[1] The Photochemical Activity and Ultraviolet Radiation (PAUR) II project (a continuation of an earlier PAUR I project) had the purpose of studying the interrelationships between changes in total ozone, tropospheric aerosols, UV radiation and photochemical activity. As part of PAUR II project, a campaign took place in Greece and Italy during MayJune 1999, with the participation of 15 European and 3 American research institutions. A variety of radiation and gaseous and aerosol atmospheric composition and optical characteristics measurements were made during the campaign. Radiative transfer models and three-dimensional (3-D) regional chemistry transport models (CTM) were applied and compared to the available data set of PAUR II. The present overview paper gives an introduction to the project and to the meteorological and environmental conditions that prevailed and outlines some results that are extensively described in the subsequent papers which form this special section. The modulation of the UVB field in the presence of different types of aerosols, its transmittance and role in the photochemistry of the particular eastern Mediterranean environment is overviewed. Using a 3-D CTM, it is shown that even a 50% reduction in Greek anthropogenic emissions has only a small effect in reducing the ozone levels over the eastern Mediterranean in summer. The environmental conditions, which prevailed during the PAUR I and PAUR II campaigns, offered cases of background conditions over the Aegean Sea as well as conditions with Saharan dust episodes and extremes in total ozone.

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