A complex case study of down to the surface intrusions of persistent stratospheric air over the Eastern Mediterranean

In this study, we present a complex case study of concurrent stratosphere-to-troposphere transport (STT) events down to the surface of a low topography region in Northern Greece, which occurred in spring 2000. Two surface stations at Livadi (850 m asl) and Thessaloniki (20 m asl) were influenced by intrusions of stratospheric air masses that persisted for a time period from 7 to 10 days within the troposphere before reaching the stations. Before reaching the surface stations in Greece, the air masses crossed the tropopause in different synoptic systems over Eastern Europe, N. America and the N. Atlantic. This is the first study, to our knowledge, that presents a down to the surface STT event in the Eastern Mediterranean. Observational evidence comes mainly from surface measurements of ozone rising by about 20 ppbv and the cosmogenic radionuclide 7Be exceeding 9 mBq m−3. Trajectory analyses, in conjunction with the evolution of the synoptic situation, were used to identify the stratospheric intrusions most likely to have impacted the surface sites and to estimate the volume fraction of surface air that has been transported from the stratosphere. The latter presented a maximum of 5.2% and allowed the investigation of the response of both ozone and 7Be to the presence of stratospheric air down to the surface.