FORMATION AND TRANSPORT OF ATMOSPHERIC AEROSOL OVERATHENS, GREECE

I. COLBECK1∗, MENG-CHEN CHUNG1 and K. ELEFTHERIADIS2

1 Department of Biological Sciences, University of Essex, Colchester, U.K.; 2 EnvironmentalRadioactivity Laboratory, Institute of Nuclear Technology, N.C.S.R. ‘Demokritos’, 15310 Ag.

Paraskevi, Attiki, Greece(∗ author for correspondence, e-mail: colbi@essex.ac.uk, fax: ++44 1206 872592)

Abstract. The effects of meteorology on ambient aerosol concentrations and aerosol transport,within the Greater Athens Area during the summer period, was investigated. Measurements of sizefractionated anions and cations were made at two sites (inland at Ag. Stefanos and on the coast atPireas) within the Greater Athens Area. The wind regime exhibited a distinct influence such thatthe sea-breeze circulation strongly enhanced the formation of secondary aerosols. For sulphate thedifference in concentration between the two sites was, on average, 8 times greater on sea-breeze dayscompared with Etesian days (warm days with NE winds). During ‘normal’ days, any differences inconcentrations were possibly due to local emissions. Elevated concentrations in the fine mode weredetected at both sites during the sea-breeze days. The sea-breeze circulation enhances the develop-ment of secondary aerosols which was clearly shown at the inland site. Nitrous acid, hydrochloricacid and particulate nitrate, sulphate and ammonium increase during sea-breeze days. Elevated levelsof nitrate, 4 µm diameter, were particularly observed on the days with a strong sea-breeze circulation.Sulphate was well correlated with both sulphur dioxide and ammonium suggesting the productionof NH4HSO4/(NH4)2SO4 aerosols, formed through the neutralisation of NH3 with sulphuric acids.Ammonium sulphate was found to be the major ammonium component in Athens.

Keywords: atmospheric aerosol, composition, size distribution

1. Introduction

Air pollution in Mediterranean urban centres is characterised by intense photo-chemical activity, high temperatures and low relative humidities during the sum-mer. The effect of these climatic conditions on atmospheric chemistry coupled withcomplex topography and specific emissions can be the generation of secondarypollutants different in composition and concentration to those measured in otherparts of the world.

Athens is one of the largest coastal urban agglomerations in the world with adense population (about four million). The Athens basin, which is about 450 km2,is open towards the sea (Saronikos Bay) to the south-west and is surrounded bymountains to the north-west, north-east and south-east. The mountains standingas physical barriers with small gaps between them. These are the routes for airmasses to enter or exit the basin by near-surface convection over the land. During

Water, Air, and Soil Pollution: Focus 2: 223–235, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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the warm period of the year, the existence of persistent northeasterly winds knownas Etesians efficiently ventilates the basin (Melas et al., 1995). There is very heavyautomobile traffic (∼1.5 million vehicles) and industrial activity within the GreaterAthens Area. Two main industrial areas lie in the Greater Athens Area, one is inthe southwest near the harbour of Pireas and the other is located outside the Athensbasin, to the west of the city.

Numerous scientific papers have been published in the literature on air pol-lution in Athens (see Ziomas, 1998, for details) and, in 1994, Athens was thefocus of an extensive experimental and modelling investigation of the chemicaland meteorological evolution of ozone and related trace gases (Ziomas, 1998, andthat issue of Atmospheric Environment). The role of the sea-breeze circulation onphotochemical pollution episodes has been investigated in many studies (e.g. Lalaset al., 1983, 1987; Cvitas et al., 1985; Prezerakos, 1986; Mantis et al., 1992; Piliniset al., 1993). Worse air pollution episodes occur during the days with a criticalbalance of different wind regimes or sea-breeze circulation. This has been invest-igated by model simulations (Kallos et al., 1993) and by experimental identification(Kambezidis et al., 1995; Klemm et al., 1998; Suppan et al., 1998).

In the Greater Area of Athens, the predominant winds blow from SSW/SWfor the early summer and from N and NE in late summer (Etesians). The SSWwind directions are associated with a sea-breeze circulation in the basin. For thesea breeze days, a weak northerly flow dominates during the late night and earlymorning hours. The sea-breeze cell develops over the Athens basin during daytimethen a strengthening sea-breeze circulation occurs in the early afternoon due tomaximisation of the sea/land temperature difference. During this time, SW windsprevail in the area.

