Abstract. 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 1200 m 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 (< 1500 m 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 > 2400 m a.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
(O3+ NO2) 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 > 2000 m a.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.