Spatial and temporal characteristics of the relationship between air quality status and mesoscale circulation over an urban Mediterranean basin

The Science of the Total Environment 217 Ž1998. 37]57

Spatial and temporal characteristics of the relationship between air quality status and mesoscale circulation over an urban Mediterranean basin P.A. Kassomenos a,U , H.A. Flocas a , S. Lykoudisa , A. Skouloudisb a

Laboratory of Meteorology, Department of Applied Physics, Uni¨ ersity of Athens, Panepistimioupolis, GR-15784, Greece b En¨ ironment Institute, JRC, ISPRA, Italy Received 1 September 1997; accepted 17 March 1998

Abstract The objective of this study is to identify and establish the day-by-day relationship between mesoscale circulation and the air quality status over the Metropolitan area of Athens for a period of 13 years and to further investigate its temporal and spatial variability. Eleven distinct mesoscale patterns are identified using a formulated methodology based on surface wind measurements. The air quality conditions are classified into seven distinct classes according to the method of the Air Quality Indicators for five main pollutants namely, O 3 , NO 2 , SO 2 , CO and black smoke ŽBS.. It was found that severe and bad air quality conditions over specific parts of the examined area are associated with the weak mesoscale patterns of southern direction or calm conditions. The good and moderate conditions are established mainly under northerly airflows. The most serious pollution problem favored even by intense northerly flow is attributed to O 3 during the warm period in the northern zone. High concentrations of O 3 , NO 2 and SO 2 in the warm period are remarkably related to the pure sea breeze circulation, especially in the central and northern zone. Q 1998 Elsevier Science B.V. All rights reserved. Keywords: Mesoscale circulation; Air quality indicators; Air quality indexing; Athens basin

1. Introduction The relationship between atmospheric circulation patterns and environmental data has been

U

Corresponding author. Tel.: q30 94 697950; fax: q30 1 7295282; e-mail: [email protected]

the subject of considerable recent research. Comrie and Yarnal Ž1992. have extensively described the methods of investigating this relationship on a climatological basis. Most of the relevant studies have tried to correlate the prevailing synoptic scale patterns with measured species concentrations over an examined area Že.g. Comrie, 1994; Dorling and Davies, 1995; Kassomenos et al.,

0048-9697r98r$19.00 Q 1998 Elsevier Science B.V. All rights reserved. PII S0048-9697Ž98.00167-3

38

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

1998. since the synoptic scale circulation is considered to play an important role on the formation of the air quality conditions through its controlling effect on the local meteorological conditions. The Metropolitan area of Athens, over the last two decades, is well known for major pollution problems. Kassomenos et al. Ž1998c. correlated the atmospheric circulation of the lower troposphere with the concentration of the main air pollutants on a climatological basis; they found that there is a strong relationship between the air quality status of the area and the synoptic scale circulation. Nevertheless, the Metropolitan area of Athens is characterised by the establishment of local circulation patterns due to the landscape variability and complicated land water distribution. Various investigators have examined the role of the locally generated air flow circulation in the development, evolution and maintenance of severe air pollution episodes in the Metropolitan area of Athens both theoretically ŽKallos et al., 1993; Kassomenos et al., 1995; Melas et al., 1995. and experimentally ŽLalas et al., 1987; Asimakopoulos et al., 1992; Ziomas et al., 1995; Helmis et al., 1987, 1996.. However, a systematic study of the day-by-day relationship of the air quality status in the Metropolitan area of Athens with the prevailing mesoscale circulation patterns has not been carried out up to now. In this study using the mesoscale flow classification scheme proposed by Kassomenos et al. Ž1998a. and the air quality indicators proposed by Kassomenos et al. Ž1998b., an attempt is made to relate the mesoscale weather patterns which occurred in the area with the air quality status in Athens on a daily basis for the period 1983]1995 and to investigate the seasonal and spatial characteristics of this relationship. In the following section, the data sets used and the prevailing mesoscale weather types in the area are briefly presented. Also, a brief description of the air quality indicators is provided. The relationship between the air quality status and the mesoscale weather patterns is analytically discussed in Section 3, while seasonal variations of this relationship are investigated in Section 4. Considerations on the spatial characteristics of

the air quality conditions and their relationship with the mesoscale circulation are presented in Section 5. Finally, the main conclusions are discussed in Section 6. 2. Data and methodology 2.1. Description of the area The Metropolitan area of Athens is situated in a small peninsula located in the southeastern edge of the Greek mainland which covers about 450 km2 with 3 600 000 inhabitants. The topography of the area is rather unfavorable because the built up area is located in a Basin, surrounded by tall and rather stony mountains Žwith heights ranging from 1400 to 500 m. from three sides and open to the sea from the south. There are small openings connecting the Metropolitan area of Athens with the Greek mainland in the north, northwest and northeast. In this area more than 1 000 000 cars are registered with nearly 25% of them with catalytic convertors. There are about 20 000 taxis and public buses with diesel engines. The industrial activities are mainly located in the west and southwest of the Basin and the nearby Thriassion plain. A monitoring network of seven surface meteorological stations is operated in the greater Athens area; five are located inside the Athens Basin while the other two are east and west of it in the nearby Mesogia and Thriassion plains respectively Žsee Fig. 1.. The main meteorological parameters measured are wind speed and direction, air temperature, humidity, cloudiness, rain, etc. Table 1 displays the coordinates and the altitude of the stations. An official monitoring network of 11 air pollution stations has operated since 1983 in the Metropolitan area of Athens. The location of the stations is given in Fig. 1. The main pollutants measured are SO 2 , CO, NO 2 , NO, O 3 and black smoke ŽBS. ŽTable 2.. 2.2. Classification of mesoscale air flow patterns The methodology used for the classification of the mesoscale flows using meteorological parame-

