Abstract. 1 Introduction

Transcription

1 On the atmospheric stability in the Athens Basin M. Petrakis,* P. Kassomenos,* S. Lykoudis,* V. Synodinou^ "National Observatory ofathens, Institute of Meteorology and Physics of the Atmospheric Environment, P.O. Box 20048, GR Athens, Greece *NRSC Democritos, Institute ofnuclear Technology and Radiation Protection, Agfa Par as kevi, Attiki, Greece Abstract Atmospheric Stability is one of the most significant factors in studying dispersion conditions in an area. The Athens Basin is characterized by major environmental problems due to physiographic characteristics and pollution sources located in and around it. The study of Stability might give useful hints about the local circulation systems. For the study of Atmospheric Stability in Athens a data base has been constructed, from two years' measurements of stability factors, at two locations in the Athens Basin, one in the National Observatory of Athens and the other in the Nuclear Research Center "Democritos". This data base was used to create the stability classes according to Pasquill-Gifford methodology. Based on the above measurements, a mass consistent model (NOABL) was applied in Greater Athens Area in order to describe the mean flow field for some of those stability classes. 1 Introduction Athens, located at the southeastern part of the mainland of Greece, is inhabited by nearly people in an area of nearly 450 Km 2. In the greater area of Athens a major number of industrial activities takes place, which in combination with the great number of vehicles (more than ) and the daily traffic degradation consist the main reasons for the serious problems concerning air quality in the area. The geography of the area is shown in Figure 1 with the three mountains surrounding the city and leaving the southern coastal area along with a narrow North-

2 178 Urban Pollution eastern passage as the only ventilation routes. The presence of these pronounced terrain features creates significant local flow fields. These local flow fields comprise the sea-breeze circulation cells, upslope/downslope flows and channeling effects [1]. Figure 1: Topography of the area of interest. Contours every 200 m starting from 100m. Due to the concentration of industrial and commercial activities in such a small area severe environmental problems with frequent high concentration of measured pollutants are quite usual. These environmental problems are associated with air-pollution episodes, especially during calm periods in the spring and summer months. 2 Atmospheric Stability The atmospheric stability is usually evaluated by identifying the atmospheric stability class. Many criteria for the classification of atmospheric conditions through the stability class evaluation have been proposed [2, 3] and/or reviewed [4, 5]. These schemes are generally applied to flat terrain.

3 Urban Pollution 179 In the present study we used the Pasquill criterion, as presented by Pasquill [6] and reported by IAEA [7], to identify the Pasquill stability classes, modified in such a way that the specific climatic conditions for the Athens area can be considered. This stability criterion has been applied at two locations with different landscape but quite close to each other. The first site is in the middle of the urban area of Athens, at the National Observatory of Athens (NOA), and the other at the east edge of the Basin, at the foot of Hymettos (1029 m). The scope of this work was to verify the validity of the above modified stability classification scheme for both urban and mountainous sites, not very distant to each other, and investigate whether an urban site can represent the whole complex region for stability purposes. 3 Topography and Instrumentation In this study meteorological data from two sites were used. The first station is that of NOA (LAT N, LON E ) which is located on a small hill, with an elevation of 107m above msl, near the center of the city. The other station (37,59*44 N, 23,49' 07 E) is located at a distance of approximately 12 Km Northeast of the center of Athens, in the Nuclear Research Center (henceforth NCRS) "Democritos" establishments. A number of continuous measurements in the last years by meteorological instruments mounted on a tower were taken. This tower is located at the foot of Hymettos at a height of 283 m above msl. The mountain is extended Southeast of the tower with various crests and slopes. The ground in the proximity of the tower is stony, covered by bushes and low trees at distances beyond 10 m (depending from the direction considered). The ground slope in the proximity is quite 20%. The tower is 84 m high and is an open structure, with an equilateral triangular cross section, of 2 m of side. It is constructed by tubular vertical legs. Three series of guys at different heights ( 24, 54 and 78 m) are used to balance the exerted wind forces. The tower is provided by 4 platforms at 18, 42, 66 and 84 m. The instruments have been mounted on tubular booms, one per platform, at a distance of approximately 4 m above the platform. Two cup anemometers, two wind direction sensors and two electrically aspirated thermometers are installed at the 18 and 84 m booms. A net radiometer has been installed on a 2 m high mast. The data are automatically collected and elaborated. The determination of Pasquill classes has been derived from the vertical temperature gradient between the two levels and the wind speed measured at the level 18m. The dataset used in this study includes hourly data covering the period from November 1989 to June 1990 from both the tower and NOA. Pasquill classes have also been computed by our modified IAEA program for both the tower and NOA. For this criterion the tower wind speed value is that measured at 18m.

