A Numerical Model for Chemical and Meteorological Processes in the Atmospheric Boundary Layer. Part II: A Case Study of the Air Quality Situation in Athens, Greece

Abstract

A three-dimensional coupled mesoscale meteorological and photochemical model has been applied to the Athens basin in Greece. The Athens area experiences episodes of very high air pollution levels a few times every year. A severe episode on 25 May 1990 was chosen for this study. On this day a high pressure system was situated over Greece, and the synoptic forcing was weak. A sea breeze developed in the basin during the day, and the pollutants were transported out of the basin through the gaps between the mountains to the north and northeast. A northward gradient in ozone concentration was present both in the observations and the model results. Unhealthily high concentrations of nitric oxides and hydrocarbons built up during the night, when it was stably stratified and the wind speeds were low. Variations in parameters, such as terrain influence and deposition processes, are performed to illustrate the sensitivity of the model results. Deposition is shown to be important for the results, although the deposition velocities over an urban area are not well known. The atmospheric flow in the Athens area is, to a great extent, determined by the local terrain. Three main features in the flow that impose consequences on the air pollution in Athens are identified.