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

39

2.2.2. Easterly flow (b) The surface winds blow from the eastern sector in all parts of the Metropolitan area of Athens with intensity less than 4 mrs during the day and even weaker during the night. 2.2.3. Strong northerly flow during the cold period (c1) The winds blow from the northern sector during daytime and night-time at speeds more than 6 mrs. 2.2.4. Weak northerly flow during the cold period (c2) The difference between this category and the previous one lies in the intensity of this flow which is less than 6 mrs. Fig. 1. The topography of the Metropolitan area of Athens. Contours are every 150 m. The air pollution monitoring stations are also shown.

ters from the monitoring network operated in the area was extensively discussed and analyzed elsewhere ŽKassomenos et al., 1998a.. Briefly, the mesoscale surface airflows in the Athens Basin can be divided into 11 categories. 2.2.1. Westerly flow (a) This category is characterized by surface winds blowing from the western sector with intensity greater than 5 mrs. The flow does not present significant diurnal variation. Table 1 Meteorological stations used in this study Žabbreviated station name, longitude, latitude, altitude. Abbreviated station name

Longituder latitude

Altitude Žabove MSL.

GMS PER NOA NFL TAT ELE SPA

37.54r23.44 37.56r23.28 37.53r23.43 38.03r23.44 38.06r23.44 38.04r23.47 37.58r23.55

10 2 107 136 237 30 130

2.2.5. Strong northerly flow during the warm period (c3) Winds blow from the northern sector in the daytime with an intensity as high as 6 mrs. This wind flow pattern presents significant diurnal variation since the wind speed diminishes significantly during the night while the direction remains unchanged. 2.2.6. Weak northerly flow during the warm period (c4) The winds blow from the northern sector with intensity less than 6 mrs during the daytime. During the night, the winds become very weak and in some cases almost calm. 2.2.7. Strong southerly flow (d1) The winds blow from the southern sector with intensity greater than 6 mrs throughout the whole 24-h period in all parts of the Metropolitan area of Athens. 2.2.8. Pure sea breeze (d2) This pattern is characterized by southerly direction in the warm period of the year within the area. This flow can reach the northern passages of the Basin, connecting it with the rest of the

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

40

Table 2 Air quality stations of the Athens Metropolitan area Station

Abbreviation

Height above surface Žm.

Longituderlatitude

Operating since

Pollutants measured

Piraeus

PEI

4

38856.6079r23838.8579

1984

SO2 , CO, BS, NO2 Ž1984]1995. O3 , NO Ž1987]1995.

Rentis

REN

4

37857.8039r23840.5039

1984

SO2 , CO, NO2 , O3 , NO Ž1993]1995. BS Ž1984]1995.

N. Smirni

SMI

4

37855.9749r23842.9009

1983

SO2 , CO, NO2 Ž1983]1995. O3 , BS, NO Ž1987]1995.

Athinas

ATH

5

37858.6819r23843.6209

1987

SO2 , CO, BS, NO2 , NO Ž1987]1995. O3 Ž1990]1995.

Aristotelous

ARI

5

37859.2969r23843.6679

1984

SO2 , CO, NO2 , NO Ž1994]1995. BS Ž1984]1995.

Patision

PAT

8

37859.9589r23843.9779

1983

SO2 , CO, BS, NO2 Ž1983]1995. O3 , NO Ž1987]1995.

Geoponiki

GEO

4

37859.0289r23842.4369

1983

SO2 , CO, NO2 Ž1983]1995. BS Ž1988]1992. O3 , NO Ž1987]1995.

Peristeri

PER

4

38800.9119r23841.7679

1989

SO2 , CO, NO2 , NO Ž1989]1995. BS Ž1991]1995. O3 Ž1990]1995.

Marousi

MAR

4

38801.8589r23847.2819

1984

SO2 , CO Ž1984]1987, 1989]1995. BS Ž1990]1992. O3 , NO2 , NO Ž1989]1995.

Lykovrisi

LYK

4

38804.1989r23846.6199

1994

CO, NO2 , O3 , NO Ž1994]1995.

Ano Liossia

LIO

8

38804.0349r23842.6979

1983

CO Ž1983]1987. SO2 , NO2 Ž1983]1995. O3 , NO Ž1987]1995.

mainland. In the night-time the winds become very weak or even calm in the northern and central part of the Metropolitan area of Athens while in the southern part a weak northerly offshore flow is established. 2.2.9. Weak sea breeze (d3) This flow pattern occurs during the warm period of the year with the characteristics of the sea

breeze. Nevertheless, the intensity of the southerly flow during the day is reduced and extends up to the central parts of the Basin. In the night-time the flow is very weak or even calms. 2.2.10. Very weak southerly flow (d4) This pattern is considerably weak with an intensity lower than 5 mrs. During the night, the winds blow from northerly directions and are very weak or almost calm within the whole Basin.

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

2.2.11. Flow without main component (e) This flow pattern is characterized by winds blowing from various directions over the Basin, with intensity less than 3 mrs. According to the classification of mesoscale patterns during the period 1983]1995 only a small percentage remained unclassified Ž10.5%.. In summary, it was found that the weak surface flows prevail throughout the whole year, summing up to 57% of the days with preference in May and June. The almost stagnant conditions prevail in one-fifth of the days annually, especially during the cold period of the year. Strong surface flows are observed with frequency 43% mainly during July and August. 2.3. Air quality indicators The air quality status regarding an air pollutant is described in terms of Air Quality Indicators ŽAQI.. This method employs a generalised and sophisticated view of air quality over the examined area rather than relying on the concentration values of the pollutants for specific stations. It should be noted that this method could be used in any urban area if site-specific threshold values are determined. Details on the methodology can be found in Kassomenos et al. Ž1998b.. In summary, the AQI are defined on the basis of basic principles concerning the effects of air pollutants on human health and vegetation. v

v

v

v v

It is necessary to protect the most sensitive groups of population. Targets should be set for achieving better air quality environment. Warnings need to be issued whenever human health is endangered. Vegetation damage should be avoided. Past air-quality conditions need to be accounted for over rolling annual periods.

