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

Abstract. We analyzed 2005–2017 data sets on ozone (O 3 ) concentrations in an area (the Vic Plain) frequently affected by the atmospheric plume northward transport of Barcelona Metropolitan Area (BMA), the atmospheric basin of Spain recording the highest number of exceedances of the hourly O 3 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 (interannual, weekly, daily and hourly) variations of concentration of O 3 and nitrogen oxides (NO and NO 2 , including remote sensing data for the latter) in April–September, and built a conceptual model for the occurrence of high O 3 episodes. Subsequently, a sensitivity analysis is done with the air quality (AQ) data to evaluate potential O 3 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 O 3 range (average −1.4 % year −1 ), as well as the well-known increase of urban O 3 (p1.8 % year −1 ), and higher urban NO decreasing slopes compared with those of NO 2 . The most intensive O 3 episodes in the Vic Plain are caused by (i) a relatively high regional background O 3 (due to a mix of continental, hemispheric–tropospheric and stratospheric contributions); (ii) intensive surface fumigation from mid-troposphere high O 3 upper layers arising from the concatenation of the vertical recirculation of air masses, but also by (iii) an important O 3 contribution from the northward transport/channeling of the pollution plume from the BMA. The relevance of the local-daily O 3 contribution is supported by (i) the 5 times larger increase of the hourly exceedances of the O 3 information threshold on weekdays compared with Sundays; (ii) the occurrence of a marked O 3 Sunday decrease of the average diurnal concentrations over the Vic Plain; and (iii) a marked increase in concentrations of NO 2 (OMI-NO 2 remote sensing) over the Vic Plain–BMA region during days with the highest diurnal O 3 concentrations compared with the lowest. We calculated the difference between the 75th percentile of O x (O 3  + NO 2 ) diurnal concentrations recorded at one of the Vic Plain AQ monitoring stations for Wednesdays minus those of the 25th percentile of O x for Sundays, equivalent to 1–2 days of emissions reductions in the BMA. A maximum decrease potential (by applying short-term measures to abate emissions of O 3 precursors) of 49 µg O 3  m −3 (32 %) of the average diurnal concentrations was determined. Obviously, structurally implemented measures, instead of episodically, would result probably in important (and larger) additional O x and O 3 decreases because not only the local O 3 coming from the BMA plume would be reduced but also the recirculated O 3 and thus the intensity of O 3 fumigation in the Plain. Therefore, it is highly probable that both structural and episodic measures to abate NO x and volatile organic compounds (VOCs) emissions in the BMA would result in evident reductions of O 3 in the Vic Plain.