A numerical simulation of the transport of surface ozone along a Mediterranean coastal area

Atmospheric dynamics and elevated ozone concentrations in the northern Adriatic

“…Pollutants that had been advected offshore during the daytime could be very easily returned over Rijeka during the night time. The model reproduced an almost equal horizontal extent of the onshore/offshore flow, which, according to Fortezza et al (), creates favourable conditions for horizontal ozone recirculation. Therefore, according to the observed trajectories (Figure ), the recirculation occurred in both the horizontal plane (between seaward and landward flow exchanges) and in the vertical plane in the thermal circulation over the bay (by the subsidence between return current and onshore breezes).…”

“…During the daytime hours (Figure parts (b) and (c) ∼1000 CET) the polluted air was close to sea level, due to subsidence in the thermal circulation cell over Rijeka. In that location, the ozone levels remained relatively high, due to stronger stratification, deficient vertical diffusion and slower destruction (Fortezza et al , ). Because the fair‐weather conditions allowed photochemical ozone production over the bay, ozone that was formed in the last few hours returned over the coastline via onshore flow in the form of a shallow (∼200–300 m deep), clockwise, afternoon eddy.…”

“…During nocturnal hours, the ozone and its precursors were presumably transported seaward from the mountain slopes by the offshore flow (i.e., superimposed land breeze and down‐slope wind). Over the urban site (Rijeka), the destruction of ozone was fast, while over the sea, O 3 levels remained high for at least 6 h. The latter was due to stronger stratification, deficiency of vertical diffusion, and slower destruction (Fortezza et al , ). In both the nocturnal and transition period (from 0800 to 1000 CET) the photochemical precursors were easily advected toward Rijeka by easterly winds and transformed into ozone.…”

“…Bastin et al , ), Italy (e.g. Fortezza et al , ; Mangia et al , ), Tunisia (e.g. Bouchlaghem et al , ) and Slovenia (e.g.…”

“…Over the northwestern Adriatic (central Mediterranean), the diurnal ozone variation is strongly affected by intense breeze circulations. Fortezza et al () have observed that coastal ozone concentrations are particularly high in the evening (from 1800 to 2200 h local time), due to the horizontal recirculation of oxidants between the land and the sea.…”