Therefore, the following concentration levels can be considered as indicative for the air quality status for each pollutant: Ža. annual mean concentration Ž CAM ., representing past annual conditions, Žb. upper Ž C UL . and lower Ž C LL . level limits, representing the proposed standard limit

41

values. C LL should be considered the one more easily encountered while C UL can be considered as 2U C LL . Žc. Short term target value Ž C TV ., related to limits for short averaging periods, or set to C LL r2, Žd. intermediate value Ž C IV . covering the range between the short term target and the annual mean values, usually set to Ž CAM q C TV .r2, and Že. alert concentration Ž CAV ., set by competent authorities for some pollutants, otherwise considered as 85% of C TV . Once these indicative values are set the AQI scale can be introduced, as described in Table 3. The indicative concentration levels used by the AQI scheme are directly calculated from each pollutant’s measurements and the AQI class can be assigned to each case. The AQI class calculated for a certain percentage of the sites is considered as representative of the domain. The selection of the percentage should allow for taking into account missing or bad values, and is highly dependent on the sites’ number. The AQI method has been applied in the Metropolitan area of Athens for CO, NO 2 , SO 2 , black smoke ŽBS. and O 3 . Black smoke is used since there are no regular measurements of PM 10 , while it has been demonstrated that PM 10 can be considered as equal to 90% of BS for Athens ŽHatzakis et al., 1986.. In accordance with the limits proposed by WHO and the EEC, 1-h daily maximum values were considered for NO 2 and O 3 , 24-h mean values for BS and SO 2 and 8-h maximum average for CO Žsee Table 4.. For each of the operating stations in the Metropolitan area of Athens and for each pollutant, the indicative concentration levels are calculated. The median Table 3 The indices of AQI scale and air quality classes Index

Air quality description

Limits

7 6 5 4 3 2 1

Severe Extreme Bad Critical Poor Moderate Good

C ) CUL CUL G C ) CLL CLL G C ) CTV CTV G C ) CAV CAV G C ) CIV CIV G C ) CAM CAM G C

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

42

Table 4 Values used in Athens for the implementation of the AQI scale. For C IV and CAM the minimum and maximum values over the examined period are given Index

O3 Ž m grm3 . 1 h max

NO2 Ž m grm3 . 1 h max

BS Ž m grm3 . 24 h avg.

SO2 Ž m grm3 . 24 h avg.

CO Žmgrm3 . 8 avg. max

CUL CLL CTV CAV

360 180 90 77

400 200 100 85

200 100 50 43

250 125 63 53

20 10 5 4.3

CIV ARI ATH GEO LIO LYK MAR PAT PEI PER REN SMI

51.0 48.8 69.4 67.0 49.3 51.2 56.4 52.9 59.3 55.5

66.7 68.7 85.7 68.9 72.8 56.8 66.4 67.4 69.5 69.1

88.9 53.9 56.3 49.8 59.2 49.7 90.1 75.0 65.6 63.6 51.3

91.6 87.8 79.3 60.7 60.3 68.6 103.0 91.4 81.3 65.7 67.9

42.5 40.4 33.0

86.3 53.2 39.9

43.0 37.8 30.2 31.9

54.2 55.8 43.4 53.1

31.2 63.0 37.8 32.0 30.5 30.5

31.9 117.0 65.9 43.2 44.1 42.8

30.5 45.9 36.0 37.8 35.1 30.4

36.8 70.6 63.1 44.1 41.9 50.7

CAM ARI ATH GEO LIO LYK MAR PAT PEI PER REN SMI

25.0 20.6 61.8 57.1 21.5 25.4 35.8 28.8 41.6 33.9

56.5 60.5 94.3 60.9 68.5 36.6 55.8 57.8 62.0 61.2

92.8 22.8 27.6 14.6 33.5 14.4 95.2 64.9 46.2 42.2 17.6

98.2 90.5 73.6 36.3 35.5 52.1 120.7 97.8 77.5 46.4 50.8

42.0 38.2 23.0

129.6 63.5 36.9

33.0 22.5 7.4 10.8

55.4 58.6 33.9 53.2

19.5 83.1 32.6 21.1 18.0 18.0

20.8 191.9 88.7 43.4 45.2 42.7

7.9 38.9 19.0 22.6 17.2 7.7

20.6 88.2 73.1 35.1 30.9 48.5

value of all available data is used as an annual mean value, CAM , and the standard limit values proposed by EEC and WHO are used as lower level limit values, C LL . On a daily basis, for each pollutant, every station has been attributed an AQI class. The overall air quality status of the Metropolitan area of Athens for the specific day is estimated as the median value of the AQI values of the 11 stations. In case there are missing values, a day is considered valid when at least four stations are valid. Thus, every station and for the Metropolitan area of Athens as a whole, each day, for each pollutant, has been attributed an AQI value, describing the air quality status regarding the specific air pollutant. It should be

3.9 3.7 2.6 2.7 2.7 2.6 4.7 3.4 3.0 3.0 2.8

4.0 5.5 3.2 2.8 2.8 3.9 6.6 4.5 4.1 3.2 3.2

3.6 3.2 0.9 1.1 1.1 0.9 5.1 2.5 1.7 1.6 1.4

3.8 6.7 2.1 1.3 1.3 3.4 8.9 4.7 3.8 2.0 2.1

mentioned that the spatial characteristics of the relationship between the mesoscale circulation and the air quality status are based on the examination of the AQI behaviour of each station separately. 3. Relationship between mesoscale categories and air quality The day-by-day cross tabulation of the mesoscale categories with the air quality classes throughout the examined period shows that the most favorable mesoscale patterns for the bad air quality conditions over the Metropolitan area of Athens are as follows.

