Drivers of divergent trends in tropospheric ozone hotspots in Spain, 2008–2019

This study aimed to investigate the causes of contrasting ozone (O₃) trends in Spanish O₃ hotspots between 2008 and 2019, as documented in recent studies. The analysis involved data on key O₃ precursors, such as nitrogen oxides (NOx) and volatile organic compounds (VOCs), among other species, along with meteorological parameters associated with O₃. The dataset comprised ground-level and satellite observations, emissions inventory estimates, and meteorological reanalysis.

The results suggest that the increasing O₃ trends observed in the Madrid area were mostly due to major decreases in NOx emissions from the road transport sector in this urban VOC-limited environment, as well as variations in meteorological parameters conducive to O₃ production. Conversely, the decreasing O₃ trends in the Sevilla area likely resulted from a decrease in NOx emissions in a peculiar urban NOx-limited regime caused by substantial VOC contributions from a large upwind petrochemical area. Unchanged O₃ concentrations in other NOx-limited hotspots may be attributed to the stagnation of emissions from sectors other than road transport, coupled with increased emissions from certain sectors, likely due to the economic recovery from the 2008 financial crisis, and the absence of meteorological variations favorable to O₃ production.

In this study, the parameters influencing O₃ varied distinctively across the different hotspots, emphasizing the significance of adopting an independent regional/local approach for O₃ mitigation planning. Overall, our findings provide valuable insights into the causes of contrasting O₃ trends in different regions of Spain, which can be used as a basis for guiding future measures to mitigate O₃ levels.