Abstract. 1 Introduction

Transcription

1 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. Petrakis^ "Department ofapplied Physics, University ofathens, 33 Ippokratous Street, GR Athens, Greece ^Institute of Meteorology and Physics of the Atmospheric Environment, National Observatory ofathens, GR 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 krn^ 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.

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

3 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 keylocations 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). Station Code Name GAR HOE EKT Table 1: Experimental layout Operational Recorded Parameters Period 18/6-28/7 U, DIR, T 23/6-29/7 U, DIR, T 24/6-29/7 U, DIR, T, Humidity A monostatic acoustic sounder at the National Observatory of Athens (NOA) for continuously monitoring the thermal structure of the atmospheric

4 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 half of the time period of the experiment. The main points drawn from the frequency distribution of the wind direction for every surface station are the following: During the night there is a western shift of the northerly flow at GAR station, probably in response to the thermal contrast between the cool inland region and the air masses affected by the presence of the Southern Evoikos Gulf to the east or due to adjustment of the surface flow to the synoptic flow. At HOE station, which is the most windy location, the wind direction distribution is essentially the same between day and night. Moreover, the two modes of the distribution lie along the major axis of the Athens Basin, confirming the well known fact that air flow in the interior of the Athens Basin is channeled. At the coast (EKT), the high frequency of the S-SW directions reflects the often occurrence of the Saronikos Gulf sea breeze. The persistence of southerly directions during night reflects the observations of the sea breeze blowing until early night (sometimes even until around midnight). In the centre of Athens (NOA), the high prevalence of northerly winds in conjunction with the reduced frequency of southerly flows in the daytime shows the effect of an opposing flow on the inland propagation of the sea breeze developed at the shoreline. The nocturnal southerly directions are explained as in the case of the coastal station. The comparison of the peaks of the day-time southerly directions at EKT and NOA stations yielded the rough estimate that about less than half of the sea breezes reach the centre of Athens during the experimental period, which is characterised by an offshore synoptic flow. 4 The Saronikos Gulf sea breeze As noted in Section 1, the present experimental period is interesting in that it offers the opportunity of observing sea breeze development under moderate ot strong offshore wind. Table 2 summarizes all sea breeze days at EKT, their duration, maximum intensity, maximum water vapour mixing ratio and additionally gives information on the larger scale flow and the time of arrival (if existing) of the sea breeze flow at NOA. The HOE measurements in the time period LST are taken to be representative of the large scale

5 Urban Pollution flow. For reference, the 0000 and 1200 GMT radiosonde wind data at 700hPa from the Hellenic Meteorological Service (HMS) station at Hellinikon near EKT are included. The link of southerly winds at HOE with NW winds aloft due to the pressure driven channeling is demonstrated. The following conclusions are drawn: On 24 out of 34 days the sea breeze managed to develop at the coast with maximum intensity ranging between 2.1 and 6.1 ms~* and water vapour mixing ratio 9-18 gkg"\ Under the conditions encountered during the experimental campaign, a typical sea breeze at the coast reaches a maximum surface wind speed of 4 ms", carrying 13 gkg~* of water vapour. The maximum intensity is attained at around LST slightly later than for the pure (under weak synoptic flow) sea breeze case. Table 2: Characteristics of the Saronikos Sea Breeze flow (wind speed in ms * and wind direction in degrees) EKT STATIC)N NOA STVITION Date Duration (LST) Max. wind speed Max. mixing ratio (gkg"') Duration (LST) Max. wind speed HOE wind speed / direction Nocturnal and noon radiosoiide data spee( 1/dir 25/6 26/6 28/6 29/6 30/6 1/7 3/7 4/7 5/7 6/7 7/7 11/7 12/7 13/7 15/7 16/7 17/7 20/7 21/7 22/7 26/7 27/7 28/ /30 5/40 4/200 5/200 3/220 7/50 7/20 7/20 3/180 7/210 9/30 6/200 4/220 6/50 7/30 9/20 10/10 9/30 6/210 12/30 8/30 9/30 13/315 6/270 21/315 18/270 15/335 14/355 17/355 10/315 8/335 10/335 20/270 9/315 6/315 8/355 4/355 2/25 5/270 4/45 8/355 9/335 7/315 14/295 22/295 12/315 17/335 14/355 17/335 14/270 18/355 12/270 16/270 9/315 9/355 8/355 9/355 5/355 11/270 1/355 10/355 9/335 The sea breeze developed at the coast even at a 12 ms" surface offshore wind at HOE. Inspection of the days when no sea breeze was formed at the coast has shown that the surface winds measured at HOE always exceeded 7 ms~* and was from There is a clear trend of the flow onset at EKT to be delayed as the offshore wind strengthens (Figure 2a). Figure 2b shows that when the sea breeze develops in the afternoon it endures for only 2-4 hours, otherwise it lasts for hours irrespective of the offshore wind intensity. Also, the

