Past and future evolution of large-scale ozone episodes in Europe: results from reanalysis and Earth system models

Extreme near-surface ozone concentrations often cluster into large episodes that last several days. They strongly depend on meteorology, precursor emissions, and ambient photochemical conditions. A new pseudo-Lagrangian algorithm has been employed to identify the spatiotemporal patterns of episodes, allowing for a good characterization of their areal extent and an assessment of their drivers. The algorithm has been used to track ozone episodes in Europe from April to September over twenty years (2003–2022) of the Copernicus Atmosphere Monitoring Service (CAMS) reanalysis as well as in the historical simulation (1950–2014) and four Shared Socio-economic Pathways (SSPs, spanning 2015–2100) of three Earth system models (UKESM1-0-LL, EC-Earth3-AerChem and GFDL-ESM4). The algorithm has also been applied to detect ozone episodes in variations of the reference scenario ssp370 of UKESM1-0-LL, either with reduced precursor emissions or with the same emissions but present-day climate.

The results from CAMS show that, despite the overall increase in the number of episodes in recent years, the frequency of large episodes has decreased following European precursor emission reductions. The 100 largest episodes mainly occurred in northern Europe during spring and in the center and south of the continent from June onwards, whereas the top 10 episodes occurred in the first years of the century associated with high temperatures and anticyclonic conditions.

Despite the decrease in large episodes in recent years, there is uncertainty regarding future episodes. Episodes of reduced size are found for SSPs with weak greenhouse forcing and low precursor emissions. In contrast, episode sizes increase in scenarios with high methane concentrations and enhanced radiative forcing, even exceeding the maximum historical size. Furthermore, the comparison of episodes in variations of the reference scenario in UKESM1-0-LL enables the exploration of the separate contributions of climate change and precursor emissions. This analysis reveals that regional precursor reductions and global methane reductions are efficient strategies to significantly decrease the size of ozone episodes across the entire continent. On the other hand, global warming has contrasting effects which are, in any case, weaker than those of precursor emissions. 

This work is distributed under the Creative Commons Attribution 4.0 License. This licence does not affect the Crown copyright work, which is re-usable under the Open Government Licence (OGL). The Creative Commons Attribution 4.0 License and the OGL are interoperable and do not conflict with, reduce or limit each other.