Long-term surface ozone variability at Mt. Cimone WMO/GAW Global Station (2165 m a.s.l., Italy)

The Mediterranean basin represents a hot spot area for short-term O3 distribution and anthropogenic contributions to it. This is why we analysed in this work the surface O3 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 O3 record at a baseline site in the Mediterranean basin. Monthly O3 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 O3 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, O3 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 O3 variability at CMN. Overall, a significant positive (95% confidence level) linear trend in monthly O3 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 O3 growth-rates have been detected at CMN during 1991 - 2011 from winter to spring and only for summer during 1996 - 2011.