6 Urban Pollution surface flow at HOE seems to be more relevant to the determination of the possibility of the sea breeze development than the synoptic flow taken at 700 hpa. 14 (a) (b) _ 12- o D D Time of onset at EKT (LST) ' 1 ' 1 ' 1 ' 1 ' 1 ' I I 1 I I I 1 I 1 I 1 I 1 I time of onset at EKT (1ST) Duration at EKT (rrs) Figure 2: Time of sea breeze (a) onset at EKT as a function of the mean LST wind speed at HOE and (b) duration at EKT as a function of the onset time for the cases with background (synoptic) northerly surface flow at HOE a O D D D 0 The cases with southerly flow at HOE should be distinguished according to whether they result from a pressure driven channeling effect or from a truly southerly geostrophic flow. Only 30/6 and 6/7 fall into the second category. Both of them were early in developing sea breezes especially the 30/6 case, where the onshore flow was stronger than 6/7. The same did not hold true for the "channeled cases", although a westerly geostrophic wind field would tend to enhance the developing mesoscale pressure gradient and the sea breeze circulation. The change of air temperature and vapour content at the onset of the sea breeze at the coast is presented as a function of time of onset in Figure 3. When the sea breeze developed in early morning, the temperature drop was small, because the negative horizontal advection of heat was compensated by the large warming rate due to insolation in comparison with the noon hours, where the temperature drop was significant. A southerly synoptic flow ("S"- marked points in the diagram) before the onset of the sea breeze had the effect of advecting cool, moist air over land, thus minimizing thermal and humidity changes when the sea breeze developed. When the sea breeze developed around LST despite strong opposing synoptic wind (the "H"-marked points in Figure 3a), the warm land masses, that were presumably advected earlier over the sea, subsequently return with the sea breeze circulation leading to minor temperature changes, but significant water vapour increase. Concerning the flow observed at NOA, Table 2 shows that in the presence of an opposing flow only in 2 cases out of 15 did the sea breeze not reach the station. However, on both these days the sea breeze lasted for only two hours at EKT. The sea breeze arrival at NOA was marked by the directional shift, a short interval of stabilized temperature and no humidity changes, at least near the surface. Anyway, with surface station data it is difficult to identify 0

7 Urban Pollution a sea breeze event at least at NOA-and probably at other inland stations when southerly winds prevail The inodif ication of the vertical profiles o wind and temperature will be discussed in the next section. The maximun i Q (a),h (b) " D D S D S, o" 8-0 Dg D S D DS D 0 s i 7-0>0 6- D to ^ D o> O) 5- H ccd O D> 3- n D S p C 2-0 'E Temperature change at onset (*C) en 1,.i* 1 co, NJ 1 -^, o 1, - 1, 1, 1- " I" D S D n ' i 1 B S B S ra S i,,,,, Time of onset at EKT (LST) Time of onset at EKT (1ST) Figure 3: The change of air (a) temperature and (b) water vapour mixing ratio after the onset of sea breeze at EKT. The points marked by "S" denote cases with southerly surface flow at HOE during LST. The "H" cases refer to strong offshore flow at HOE station. wind speeds measured at NOA are comparable if not greater than at the coast, but this could be partly attributed to the speed-up effect of the hill where the NOA station operates. Also, there is usually a drop in the intensity of the opposing flow during the hour before the arrival of the sea breeze. The next point that is important to be resolved is the possibility of the Saronikos Sea breeze to reach HOE or GAR. Prezerakos [6] studied pure sea breeze days (weak synoptic conditions) over the Athens Basin using surface data over a 25-year period. Among the meteorological surface stations he used was the one at Tatoi Airport, approximately 2-3 kms NW of HOE. For July he found that when the Saronikos sea breeze had developed at the coast, it was observed at Tatoi between LST and LST with probabilities 17% and 19%, respectively. In 55% of the cases it was intermittent as concluded by 3-hours observations. Those frequencies of occurrence imply that we should be able to identify some sea breezes at HOE as well, although the period selected was windy with NE offshore wind. The analysis excluded the days with southerly flow and concentrated on the days when the sea breeze flow lasted for more than 3 hours. Three cases (28/6, 3/7 and 27/7) were found with some evidence of sea breeze arrival at HOE; on all of them the flow passed NOA at 1100 LST. On the 28/6, the surface early morning flow was from NE blowing at 5 ms~* and on the 3/7 it was from the same direction at 7 ms~*. As seen in Table 2, the sea breeze developed early at EKT, but the relatively strong opposing flow significantly delayed (by 3 hours) its arrival at NOA as compared to 30/6 when it arrived within 30 min. As a result, it reached HOE at 1300 LST on 28/6 and at 1800 LST on 3/7. The time evolution on 28/6 is shown in Figure 4. The 27/7 case is analysed in section 5. Figure 4 shows that prior to the arrival of the flow at HOE the wind was blowing from 60 at 5 ms~\ then the velocity

