Headline: Influence of synoptic and local meteorological conditions on surface ozone concentrations over Europe

Climate change is expected to alter global, regional and local meteorological conditions and as a result, the changes in the climate system will play an essential role on future air quality. Tropospheric ozone is considered one of the most harmful pollutants and it is strongly dependent on weather conditions. Therefore, understanding the impacts of near-term climate change on ozone concentrations is crucial for developing effective air quality policies. This dissertation focuses on the analysis of the influence of synoptic and local meteorological conditions on ground-level ozone over Europe and it provides a comprehensive spatial characterization of the most important meteorological key-driving factors of surface ozone concentrations over the whole domain. For this purpose two approaches are proposed: i) a weather types classification and ii) regression methods. Firstly, large-scale atmospheric circulation is examined through a weather types classification, implemented grid cell-by-grid cell over Europe. The ability of a suite of global climate models to reproduce realistic synoptic patterns in the present climate is evaluated against two reanalysis products. Additionally, the association between weather types and anomalies of maximum and minimum temperatures is investigated. In general, the models are able to capture realistic synoptic patterns when compared to the reanalyses. However, some limitations to reproduce the frequencies of certain weather types, such as low flow conditions over South Europe in summer and autumn are found. The projected changes in the frequency of weather types under future climate scenarios reveal an increase of anticyclonic days and warmer conditions affecting the British Isles in summer, and more westerlies and consequently mild winter conditions over Central Europe. As a result of a projected increase of low flow conditions over the Mediterranean basin, stagnant situations would become more frequent, favouring episodes of air pollution. Further analysis indicate that changes in the frequency of weather types represent a minor contribution of the total change of projected European temperatures. Thus, the temperature changes could be attributed to the so-called within-type variations (changes of the weather types themselves). In the context of climate change, that implies that global warming would also affect the characteristics of some weather types over time (i.e., within-type variations) that are associated with warmer temperatures under future conditions. Secondly, the classification of weather types provides an easy physically interpretable framework for assessing the impacts of synoptic conditions on ozone concentrations. A synoptic-regression approach is developed to investigate the effect of both, synoptic and local meteorological conditions on surface ozone over the European domain. It is shown that local meteorological conditions are generally dominant factors influencing surface ozone variability, rather than the synoptic conditions. The results reveal distinctive regional and seasonal patterns of the most influential ozone drivers. In particular, local meteorological conditions have a strong influence over Central and East Europe, where maximum temperature becomes the most important driver of surface ozone in summer and relative humidity along with surface solar radiation in spring. Finally, a multi-model assessment examines the capability of a set of state-of-the-art air quality models to reproduce the observed relationship between meteorological variables and surface ozone. The results show distinctive seasonal and regional performances in the statistical models developed for each dataset (i.e. observations and model outputs). Overall, the air quality models are in better agreement with observations over the regions referred to as internal regions: England, France, Mid-Europe, North Italy and East Europe. On the contrary, they present more limitations over the rest of the regions, referred to as the external regions: Inflow, Scandinavia, Iberian Peninsula, Mediterranean and the Balkans. There is a larger meteorological contribution in the internal regions, especially in summer where the local meteorology plays an important role in photochemical processes. A minor meteorological effect is found in the external regions, probably due to a major influence of the dynamical processes that are not captured by the statistical models. Most of the air quality models tend to overestimate the sensitivity to maximum temperature and solar radiation and none of them are able to capture the strength of the observed relationship between ozone and relative humidity appropriately. Here, dry deposition schemes may be a key for the underestimation of such relationship. Further analysis of the slopes of the ozone-temperature relationship indicates that the air quality models capture the observed relationship between ozone and temperature in most of the internal regions in summer, while in spring they overestimate it in most of the European regions.