Headline: Twenty years of ozone air quality in Europe: trends in models and measurements

The EURODELTA-Trends exercise is a multi-model experiment in which seven regional models performed anair quality hindcast over Europe for the 1990-2010 period at regional-scale resolution (25km). This twenty-yearlookback was designed to complement an investigation of observed European air quality trends over the sametime period, undertaken by the European Monitoring and Evaluation Program Task Force on Measurements andModeling (EMEP-TFMM). Observations at rural ground-based monitoring stations indicate that peak episodicozone, represented by, e.g., 98th percentile of maximum daily 8-hour average ozone (MDA8), have decreased in Europe over the 1990-2010 period. Annual average ozone, on the other hand, was increasing during the 1990-2000 period, but showed a decreasing trend over the 2000-2010 period. Here, the first results of the Eurodelta-Trends exercise for ozone will be presented, with a focus on (1) the capability of the participating models to reproducethe observed trends in European ozone between 1990 and 2010 and (2) the assessment of trends causes, such aschanges in precursor emissions and/or meteorology.Seven regional models, including six regional Chemistry-Transport Models (EMEP-MSCW, Chimere, CMAQ,LOTOS-EUROS, MINNI, and Polyphemus) and the online coupled model WRF-Chem, participated in theEURODELTA-Trends exercise, which builds upon previous iterations of the CITYDELTA and EURODELTAprojects (Thunis et al., 2007; Bessagnet et al., 2014; Cuvelier et al., 2007). Model simulations for EURODELTATrendsincluded a number of time-slice sensitivity experiments for the years 1990, 2000, and 2010, designed toisolate the contribution of European emission changes, boundary conditions (i.e. extra-European influence), andmeteorology on surface ozone concentrations. Four of the participating models performed a full 21-year hindcastfor 1990-2010. A second 21-year simulation was performed using 2010 emissions for the whole time period, inorder to investigate the role of meteorological variability in the modeled trends. Based on these model results, ourunderstanding of tropospheric ozone drivers in Europe will be discussed.