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

3.1. Weak sea breeze Being characterized by southerly flow during the daytime, this flow is not strong enough to reach the foothills of the mountains located in the northern part of the Metropolitan area of Athens and pass through the northern passages towards the other parts of the Greek mainland. Therefore, its horizontal extent does not allow the ventilation of the air within the Basin. Furthermore, the vertical depth of the airflow is small enough to favor the accumulation of air pollutants released or transformed in the lower tropospheric layers.

northerly flows during both the cold and warm period. More specifically, the analysis for each pollutant separately shows that: v

3.2. Very weak southerly flow It is responsible for air quality problems during the cold period of the year because the lower solar intensity, the increased moisture content of the soil and the denser vegetation pattern during this period as compared to the warm period favor the latent heat in the lower troposphere rather than the sensible heat release, thus, preventing the development of an intense breeze cell ŽKallos and Kassomenos, 1992.. Therefore, the horizontal penetration of this southerly flow is limited, extending approximately up to the middle of the Basin while its vertical extent is significantly reduced.

v

3.3. Almost calm conditions These cause very slow horizontal dispersion of air pollutants and limited vertical extent of the mixing layer due to the associated very weak airflow of various directions. It should be mentioned that the frequency of this category is in general high in all the classes for all the pollutants. This might be associated with the limitations of the density and the distribution of the stations used in this study, since a denser and more evenly distributed network would have produced a better classification of the airflows. Good or moderate air quality conditions are more commonly established over the examined area for all the pollutants under the strong

43

v

For O 3 , cold period’s strong northerly flow is associated with 38% and 33% of the days characterized by good and moderate air quality conditions, respectively. On the contrary, warm period’s strong northerly flow favors the bad air quality conditions with a frequency of 30% ŽFig. 2.. This is possibly due to the fact that during the warm period O 3 concentrations are high even over non-urban areas while the strong northerly flow seems to be responsible for the transportation of O 3 over the Metropolitan area of Athens. The close relationship of warm period’s strong northerly flow with high levels of O 3 is not observed with any other of the examined pollutants. Extreme air quality conditions due to O 3 occur mainly under weak sea breeze Ž32%. and almost calm conditions Ž32%. ŽFig. 2.. With respect to NO 2 the extreme and bad conditions are mainly favored by almost calm conditions Ž42% and 24%, respectively., while they are significantly related to the southerly type flows as weak sea breeze and very weak southerly flow Žsee Fig. 3.. Warm period’s strong northerly flow is associated with bad air quality conditions with a low frequency Ž17%., as compared to O 3 . On the contrary, the strong northerly flows during both cold and warm periods are mainly associated with good or moderate air quality conditions Ž82% and 52%, respectively.. SO 2 presents a very low frequency of bad or extreme air quality conditions Žless than 10% of the days annually ., which are mostly related to the very weak southerly flow, almost calm conditions and weak northerly flow during the cold period. The strong northerly flow during the warm period is responsible for the formation of good or moderate conditions. The concentrations of SO 2 are generally low due to a number of rectification measures imposed by the authorities in the last decade implying the use of petrol with low SO 2 concentration.

44

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

Fig. 2. Relative frequency distribution of the AQI classes within the mesoscale categories for O 3 throughout the whole period. v

With respect to CO, only 3% of the examined days are characterized by severe conditions. Similarly with SO 2 , the very weak southerly

flow, almost calm conditions and weak northerly flow during the cold period are responsible for the occurrence of the extreme

Fig. 3. As Fig. 2, but for NO 2 .

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

v

conditions ŽFig. 4.. Good or moderate conditions appear in almost 45% of the examined days, mainly related to the warm period’s strong northerly flow. With respect to BS, almost 30% of the days assigned to extreme air quality conditions and 40% of the days assigned to severe are characterized by almost calm conditions. Very weak southerly flow is also responsible for the occurrence of severe and extreme events. Good and moderate conditions are mainly attributed to the warm period’s strong northerly flow.

4. Seasonal variations In order to investigate the seasonal variability of the relationship between the mesoscale air flow and the air quality, the year was divided in two main periods according to Lykoudis et al. Ž1996.: the warm period which extends from May to September being characterized by strong photochemical activity and the cold period extending from November to March with no or less pho-

45

tochemical activity. October and April form the transient period of the year. O 3 presents strong seasonal variability due to its photochemical formation; thus almost 83% of the days assigned to classes bad and extreme are confined in the warm period of the year. According to Fig. 5a warm period’s strong northerly flow is the main contributor to the overall accumulation of O 3 since it accounts for 37% of the bad events. Since this mesoscale category is mainly associated with the sub-synoptic scale flow, namely ‘Etesians’, over the Aegean sea during the warm period ŽKassomenos et al., 1998a., it could be supported that the ‘Etesians’ favor ozone transportation from the north over the Metropolitan area of Athens. The formation of extreme conditions due to O 3 in the warm period is also favored by weak sea breeze Ž36%. and almost calm conditions Ž26%.. An interesting seasonal feature of O 3 is the remarkable contribution of the pure sea breeze flow to the accumulation of high concentrations, since it is responsible for 20% of the extreme events. This can be attributed to the fact that in

Fig. 4. As Fig. 2, but for CO.

