Revisiting the three largest surface ozone episodes in Europe during 2003–2022 using circulation analogues

Abstract. Surface ozone episodes in Europe are controlled by a combination of chemical and meteorological processes, although their relative contributions to extreme ozone events remain poorly quantified. In this study, we apply, for the first time, a flow analogue methodology to investigate the three largest European ozone episodes identified in the CAMS reanalysis during 2003–2022: August 2003 and July 2006 over western Europe, and July–August 2010 over western Russia. Circulation analogues are identified from daily 500 hPa geopotential height anomaly patterns, and the associated ozone exceedances are used to quantify the contribution of large-scale atmospheric circulation to each event. The analogue reconstruction captures approximately 55 % of the observed ozone exceedances during the July 2006 episode, increasing to 76 % when the analogue pool is restricted to the higher emission period (2003–2012). This highlights the importance of precursor availability under favourable circulation conditions. In contrast, circulation analogues reproduce up to 47 % of the observed exceedances during the August 2003 episode, when a severe heatwave was profoundly amplified by land–atmosphere and soil moisture feedbacks. For the July–August 2010 episode, circulation alone explains around 41 % of the observed ozone exceedances. However, conditioning the analogue selection on regional fire occurrence increases the reconstruction to about 84 %, demonstrating the critical role of biomass burning emissions. Overall, our results show that persistent anticyclonic circulation provides the dynamical framework for extreme ozone episodes in Europe, while their magnitude depends strongly on precursor availability and, in some cases, on additional processes such as biomass burning.