2005–2017 ozone trends and potential benefits of local measures as deduced from air quality measurements in the north of the Barcelona Metropolitan Area
Authors: Massagué Jordi , Carnerero Cristina , Escudero Miguel , Baldasano José María , Alastuey Andrés , Querol Xavier 
Published: February 2019

Abstract. 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 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 (interannual, weekly, daily and hourly) variations of concentration of O3 and nitrogen oxides (NO and NO2, including remote sensing data for the latter) in April–September, and built a conceptual model for the occurrence of high O3 episodes. 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 range (average −1.4 % year−1), as well as the well-known increase of urban O3 (&amp;plus;1.8 % year−1), 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); (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 relevance of the local-daily O3 contribution is supported by (i) the 5 times larger increase of the hourly exceedances of the O3 information threshold on weekdays compared with Sundays; (ii) the occurrence of a marked O3 Sunday decrease of the average diurnal concentrations over the Vic Plain; and (iii) a marked increase in concentrations of NO2 (OMI-NO2 remote sensing) over the Vic Plain–BMA region during days with the highest diurnal O3 concentrations compared with the lowest. We calculated the difference between the 75th percentile of Ox (O3 + NO2) diurnal concentrations recorded at one of the Vic Plain AQ monitoring stations for Wednesdays minus those of the 25th percentile of Ox 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 O3 precursors) of 49 µg O3 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 Ox and 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 NOx and volatile organic compounds (VOCs) emissions in the BMA would result in evident reductions of O3 in the Vic Plain.