Influence of stratospheric air masses on radiotracers and ozone over the central Mediterranean.

This work aims at determining the characteristics and the preferred source regions of stratospheric air masses at the Mt. Cimone World Meteorological Organization-Global Atmosphere Watch station located in the Italian northern Apennines (44°110N, 10°420E, 2165 m above sea level) , a site which is considered representative of Southern Europe-Mediterranean free troposphere. Source regions of high concentrations of 7Be, 7Be/210Pb, and O3 and regions originating high potential vorticity values are analyzed by means of trajectory statistical methods applied over the time series collected in the period of 1998-2011 to this aim. Our analysis points out the frequent occurrence of stratosphere-to-troposphere exchange in the North America and northern Atlantic region, which can be linked to the usual location of the polar-front and subtropical jet streams in these two regions, but also to the tropopause discontinuity and to the high cyclogenetic activity in the Atlantic region. High 7Be concentrations associated with trajectories of high-latitude origin descend from high altitudes to the study site, while at midlatitudes trajectories arrive from relatively lower levels. The geographical distribution of the tropopause crossing of air masses before arriving the study site points out particularly to Greenland. Our results also suggest the importance of the interaction between upper level disturbances and the Alps mountain range as a driver for stratosphere-to-troposphere exchange. In this case, the time between the tropopause crossing and the arrival at the elevated study site is less than 24 h and frequently below 6 h. The Atlas range in northern Africa plays a similar role though with lower influence.