The 100 largest surface ozone episodes in Europe during 2003–2022: role of meteorology and emissions

Large-scale ozone episodes over Europe are influenced by complex interactions between meteorology and precursor availability, whose relative importance varies across regions and seasons. This study investigates the spatial distribution and drivers of the 100 largest ozone episodes identified in the Copernicus Atmosphere Monitoring Service (CAMS) global reanalysis over Europe during April–September 2003–2022.First, ozone extremes are identified as exceedances of the local 95th percentiles of daily ozone maxima. A semi-Lagrangian algorithm is employed to merge them as daily patterns and then into ozone episodes if they are connected in space and time. This enables a robust characterisation of their spatial extent and temporal evolution. We find that large ozone episodes mainly affect regions north of around 48° N in Apr-May and south of 54° N in Jun-Sep, clustering in three key regions: the British Isles (BRIT) and Eastern Europe (EEU) during Apr-May, and a large region that covers Central Europe (CEU) in Jun-Sep. Additional meteorological and chemical data, as well as an algorithm for the identification of atmospheric blocking and subtropical ridges, are used to assess the role of meteorological processes and precursor emissions in the formation of ozone episodes in these regions and seasons.In EEU, ozone episodes are favoured by well-defined anticyclonic conditions, although elevated precursor concentrations, frequently linked to biomass burning, are also required. In contrast, large episodes affecting BRIT occur under atypical synoptic conditions characterized by negative anomalies of 500 hPa geopotential height and daily maximum temperature at 2 m as well as stronger than usual winds. The potential reasons for these unexpected results are discussed. In CEU, we identify significant north-south differences: episodes in northern CEU are strongly influenced by persistent blocks and ridges, while those in the south are associated with weaker synoptic forcing, with enhanced subsidence as the main contributing mechanism. These findings are relevant for future air quality assessments as they demonstrate that the occurrence of large-scale ozone episodes in Europe is driven by region-specific combinations of meteorological conditions and precursor availability.