4 180 Urban Pollution 4 Methodology and Discussion In the present study the Pasquill [2] criterion is used, in slightly modified form, to calculate the atmospheric stability class based on wind velocity, solar radiation and cloud cover data [22]. The modification concerns the following scheme. Instead of Pasquill initial values the following intervals of wind speed were used: <2, 2-2.5, 2.5-3, 3-4, 4-6, >6 m/s. For the solar radiation and cloud cover the following intervals (in langleys/h) and in octals have been used: Solar Radiation :>50, , , , <12.5, and Cloud cover : >=4, <=3. The reason for the finer subdivision intervals of the wind speed and solar radiation data (RD) is that, according to the IAEA tables, with the initial division for Pasquill stability classification, two values of stability class for the same interval can be computed and as a result this can sometimes produce dubious results. For example for wind speed < 2m/s and 50>RD>=25 langleys/h we have both A and B values for the stability class. In dispersion calculations though one has to be able to define one stability class per interval for further calculations. Since large values of wind speed and small values of solar radiation correspond to stable conditions the wind speed and solar radiation were subdivided into smaller intervals in such a way that increased wind speeds and reduced solar radiation values, correspond to a more stable atmospheric condition. Thus the stability classification schemes for day and night conditions are computed in Synodinou et al. 1995, Tables 1 and 2, [8]. Based on the above mentioned tables the frequency of appearance of the stability classes in the tower are presented in Table 1. Table 1. Frequency of occurrence of stability classes estimated with the modified Pasquill criteria. Stability Class A B C D E F Frequency of Appearance (%) From Table 1 it is apparent that more than 70% of the hours were assigned to stability classes D, E and F. These stability classes are associated with the establishment of local circulations. Due to the significance of the local circulations and their association with the air pollution episodes in Athens this study is concentrated in the

5 Urban Pollution 181 correlation of the extracted stability classes, with the prevailing mean wind flow field in the area. In order to describe in a better way the flow field in the Attic peninsula the NOABL model was used for different stability classes associated with the establishment of local circulations (e.g. D-F). Neutral Atmospheric conditions (Stability class D). For the convenient presentation of wind flow two cases were discussed. One with SW and another with NE wind of 2 m/sec surface wind. These directions represent the prevailing wind directions in Athens. When the mean wind is from Northeastern direction, the wind flow over the Athens basin is from the same directions too. The winds are very light over the mainland (less than 1 m/s) while over the sea speed up (but from the same direction). It is apparent that over the mountains the wind is stronger compared to the wind over the plane, but still remains light enough. A slight modification of the wind flow is evident near the foothills of Parnitha. With a SW direction the wind speed over the sea retains the initial value, while over the Athens basin is gradually diminishing. There is no significant veering of the wind flow when it approaches the foots of the mountains, but over the Hymmetus becomes stronger (Figure 2) (a) (b) Figure 2 : Wind flow for Neutral Atmospheric Conditions for (a) NE, (b) SW mean wind flow.