46

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

Fig. 5. Relative frequency distribution of the AQI classes within the mesoscale categories for O 3 for Ža. warm period and Žb. cold period.

the warm period the northerly land breeze during the night and early morning hours transports the photochemically formed O 3 and NO 2 from the Athens Metropolitan area southwards over the

Saronic Gulf, while the intense sea breeze flow carries them back to the examined area during the daytime. Extreme air quality conditions due to O 3 are

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

not observed in the cold period while bad air quality conditions occur in only 7% of the days under very weak southerly flow and almost calm conditions ŽFig. 5b.. The frequency of the good and moderate air quality conditions become as high as 70% in the cold period with strong northerly flow being the most significant contributor. However, the frequency of poor and critical conditions is increased in the cold period and especially in the transient months, under mainly weak southerly flow. Therefore, it is suggested that high O 3 concentrations could be found even in the non-photochemical period under the aforementioned favourable meteorological conditions. This will be further investigated on a spatial basis. As can be seen in Fig. 6, NO 2 has weaker seasonal variability when compared to O 3 , accounting for the occurrence of extreme and bad events with a frequency of 60% in the warm period and 47% in the cold one. The lower frequency of the bad and extreme events due to NO 2 in the warm period as compared to O 3 could be associated with the overall decrease of the NO 2 production in the warm period over the Metropolitan area of Athens as a result of the reduction of the industrial activity and the vacancies of Athenians in the summer ŽViras et al., 1996.. Weak southerly flows contribute to the formation of extreme and bad air quality conditions in both the warm and cold period of the year. Specifically, almost 60% and 90% of the days assigned to classes bad and extreme, respectively ŽFig. 6a., are associated with very weak southerly flow and almost calm conditions in the cold period. The same patterns favour the formation of the bad and extreme conditions in the transient months, with lower frequencies Ž42% and 74%, respectively.. In the warm period, again weak sea breeze and almost calm conditions mostly favor the occurrence of bad and extreme conditions, with lower percentages 30 and 60%, respectively ŽFig. 6b.. It should be noted that the pure sea breeze circulation is an important contributor to the occurrence of extreme conditions Ž26%. as was found for O 3 . It is worth noting the influence of the mesoscale pattern westerly flow in the frequent occurrence of bad conditions due to NO 2 in the warm

47

period which can be attributed to its transportation from the nearby industrial area of the Thriassion plain, or the western suburbs of Athens where the most important industrial activity and the main road axes of the Metropolitan area of Athens are located. The transportation of polluted air masses from the Thriassion plain towards the Basin through the passage of mountain of Egaleo Žsee Fig. 1. in the warm period of the year is favored by the prevailing weak synoptic circulation as supported by Asimakopoulos et al. Ž1992.. Since the SO 2 is mainly associated with central heating, the good air quality conditions are predominant in the warm period, while its accumulation is favored in the cold period. Almost calm conditions, very weak southerly flow and cold period’s weak northerly flow account for 95% of extreme events in the cold period. In the warm period and the transient months, extreme conditions do not occur whereas almost 50% of the bad events are associated with weak sea breeze and almost calm conditions ŽFig. 7.. Pure sea breeze flow contributes significantly to the overall accumulation of SO 2 in the warm period, Ž33% of the days assigned to class bad are related with this pattern. because it transports the SO 2 that is considerably produced in the southern zone either by the harbor activities or by the small industries located there. Similarly bad and extreme events with respect to CO are confined to the cold period of the year. Fig. 8 shows that the influence of almost calm conditions and very weak southerly flow seems to be crucial in the accumulation of CO in this period since it accounts for 50% and 70% of the days being characterized by bad and extreme conditions, respectively. Also, in the cold period, the weak northerly flow is significantly related to the formation of extreme air quality conditions. In the warm period, weak sea breeze plays the most important role in the occurrence of bad events, while no extreme events are observed. In the transient months, the bad and severe conditions are attributed to almost calm conditions, weak sea breeze and very weak southerly flow. The accumulation of BS is confined in winter, under very weak southerly flow and almost calm

48

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

Fig. 6. As Fig. 5, but for NO 2 .

conditions. On the contrary, in the warm period and transient month’s weak sea breeze is the main contributor to the formation of problematic

air quality, accounting respectively for 60% and 34% of the days assigned as severe air quality conditions.

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

49

Fig. 7. Relative frequency distribution of the AQI classes within the mesoscale categories for SO 2 for the cold period.

5. Spatial variations The Metropolitan area of Athens presents significant spatial variability in terms of the population density, land cover and human activities. Taking in mind the description of the area ŽSection 2.1. three main zones parallel to the seashore are distinguished Žsee Fig. 1.: southern Žwhich is represented by PEI, REN, and SMI stations., central Žrepresented by PAT, ATH, ARI, GEO, and PER stations. and northern Žstations LIO, LYK, and MAR.. The central zone with mean height of 80 m above sea level is characterized by heavy saturated traffic practically throughout the whole day. The southern zone being located along the coast is a mixed residential and industrial area at a mean height of 40 m above sea level. The north zone Žpractically northeasterly . is primarily an area of residence being located at more than 150-m mean height above sea level. The central zone could be also distinguished in western and eastern parts along the Kifissos River. The western part of the Athens Basin that is mixed, concentrating residential, industrial and

heavy traffic activities because of the main highways passing through this area and the nearby Thriassion plain. However, due to the absence of adequate air quality network in the western part, it is difficult to draw safe conclusions. In this study, PER station is examined in order to get some indications on the spatial characteristics of the western part of the Athens Metropolitan area. 5.1. SO 2 The whole Athens Metropolitan area is characterized by good or moderate conditions throughout the whole year, except the central zone that is exposed to poor conditions: 90%, 80%, 75% and 50% of the days are attributed to good and moderate classes, respectively in the northern, southern, western, and central zone. During the cold period of the year when the high concentration of SO 2 is greatly favoured, the spatial distribution of the SO 2 concentrations indicates clearly the techniques used for heating in the various parts of the Metropolitan area of Athens. This is mainly due to the absence of significant SO 2 in the area because there is no

50

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

Fig. 8. As Fig. 7, but for CO.

