Observational evidence of an ozone episode over the Greater Athens Area

Research flights have been performed over the Greater Athens Area (GAA) and southwards over the Island of Aegina and east of Peloponnisos in order to investigate the evolution of an ozone episode over GAA and the transportation of the urban pollution plume southwards from the Athens region. During the 3 day period of 6 July to 8 July 1994, the GAA was under the influence of an ozone episode with ground-level noontime concentrations of more than 120 ppbv. Upper-air ozone concentrations measured during the flights were as high as 135 ppbv. The interaction of the weak synoptic conditions over the area along with the development of a mesoscale thermal circulation created poor dispersion conditions during the period of interest and resulted in elevated ozone levels. The primary pollutants emitted in the GAA during the night and early morning hours, were funneled out to the Saronic Gulf and southwards along the southwestern Aegean Sea, near the coast of east Peloponnisos. Under the influence of strong sunlight these primary pollutants continued to undergo photochemical reaction giving rise to elevated ozone levels tens of kilometers downwind of the pollution emission sources. Further evidence of the photochemically-aged air masses was the high correlation (R2 = 0.8) observed between NOy, and ozone. The ozone production efficiency in these transported air masses reached a value of close to six.