Aircraft measurements and model simulations of the air quality in Athens, Greece

Simulations with a three-dimensional time-dependent coupled photochemical and meteorological model are compared with aircraft measurements. The aircraft measurements were taken during the experimental phase of Mediterranean Campaign of Photochemical Tracers – Transport and Chemical Evolution (MEDCAPHOT-TRACE) which took place in Athens, Greece, in August–September 1994. Two days with very different circulations are presented. The first case, 12 September, is a day with rather strong synoptic forcing and the Athens Basin is well ventilated by northerly winds. The emissions from Athens are found as polluted air south of the city, over the sea. The second case is 15 September, a day when the synoptic forcing was weak and several sea-breeze systems developed in the area. Two aircraft missions were performed on this day, one at daybreak and the other in the early afternoon. Low internal boundary-layer heights caused high pollution concentrations in and over the city of Athens. Model output is extracted with the space and time information from the aircraft, i.e. a virtual flight is performed in the model. Model results and the experimental observations are compared; the general conclusion is that the model performed well on the two simulated days. The flow is complex on 15 September and both the model simulation and the observations show increasing concentration of ozone and other pollutants towards north in the basin. In the gap between two mountains in the north, there is an opposing wind that halts the sea-breeze. This creates a convergence zone which in turn causes vertical motions that transport polluted air up to above 2000 m, i.e. in this case to the free troposphere.