industrial activities while the techniques used for heating are mainly based on electric power. Consequently, only 1% of poor and 7% of critical air quality conditions are identified in this zone being associated with very weak southerly flow and almost calm conditions while no bad events are observed. On the other hand, the situation is completely different in the central zone where 43% and 63% of the days are characterized by bad and extreme air quality conditions in ATH and PAT station, respectively. This is due to the use of more traditional techniques of heating based on petrol combustion. The majority of these days were associated with very weak southerly flow and almost calm conditions Žabout 70%., while a significant percentage is related to the cold period’s weak northerly flow. In the southern zone there is clear difference between PEI in which 42% of the days are characterized by bad, extreme and severe conditions while the corresponding percentage for SMI is 13%. It is unambiguous that this difference is mainly due to the fact that SMI is a mainly residential area in which old techniques for cen-

tral heating are used, while PEI combines residential, light industrial and harbor activities that enhance the SO 2 problem during winter. The main air quality patterns associated with these conditions are very weak southerly flow and almost calm conditions Ž52%.. The contribution of the northerly flow categories is also significant Ž44%. implying that the SO 2 being produced in the central zone is likely to be transported southwards. Station PEI presents high levels of SO 2 concentration, even in summer, due to the ship traffic, which is not the case for the other station of the southern zone ŽSMI.. More specifically, the frequency of bad air quality conditions in PEI is as high as 31% and 15% in the cold and warm period, respectively. Moreover, the extreme air conditions in the warm period are associated with weak sea breeze and almost calm conditions Žsee Fig. 9.. In the station ATH of the central zone the frequency of extreme conditions drops to 6% in the warm period while the PAT station presents significant percentages of bad conditions in summer Ž27%. that occur mainly under pure and

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

51

Fig. 9. Frequency distribution of the AQI classes for the station of PEI within the mesoscale categories for SO 2 in the warm period.

weak sea breeze or calm conditions. Pure and weak sea breeze conditions may be responsible for the transportation of SO 2 from the southern to central zone during the warm period. Of interest are those bad air quality conditions in the western suburbs occurring in almost 10% of the days throughout the year, without any seasonal preference. Since these conditions are mainly associated with westerly flow, it is suggested that there is transportation of SO 2 from the industrial area of the Thriassion plain to the western suburbs of Athens throughout the whole year. 5.2. CO Since the emissions of CO in the Metropolitan area of Athens are primarily caused by cars ŽViras et al., 1996., the highest concentration is observed in the so-called ‘traffic stations’ of the central zone PAT and ATH. In particular, in PAT 53% of the cold period days are characterized by extreme and severe air quality conditions while in ATH the corresponding frequency decreases to 24%. This is due to the fact that the ATH station

is located in the inner ring of Athens in which daily rectification traffic measures are imposed. In the warm period, the extreme conditions are markedly reduced in the traffic stations Ž20% in PAT and 3% in ATH. which is attributed to the cold start of the car engines in combination with the reduction of the traffic ŽViras et al., 1996.. Of interest is that no particular mesoscale pattern favours the extreme conditions that are formed in the traffic stations Žsee Fig. 10., because the CO accumulates locally along the road axes while its concentration reduces sharply away from them, rather independently of the meteorological conditions. This is especially evident in ATH during the warm period of the year. Secondary hot spots of CO concentration are identified in the southern zone and the western suburbs of the Metropolitan area of Athens. The southern zone experiences high frequency of bad and extreme conditions in the cold period Ž71%. which decreases substantially in summer Ž24%.. It should be noted that the concentration levels of CO in summer are especially high in PEI rather than in SMI because of the heavier traffic of the cars around the harbor area. The bad conditions

52

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

Fig. 10. Frequency distribution of the AQI classes for the station of ATH within the mesoscale categories for CO in the cold period.

in the southern zone occur under strong northerly flow for both cold and warm period, suggesting that the CO being produced by the heavy traffic in the main traffic axes at the north is likely to be transported southwards. In the western part of the Basin, the bad and extreme conditions occur in 38% and 12% of the days in the cold and warm period, respectively, mainly under almost calm conditions and very weak southerly flow. On the contrary, the northern zone is not characterized by serious pollution problems due to CO, even in the cold period when the frequency of the good and moderate conditions is as high as 64%. During winter 20% of the days could be characterized by bad and extreme air quality conditions being associated with almost calm conditions and very weak southerly flow Žmore than 80% of the days.. In summer the frequency of the bad and extreme conditions becomes as low as 3%. This difference between the CO levels between the cold and the warm period of the year can be also attributed to the cold start of the car engines and less traffic on the main road axes that cross the northern zone.

5.3. O 3 In general, the Metropolitan area of Athens presents serious problems of photochemical pollution in the warm period. However, the problem is significant in winter as well, since bad conditions due to O 3 are formed on average in 10% of the winter days in all the stations. The highest levels of O 3 concentration are found in the urban background stations while the O 3 concentration is reduced in the traffic stations of the central and southern zone: 70% of the days throughout the year are characterized by good or moderate air quality conditions in the central and southern zone while in the northern zone the corresponding frequency reduces to 50%. Consistent with Pilinis et al. Ž1993., hot spot of O 3 is found in the northern zone, where the frequency of extreme and bad air quality conditions becomes as high as 25% and 70% respectively, in the warm period. These conditions are mainly favored by calm conditions and weak sea breeze while a significant percentage Ž15%. is associated with the warm period’s strong northerly