Despite their importance in the atmospheric environment, ambient aerosols havebeen poorly studied in Greece. Most work has concentrated on the chemical speci-ation of urban aerosol (Koliadima et al., 1998; Eleftheriadis et al., 1998; Ziomas etal., 1995; Kirkitsos and Sikiotis, 1993; Tsitouridou and Samara, 1993). Previouslywe have investigated the concentration of common ionic species in the atmosphericaerosol at the centre of Athens, as well as that of secondary gaseous pollutantssuch as nitric and hydrochloric acids (Eleftheriadis et al., 1998). It was shownhow the build-up of atmospheric aerosol and acidic species in the centre of thecity was dependant upon the wind flow patterns. In this article we present furtherinvestigations of the impact of wind regime on aerosol concentrations together withsize fractionated concentrations of atmospheric aerosols at two sites: one upwindand one downwind of Athens.

2. Methodology

Two sampling sites were selected, located at the coastal and inland boundaries ofthe Athens Metropolitan area along the NE-SW axis. The distance between the two

FORMATION AND TRANSPORT OF ATMOSPHERIC AEROSOL OVER ATHENS 225

Figure 1. Map of the Greater Athens Area displaying the sampling locations.

sites was 22 km. The southwesterly wind coincides with the main surface windflow pattern over the summer period. The coastal site was located on the top ofCastella hill 90 m a.s.l. in Pireas and the inland site on the edge of the Ag. Stefanosresidential area (Figure 1).

Sampling was performed over 24 hr periods with an annular denuder system anda May cascade impactor. Three annular denuder tubes were used for SO2, HNO3,HCl, HONO and NH3 measurements. Air was drawn through the annular denuderby means of a vacuum pump operating at 10 L min−1. The first and the seconddenuder tubes were coated with 1% Na2CO3 and glycerol in a methanol:H2O(1:1) solution. Determination of ammonia was made by the third denuder coatedwith 1% citric acid in a methanol solution. A PTFE filter pack with three filtersin series followed the annular denuder tube: Teflon (0.45 µm Whatman), Nylon(Nylon 66, 0.45 µm Sartorius) and Whatman 41 filters, in 47 mm diameter, wereused in tandem. Nylon filters were washed with de-ionised water then dried in a

226 I. COLBECK ET AL.

vacuum desiccator prior to use in order to reduce the background levels of nitrateand chloride. The Whatman 41 filters were impregnated with 5% phosphoric acidsolution for ammonia collection and then dried in a vacuum desiccator prior touse. The cascade impactor sampled at 5 L min−1 resulting in size cuts of 0.5, 1.0,2.0, 4.0, 8.0, 16.0 and 32.0 µm aerodynamic diameter. Standard microscopy slides(76 × 26 mm, thickness = 1.0 mm) were coated with 1% silicone oil in hexanesolution. One slide was used for each stage. A Teflon filter (47 mm diameter, poresize 0.45 µm) was used as the back up filter.

All samples were analysed for Cl−, NO−3 , SO2−

4 , NH+4 , Na+, K+, Mg2+ and

Ca2+ by ion chromatography (Dionex 2000i/SP). Over the three-week period from17 July to 6 August 1997, seventeen samples were obtained for each site (nosamples were obtained on four days during this period). Meteorological paramet-ers, such as wind speed and direction and relative humidity, were provided byroutine measurements of the Greek Meteorological Service. These data were takenat Elliniko and Dekeleia, 6 and 7 km from the inland and coastal sites, respectively.They were selected as the closest representative measurements for the ambient airat the sampling sites.

Over the sampling period, the temperature was in the range of 20–37 ◦C andrelative humility ranged from 20–80% with the highest values occurring for only afew hours of the night. Under these conditions, nitric and hydrochloric acid in thegaseous phase may be responsible for artifact formation on filter packs or impactorsubstrates. The methodology used here did not provide artifact prevention for theimpactor by means of an upstream denuder (Wall et al., 1988). However, massconcentrations of ionic species obtained by the integrated impactor samples can beexamined against those obtained by the denuder filter pack, which are free fromartifact formation. From the sampling characteristics of the denuder system a 50%aspiration efficiency cut off for its inlet at 8 µm was obtained. In Figure 2, theresults for denuder versus impactor concentrations are plotted for both sites. Alarge discrepancy is observed for Cl− and Na+ whilst NO−

3 , SO2−4 and NH+

4 showrelatively good agreement (correlation coefficients = 0.23, 0.31, 0.66, 0.53 and0.69, respectively). For the first two species it is more likely that they are part ofNaCl which was inefficiently sampled by both devices. It is concluded that artifactformation does not make a significant contribution to the mass concentration of theexamined species sampled by the impactor.

