Abstract. Ground level and vertical measurements (coupled with modelling) of ozone (O3), other gaseous pollutants (NO, NO2, CO, SO2) and aerosols were carried out in the plains (Vic Plain) and valleys of the northern region of the Barcelona Metropolitan Area (BMA) in July 2015; an area typically recording the highest O3 episodes in Spain. Our results suggest that these very high O3 episodes were originated by three main contributions: (i) the surface fumigation from high O3 reservoir layers located at 1500–3000 m a.g.l., and originated during the previous day(s) injections of polluted air masses at high altitude; (ii) local/regional photochemical production and transport (at lower heights) from the BMA and the surrounding coastal settlements, into the inland valleys; and (iii) external (to the study area) contributions of both O3 and precursors. These processes gave rise to maximal O3 levels in the inland plains and valleys northwards from the BMA when compared to the higher mountain sites. Thus, a maximum O3 concentration was observed within the lower tropospheric layer, characterised by an upward increase of O3 and black carbon (BC) up to around 100–200 m a.g.l. (reaching up to 300 µg/m3 of O3 as a 10-s average), followed by a decrease of both pollutants at higher altitudes, where BC and O3 concentrations alternate in layers with parallel variations, probably as a consequence of the atmospheric transport from the BMA and the return flows (to the sea) of strata injected at certain heights the previous day(s). At the highest altitudes reached in this study (900–1000 m a.g.l.) during the campaign, BC and O3 were often anti-correlated or unrelated, possibly due to a prevailing regional/hemispheric contribution of O3 at those altitudes. In the central hours of the days a homogeneous O3 distribution was evidenced for the lowest 1 km of the atmosphere, although probably important variations could be expected at higher levels, where the high O3 return strata are injected according to the modelling results and free sounding data. Relatively low concentrations of ultrafine particles (UFP) were recorded in the 100–200 m a.g.l. atmospheric layer where concentrations of O3 were high; and nucleation episodes were only detected into the boundary layer. Two types of O3 episodes were identified: Type A) with major exceedances of the O3 information threshold (180 µg/m3 on an hourly basis) caused by a clear daily concatenation of local/regional production with accumulation (at upper levels), fumigation and direct transport from the BMA (closed circulation); and Type B) with regional O3 production without major recirculation (neither fumigation) of the polluted BMA/regional air masses (open circulation), and relatively lower O3 levels. The interpretation of OX (O3 + NO2) experimental data from strategically selected monitoring sites on the coast and inland, together with the photochemical modelling results have allowed to study the O3 phenomenology associated with the onset and development of severe episodes in the region of Catalonia in NE Spain. To implement potential O3 control and abatement strategies two major key tasks are proposed: (i) meteorological forecasting, from June to August, to predict recirculation episodes so that NOX and VOCs abatement measures can be applied before these episodes start; (ii) sensitivity analysis with high resolution modelling to evaluate the effectiveness of these potential abatement measures of precursors for O3 reduction.
Supplementary material to "Phenomenology of the highest ozone episodes in NE Spain"