Long-Term Trends in Surface Ozone Over Ireland: Insights from long-term measurement dataset and advanced model contributions.

This study extends previous research analysing the long-term measurement records of surface ozone (O₃) across Ireland, focusing on the Mace Head atmospheric research station. Due to its proximity to the Atlantic, Ireland plays a vital role in O₃ monitoring, influenced by local and long-range pollutant transport. Using innovative trajectory analysis techniques, exceedances of O₃ concentrations linked to different pollution sectors were identified, revealing distinct seasonal patterns. The findings show a significant rising trend in surface O₃ at urban sites over the past two decades but without a similar trend at coastal sites. The highest O₃ levels and exceedances at coastal sites, less influenced by local emissions, are heavily influenced by meteorological processes, including transboundary pollution and stratospheric intrusion. Observations at coastal sites reveal seasonal cycles with a springtime maximum. At Mace Head, a declining trend is observed in springtime O₃ levels, contrasted with a rising trend during the winter months. When examining data from the clean-air sector, similar springtime declines are observed; however, a rise in winter is not seen, implying that the rising wintertime trends are a response to decreasing European emissions and the weekend effect. It is found that during the spring season, exceedances correlate with high maxima. To complement these observations, advanced modelling is used to quantify O₃ contributions from various sources, elucidating key drivers behind the observed changes. The analysis indicates that European emissions play a significant role during the summer months, while North American emissions are comparable during other seasons. The elevated springtime O₃ levels are primarily attributed to stratospheric transport, influences from westerly transboundary air pollution, and nitrogen oxides from lightning activity. Trend analysis reveals that reductions in baseline O₃ levels and early-spring exceedances require targeted methane mitigation, while overall emission reductions are essential to curb exceedances across seasons. Late-spring and summer exceedances can be effectively reduced by addressing European and local pollution sources. This research highlights the importance of seasonal factors in air quality management across Ireland, emphasizing the need for a multi-faceted approach to control O₃ levels and reduce exceedances through global and regional emission reductions.