Spatiotemporal variations of tropospheric ozone in Spain (2008-2019)

This study aims to support the development of Spain's Ozone Mitigation Plan by evaluating the present-day spatial variation (2015-2019) and trends (2008-2019) for seven ground-level ozone (O₃) metrics relevant for human/ecosystems exposure and regulatory purposes. Results indicate that the spatial variation of O₃ depends on the part of the O₃ distribution being analyzed. Metrics associated with moderate O₃ concentrations depict an increasing O₃ gradient between the northern and Mediterranean coasts due to climatic factors, while for metrics considering the upper end of the O₃ distribution, this climatic gradient tends to attenuate in favor of hotspot regions pointing to relevant local/regional O₃ formation. A classification of atmospheric regions in Spain is proposed based on their O₃ pollution patterns, to identify priority areas (or O₃ hotspots) where local/regional precursor abatement might significantly reduce O₃ during pollution episodes. The trends assessment reveals a narrowing of the O₃ distribution at the national level, with metrics influenced by lower concentrations tending to increase over time, and those reflecting the higher end of the O₃ distribution tending to decrease. While most stations show no statistically significant variations, contrasting O₃ trends are evident among the O₃ hotspots. The Madrid area exhibits the majority of upward trends across all metrics, frequently with the highest increasing rates, implying increasing O₃ associated with both chronic and episodic exposure. The Valencian Community area exhibits a mixed variation pattern, with moderate to high O₃ metrics increasing and peak metrics decreasing, while O₃ in areas downwind of Barcelona, the Guadalquivir Valley and Puertollano shows no variations. Sevilla is the only large Spanish city with generalized O₃ decreasing trends. The different O₃ trends among hotspots highlight the need for mitigation measures to be designed at a local/regional scale to be effective. This approach may offer valuable insights for other countries developing O₃ mitigation plans.