0 1995 by John Wiley & Sons, Ltd. Received ... situated on the top of a hill 107 m a.m.s.1. near the Acropolis far from the traffic and industrial areas of the city ...
ENVIRONMETRICS, VOL.6,349-361 (1995)
STATISTICAL EVALUATION OF SELECTED AIR POLLUTANTS IN ATHENS, GREECE H. D. KAMBEZIDIS Institute of Meteorology and Physics of the Atmospheric Environment, National Observatory of Athens. P.O. Box 2004%. GR-11%10 Athens. Greece
R. TULLEKEN Department of Air Pollution, Agricultural University, P.O. Box 8129, NL-6700 EV Wageningen, The Netherlandr
G . T. AMANATIDIS CEC DGXII, Rue de la Loi 200. B-1049 Brussels, Belgium
A. G . PALIATSOS Department of General Mathematics, Technological and Education Institute of Piraeus. P. Ralli and Thivon 250, GR-122 44 Athens, Greece
AND D. N. ASIMAKOPOULOS Laboratory of Meteorology. Department of Applied Physics, University of Athens, Ippokratous Str. 33. GR-106 80 Athens. Greece, and Institute of Meteorology and Physics of the Atmospheric Environment, National Observatory of Athens, P.O. Box 20048, GR-118 I0 Athens, Greece
SUMMARY A statistical evaluation of the air pollution situation in Athens for the year 1982 is presented. This analysis refers to NOz, NO, O3 and SO, concentration levels measured at the National Observatory of Athens (NOA) with a fully automatic monitoring station. The seasonal, weekly and daily variations of the above pollutants are in agreement with the corresponding activities in the city which are responsible for these emissions. Auto- and cross-correlation techniques are applied to the pollutants for January and July. It is found that all pollutants exhibit a well defined oscillatory behaviour because of their diurnal peaks in both months, except for NO2 in January whose auto-correlation function shows no regular shape. For the same reason the January NO-NO2 cross-correlation function behaves in the same way; the pairs NO-03 and N02-03 exhibit a clear oscillatory pattern. The spectral estimate method is also employed to the above pollutants for the two months in question. The energy content of 03,for instance, in July shows to be concentrated around a frequency corresponding to a periodicity of 24 hours. KEY WORDS
air pollution; statistical evaluation; urban air pollution; air pollutant statistics
1. INTRODUCTION
Air pollution is a serious problem in many heavily populated and industrialized areas in the world. For this reason many studies about individual air pollutants have appeared in the international literature. These studies tackle the problem in the area of concern and try to suggest CCC 1180-4009/95/040349- 13 0 1995 by John Wiley & Sons, Ltd.
Received November 1992 Revised September 1994
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Figure I . Map of the Athens basin. Residential areas are indicated by vertical lines and industrial areas by horizontal lines. Also indicated are the peaks of major mountains with their elevation in metres a.m.s.1. as well as the 100 m (dashed line) and 500 m (dot-dashed line) contours. K: Keratsini power plant
solutions. Statistical assessment of air quality and use of dispersion models are tools for all air pollution studies. Athens is a city of 3.5 million people (census of March 1991) located in an oblong basin of approximately 450 km2 area. It is surrounded by high mountains on three sides and the sea on the south, bisected by a series of small hills (Figure 1). Because of the concentration of industrial and commercial activities in this relatively small area, severe environmental degradation has taken place with high concentrations of measured pollutants and a serious visibility reduction with frequent appearance of a ‘brown’ cloud known in Greece as ‘nephos’. Many studies on this phenomenon have been carried out, e.g. by Giisten et al. (1988), Kambezidis et al. (1986), Kambezidis and Papanikolaou (1988), Lalas et al. (1987), Viras and Siskos (1992), and Zerefos et al. (1990). The Institute of Meteorology and Physics of the Atmospheric Environment (IMPAE, ex Meteorological Institute) of NOA is the first establishment in Athens which started monitoring
ATHENS AIR POLLUTANT STATISTICS
