Pergamon Atmospheric Environment Vol. 29, No. 24, pp. 3619-3632, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1352-2310/95 $9.50 + 0.00 1352-2310(95) 00199-9 NUMERICAL SIMULATION OF PHOTOCHEMICAL SMOG FORMATION IN ATHENS, GREECE--A CASE STUDY N. MOUSSIOPOULOS,# P. SAHMt and CH. KESSLER:~ tLaboratory of Heat Transfer and Environmental Engineering, Aristotle University Thessaloniki, 54006 Thessaloniki, Greece; $Institut fiir Technische Thermodynamik, Universit/it Karlsruhe, 76128 Karlsruhe, Germany (First received 15 May 1993 and in final form 20 March 1995) Abstract--High emission levels and the unfavourable topography are the main reasons for the alarming photochemical! air pollution levels in Athens. An analysis of available air quality data proves that air pollution levels in Athens are largely affected by local wind circulation systems. The most frequent of these systems is dominated by the phenomenon of the sea breeze. Severe air pollution episodes occur, however, primarily under synoptic situations leading to stagnant conditions in the atmosphere over Athens. Photosmog folxnation in the Athens Basin is studied with the photochemical dispersion model MARS. The implicit solution algorithm incorporated in MARS is characterized by a variable time increment and a variable order. This solver allows avoiding unnecessary operator splitting by a coupled treatment of vertical diffusion and chemical kinetics. In this paper, MARS is used to analyse the situation on 25 May 1990, a day for which very high air pollution levels were reported in Athens. The simulation results elucidate the characteristics of a photosmog episode under stagnant conditions in Athens. In general, the model results reproduce satisfactorily the observed air pollution patterns. Key word index: Urban air pollution, photosmog modelling, numerical algorithms. 1. INTRODUCTION As a consequence of population growth and indus- trialization, about 40% of the Greek population, 50% of the registered Grcek cars and 50% of the Greek industrial activities are concentrated in the Greater Athens Area (GAA). The associated high anthropo- genic emissions in conjunction with the topograph- ical and meteorological features of the GAA result in high air pollution levels. The visual results of atmo- spheric pollution, being called "Nephos" (a brown cloud over the city), made their appearance in the 1970s. Alarmingly elevated pollutant concentrations already threaten public health and at the same time cause irreparable damage to invaluable ancient monuments (Skoulikidis, 1983). In the last decades, sulphur dioxide and smoke (i.e. suspended particulates with a diameter less than 2/~m) were considered to be the most critical constitu- ents of the Athenian smog. As far as sulphur dioxide is concerned, in the period 1974-1985 a constant re- duction of pollution levels has been achieved with corrective interventions which focused on limitations in the use of heavy oil, reductions of diesel fuel sulphur content and periodic controls of combustions. Al- though during the last years an increasing trend of sulphur dioxide concentrations was observed in Athens, the corresponding pollution levels remain relatively low (YPEHODE, 1994). On the contrary, the smoke pollution levels in Athens remained high, in spite of a noticeable de- crease in the mean annual smoke concentrations achieved in the period 1984-1989 by a regular control of industrial furnaces. Mean smoke concentrations of the order of 150/~gm -s until 1990 forced the authori- ties to additional measures for a further reduction of smoke emissions which proved to be successful: in the year 1993 the mean concentrations were below the air quality standard of 80 #g m-3 (YPEHODE, 1994). From the above, it is clear that the antipollution strategy implemented in Athens before 1990 was to a large extent successful as far as sulphur dioxide and smoke are concerned. Unfortunately, these pollutants have long since ceased to be the main characteristic of the Athenian smog. At present it is generally realized that the Athenian smog is predominated by photo- chemical oxidants (cf. Mantis et al., 1992), that is chemical substances formed in the atmosphere under the influence of solar radiation, when nitrogen oxides and hydrocarbons (and to a lesser extent carbon mon- oxide) are present (Giisten, 1986). Until recently, the concentration levels of the main photochemical oxidants, i.e. nitrogen dioxide and ozone, have been dramatically increasing. As an example, the monthly 3619