The present paper presents some of the results from the EU-funded Photochemical Activity and solar Ultraviolet Radiation (PAUR) and Photochemical Activity and solar Ultraviolet Radiation Modulation Factors (PAUR II) projects that are relevant to the photochemistry of the Eastern Mediterranean ender variable conditions. Compared to S-SW flows, northern flows result in higher ozone values above the PBL. Usually during N flow the aerosol optical depth is also lower, thus resulting in higher NO2 photodissociation rates. In one case of northern flow, the Etesian synoptic regime, N-NE flows are associated with high wind speeds and result to even lower aerosol optical depth. S-SW flow during springtime in some cases brings air loaded with Saharan dust particles over the region that is poor in tropospheric ozone and precursors. Often, these cases are associated with southern stratospheric circulation, resulting in reduced total ozone amounts and hence increased ozone photolysis rates, whereas the dust load reduces nitrogen dioxide photolysis rates. Results are presented from a spectral radiation model, which simulates the spetral UV radiation during some of the above cases. Measurement results from the Athens basin and rural Aegean sites are presented, that compare the ozone levels between normal N now conditions and Etesian flow. (C) 2000 Elsevier Science Ltd. All rights reserved.