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air pollution with a network of six semi-automatic stations in 1968. This network was in operation until 1984. In late 1981 IMPAE started the operation of a fully automatic monitoring station. This effort was continued until late 1986 with some intermissions in the period 1983-1986. 1982 was therefore a year with complete records from the automatic station. NOA is situated on the top of a hill 107 m a.m.s.1. near the Acropolis far from the traffic and industrial areas of the city (see Figure 1). Its location is almost ideal for taking air pollutant average concentration measurements. The aim of this study is to give a statistical evaluation of NO2, NO, O3and SO2 from the NOA automatic station for the year 1982. This year was chosen because of the uninterrupted and reliable data from this automatic station, the first installed in the basin. Moreover, the data sets from NOA are less affected by local air pollution sources, thus giving an illustration of the mean situation. It has been reported (Zerefos et al. 1989, 1990; Ziomas et al. 1989) that air pollutants in Athens show an increasing trend until almost 1990, e.g. NO2 20 per cent per year, SOz 10 per cent per year and O3 25 per cent per year in the period 1984- 1989 on average from four stations in the Athens basin. Nevertheless their annual features are not anticipated to change drastically with time. This is so because the state-imposed measures for emission control affect air pollutants concentration levels, but they do not influence their qualitative characteristics in general. Two methods have been employed in this paper. The first is the application of auto- and crosscorrelation techniques to the air pollutants data sets for January (mid-winter month) and July (mid-summer month). These correlation functions reveal periodicities for one (auto-) or a pair (cross-) of the air pollutants up to almost one week (144 hours). Kambezidis and Papanikolaou (1988) and Lalas et al. (1982) have applied these techniques to SO2 only; this work extends the idea to more pollutants. The second method of the paper is the employment of the spectral estimate technique in the Athens air pollution for January and July 1982. This gives information about the air pollutant energy content distribution in the time domain. Both methods, though applied in the Athens case, are of more generalized interest to air pollution workers. The months of January and July were chosen throughout this paper to contrast the air pollution patterns from the climatic point of view. Additionally, maximum values of individual pollutants are recorded in Athens during the winter and summer periods (PERPA 1980, 1989). 2. CLIMATE AND AIR POLLUTION SOURCES IN THE ATHENS BASIN The climate of Athens is mediterranean with hot, dry summers and wet, mild winters. The average daily winter (December, January, February) temperature is 9.9"C and 25.8"C for the summer (June, July, August). The average annual rainfall is about 418 mm; most of it occurs in early October and the winter months, but in the summer some infrequent, severe local thunderstorms of short duration may also give appreciable precipitation. Global solar radiation is rather strong with average diurnal values (on a horizontal surface) of the order of 22 MJ m-* in the summer and 8 MJ m-2 in the winter. Sea breeze cells develop along the main SW-NE axis of the basin in late spring and the summer. The effect of sea breeze circulation to air pollution of cities situated near coastal regions is a rather complicated phenomenon, especially when the terrain is not flat. This effect has been investigated by various researchers (Ganor et al., 1978; Kambezidis et a[., 1994; Lalas et al., 1983; Lyons and Cole, 1976; Steinberger and Ganor, 1980) and was the subject of a recent international NATO/CCMS Pilot Study Workshop (Melas et al., 1995). Table I gives comparative NO,( = NO2 + NO) and SO2 emission rates in the Athens basin for 1973, 1978 and 1983. The total annual emission for SO2 decreases from 1973 to 1983. This is so because heavy residual oil with 3.5 per cent sulphur content was progressively replaced by diesel
H.KAMBEZIDIS ET AL.
