2005–2017 ozone trends and potential benefits of local measures as deduced from air quality measurements in the north of the Barcelona metropolitan area

We analyzed 2005–2017 data sets on ozone (O₃) concentrations in an area (the Vic Plain) frequently affected by the atmospheric plume northward transport of the Barcelona metropolitan area (BMA), the atmospheric basin of Spain recording the highest number of exceedances of the hourly O₃ information threshold (180 µg m⁻³). We aimed at evaluating the potential benefits of implementing local-BMA short-term measures to abate emissions of precursors. To this end, we analyzed in detail spatial and time variations of concentration of O₃ and nitrogen oxides (NO and NO₂, including OMI remote sensing data for the latter). Subsequently, a sensitivity analysis is done with the air quality (AQ) data to evaluate potential O₃ reductions in the north of the BMA on Sundays compared with weekdays as a consequence of the reduction in regional emissions of precursors.

The results showed a generalized decreasing trend for regional background O₃ as well as the well-known increase in urban O₃ and higher urban NO decreasing slopes compared with those of NO₂. The most intensive O₃ episodes in the Vic Plain are caused by (i) a relatively high regional background O₃ (due to a mix of continental, hemispheric–tropospheric and stratospheric contributions); by (ii) intensive surface fumigation from mid-troposphere high O₃ upper layers arising from the concatenation of the vertical recirculation of air masses; but also by (iii) an important O₃ contribution from the northward transport/channeling of the pollution plume from the BMA. The high relevance of the local-daily O₃ contribution during the most intense pollution episodes is clearly supported by the O₃ (surface concentration) and NO₂ (OMI data) data analysis.

A maximum decrease potential (by applying short-term measures to abate emissions of O₃ precursors) of 49 µg O₃ m⁻³ (32 %) of the average diurnal concentrations was determined. Structurally implemented measures, instead of episodically, could result in important additional O₃ decreases because not only the local O₃ coming from the BMA plume would be reduced, but also the recirculated O₃ and thus the intensity of O₃ fumigation in the plain. Therefore, it is highly probable that both structural and episodic measures to abate NO_x and volatile organic compound (VOC) emissions in the BMA would result in evident reductions of O₃ in the Vic Plain.