3. Results and Discussion

The 24 hr average concentrations for atmospheric aerosol and gaseous species atAg. Stefanos and Pireas are shown, respectively, in Figures 3 and 4. It is evident thatmuch higher concentrations of all the aerosol species were found at Ag. Stefanosduring the first half of the sampling period, while similar or slightly lower concen-trations were observed during the remainder of the period. This is clearly shown

FORMATION AND TRANSPORT OF ATMOSPHERIC AEROSOL OVER ATHENS 227

Figure 2. Mass concentrations measurend in parallel by a May Cascade impactor (<8 µm) anddenuder/filter pack.

in Figure 5 where the difference in concentration between the two sites is plotted.The concentration patterns at the two sites can be explained by the dominant windflow patterns present in the area during the measurements.

Following the work of Suppan et al. (1998) each day was classified accordingto the wind pattern:

• Etesian days: strong northerly winds, which suppress the land/sea-breeze andlead to low pollution levels inside the basin.

• Sea-breeze days: weak northerly winds and a fully developed sea-breeze cellaffecting the whole basin of Athens.

• Normal days: moderate northerly winds and development of a weak sea-breezecell near the coast line.

Table I shows the classification of the days into the three categories. Two typicalsituations dominated during this period. During the first half of the campaign, 17–26 July 1997, southwesterly winds, due to the sea-breeze circulation, were domin-ant. From 29 July 1997 onward, the Etesian winds prevailed in the Athens basin.For typical sea-breeze days higher pollution levels are observed at Ag. Stefanoswhilst lower concentrations coincide with cleaner northeasterly winds (see Fig-ure 5). During 30–31 July 1997, the concentrations detected at both sites weresimilar. This was due to the northerly winds with relatively higher speeds (∼6 ms−1).

228 I. COLBECK ET AL.

Figure 3. Ambient aerosol concentrations during the campaign.

When a strong sea-breeze circulation occurred, high levels of secondary pol-lutants, such as HNO3, NO−

3 and SO2−4 were detected. For instance, during the

period of 24–25 July, there was very weak northerly winds or calm states that en-hanced the formation of secondary aerosols. The pollutants were then transportedby the southwesterly sea-breeze and reached the NE of the Athens basin, e.g. Ag.Stefanos. This is confirmed by the higher levels of nitrous acid that were observed

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Figure 4. Ambient gaseous concentrations during the campaign.

230 I. COLBECK ET AL.

Figure 5. Difference in the concentrations of aerosol and gaseous pollutants between Ag. Stefanosand Pireas.

FORMATION AND TRANSPORT OF ATMOSPHERIC AEROSOL OVER ATHENS 231

TABLE I

Classifications of the days into meteorological conditions

Sea-breeze days Normal days Etesian days

17–26 July | || 27–28 July || | 29 July–1 August

2 August 3 August 4–7 August

(10 days) (3 days) (7 days)

TABLE II

Increased pollutant levels during sea-breeze days over the basin (displayed by the ratiosof differences between the two sites on sea-breeze days to those on Etesian days)

�Sea/�Etesian HONO HNO3 HCl SO2 Cl− NO−3 SO2−

4 NH+4

Average 3.3 2.0 7.4 2.0 1.8 8.8 8.2 10.5

Max. 5.9 4.9 17.0 7.5 4.8 32.0 20.8 22.9

during these sea-breeze days (Figure 5). Nitrous acid, an important secondary airpollutant, promotes photochemical reactions due to its photolytic generation ofhydroxyl radicals (OH·). This promotes the production of nitric acid:

HONO + hν (λ < 400 nm) → OH· + NO

2HONO → NO + NO2 + H2O

NO2 + OH· + M → HNO3 + M

Danalatos and Glavas (1999) indicated that this gas phase formation is predomin-ant, with a maximum diurnal value in the afternoon during the summer.