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

flow, consistent with Kassomenos et al. Ž1998b.. Of interest is the contribution of pure sea breeze to the development of bad and extreme conditions Ž20%.. Therefore, it can be seen that the accumulation of O 3 in the northern zone may be caused from: Ža. local production of O 3 , Žb. transportation of O 3 from the central and southern zone with southerly flow, Žc. photochemical transformation of the primary pollutants that are produced in the central and southern zone and transported northwards, and Žd. transportation of O 3 that is produced out of the Metropolitan area of Athens with strong northerly flow. In winter, the frequency of the extreme and bad conditions over the northern zone reduces to 3% and 25%, respectively. These conditions are mainly related to calm conditions and very weak southerly winds Žmore than 90% of the days.. The southern zone is characterized by the predominance of bad and extreme conditions in the warm period. In PEI almost 57% of the days are attributed to bad and extreme conditions, associated with the sea breeze of the Saronic Gulf Žpure and weak, more than 65% of the days. or the warm period’s strong northerly flow Ž11%.. However, in the other station of the southern zone, SMI, the frequency of the bad and extreme conditions alters to almost 80%. This could be attributed to the difference of the traffic density between the two stations: since NO destroys O 3 in the photochemical circle, the reduced production of NO by the traffic in the urban station of SMI as compared to the traffic station of PEI further supports the higher concentration of O 3 in SMI. Only 15% of the days characterised as bad and extreme conditions are associated with almost calm conditions. On the contrary, northerly flows Žstrong and weak. favour the accumulation of O 3 in this area in 30% of the warm period days. Since the pure sea breeze does not present as strong a relationship to the formation of bad and extreme conditions over the southern zone as was found for the northern zone, there is supporting evidence of O 3 transportation northwards by the pure sea breeze. During winter bad conditions

53

appear in only 8% of the days in the southern zone, mainly associated with calm conditions and very weak southerly flow. In the central zone almost 60% of the days during the warm period and 10% during winter are characterised by bad or extreme air quality conditions. During summer pure sea breeze Ž30%., weak sea breeze and northerly flows are also responsible for high O 3 concentrations ŽFig. 11.. The penetration of the sea breeze and the intensity of the warm period’s strong northerly flow in the warm period seem to favor the transportation of O 3 to the central zone, however with lower frequency as compared to the southern zone. In the cold period almost 95% of the days characterized as bad and extreme are associated with almost calm conditions and very weak southerly flow. Similar results were also found in the PER station representing the western suburbs of Athens. In winter, the bad air conditions are formed for 10% of the days under the weak air flow patterns. 5.4. NO2 NO 2 emissions are mainly due to the traffic either directly or through the transformation of NO. The accumulation of NO 2 in all the zones of the Metropolitan area of Athens is related to the weak southerly airflow patterns and almost calm conditions. The highest levels are found in the central zone and the western suburbs, with no seasonal preference, since in the cold period the accumulation of NO 2 is favored mainly by the heavy traffic in the city center while in the warm period is mostly associated with the photochemical character of NO 2 . The frequency of bad and extreme air conditions due to NO 2 decreases substantially in the southern zone and more markedly in the northern zone, without presenting any significant difference between the cold and warm period. In the western part of the Basin 63% and 68% of the days are characterized as having bad, extreme and severe air quality, respectively in the warm and cold period. It is worth mentioning the

54

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

Fig. 11. Frequency distribution of the AQI classes for the stations of ATH, PAT and SMI within the mesoscale categories d2, d3, c3 and c4 for O 3 in the warm period.

significant percentage of high concentrations associated with westerly flow; a relationship that was similarly observed for SO 2 as well ŽFig. 12.. In the Northern part of the Basin the overall percentage of bad and extreme conditions do not generally differ between the cold and warm period. These conditions are associated with weak flows for almost 90% of the days for the cold period Žalmost calm conditions and very weak southerly flow. and 80% for the warm period Žalmost calm conditions and pure and weak sea breeze.. A significant percentage Ž5%. of bad and extreme conditions is associated with strong northerly flows especially in the warm period. In the central zone only the station PAT presents a slight predominance of the bad conditions in the cold period because, unlike the stations ATH and ARI, it is located out of the traffic ring where the movement of cars is controled by restrictive measures. More specifically, 91% of the days in winter and 97% in the warm period are attributed to bad, extreme and severe conditions in this station while the corresponding percent-

ages for ATH station are 80% for the warm period and 64% for the cold period. These are mainly associated with almost calm conditions and very weak southerly flow in winter and pure and weak sea breeze and almost calm conditions during summer. Only a small percentage of bad conditions are associated with northerly flows. In the southern zone the traffic station PEI presents different behavior than the urban background station of SMI: in PEI 87% and 73% of the days, respectively, in the warm and cold period are associated with bad and extreme conditions while in SMI the corresponding frequencies are as low as 30% and 17%. The increased NO 2 concentration in PEI during summer is due to the enhanced traffic of the harbor. In the southern zone, more than 80% of the days throughout the whole year are associated with very weak sea breeze, pure sea breeze and calm conditions. It should be noted that the contribution of the pure sea breeze to the formation of bad and extreme conditions intensifies from the southern towards the northern zone, as was found for O 3 .

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

55

Fig. 12. Frequency distribution of the AQI classes for the station of PER within the mesoscale categories for NO 2 throughout the whole year.

5.5. Black smoke (BS) The main source of BS is the incomplete combustion in the diesel motors of taxis, buses and light duty vehicles. Central heating and industry cause lower emissions. The hot spots of the BS concentration are found in the city center and the harbor. In the central zone, the traffic stations PAT and ARI present the highest frequency of extreme and severe conditions. This frequency is reduced in the station ATH of the city center because of the restrictive traffic measures. It should be noted that critical conditions occur in the whole central zone in 30]35% of the days throughout the whole year, without any seasonal preference. In the harbor Žstation PEI., the frequency of extreme and severe conditions is 20% and 6% in the cold period and warm period, respectively, while at a distance 1]2 km to the north Žstation REN. the corresponding frequencies reduce almost by half. It should be noted that the extreme and severe conditions in the southern zone are

associated with the warm and cold period strong northerly flows, in 35% of the warm period days and 15% of the cold period days, respectively. The western part of the Basin is characterized by 30% of critical conditions in the cold period with a remarkable contribution of the cold period’s strong northerly flow. 6. Concluding remarks From the above discussion the following concluding remarks could be drawn: v

v

v

good and moderate conditions over the Metropolitan area of Athens are established mainly under the strong northerly airflows, in the cold and warm periods; bad and severe conditions are mainly associated with the weak southerly flows, weak sea breeze and very weak southerly flow as well as with almost calm conditions; hot spots regarding SO 2 and BS concentration are the central zone and the harbor, whereas for CO the traffic stations of central and

