Phenomenology of summer ozone episodes over the Madrid Metropolitan Area, central Spain

Various studies have reported that the photochemical nucleation of new ultrafine particles (UFPs) in urban environments within high insolation regions occurs simultaneously with high ground ozone (O3) levels. In this work, we evaluate the atmospheric dynamics leading to summer O3 episodes in the Madrid air basin (central Iberia) by means of measuring a 3-D distribution of concentrations for both pollutants. To this end, we obtained vertical profiles (up to 1200m above ground level) using tethered balloons and miniaturised instrumentation at a suburban site located to the SW of the Madrid Metropolitan Area (MMA), the Majadahonda site (MJDH), in July 2016. Simultaneously, measurements of an extensive number of air quality and meteorological parameters were carried out at three supersites across the MMA. Furthermore, data from O3 soundings and daily radio soundings were also used to interpret atmospheric dynamics. The results demonstrate the concatenation of venting and accumulation episodes, with relative lows (venting) and peaks (accumulation) in O3 surface levels. Regardless of the episode type, the fumigation of high-altitude O3 (arising from a variety of origins) contributes the major proportion of surface O3 concentrations. Accumulation episodes are characterised by a relatively thinner planetary boundary layer ( 2400ma.s.l.). This orographic-meteorological setting causes the vertical recirculation of air masses and enrichment of O3 in the lower tropospheric layers. When the highly polluted urban plume from Madrid is affected by these dynamics, the highest Ox (O3CNO2) concentrations are recorded in the MMA. Vertical O3 profiles during venting episodes, with strong synoptic winds and a deepening of the planetary boundary layer reaching > 2000ma.s.l., were characterised by an upward gradient in O3 levels, whereas a reverse situation with O3 concentration maxima at lower levels was found during the accumulation episodes due to local and/or regional production. The two contributions to O3 surface levels (fumigation from high-altitude strata, a high O3 background, and/or regional production) require very different approaches for policy actions. In contrast to O3 vertical top-down transfer, UFPs are formed in the planetary boundary layer (PBL) and are transferred upwards progressively with the increase in PBL growth. © Author(s) 2018.Acknowledgements. The present work was supported by the Spanish Ministry of Agriculture, Fishing, Food and Environment, the Madrid City Council and Madrid Regional Government, and by the Ministry of Economy, Industry and Competitiveness and FEDER funds under the project HOUSE (CGL2016-78594-R) and the Gen-eralitat de Catalunya (AGAUR 2017 SGR41). Part of this research was supported by the Korea Ministry of Environment through “The Eco-Innovation project”. Support is also acknowledged from the CUD of Zaragoza (project CUD 2016-05), UPV/EHU (UFI 11/47, GIU 16/03), the project PROACLIM CGL2014-52877-R, the City Council of Majadahonda for logistic support, AEMET for providing surface meteorological data, and data from radio soundings and ozone free soundings. We thank Alava Ingenieros, TSI, Solma Environmental Solutions, and Airmodus for their support and María Díez for her computer support in the treatment of radiosonde data. Dr. Escudero has been awarded a José Castillejo grant from the Spanish Ministry of Education and Science (CAS17/00108).Peer reviewe