On the origin of the highest ozone episodes in Spain

Abstract The 2000–2015 occurrences of the highest ozone (O 3 ) 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 16 years mean O 3 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 O 3 titration intensity due to the preferential abatement of NO vs NO 2 , as supported from the neutral trend of O X (NO 2  + O 3 ) at these sites. We found that the exceedances of the hourly information threshold for O 3 (> 180 μg/m 3 ) 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 O 3 might be relevant during the highest O 3 episodes in the Western Mediterranean, our results evidenced that in the above specific areas, regional-local O 3 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 O 3 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 O 3 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 O 3 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 O 3 RB caused by the urban plume transport.