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Table I. NO, and SO2 annual emission rates (t a-') ~
~
Air pollutant NO,
Year
Central heating Vehicles
1973
1978
1983
4,860 (17.2) 15,312 (54.0)
3,322
1,391 (5.4)
(9.3)
22,763
17,400 (67-0)
(28.8)
(63.7) 9,626 (27.0)
28,322 (100.0)
35,711 (100.0)
Central heating
14,050
11,999 (1 6.9) 7,668
3,690 (20-7)
Vehicles
(1 6.4) 12,245
(10.8) 5 1,262 (72-3) 70,929 (100.0)
(7.9) 12,696
Industry
8,150
Total
so2
~~
~
Source
Industry
(14.3)
59,410
(69.3) Total
85,705
(100.0)
7,181 (27.6) 25,972 (100.0)
1,410 (7 1.4)
17,796 (100.0)
The numbers in parentheses are percentage contribution of each source to the total amount. 1973 and 1983 values are from PERPA (1980, 1989) records, while 1978 data are OECD (1983) estimates.
of 0.5 per cent sulphur content in the period 1976-1982. Heavy residual oil was banned in November 1977 but this action gave only a slight decrease in the percentage for industry as far as SO2 was concerned. This may be attributed to the growth in local industry of about 11 per cent (Kambezidis and Papanikolaou, 1989; PERPA 1989). The increase in the number of vehicles, on the other hand, is appreciable, but its added contribution is partially offset by the alternate weekend driving regulation put in effect in 1979 and the modified day-after-day driving regulation put in power in 1981. (Under the first regulation, cars with license plates ending in odd numbers were allowed to operate one weekend, with even-numbered plates the next weekend; in this way on any weekend only half of the cars were in operation. Under the second regulation, on any day of the week cars with plates ending in digits 1-5 were allowed to operate, while the next day cars with plates ending in digits 6-0 operated. On weekends all cars were free. From 18 January 1988 a new modification of the previous regulations was put in effect; cars with even-numbered plates were allowed to operate on even dates and odd-numbered plates on odd dates.) The state has also planned to issue an 'exhaust gas control card' for every car in the Athens basin commencing October 1994. 3. DATA COLLECTION
As was mentioned in the introduction, an automatic (MONITOR LABS) monitoring station was installed at NOA to measure the basic air pollutants. The system comprised the following measuring units: (i) SO2 analyser (model 8850); (ii) NO, analyser (model 8840); (iii) O3analyser (model 8410E), and (iv) CO analyser (model 83 10). All these units were calibrated every midnight via a calibrator (model 8500); the above pollutants were measured with a sampling rate of 1 scan min-' . These digitized measurements were preprocessed via a data logger (model 7000) and then recorded on magnetic tape for further processing. There was a continuous record on chart recorder for visual display and back-up purposes. The system maintenance was regular according to the manufacture directions to assure compatibility of the measurements.
ATHENS AIR POLLUTANT STATISTICS
353
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.~ n u a variation l of monthly means in (a) NO, NO2, 03,and (b) SO2concentrations at
IOA for 1982
The one-minute measurements were used in this study to derive hourly means of NO2, NO, O3 and SO2 for the whole of 1982. This year was the first without Keratsini power plant (Figure 1) operating. Its cessation caused a drastic decrease in SO2 levels in the Athens basin (OECD 1983). 4. RESULTS AND DISCUSSION
The seasonal variation of 03,NO and NO2 observed at NOA during 1982 is shown in Figure 2(a). 0,presents a strong seasonal variation with a maximum during the summer. In the presence of NO,, HC and intense solar insolation, photochemical production of O3becomes the dominant process during this period (Seinfeld 1975). In the cold period of the year (October to April) NO, present maximum values when NO and NO2 emissions are higher. In addition, the lower mixing height during winter contributes significantly to higher NO, concentrations. The residence time of NO in the atmosphere during the summer is low and therefore this pollutant is rapidly converted to NO2 and consequently exhibits a minimum value (Stern et al., 1973). Conversely, during the wintertime, NO becomes the dominant oxidant because it is not easily transformed to NO2 (see also the daily variation in Figure 3(a)). The seasonal variation of SO2 presents maximum values during the cold period because of the higher discharge rates from the central
H. KAMBEZIDIS ET A L .
354
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..........................