8 Urban Pollution 30 28/6/93 4:00 8:00 12:00 16:00 20:00 0:00 4:00 I 6 - "8., <D 4 - Q. T3 I 2^ (b) 4:00 8:00 12:00 16:00 20:00 0:00 4:00 4:00 8:00 12:00 16:00 20:00 0:00 4:00 Time (LSI) Figure 4: Time series of (a) temperature and water vapour mixing ratio (only at EKT), (b) wind speed and (c) direction at the surface stations for 28/6/93. dropped to 3 ms~*, the direction gradually shifted to 220 and the temperature dropped by 1 C. It is not clear if the wind regime after 1700 LST was still affected by the sea breeze. An interesting feature of the figure is the backing at EKT and veering at HOE. Similar patterns are seen in the 3/7 case (not shown). On the other hand, the GAR station is not affected by the Saronikos sea breeze, except for indication for an early afternoon arrival on 27/7, which is discussed in section 5. Therefore, under the experimental conditions, it seems that the opening between the Hymettos and Penteli Mts is not affected by the N-S thermal contrast during daytime.

9 Urban Pollution 5 Analysis of a case study (27/7/93) On the 27th of July 1993 the synoptic conditions over Athens Basin induced a weak northwest flow according to the nocturnal radionsonde at Hellinikon (not shown here) that was also apparent in the early morning surface wind direction at EKT, GAR and HOE (Figures 5b and 5c). The coastal wind backed slowly to south directions, reaching 210 at 1200 LST indicating the onset of the Saronikos sea breeze flow. According to the noon radiosonde profiles (also not shown here) the sea breeze flow was confined below 1000 m above which the synoptic northwest flow persisted. 27/7/93 ~~i i i i i i i i i i i r koo 8:00 12:00 16:00 20:00 4:00 i 1 -o ; ~i r 4:00 8:00 12:00 16:00 20:00 0:00 4: l I I I I I I I I I I 4:00 8:00 12:00 16:00 20:00 Time (LST) 0:00 4:00 Figure 5: As in Figure 4 for 27/7.793.

10 10 Urban Pollution Due to the sea breeze cool air advection, the EKT temperature did not rise in early morning (in contrast to the inland GAR and HOE stations) and the water vapour mixing ratio was as high as 14 gkg (Figure 5a). The wind direction at the rest surface stations changed to south directions with a time delay relative to the coast. Thus, it seems that the sea breeze arrived at NOA, GAR, and HOE at about 1130, 1430, 1900 LST, lasting until 2100, 2000 and 2100 LST, respectively. Actually, the GAR wind direction data snowed that the sea breeze endured for only half an hour ( LST: south to southwest wind directions) and after that time period it apparently interacted with the Evoikos Gulf sea breeze (southeast wind directions). The arrival of the Saronikos sea breeze at the surface stations was combined with an increase of wind speed up to 6 ms~* at GAR, 5 ms~* at NOA, and a delay of the fall off of wind speed at the afternoon (4 ms") at HOE, a stabilization of the temperature at NOA, and a temperature decrease at GAR (from 32 to 30 C) and HOE (rapid fall). After 1500 LST the intensity of the sea breeze flow at the coast increased (5 ms~* at 1800 LST). Later, the wind there shifted to east directions and declined after 2200 LST, indicating the destruction of the flow. (a) wind speed (m/s) (b) wind direction (deg) Time (LST) (c) potential temperature (*K) Time (LST) (d) water vapour mixing ratio (g/kg) Time (LST) Time (LST) Figure 6: Time-height plot of (a) wind speed, (b) wind direction, (c) potential temperature and (d) water vapour mixing ratio obtained by the balloon flights at NOA on 27/7/93.

