Abstract. We present an analysis of long-term trends in surface ozone (O3) across Ireland, with specific focus on the Mace Head atmospheric research station, representative of Northern hemispheric background atmospheric conditions. Surface O3 dataset was characterised using advanced trajectory analysis and seasonal decomposition, 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, which are less susceptible to influence from local and easterly emissions but heavily influenced by transboundary pollution and stratospheric intrusion. At Mace Head, springtime O3 levels exhibit a declining trend, whereas wintertime levels show a rising trend. Focussing on the clean sector, the springtime decline remains significant, but without corresponding clean sector rising wintertime trends, implying the rising winter trends occur in response to declining local, United Kingdom (UK) and 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 predominantly attributed to stratospheric transport, hemispheric and long-range transport and lightning NOx. The complementary trend and sectoral observational analysis reveal a decline in total spring-time concentrations, with a more rapid decline in exceedances from the UK and continental sector. 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.
Korhale et al. (Mon,) studied this question.