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Figure 3. Daily variation of hourly means in NO, NO2, and O3 concentrations for (a) January and (h) July at NOA for 1982
heating systems. These higher emissions are confined below lower mixing heights and therefore high SO2 concentrations are observed (Figure 2(b)). Mean daily variation of the above air pollutants is shown in Figure 3. Peaks of NO at 09.00 and 21.00 LST appear in January (Figure 3(a)). The morning and evening peaks are due to the traffic jams in the city centre and the various activities in the town. A small peak in 0 3 appears at 14.00 LST, i.e. with a time lag of 5 hours from the NO peak. However, its values are low because of the low photochemical production during the cold period of the year. The NO2 does not exhibit a daily variation because NO is not rapidly transformed to NO2 during wintertime, as already explained, In July, the morning NO peak occurs at 07.00 LST because the activities in the city start earlier in association with the longer daylight period; the secondary maximum disappears (Figure 3(b)) because of the low NO, emissions during the afternoon in summer. The transformation rate of NO to NO2 is now high. Because of the high solar insolation during the daytime period, the photochemical O3production rate is high and this air pollutant exhibits a broad maximum. Such a broad maximum O3 concentration level has also been reported by Giisten et al., (1988) during the summer of 1984 not only for NOA, but also at the top of the hill
ATHENS AIR POLLUTANT STATISTICS 0.14
355
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Figure 4. Daily variation of hourly means in SO2 for January and July at NOA for 1982
of Lykabetus (300 m high) near the city centre, at the Piraeus harbour as well as on the north side of Aegina island (30 km to the south of central Athens, see Figure 1). This study claims an O3 transport attributed to the sea breeze flow which appears quite often in Athens (more than 30 per cent of the days in the spring and summer months (Catsoulis 1983; Prezerakos 1986). Similar results have also been found in a study by Lalas et af., (1983). In the case of SO2 (Figure 4) January exhibits two maxima at 09.00 and 21.00 LST. The evening peak is greater than the morning one since during night-time more central heating systems are in operation than in the morning. The July morning maximum is mostly attributed to the traffic jam in the city centre. The SO2 levels during July remain low, because the central heating systems are not in operation. The auto-correlation functions (ACF) for each of the pollutants for the months of January and July are shown in Figure 5. The maximum time lag in all cases is 144 hours, i.e. 6 days. January ACF for NO2 does not present any periodicity, since this pollutant does not show any significant daily variation (Figure 3(a)).It is noteworthy here that in all cases of NO2 and NO ACFs, their auto-correlation coefficient (ACC) is almost positive. This fact may indicate the presence of a long-term component in their time series. To the contrary, January NO (Figure 5(b))shows an oscillatory behaviour with a period of 12 hours due to its two daily peaks which are half a day apart (see Figure 3(a)). The July NO2 pattern changes to a 24-hour periodicity (Figure 5(a)) apparently due to its sole (morning) peak (see also Figure 3(b)). Another important feature is the big amplitude of the ACFs in the warm period. This fact is attributed to the absence of a secondary maximum that appears during the cold months (e.g. January) in the daily series; corollary of this absence is the change in the ACF’s frequency (pattern). Figure 5(c) gives the ACFs of SO2 for January and July. During the winter months the central heating switching-on status occurs twice a day, resulting in a 12-hour periodicity. In the warm period of the year a 24hour oscillation in the ACF occurs due to the morning SO2 peak. This means that during the warm period (e.g. July) a daily maximum value in SOz concentration is always anticipated early in the morning, mainly because industry starts. In the case of O3 (Figure 5(4) the ACF in the warm period exhibits a sinusoidal ‘pendulum decay’ pattern during the 6-day period. Its ACF exhibits a 24-hour oscillation due to the intense diurnal cycle of the solar insolation. The
