Pergamon Atmospheric Environment Vol. 27B, No. 4, pp. 353~370, 1993 Copyright © 1994 Elscvicr Scicn~ Ltd Printed in Great Britain. All rights reserved 0957-1272/93 $6.00 + 0.00 MODELING OF PHOTOCHEMICAL POLLUTION IN ATHENS, GREECE. APPLICATION OF THE RAMS-CALGRID MODELING SYSTEM CHRISTODOULOS PILINIS,* PAVLOS KASSOMENOS a n d GEORGE KALLOS University of Athens, Laboratory of Meteorology, Ippocratous 33, Athens 106 80, Greece (First received 21 October 1992 and in final form 30 April 1993) Almtmet--The causes of the poor air quality in Athens, Greece during the severe episode of 25-26 May 1990 has been studied, using a prognostic model (RAMS) and a three-dimensional Eulerian air quality model (CALGRID). The modeling effort indicates that the main urban area of Athens exhibited high concentrations of nitrogen oxides, the main sources of which are automobiles, while the NNE suburban area exhibited high ozone concentrations, the product of photochemical activity of the primary pollutants that were transported by the sea-breeze. The application of the models also demonstrated the need for an accurate emission inventory for improved predictions of the pollutant concentrations. It was also found that a 50% reduction of the nitrogen oxide emissions will increase the ozone levels in the downtown area substantially. Key word index: Photochemical modeling, ozone, mesoscale modeling, air pollution. INTRODUCTION The problem of air pollution in Athens has been of great concern for the last two decades, because the imposed state and Commission of the European Communities (CEC) limits on 03 and NO2 have been violated for a significant number of days, during all seasons every year (Kallos and Kassomenos, 1992; Kallos et al., 1993). Despite this fact, the air quality in Athens cannot be considered as the worst among other cities in Europe and the American continent. It is well known that the relatively high concentrations of air pollutants in the Greater Athens Area (GAA) are mainly due to a combination of factors, i.e. the physiographic characteristics of this region, the meteorological conditions and the sources of air pol- lutants (e.g. Lalas et al., 1982, 1983, 1987; Gusten et al., 1988; Katsoulis, 1988; Kallos et al., 1993). The concentrations of air pollutants within the Athens Basin are affected by the weather conditions occurring in the region as they are modulated by the regional and local physiographic characteristics. The role of some mesoscale circulations in the GAA on the formation of air pollution episodes (violation of the state imposed limits) has been the subject of several studies (e.g. Lalas et al., 1982, 1983, 1987; Prezerakos, 1986; Flassak and Moussiopoulos, 1989; Kallos et al., 1993). Although, most of these studies have empha- sized the role of the sea (land)-breeze mechanisms in the GAA, it has been found, recently, that this is not always the case (Kallos et al., 1993). In that study, the case-by-case analysis of the 80 worst air pollution episodes that occurred in Athens during the time period 1983-1990 showed that the worst air pollution episodes in Athens occurred during the days where the synoptic and local circulations in the Athens Basin were in critical balance and/or with warm advection in the lower troposphere. In contrast to the extensive experimental effort, and the limited atmospheric modeling that has been done to isolate the weather patterns that lead to high pol- lutant concentrations, the air quality modeling effort that relates the pollutant levels to emissions of pri- mary species is very limited (Moussiopoulos, 1990). As a consequence, the effects of the different state actions, e.g. restrictions of the traffic in the center of the city, the use of cars with catalytic converters etc., have not been evaluated. Besides there is the suspicion, that, while the traffic restriction reduces locally the genera- tion of primary pollutants, it may result in higher ozone concentrations, because of the smaller doses of NO, a major sink of ozone in Athens (Lalas et al., 1987). The purpose of our research program is to study in a systematic manner the major causes of the air qual- ity degradation in Athens, using state of the art atmo- spheric and photochemical modeling. In this paper the first results of this modeling effort are presented and several characteristic features of the photochemi- cal air pollution in Athens are explained. DESCRIPTION OF THE AREA *Current address: Environmental Quality Laboratory, California Institute of Technology, Pasadena, CA 91125, U.S.A. The cities of Athens, Piraeus and their suburbs are located in an oblong basin surrounded by high moun- tains from three sides and open to the sea from the 353