| Summary: | The Mediterranean basin represents a hot spot area for short-term O 3 distribution and anthropogenic contributions to it. This is why we analysed in this work the surface O 3 variability observed at Mt. Cimone WMO/GAW global station (CMN, 44°12′ N, 10°42′ E, 2165 m a.s.l., Italy) from 1991 to 2011. The measurements performed at this mountain observatory represent the longest surface O 3 record at a baseline site in the Mediterranean basin. Monthly O 3 averages at CMN show a typical seasonal cycle characterised by a winter minimum and a spring – summer maxima. The shape of the mean annual variation of O 3 is well comparable with those observed at other four baseline sites in the Alps and in the Mediterranean region: Jungfraujoch – Swiss Alps, Sonnblick – Austrian Alps, Mt. Krvavec– Slovenia and Giordan Lightouse – Island of Gozo, Malta. In general, O 3 levels at CMN show higher values during warm months, which is likely to be related both to vertical transport of polluted air-masses at regional and continental scales and to enhanced photochemistry. Here, we also investigate the influence of specific atmospheric processes (i.e. the occurrence of heat-waves, North Atlantic Oscillation, thermal transport of air-masses from the regional PBL and stratospheric intrusions) in affecting O 3 variability at CMN. Overall, a significant positive (95% confidence level) linear trend in monthly O 3 mole fraction was observed over the period 1991–2011 (0.21 ± 0.10 nmol/mol yr −1 ) while no trend (-0.02 ± 0.12 nmol/mol yr −1 ) was detected for the 1996–2011, when measurements were carried out by an homogeneous experimental set-up. On a seasonal basis, a positive trend has been observed for 1996–2011 (0.34 ± 0.32 nmol/mol yr −1 ) only for spring. Significant decreases of the seasonal O 3 growth-rates have been detected at CMN during 1991–2011 from winter to spring and only for summer during 1996–2011.
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