11 Urban Pollution 11 Figure 7: The facsimile record by the acoustic sounder operating at NOA 27/7/93 during (a) LST and (b) LST. on & 2> I downdraft ^ 27/7/93 323m updraft 187 m 85 m 51m :30 10:42 10:54 11:06 11:18 11:30 Time (LST) Figure 8: The vertical velocity data by the acoustic sounder operating at NOA on 27/7/93 for the time period 1030 to 1130 LST (the sea breeze front passed by the station at 1100 LST)

12 12 Urban Pollution The evolution of the sea breeze was also depicted in the rawinsonde data obtained from the balloon flights at NOA station (Figures 6a-6d). The early morning surface temperature inversion was slowly destroyed from below and the wind aloft was northerly. The sea breeze arrival at about 1100 LST was evident in the wind direction ( ) along with a low level jet of 6 ms~* below 100 m (it is noted that the NOA station is on the top of an 110 m high hill). The inflow of cool air caused the formation of a stable layer above a convective 50 m deep layer. The sea breeze cell extended up to 650 m at midday (Figures 6a-6b). The water vapour mixing ratio (Figure 6c) did not seem to increase significantly with the arrival of the sea breeze (probably because of the 6 km inland penetration of the sea air and the corresponding drying-out) until late afternoon (1900 LST), when the thermal structure of the atmosphere became almost neutral. The sea breeze front arrival (exactly at 1100 LST) was evident in the facsimile record of the acoustic sounder as a thermal structure extending up to 400 m (indicated by an arrow in Figure 7a). The vertical velocity offshore "' --.._ (a) city centre (_.. \ ( D S wind speed (m/s) 700 -T C " / wind direction (deg) wind speed (m/s)?nn -T nonn Uouu-ujuu nnnn LO, OTi city centre LST / LST n wind direction (deg) Figure 9: The rawinsonde data: (a) wind speed and (b) wind direction obtained by the balloon flights at the open sea (offshore), near Aegina island, and the corresponding inland (city centre) ones on 27/7/93 during the time periods LST, LST and LST.

13 Urban Pollution 13 measurements by the acoustic sounder showed a downdraft (1.5 ms~*) just before ^the arrival of the sea breeze front and an extended updraft (2.5 ms") just after (Figure 8), like a gravity current as described by Simpson et al [7]. The plumes' activity changed to a surface shear layer after 1500 LST (the convective surface layer was suppressed) according to the facsimile record in Figure 7b. As vertical mixing presumably ceased after sunset, an elevated echo layer remained distinguishing the upper boundary of the formerly inflow region from the large scale structure. Figures 9a-9d includes vertical profiles over the city centre (NOA) and offshore (see Figure 1) for about the same time period. In early morning ( LST), a 200 m deep, stable surface layer of NW winds was found over the city, featuring a low-level jet, while northeast winds prevailed aloft in a less stable layer. On the other hand, over the sea, the wind profile was uniform from northerly directions. The air temperatures over the two areas approached each other at around 600 m. Later, in mid-morning ( LST), the near-surface stability over the city was eroded from the thermal activity and the layer of NW winds became deeper 700 g g 400- JC. o> j! 300- offshore (c) 700 f 300- g 400- g> potential temperature ( K) offshore (d) 700 i i i i r water vapour mixing ratio (g/kg) g 400 o> J potential temperature ( K) city centre city centre water vapour mixing ratio (g/kg) Figure 9 (continue): The rawinsonde data: (c) potential temperature, and (d) water vapour mixing ratio obtained by the balloon flights at the open sea and the city centre.