356
H. KAMBEZIDIS ET AL.
Figure 5. ACC for (a) NO2, (b) NO, ( c ) SO2, and (d)O3 at NOA in January and July 1982
minimum values of ACC exhibit also a 24-hour periodicity which is due to the daily pattern of O3 that appears in the warm period of the year (see Figure 3(b)).The appearance of this dominating broad maximum which is extended throughout the daylight period of the day modifies the ACF to resemble a 'pendulum oscillation' with no amplitude loss even after the sixth day. In the cold period (e.g. January) NO and O3 show two maxima. For this reason, the 0 3 ACF exhibits a double oscillation pattern within the day. Lalas et al., (1982) have found a 7-day periodicity for the ACF of SO2 in the period 1974-1979, while Kambezidis and Papanikolaou (1988) found a 6-day period during 1973-1982; both results come from the analysis of the data sets from the NOA network with the six semi-automatic stations in Athens. Figure 6 gives the cross-correlation function (CCF) for some pairs of air pollutants. The NONO2 CCF for January (Figure 6(a))shows no particular behaviour because of the constancy in NO2 and the strong diurnal variation in NO; their cross-correlation coefficient (CCC) is therefore slightly negative or near to zero. In July when NO (Figure 3(b)) has a diurnal one-peak cycle, NO2 exhibits a secondary peak at around local midnight. Their CCF exhibits a well defined 24hour period oscillation with a hint of a secondary peak which differs from the main one a time lag of 8 hours in the first 48 hours (2 days). Afterwards, the main peak becomes progressively dominant. Figure 6(b)shows the CCF in the case of NO-03. In January, NO and O3exhibit a 12hour oscillatory behaviour because of their diurnal cycles (Figure 3(a)). In July, NO and O3 exhibit a daily cycle which is attributed to the 24-hour difference between NO and O3maximum
ATHENS AIR POLLUTANT STATISTICS
-0.8
.
dc
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7i
m
Tlme la# (houn)
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and minimum values (07.00 LST, see Figure 3(b)).Furtheremore, the N02-03CCF (Figure 6(c)) can be interpreted as for the NO-03 case; that is, in January their CCC remains around zero, while in July a strong 24-hour oscillation occurs. The Blackman and Tukey (1958) method of spectral analysis was applied to the time series of the four pollutants for January and July separately in order to reveal periodicities in the energy content. Figure 7 shows the spectral estimates (SE) of the pollutants in consideration in the periods range: infinity-2 hours. (The units in the horizontal axis in these figures were originally in lags from 0 to 240. The lags were then converted into time, in hours, via the relation: time = 2 x 240/number of lag, which for lag = 0 gives time = Infinite hours and for lag = 240, time = 2 hours.) The smooth solid and dashed lines in each figure represent the upper 95 per cent confidence limits for the spectra; the 95 per cent solid line refers to the 'dashed' spectrum and the 95 per cent dashed line to the 'solid' spectrum. The spectral peaks standing out of their respective confidence limits imply confidence higher than 95 per cent and are therefore of major statistical significance. The NO2 SEs (Figure 7(a))in both months, but more pronounced in July, show significant contribution to their energy content at periods of 24, 12, 8 and 6 hours, respectively. This is in agreement with the July NOz ACF (see Figure 7(a))where the ACC (Figure 5(a)) is higher at the period of 24 hours. The energy content of the January NOz SE is less than its July counterpart because NO2 has a low variability in winter. Periods of less than 6 hours in both months are
H.KAMBEZIDIS E T A L .