6 182 Urban Pollution Moderate Stable Atmospheric Conditions (Stability Class E). With a mean wind flow from Northern directions (2 m/s, surface wind) the wind over the Basin is very light (less than 1 m/s) and from Northern directions. There is a speed up of the wind flow over the Hymmetos and Parnitha mountains and over the Saronic Gulf as well. When the mean wind flow is from SW directions the wind over the Athens basin is from southwestern or western directions very light in magnitude (less than 1 m/s). There is also a strengthening of the wind flow over the mountains and especially over Hymettos. A slight veering of the flow is apparent at the seashore and near the northern foothills of the Hymettos, near the Agia Paraskevi opening as well. Over the sea the wind speed increases (Figure 3) (a) (b) Figure 3: Wind flow for Stable Atmospheric Conditions for (a) NE, (b) SW mean wind flow. Very Stable Atmospheric Conditions (Stability Class F). When the mean flow is from Northern directions over the Athens Basin the wind speed approaches calmness in the Northern part of the Basin and very light in the central and southern part. Near the openings of Agia Paraskevi and Ano Liossia there is a considerably different wind flow. In the passage of Ano Liossia the wind flow is from eastern directions while the same is true for the passage of Agia Paraskevi. Thus the wind flow enters Athens Basin coming from the Mesogia plain, while air masses are leaving Athens Basin to the Triassion Plain. Over the mountains and the

7 Urban Pollution 183 sea the wind speed is getting stronger. Some anavatic winds are also evident in the Hymettos mountain. When the mean wind flow was from SW directions air masses are entering from the Thriassion plain to the Athens basin while, in the same time, air masses are leaving it, moving to the Mesogia Plain through the two passages mentioned above. Over the Athens basin the winds are very light and veering, driven by the topography, at the southern foothills of Hymettos near the seashore as well as at the northern part of the Basin (Figure 4). There is also a veering at the seashore when the wind flow approaches it. A speed up of the wind flow approaches it. A speed up of the wind flow over the mountains even in the hilly ones of Western Attica is also evident (a) (b) Figure 4: Wind flow for Very Stable Atmospheric Conditions for (a) NE, (b) SW mean wind flow. 5 Conclusions The NOA and the NRC are situated in such way -center of the city and Northeastern part of it- so that measurements of the different meteorological parameters and calculation of the atmospheric stability can give a general idea about pollutant dispersion. The correlation of stability classes and wind direction is of primary importance to the study of pollutant transport since the location of NCR

8 184 Urban Pollution is near the Northeastern opening of Attica and this passage presents a ventilation opening for the basin of Athens. For this reason the mean flow fields are simulated for those stability classes associated with the establishment of local circulations in the area. Pollutants emitted in the center of Athens under favorite meteorological conditions and Southern sea breeze could be transported toward the North or North-Eastern area of Athens basin and either accumulate there or escape through the passage to the plain of Mesogia. The topography of the area around Athens -elevated terrain in most places- is a crucial factor for the transport of air pollutants. The planning of a new airport though in the plain of Mesogia presents a future risk for the city of Athens under very stable atmospheric conditions and northern mean wind flow. Acknowledgments We acknowledge IAEA which provided to a part of the equipment of the tower, NCRS "Democritos" for the construction of the tower and the offer of the rest of the equipment and the personnel of NOA who kindly offer the NOA meteorological measurements. References [1] Carapiperis I. and Katsoulis B. On the study of sea-breeze in Athens during winter. (1977). Bull Greek Met. Soc [2] Pasquill F. The estimation of the dispersion of windborne material. (1961). Met. Mag. Vol. 90. pp [3] Turner D.B. A diffusion model for an urban area. (1964). /. Appl Met. Vol3. pp [4] Pasquill F. Atmospheric Diffusion. (1974). 2nd Edition John Wiley. New York. pp [5] Gifford F.A. Turbulent Diffusion-Typing Schemes: A Review. (1976). Nucl Saf. Vol. 17. pp [6] IAEA Safety Guides. Atmospheric Dispersion in Nuclear Power Plant Sitting. (1980). IAEA Safety Series No 50-SG-S3. [7] Synodinou V., M. Petrakis, P. Kassomenos and S. Lykoydis (1995). Atmospheric Stability and Atmospheric Circulation in the Athens Basin. Nuovo Cimento C. Under Revision.