14 14 Urban Pollution and well-mixed. A shallow sea breeze flow (below 100 m) had developed offshore. In the early afternoon ( LST), the sea breeze circulation covered 600 m and at least 500 m offshore and over the city, respectively. An interesting feature was the drying and warming aloft over the sea relatively to the city. This could be the effect of subsidence over the sea, caused by the removal of cool air by the sea breeze. According to the above, the balloon flights at the open sea reveal a different sea breeze cell which seems to dominate over synoptic flow after 1300 LST with a wind shift to south directions up to 600 m and a low level jet (below 100 m). Thus, the onset of the sea breeze flow observed earlier at the EKT station seems to correspond to a small local sea breeze cell (like the ones observed by Physick & Byron-Scott [5] in south Australia), while the regional (Saronikos Gulf) cell is delayed and possibly correspond to an increase of the wind speed at EKT and a change in the thermal structure at NOA after 1500 LST (see Figures 6c and 8b, respectively). 6 Conclusions The main objective of the oresent study was to identify the features of the development of the Saronikos Gulf sea breeze against an opposing (offshore) synoptic flow as well as important topographic and thermal effects on the evolution of sea breeze circulation. It is seen that for 70% of the experimental days the sea breeze developed at the coast and it is significantly delayed by the strongest offshore wind cases. In such cases, the frontal characteristics are enhanced. A typical sea breeze peaks at around LST at the coast When the sea breeze developes in early morning, the temperature drop is small, because the negative horizontal advection of heat is compensated by the large warming rate due to insolation in comparison with the noon hours, where the temperature drop is significant. A southerly synoptic flow before the onset of the sea breeze has the effect of advecting cool, moist air over land, thus minimizing thermal and humidity changes when the sea breeze developes. When the sea breeze developes around LST despite strong opposing synoptic wind, the warm land masses, that were advected earlier over the sea, subsequently return with the sea breeze circulation leading to minor temperature changes, but significant water vapour increase. Also, when the sea breeze develops in the afternoon it endures for only 2-4 hours, otherwise it lasts for hours irrespective of the offshore wind intensity. Roughly, half of the sea breezes reach the centre of Athens city and they usually persist until midnight. The onset of the sea breeze is marked by a directional shift, a short interval of constant air temperature and no humidity changes, at least near the surface. The sea breeze direction at the coast is observed to continuously back in the course of the day, whereas in the centre of Athens it blows from the south-southwest directions. Ten percent of the observed cases at the coast reach the northern part of the Athens Basin in the late afternoon. Also, it seems that the opening between the Hymettos and Penteli Mts is not affected by the N-S thermal contrast during daytime under the certain experimental conditions (etesians with N-NE wind direction). Finally, the mechanism of pressure-driven channeling is proposed to be important in producing low level southerly flows inside the basin in the

15 Urban Pollution 15 presence of west-northwest synoptic flow. This situation complicates the depiction of sea breeze flows in the city centre. The balloon flights at the open sea reveal a regional sea breeze cell which seems to dominate over synoptic flow after noon, while the onset of the sea breeze flow observed earlier at the coast corresponds to a small local sea breeze cell References 1. Asimakopoulos, D.N., Deligiorgi, D.G., Drakopoulos, C, Helmis, C.G., Kokkori, K., Lalas, DP., Sikiotis, D. & Varotsos, C An experimental study of nighttime air-pollutant transport over complex terrain in Athens, Atmospheric Environment^ 1992, 26B, No 1, Kallos, G., Kassomenos, P. & Pielke, R.A. Synoptic and mesoscale weather conditions during air pollution episodes in Athens, Greece, Boundary Layer Meteorology, 1993, 62, Lalas, D.P., Asimakopoulos, D.N., Deligiorgi, D.G. & Helmis, C.G., Sea breeze circulation and photochemical pollution in Athens, Greece, Atmospheric Environment, 1983,17, Lalas, D.P., Tombrou-Tsella, M., Petrakis, M, Asimakopoulos, D.N. & Helmis, C.G. An experimental study of horizontal and vertical distribution of ozone over Athens, Atmospheric Environment, 1987,12, Physick, A.L. & Byron-Scott, R.A.D. Observations of the sea breeze in the vicinity of a gulf, Weather, 1977, 32, Prezerakos, N.G. Characteristics of the sea breeze in Attika, Greece, Boundary Layer Meteorology, 1986, 36, Simpson, I.E., Mansfield, D.A. & Milford, J.R Inland penetration of sea breeze fronts, Quart J. Roy. Meteor. Soc^ 1977,103, Soilemes, A.T., Helmis, C.G., Papageorgas, P.G. & Asimakopoulos, D.N. A tethered balloon profiler system, Meas. Sci. TechnoL, 1993, 4,