Decadal O-3 Variability At The Mt. Cimone Wmo/Gaw Global Station (2,165 M A.S.L., Italy) And Comparison With Two High-Mountain "Reference" Sites In Europe

Tropospheric ozone (O3) is a greenhouse gas as well as a harmful air pollutant with adverse effects on human health and vegetation: The observation and attribution of its long-term variability are key activities to monitor the effectiveness of pollution reduction protocols. In this work, we present the analysis of multi-annual near-surface O-3 (1996-2016) at the Mt. Cimone (CMN, Italian northern Apennines) WMO/GAW global station and the comparison with two "reference" high-mountain sites in Europe: Jungfraujoch (JFJ, Swiss Alps) and Mt. Zugspitze (ZUG/ZSF, German Alps). Negative O-3 trends were observed at CMN over the period 1996-2016 (from -0.19 to -0.22 ppb yr(-1)), with the strongest tendencies as being observed for the warm months (May-September: - 0.32 ppb yr(-1) during daytime). The magnitude of the calculated O-3 trends at CMN are 2 times higher than those calculated for ZUG/ZSF and 3-4 times higher than for JFJ. With respect to JFJ and ZUG/ZSF, higher O-3 values were observed at CMN during 2004-2008, while good agreement is found for the remaining periods. We used Lagrangian simulations by the FLEXPART particle dispersion model and near-surface O-3 data over different European regions, for investigating the possibility that the appearance of the O-3 anomalies at CMN could be related to variability in the atmospheric transport or in near-surface O-3 over specific source regions. Even if it was not possible to achieve a general robust explanation for the occurrence of the high O-3 values at CMN during 2004-2008, the variability of (1) regional and long-range atmospheric transport at CMN and (2) European near-surface O-3 could motivate the observed anomalies in specific seasons and years. Interestingly, we found a long-term variability in air mass transport at JFJ with enhanced (decreased) contributions from Western European (intercontinental regions).