358
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Figure 7. SE for (a) NOz, @NO, (c) SO,, and (d)0,at NOA in January and July 1982. Smooth curves represent 95 per cent confidencelimits for the spectra. Each spectrum with the correspondingconfidence curve constitute a 'solid-dashed curve' pair. INF at the beginning of the horizontal axes means infinity
considered insignificant as far as the 95 per cent confidence limits are concerned. The NO SE (Figure 7(b)) shows one order of magnitude lower energy content in summertime than in wintertime; this is consistent with its greater variability during the winter months than in the summer. As far as the significant periodicities are concerned, these are of 12,8,6 hours in January and 24, 12,8,6,4.8and 4 hours in July. This indicates that finer NO structures are present within
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ATHENS AIR POLLUTANT STATISTICS
359
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a single day in both seasons, particularly in the summer, probably as a result of the local source variations (vehicles). An interesting feature is observed in the SOz SE (Figure 7(c)). Significant winter periodicities are those of 12 and 6 hours and summer ones of 24, 5.6, 4.4, 4, 3.8 and 3.4 hours. The winter pattern is strictly attributed to the dominating operation of the central heating systems. In the summer, these systems do not operate, and therefore the fine periodicity pattern may be attributed to the variation of other sources (fires from nearby forests, bakeries and small enterprises using residual oil as energy source). The O3SE (Figure 7(4) shows periodicities of 24,
360
H.KAMBEZIDIS ET AL.
12,8 and 6 hours in July related to the sea breeze/etesians interchangeability. (Etesians are rather strong summer northerly winds.) The January SE shows a sole significant peak at 12 hours related to the dominant daily O3 pattern, as shown in Figure 3(a), on clear-sky days and not influenced by any weather systems, due to lack of particular wind circulations which may give finer pattern, as is the summer case. 5. CONCLUDING REMARKS
This study presented some statistical evaluation of NO2, NO, O3and SO2 for the year 1982 from an automatic air pollution monitoring station installed at the NOA site. This was done to study in depth the above pollutants since the weather patterns throughout any year present a regular cycle affecting the air pollutant concentrations quantitatively. 1982 was the first year with continuous reliable data from the automatic station, while the NOA site ensures measurements reflecting the average air pollution situation in the area. For the above reasons the results of this study may be useful for the problem of Athens air pollution in the future. The seasonal variation of NO, was found to exhibit maximum values during wintertime because of the higher emission rates and the lower atmospheric mixing height. SO2 emission rates are also higher during winter due to the central heating operation. O3takes up maximum values during summertime because of its photochemical production due to the higher solar insolation in the area. In the case of the daily variation in NO, two peaks appear at about 09.00 and 21 .OO LST in January due to the traffic jam in the city centre. This double peak is confined during July to a smaller one observed at about 07.00 LST; the latter is possibly caused by the fewer activities in the city during the summer. An O3 peak appears 5 hours later than the NO one in both months because of the time delay in the atmospheric photochemical reactions. The broad O3maximum in July is caused by the intense solar insolation. As far as the SO2 is concerned, this shows a similar behaviour to NO. The ACF was employed to establish any periodicities in the air pollutants concentrations in a time scale of 6 days. All pollutants exhibit a well defined oscillatory behaviour because of their diurnal peaks in both January and July except for NO2 in January whose ACF exhibits no regular pattern. For the same reason the January NO-NO2 CCF behaves in the same way while the pairs NO-03 and N02-03exhibit a clear oscillatory pattern. Finally, the SEs of the above pollutants were examined for January and July 1982. The most important SE is that for O3 in July with a peak having a period of almost one day. REFERENCES Blackman, R.B. and Tukey, J.W. The Measurement of Power Spectra, Dover Publications, New York, 1958. Catsoulis, B.D. ‘Observations of the summer sea breeze at Athens, Greece’, Pure and Applied Geophysics, 103, 150-156 (1983).
Ganor, E., Beck, Y, and Donagi, A. ‘Ozone concentrations and meteorological conditions in Tel Aviv, 1975’, Atmospheric Environment, 12, 1081-1085 (1978).