The degree of enhanced pollutant levels by the sea-breeze circulation is ex-amined in Table II by using the differences of the measured concentrations betweenthe two sites. The results clearly show that secondary pollutants are dramaticallyincreased during the sea-breeze days together with the efficient clearance of pollu-tion by the Etesian winds over the basin. On average, primary SO2 on sea-breezedays is double that on Etesian days. HCl exhibits a high degree of intensificationand as well the secondary particulate NO−

3 , SO2−4 and NH+

4 .The illustrations of particulate SO2−

4 , NO−3 and Cl−, in measured size ranges

under three different wind regimes, are presented in Figure 6. During the Etesian

232 I. COLBECK ET AL.

days, the well-mixed atmosphere leads to a relatively small difference between thetwo sites. Slightly higher concentrations of nitrate and sulphate were measured atPireas. The increment was exhibited predominantly within the size range of 1–2 µm diameter. This suggests that the northerly wind might have transported thesecondary particles, formed in the centre of Athens, to the Saronic Gulf. Duringnormal days, northerly winds prevail at Ag. Stefanos and southerly winds dom-inant over the coastal area. The difference in size distribution between the twosampling stations on the normal days is due to the local emissions at each site.Fine sulphate was higher at Pireas than at Ag. Stefanos. This may possibly beassociated with industrial emissions, however a firm conclusion can not be madefrom the few samples classified as normal days in this survey. During the sea-breezedays, elevated concentrations of the fine mode particles were detected at both sites.A significant increase in the condensation mode for sulphate (d < 0.5 µm) wasevident as the sea-breeze cell developed. Over this period, particulate nitrate in boththe fine and coarse modes also increased. Elevated levels of nitrate, 4 µm diameter,were particularly observed on the days with a strong sea-breeze circulation. Theformation of nitric acid with marine aerosols is assumed as the main reaction.

The ion balance between measured ions at both sites is shown in Figure 7. Thelarge deficiency of anions may be due to carbonate/bicarbonate ions (CaCO3 is amajor component of loose soil and dust in Greece) and also due to the possibleexistence of other earth materials such as CaO, MgO, Na2O and K2O. It was foundthat correlations amongst the measured species were generally significantly higherat Ag. Stefanos than that at Pireas. Sulphate showed good correlations betweenits precursor SO2 as well with ammonium (r = 0.94 and 0.76, respectively). Thissuggests the production of NH4HSO4/(NH4)2SO4 aerosols, formed through theneutralisation of NH3 with sulphuric acids. The molar ratio of ammonium to sulph-ate obtained was (NH+

4 /SO2−4 ) 1.3±0.4 at Ag. Stefanos and 1.5±0.4 at Pireas. This

indicates that the components of ammonium sulphate aerosol were mainly presentas NH4HSO4 and (NH4)3H(SO4)2 during this period.

4. Conclusions

The wind regimes exhibit a distinct effect on ambient aerosols in Athens. Analysisof the size distributions, for different wind regimes indicated that on Etesian daysconcentrations of all species are marginally higher at Pireas. During ‘normal’ days,the size distributions were similar and any difference in the distribution betweenthe two sampling sites on such days maybe due to the local emissions. Elevatedconcentrations in the fine mode were detected at both sites during the sea-breezedays. The sea breeze circulation enhances the development of secondary aerosolswhich was clearly shown at the inland site. Nitrous acid, hydrochloric acid andparticulate nitrate, sulphate and ammonium increase during sea-breeze days. Elev-ated levels of nitrate, 4 µm diameter, were particularly observed on the days with

FORMATION AND TRANSPORT OF ATMOSPHERIC AEROSOL OVER ATHENS 233

Figure 6. Concentrations of sulphate, nitrate and chloride, as a function of particle size, duringsea-breeze, Etesian and normal days.

a strong sea-breeze circulation. The formation of nitric acid with marine aerosolswas assumed as the main reaction. Ammonium sulphate was found to be the majorammonium component in Athens. As only seventeen days data were collected inthe present study, further evaluation involving more samples will be required for abetter understanding and confirmation of the results.

234 I. COLBECK ET AL.

Figure 7. Ion balance of ambient aerosols.

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