Phenomenology of high-ozone episodes in NE Spain

Ground-level and vertical measurements (performed using tethered and non-tethered balloons), 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 mg-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 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 mg^-3 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 with the tethered balloons (900-1000 m a.g.l.) during the campaign, BC and O₃ were often anti-correlated or unrelated, possibly due to a prevailing regional or even 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 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 O₃ episodes were identified: type A with major exceedances of the O₃ information threshold (180 μg mg^-3 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 (or fumigation) of the polluted BMA/regional air masses (open circulation), and relatively lower O₃ levels, but still exceeding the 8 h averaged health target. 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 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 O₃ reduction.