Pergamon www.elsevier.com/locate/asr zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Adv. Space Res. Vol. 27, No. 12, pp. 1955-1963,200l 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-1177/01 $20.00 + 0.00 PII: SO273-1177(01)00279-4 CHANGES IN SURFACE UV SOLAR IRRADIANCE AND OZONE OVER THE BALKANS DURING THE ECLIPSE OF AUGUST 11,1999 C.S. Zerefos’, D. S. Balis’, P. Zanis’, C. Meleti’, A.F. Bais’, K. Tourpali’, D. Melas’, I. Ziomas’, E. Galani’, K. Kourtidis’, A. Papayannis* and Z. Gogosheva3 ’ Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Greece 2 National Technical University of Athens, Greece 3 Institute for Astronomy, Stara Zagora, Bulgaria ABSTRACT Intensive measurements of W solar n-radiance, 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 moon’s 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 n-radiance at the shorter wavelengths. A 20- minute oscillation of erythemal W-B solar n-radiance 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 lo-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. 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION A solar eclipse being a photolytical perturbation of the atmospheric environment provides an excellent opportunity to investigate the observed changes in solar W n-radiances, in total columnar ozone, in surface ozone or other surface chemical parameters and in meteorological parameters at sites located near the path of the moon’s shadow. In the literature there have been several studies that report effects of a solar eclipse on total ozone column (Zerefos et al., 2000 and references therein). Bojkov (1968) reported results from Dobson spectrophotometric observations performed in Sofia, Bulgaria during the solar eclipse of May 1966 and concluded that an increase of 14 D.U. was observed at the maximum phase of the eclipse. In that paper a critical review of total ozone measurements during a solar eclipse is presented and it was emphasized that similar results were also reported in other studies based on Dobson ozone observations. More recently Chakrabarty et al., (1997) reported that total ozone measurements performed with a Dobson spectrophotometer during the solar eclipse of 24 October 1997 over Ahmedabad, India, showed a sharp fall in the ozone column 10 minutes before the maximum obscuration of the sun, followed by a sharp rise 10 minutes after. Mims and Mims (1993) using a portable filter radiometer took observations during the total solar eclipse of July 1991. They reported the occurrence of two ozone minima (down to -7% of the pre-eclipse values) on either side of totality, as well as two maxima (3% higher than the post-eclipse observations) on either side of totality. It should be noted that the majority of 1955