New classification scheme for ozone monitoring stations based on frequency distribution of hourly data

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.

On the origin of the highest ozone episodes in Spain

The 2000-2015 occurrences of the highest ozone (O₃) pollution episodes in Spain were evaluated to investigate their origin. To this end, data series available for urban and regional background (UB and RB), traffic (TR) and industrial (IN) sites were analysed separately and intercompared. Results evidenced that during these 16years mean O₃ levels in the RB sites did not change significantly, and remained constantly high. However, there is a clear increase at the TR and UB sites. Although sensitivity analysis is needed to interpret the cause of this increasing trend, this might be caused probably by the lower O₃ titration intensity due to the preferential abatement of NO vs NO₂, as supported from the neutral trend of O_X (NO₂+O₃) at these sites. We found that the exceedances of the hourly information threshold for O₃ (>180μg/m³) are recorded mostly at UB and IN sites located in seven areas of Spain (specific hotspots or at the tail end of large urban plumes), and that these increased during summer heatwaves (i.e. 2003 and 2015). Although the external contribution of regional-to-subcontinental transported O₃ might be relevant during the highest O₃ episodes in the Western Mediterranean, our results evidenced that in the above specific areas, regional-local O₃ production decisively contributes to the exceedances of the information threshold. Also that the human protection threshold and the AOT40 are more frequently exceeded in the Central, Southern and Mediterranean sides of the Iberian Peninsula. The design of effective episode abatement measures is quite complex in those conditions, due to both the nonlinearity of the chemical processes of O₃ formation and destruction, and to the interplay with the complex meteorological setting, causing frequent recirculation and in situ aging of air masses. However, the combination of meteorological forecasting of the main recirculation processes and sensitivity analysis of NO_X/VOC emission abatement measures might be powerful tools to evaluate the effectiveness of potential O₃ mitigation strategies. Finally we would like to highlight that the current UB, RB, IN and TR classification (somewhat subjective) is not adequate to interpret the origin of O₃ exceedances in complex areas of Southern Europe. Thus, a UB station recording exceedances, and located in a small city in the tail end of an urban plume of a large city, receives not only the contribution from its own UB, but mainly from the specific high O₃ RB caused by the urban plume transport.