Surface Ozone Distribution & Trends Over Ireland : Insights from long-term measurement record and source attribution modelling

Abstract. Surface ozone (O₃) pollution is assessed across Ireland with a focus on long-term trends with a specific focus on the Mace Head atmospheric research station which monitors background O₃ advected into Europe via prevailing South Westerlies. Using innovative trajectory analysis, O₃ concentrations, exceedances and were identified by sectors, revealing distinct seasonal and spatial patterns. Findings show a significant rising trend in surface O₃ at Irish urban sites over the past two decades but without a similar trend at coastal sites. Highest O₃ levels and exceedances were observed at remote coastal sites, less influenced by local emissions, and heavily influenced by meteorological processes, including transboundary pollution and stratospheric intrusion. At Mace Head, springtime O₃ levels show a declining trend, with a rising winter-time trend. Looking only at the clean sector, the springtime decline remains significant; but without rising wintertime trends, implying the rising winter trends are a response to declining European emissions. Advanced modelling tools are used to quantify O₃ source contributions, elucidating key drivers behind the observed changes. Characteristic springtime O₃ maxima at Mace Head are attributed to stratospheric transport, influences from westerly transboundary air pollution, and lightning NO_x. Combined trend and sectoral observational analysis reveals that total spring-time concentrations are in decline, with exceedances from the UK & continental sector declining at a greater rate. This research highlights the importance of seasonal factors in air quality management across Ireland, emphasising the need for a multi-faceted approach to control O₃ levels and reduce exceedances through global and regional emission reductions.