Giisten, H., Heinrich, G., Cvitas, T., Ruscic, B., Lalas D. P. and Petrakis M. ‘Photochemicalformation and transport of 0,in Athens, Greece’, Atmospheric Environment, 22, 1855-1861 (1988). Kambezidis, H.D., Kassomenos, P. and Kiriaki, E. ’Smoke concentration levels in a monitoring network in Athens, Greece’, Atmospheric Environment, 20,601- 604 (1986). Kambezidis, H.D. and Papanikolaou,N. S. ‘SOzconcentrationlevels from a monitoring network in Athens, Greece’, Atmospheric Environment, 22, 2557-2568 (1988). Kambezidis, H.D., Peppes, A.A. and Melas, D. ‘An environmental experiment over Athens urban area under sea breeze conditions’, Atmospheric Research (to appear), (1995).
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Lalas, D.P., Veirs, V. R., Karras, G. and Kallos, G. ‘An analysis of the SO2 concentration levels in Athens, Greece’, Atmospheric Environment, 16, 531-544 (1982). Lalas, D.P., Asimakopoulos, D. N., Deligiorgi, D.G. and Helmis, C.G. ‘Sea-breeze circulation and photochemical pollution in Athens, Greece’, Atmospheric Environment, 17, 1621-1632 (1983). Lalas, D.P., Tombrou-Tsella, M., Petrakis, M., Asimakopoulos, D.N. and Helmis, C.G. ‘An experimental study of the horizontal and vertical distribution of O3 over Athens’, Atmospheric Environment, 21,26812693 (1987). Lyons, W.A. and Cole, H.S. ‘Photochemical oxidant transport: mesoscale lake breeze and synoptic scale aspects’, Journal of Applied Meteorology, 15, 733-743 (1976). Melas, D., Kambezidis, H.D., Walmsley, J.L., Moussioloulos, N., Bornstein, R.D., Klemm, O., Asimakopoulos, D.N. and Schiermeier, F.A. ‘Summary of the NATO/CCMS Pilot Study Workshop on: Air pollution transport and diffusion over coastal urban areas’, Atmospheric Environment (to appear), (1995). OECD Environmental Policies in Greece, OECD, Paris, 1983. PERPA (Athens Environmental Pollution Control Project) ‘Air pollution in Athens area’, Technical Reports, Athens, 1980 and 1989. Prezerakos, N.G. ‘Characteristics of the sea-breeze in Attica, Greece’, Boundary-Layer Meteorology, 36, 245-266 (1986). Seinfeld, J.H. Air Pollution Physical and Chemical Fundamentals’. McGraw-Hill, 1975, pp. 523. Steinberger, E.H. and Ganor, E. ‘High ozone concentrations at night in Jerusalem and Tel Aviv’, Atmospheric Environment, 14,221-225 (1980). Stern, A.C., Wohlers, H.C., Boubel, R.W. and Lowry, W.P. Fundamentals of Air Pollution, Academic Press, 1973, pp. 492. Viras, L.G. and Siskos, P.A. ‘Air pollution by gaseous pollutants in Athens, Greece’, in: Nriagu, J.O. (ed), Gaseous Pollutants: Characterisation and Cycling, Wiley, 1992. Zerefos, C.S., Bais, A.F., Ziomas, I.C., Paliatsos, A.G., Amanatidis, G.T. and Tourpali, K. ‘Review of trends in air pollutants in Greece’, WMO Technical Conference on Monitoring and assessment of changing composition of troposphere, Sofia, October WMO-No 724, 1989, pp. 47-49. Zerefos, C.S., Ziomas, I.C., Bais, A.F., Mantis, H., Pepapis, C., Amanatidis, G.T., Paliatsos, A.G. and Roemer, M. ‘Air pollution evolution in the Athens basin’, Informative Bulletin of the Technical Chamber of Greece, 1598, 37-97 (in Greek) (1990). Ziomas, I.C., Zerefos, C.S., Bais, A.F., Proyou, A.G., Amanatidis, G.T. and Kelessis, A.G. ‘Significant increasing trends in surface ozone in Greece’, Environmental Technical Letters, 10, 107 1-1082 (1989).
