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 (O-3) 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 Majada-honda 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 O-3 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 O-3 surface levels. Regardless of the episode type, the fumigation of high-altitude O-3 (arising from a variety of origins) contributes the major proportion of surface O-3 concentrations. Accumulation episodes are characterised by a relatively thinner planetary boundary layer (<1500m at midday, lower in altitude than the orographic features), light synoptic winds, and the development of mountain breezes along the slopes of the Guadarrama Mountain Range (located W and NW of the MMA, with a maximum elevation of >2400ma.s.l.). This orographic-meteorological setting causes the vertical recirculation of air masses and enrichment of O-3 in the lower tropospheric layers. When the highly polluted urban plume from Madrid is affected by these dynamics, the highest Ox (O-3 + NO2) concentrations are recorded in the MMA. Vertical O-3 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 O-3 levels, whereas a reverse situation with O-3 concentration maxima at lower levels was found during the accumulation episodes due to local and/or regional production. The two contributions to O-3 surface levels (fumigation from high-altitude strata, a high O-3 background, and/or regional production) require very different approaches for policy actions. In contrast to O-3 vertical top-down transfer, UFPs are formed in the planetary boundary layer (PBL) and are transferred upwards progressively with the increase in PBL growth.