A 6-year analysis of stratospheric intrusions and their influence on ozone at Mt. Cimone (2165 m above sea level)

In this paper we present a study on stratospheric intrusion (SI) events recorded at a high mountain station in the Italian northern Apennines. Six years (1998–2003) of surface ozone and beryllium-7 concentration measurements as well as relative humidity values recorded at the GAW Mt. Cimone research station (44°11'N, 10°42'E; 2165 m asl) were analyzed. Moreover, three-dimensional backward trajectories calculated by the FLEXTRA model and potential vorticity values along these trajectories were used. In order to identify SI and evaluate their contribution to the tropospheric ozone at Mt. Cimone, a statistical methodology was developed. This methodology consists of different selection criteria based on observed and modeled stratospheric tracers as well as on tropopause height values recorded by radio soundings. On average, SI effects affected Mt. Cimone for about 36 days/year. The obtained 6-year SI climatology showed a clear seasonal cycle with a winter maximum and a spring-summer minimum. The seasonal cycle was also characterized by an interannual variation. In particular, during winter (autumn), SI frequency could be related to the intensity of the positive (negative) NAO phase. In order to separate direct SI from indirect SI, a restrictive selection criterion was set. This criterion, named Direct Intrusion Criterion (DIC), requested that all the analyzed tracers were characterized by stratospheric values. Direct SI affected Mt. Cimone for about 6 days/year, with frequency peaks in winter and early summer. At Mt. Cimone, SI contribution to background ozone concentrations was largest in winter. On average, an ozone increase of 8% (3%) with respect to the monthly running mean was found during direct (indirect) SI. Finally, the typical variations of stratospheric tracers during SI events were analyzed. The analysis of in situ atmospheric pressure values suggested that direct SI were connected with intense fronts affecting the region, while indirect SI were possibly connected with subsiding structures related with anticyclonic areas.

Citation proposal
American Geophysical Union (AGU) - Paolo Cristofanelli (Institute of Atmospheric Sciences and Climate (ISAC) - CNR) - Paolo Bonasoni (Institute of Atmospheric Sciences and Climate (ISAC) - CNR) - Laura Tositti (Environmental Radiochemistry Laboratory, Chemistry Department, Bologna University,) - Ubaldo Bonafè (Institute of Atmospheric Sciences and Climate (ISAC) - CNR) - Francesco Calzolari (Institute of Atmospheric Sciences and Climate (ISAC) - CNR) - Franco Evangelisti (Institute of Atmospheric Sciences and Climate (ISAC) - CNR) - Silvia Sandrini (Environmental Radiochemistry Laboratory, Chemistry Department, Bologna University) - Andreas Stohl (Norwegian Institute for Air Research) (2006) . A 6-year analysis of stratospheric intrusions and their influence on ozone at Mt. Cimone (2165 m above sea level). https://geoportal.mountaingenius.org/geonetwork/srv/api/records/2137249c-fddc-43f0-adf0-430444112241