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57

56

v

v

v

v

v

southern zone, with a clear preference in the cold period. High levels of NO 2 are mainly found in the central zone, independently of the season; the most serious pollution problem in the Metropolitan area of Athens is the high concentration of O 3 in the warm period, affecting principally the northern zone. The problem is also substantial in the cold period; the high O 3 concentration is usually favored by the strong northerly flow during the warm period; high concentrations of O 3 and NO 2 over the Metropolitan area of Athens in the warm period are remarkably related to the pure sea breeze circulation. This relationship intensifies from the southern towards the northern zone; SO 2 is transported from the central to the southern zone under northerly flow conditions during the cold period, while pure sea breeze conditions may transport SO 2 from the southern to the central and even to the northern zone. SO 2 seems also to be transported from the industrial area of the Thriassion plain to the western suburbs of Athens throughout the year; the mesoscale classification can be also used in other areas, if surface observations are available, however leading to the selection of different mesoscale categories depending on the physiographic characteristics of the area. It is worth mentioning that the denser and more evenly distributed network is employed, the more satisfactory results are expected.

Acknowledgements

This work is partly funded by DG XIII ŽProgram EMMA Contract No. EN 1005.. The authors would like to thank authorities of Division of Atmosphere and Noise, Ministry of Environment, Physical Planning and Public Works for the data used in this study.

References Asimakopoulos DN, Deligiorgi D, Dracopoulos C et al. An experimental study of night-time air-pollutant transport over the complex terrain of Athens. Atmos Environ 1992;26B:59]71. Comrie AC, Yarnal B. Relationships between synoptic scale atmospheric circulation and ozone concentrations in Metropolitan Pittsburgh, Pennsylvania. Atmos Environ 1992;26:301]312. Comrie AC. A synoptic climatology of rural ozone pollution at three forest sites in Pennsylvania. Atmos Environ 1994;28:1601]1614. Dorling SR, Davies TD. Extending cluster analysis } synoptic meteorology links to characterise chemical climates at six northwest European-monitoring stations. Atmos Environ 1995;29:145]167. Hatzakis A, Katsougianni K, Kalandidi K, Day A, Trihopoulos D. Short term effects of air pollution on mortality in Athens. Int J Epidemiol 1986;15:73]81. Helmis CG, Asimakopoulos DN, Deligiorgi DN, Lalas DP. Observations of sea breeze fronts near the shoreline. Bound Layer Meteorol 1987;38:395]410. Helmis CG, Asimakopoulos DN, Papadopoulos C, Kassomenos P, Kalogiros J, Papageorgas P. Air mass exchange between Athens basin and Mesogia Plain, Attica, Greece. Atmos Environ 1996: accepted for publication. Kallos G, Kassomenos P. Weather conditions during air pollution episodes in Athens, Greece: an overview of the problem. Proceedings of the 19th ITM of NATO-CCMS on Air Pollution Modelling and its Application. Greece: Ierapetra, 1992:77]102. Kallos G, Kassomenos P, Pielke R. Synoptic and mesoscale circulations associated with air pollution episodes in Athens, Greece. Bound Layer Meteorol 1993;63:163]184. Kassomenos P, Kotroni V, Kallos G. Analysis of climatological and air quality observations from greater Athens area. Atmos Environ 1995;29B:3671]3688. Kassomenos P, Flocas HA, Lykoudis S, Petrakis M. Analysis of mesoscale patterns in relation to synoptic conditions over an urban Mediterranean basin. Theor Appl Climatol 1998a;59:215]229. Kassomenos P, Skouloudis AN, Lykoudis S, Flocas HA. AirQuality Indicators for uniform indexing of atmospheric pollution in large metropolitan areas. Atmos Environ 1998b: accepted for publication. Kassomenos PA, Flocas HA, Skouloudis AN, Lykoudis S, Asimakopoulos V, Petrakis M. Relationship between air quality indicators and synoptic scale circulation at 850 hPa at Athens. Environ Technol 1998c;19:13]24. Lalas DP, Tombrou-Tsella M, Petrakis M, Asimakopoulos DN, Helmis CG. An experimental study of the vertical distribution of ozone over Athens. Atmos Environ 1987;12:2681]2693. Lykoudis S, Kassomenos P, Petrakis M. A method to estimate the onset of the warm period of the year. 3rd Conference

P.A. Kassomenos et al. r The Science of the Total En¨ ironment 217 (1998) 37]57 on Meteorology, Climatology and Physics of the Atmospheric Environment, 1996:260]263. Melas D, Ziomas IG, Zerefos CS. Boundary layer dynamics in an urban coastal environment under sea breeze conditions. Atmos Environ 1995;29B:3605]3617. Pilinis C, Kassomenos P, Kallos G. Modeling of the photochemical pollution in Athens, Greece. Application of the RAMS-CALGRID modeling system. Atmos Environ 1993;27B:353]370.

57

Viras LG, Paliatsos AG, Fotopoulos AG. Nine-year trend of air pollution by CO in Athens, Greece. Environ Monit Assess 1996;40:203]214. Ziomas IC, Melas D, Zerefos CS, Paliatsos A, Bais AF. Forecasting peak pollutant levels using meteorological variables and indices. Atmos Environ 1995;29:3605]3617.