Sampling of an STT event over the Eastern Mediterranean region by lidar and electrochemical sonde

A two-wavelength ultraviolet (289–316 nm) ozone Differential Absorption Lidar (DIAL) system is used to perform ozone measurements in the free troposphere in the Eastern Mediterranean (Northern Greece). The ozone DIAL profiles obtained during a Stratosphere-toTroposphere Transport (STT) event are compared to that acquired by an electrochemical ozonesonde, in the altitude range between 2 and 10 km. The measurement accuracy of these two instruments is also discussed. The mean difference between the ozone profiles obtained by the two techniques is of the order of 1.11 ppbv (1.86%), while the corresponding standard deviation is 4.69 ppbv (8.16%). A case study of an STT event which occurred on 29 November 2000 is presented and analyzed, using ozone lidar, satellite and meteorological data, as well as air mass back-trajectory analysis. During this STT event ozone mixing ratios of 55–65 ppbv were observed between 5 and 7 km height above sea level (a.s.l.). Stratospheric air was mixed with tropospheric air masses, leading to potential vorticity (PV ) losses due to diabatic processes. The ozone DIAL system can be used for following STT events and small-scale mixing phenomena in the free troposphere, and for providing sequences of vertical ozone profiles in the free troposphere.