Ground-level and vertical measurements (performed using tethered and non-tethered balloons), coupled with modelling, of ozone (O 3), other gaseous pollutants (NO, NO 2 , CO, SO 2) 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 O 3 episodes in Spain. Our results suggest that these very high O 3 episodes were originated by three main contributions: (i) the surface fumigation from high O 3 reservoir layers located at 1500–3000 m a.g.l. (according to modelling and non-tethered balloon measurements), and originated during the previous day(s) injections of polluted air masses at high altitude; (ii) local/regional pho-tochemical 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 O 3 and precursors. These processes gave rise to maximal O 3 levels in the inland plains and valleys north-wards from the BMA when compared to the higher mountain sites. Thus, a maximum O 3 concentration was observed within the lower tropospheric layer, characterised by an upward increase of O 3 and black carbon (BC) up to around 100–200 m a.g.l. (reaching up to 300 µg m −3 of O 3 as a 10 s average), followed by a decrease of both pollutants at higher altitudes, where BC and O 3 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 with the tethered balloons (900–1000 m a.g.l.) during the campaign, BC and O 3 were often anti-correlated or unrelated, possibly due to a prevailing regional or even hemispheric contribution Published by Copernicus Publications on behalf of the European Geosciences Union. 2818 X. Querol et al.: Phenomenology of high-ozone episodes in NE Spain of O 3 at those altitudes. In the central hours of the days a homogeneous O 3 distribution was evidenced for the lowest 1 km of the atmosphere, although probably important variations could be expected at higher levels, where the high O 3 return strata are injected according to the modelling results and non-tethered balloon data. Relatively low concentrations of ultrafine particles (UFPs) were found during the study, and nucleation episodes were only detected in the boundary layer. Two types of O 3 episodes were identified: type A with major exceedances of the O 3 information threshold (180 µg m −3 on an hourly basis) caused by a clear daily concatena-tion of local/regional production with accumulation (at upper levels), fumigation and direct transport from the BMA (closed circulation); and type B with regional O 3 production without major recirculation (or fumigation) of the polluted BMA/regional air masses (open circulation), and relatively lower O 3 levels, but still exceeding the 8 h averaged health target. To implement potential O 3 control and abatement strategies two major key tasks are proposed: (i) meteorological forecasting, from June to August, to predict recirculation episodes so that NO x and VOC 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 O 3 reduction.