1999 at Thessaloniki, Greece and Stara Zagora, Bulgaria, located very close to the footprint of the moons shadow during the solar eclipse with the maximum coverage of the solar disk reaching about 90% and 96% respectively. It is shown that during the eclipse the diffuse component is reduced less compared to the decline of the direct solar irradiance at the shorter wavelengths. A 20-minute oscillation of erythemal UV-B solar irradiance was observed before and after the time of the eclipse maximum under clear skies, indicating a possible 20-minute fluctuation in total ozone presumably caused by the eclipse induced gravity waves. The surface ozone measurements at Thessaloniki display a decrease of around 10–15 ppbv during the solar eclipse. Similarly, ozone profile measurements with a lidar system indicate a decrease of ozone up to 2 km during the solar eclipse. The eclipse offered the opportunity to test our understanding of tropospheric ozone chemistry. The use of a chemical box model suggested that photochemistry can account for a significant portion of the observed surface ozone decrease.

Evaluation of ozone column measurements over Greece with NILU-UV multi-channel radiometers

Measurements of total ozone over Greece have been performed with NILU-UV multi-channel radiometers since 2005. In this study, the measurements at two stations, Thessaloniki and Athens, of the ultraviolet (UV) monitoring network are compared with ozone values derived from Brewer spectroradiometers at the same sites. The overall percentage difference on daily averages (including measurements for solar zenith angle below 70˚) is below (1 ± 3.5)% at both sites. The cloud modification factor (CMF), specified as the ratio of measured irradiance with a model-derived cloud-free value, is being used to examine the effect of cloudiness on the retrieved ozone abundances. During days with average CMF values above 0.7, the difference in total ozone measurements is within ± 5%. The use of momentary ozone values from the radiometers with this limitation in CMF reduces the difference of the average values to (0.0 ± 2.2)% for Athens and (0.21 ± 2.8)% for Thessaloniki.

Factors affecting solar ultraviolet irradiance measured since 1990 at Thessaloniki, Greece

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.