Athens, Greece - WIT Press Environment, National Observatory of Athens, GR-11810 Athens, Greece Abstract

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Text of Athens, Greece - WIT Press Environment, National Observatory of Athens, GR-11810 Athens, Greece...

  • Features of the Athens Basin wind flow in

    view of recent experimental work

    D.N. Asimakopoulos,* C.G. Helmis,* K.H. Papadopoulos,*

    J.A. Kalogiros,* A.T. Soilemes," M. Petrakiŝ

    "Department of Applied Physics, University of Athens,

    33 Ippokratous Street, GR-10680 Athens, Greece

    Înstitute of Meteorology and Physics of the Atmospheric

    Environment, National Observatory of Athens, GR-11810

    Athens, Greece

    Abstract

    According to past experience, the nearly stagnant conditions caused by the presumed equilibrium between the Saronikos Gulf sea breeze and an opposing synoptic flow is identified as the principal mechanism leading to high pollution episodes in Athens during the summer. Previous experimental work could not answer several questions related to air mass flow over the "transitional" areas of the Athens Basin such as the offshore, the coastal regions and the natural openings. In this context, recent experimental work focuses on the inland propagation of the southerly sea breeze from the coast to the northern part of the basin mainly under moderate northerly background winds. An experimental campaign was designed to cover the basic key-locations revealing the wind flow over the Athens Basin. With the aid of a network of meteorological stations, two tethered balloons and a high range acoustic sounder operated over a two months summer period it is attempted to address some features of the wind flow over the Athens Basin. Results from this experimental campaign are presented and discussed.

    1 Introduction

    The Athens Metropolitan Area (AMA) is a region of well-known air pollution problems caused by the concentration of industrial, transport and service activities in an area of 450 krn̂ inhabited by nearly 4 million people. The main residential and service activities area is found in the Athens Basin (Figure 1), a coastal valley washed by the Saronikos Gulf to the south. The Athens Basin is surrounded by the Hymettos Mt to the east, the Penteli and Parnitha Mts to the north and the Egaleo Mt to its west. The main pollutant source in the city centre is automobile traffic, while the west part of the basin is significantly affected by the industrial emissions.

    Transactions on Ecology and the Environment vol 6, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541

  • 2 Urban Pollution

    In such an area of complex topography, featuring sea to land, urban to rural and mountain to plain transitions, the contribution of local flows to the observed wind and thermal field is significant and actually dominates under weak synoptic pressure gradient The numerical and observational studies concerning air quality and atmospheric transport and diffusion mechanisms in AMA expectedly dealt with such periods of weak synoptic flow. Specifically, the pollutants' advection downtown Athens by the southerly Saronikos Gulf sea breeze and the transport from the Triassion Plain to the city centre are main factors of the air quality problem. In order to understand the different mechanisms of pollutant transport and advection, a number of field campaigns were organised in the recent years in the AMA. The first campaign by Lalas et aL [3] dealt with the Saronikos Gulf sea breeze circulation and provided strong evidence that air pollutants recirculate in the daily cycle of sea and land breeze, enhancing the next day's pollution levels. The second campaign by Lalas et aL [4] addressed the issue of the horizontal and vertical distribution of ozone over Athens and confirmed that ozone generated over the sea, significantly enhancing the ground concentrations at the coastal areas, but slightly at the city centre. The third campaign by Asimakopoulos et aL [1] studied the transport mechanisms from the industrial area of Thriassion Plain to the city of Athens using tracer gas and tetroon tracking techniques.

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    Figure 1: The Athens Metropolitan Area and the locations of all surface stations (height contours of 200 m).

    Transactions on Ecology and the Environment vol 6, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541

  • Urban Pollution 3

    Two advection paths were identified, the straightforward one through the northern opening between Mts Egaleo and Parnitha and the second one through the Daphni opening of Egaleo Mt (see Figure 1).

    In the course of the South European Cycles of Air Pollution (SECAP) program, an experimental campaign was designed to cover the basic key- locations determining the surface wind flow over the Athens Basin in relation to the crucial questions that have been appraised during past experimental and theoretical studies. Previous experimental work, as outlined in the preceding paragraphs, could not answer several questions related to air mass flows over the transitional areas of the Athens Basin such as the offshore, the coastal regions and the topographic openings of potential ventilation of the basin. The experimental conditions and period were such as to allow the analysis of the wind flow in the Athens Basin during the expected stagnant conditions caused by the presumed equilibrium between the Saronikos Gulf sea breeze and an opposing synoptic flow [2]. This mechanism is one of the two principal mechanisms leading to high pollution episodes in Athens and the major one during the warm period. According to this process, the prevalence of the local summer northerly winds (etesians) would delay or inhibit the onset of the southerly Saronikos Gulf sea breeze. The latter is rather insignificant since the domination of the moderate synoptic flow would improve air quality over the Athens Basin. The main objective of the present study is to assess the features of the former case. In particular, the study focuses on the inland propagation of the southerly sea breeze against a moderate offshore wind. It is worth mentioning that vertical profiles of the sea breeze over the Saronikos Gulf are presented, the first ones ever obtained in this area.

    2 Experimental setup

    Measurements span the period from 18/6/93 to 29/7/93. The surface meteorological stations installed are given in Table 1, and Figure 1 depicts their exact locations. Data were stored as means and standard deviations over 10 min intervals. In addition to the surface stations the following systems were used: • Two tethered meteorological profiler systems each consisting of a 4.3 m^ balloon and an instrumented measuring package for measuring profiles of wind speed, direction, temperature and humidity in the lowest 800 m of the atmospheric boundary layer over selected periods [8]. The periods of interest were those with a prevailing northerly, offshore wind (the warm period etesians' wind regime).

    Table 1: Experimental layout Station

    Code Name GAR HOE EKT

    Operational Period

    18/6 - 28/7 23/6 - 29/7 24/6 - 29/7

    Recorded Parameters

    U, DIR, T U, DIR, T

    U, DIR, T, Humidity

    • A monostatic acoustic sounder at the National Observatory of Athens (NOA) for continuously monitoring the thermal structure of the atmospheric

    Transactions on Ecology and the Environment vol 6, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541

  • 4 Urban Pollution

    boundary layer and the vertical component of the wind covering heights up to 800 m above the ground.

    The above data were supplemented by the hourly means of barometric pressure, air temperature, humidity, wind speed and direction measured by the permanent surface meteorological station operated by the Institute of Meteorology and Physics of the Atmospheric Environment of the National Observatory of Athens (NOA) on a small hill (110 m high) in the centre of Athens, close to the acoustic sounder antenna. The two balloon systems were flown on July, 27 at NOA and in the sea area northeasterly of Aegina island from a small sea vessel.

    3 General features of the wind flow during the experimental period

    The synoptic conditions were typical of the season studied with N-NE winds (the 'etesians') for more than hal