An experimental study of nightime air-pollutant transport over complex terrain in Athens

The Thriassion plain is a heavily industrialized area to the west of the Athens basin, separated from it by Mount Aegaleo, a 468-m high ridge about 15 km long. Three field experiments were performed to determine the possibility of air pollutant transport into the Athens basin. Sulphur hexafluoride (SF6) was released from one of the stacks of the Hellenic Oil Refineries, situated in the eastern part of the Thriassion plain, together with several releases of tetroons. These experiments revealed two mechanisms of air mass transport from the Thriassion plain, a daytime mechanism, when the air mass is transported along the Mount Aegaleo ridge and through the passage between Mount Aegaleo and Mount Parnitha to the north, and a nighttime mechanism, when transport occurs over Mount Aegaleo. SF6 was released only during the night and although in all three occassions it reached the western suburbs of Athens, it did so by different simultaneous mechanisms. Furthermore its advection-diffusion once in the basin showed large variability, whose cause was the details of the flow field as it developed under the influence of the thermal stratification. The experiments demonstrate the complexity of the diffusion of air pollutants in complex terrain and the influence of both the details of the flow field and the stratification in determining the local ground-level concentrations. They also point out the need for simultaneous modelling of both factors, for the correct computation of pollution levels.