Decadal O3 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 O3 (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 O3 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 O3 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 O3 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 O3 data over different European regions, for investigating the possibility that the appearance of the O3 anomalies at CMN could be related to variability in the atmospheric transport or in near-surface O3 over specific source regions. Even if it was not possible to achieve a general robust explanation for the occurrence of the high O3 values at CMN during 2004-2008, the variability of (1) regional and long-range atmospheric transport at CMN and (2) European near-surface O3 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).