Insights into the 100 largest European surface ozone episodes during spring–summer 2003–2022

In this study, we investigate the spatial distribution and drivers of the 100 largest ozone episodes identified in the Copernicus Atmosphere Monitoring Service global reanalysis over Europe during April-September 2003-2022. Using a semi-Lagrangian algorithm to detect large-scale ozone episodes and an atmospheric blocking and subtropical ridge identification method, we analyse the role of meteorological processes and precursor emissions in three key regions: the British Isles (BRIT) and Eastern Europe (EEU) during April-May, and Central Europe (CEU) in June-September, revealing some regional differences. The 28 days with ozone episodes in EEU are associated with anticyclonic conditions but also require elevated concentrations of precursors, often linked to wildfire activity. In contrast, episodes affecting BRIT are characterized by negative anomalies of 500 hPa geopotential height and daily maximum temperature at 2 m as well as stronger than usual winds that ventilate the region. Out of a total of 38 days with episode there, one-quarter are associated with easterly flow and changes in ozone formation sensitivity and three-quarters present varying dynamical conditions. In CEU, we identify significant north-south differences: while 36 out of a total of 60 days with episodes in northern CEU are strongly influenced by blocks and ridges, the 29 days with episodes restricted to the south are affected by weaker synoptic forcing and enhanced subsidence. 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.