Atmospheric Research 89 (2008) 382–390

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Interaction patterns of major photochemical pollutants in Istanbul, Turkey

Ulaş İma, Mete Tayanç b, Orhan Yenigün a,⁎a Bogazici University, Institute of Environmental Sciences, İstanbul, Turkeyb Marmara University, Faculty of Engineering, Department of Environmental Engineering İstanbul, Turkey

a r t i c l e i n f o

⁎ Corresponding author. Tel.: +90 212 359 69 46; faE-mail addresses: ulasim@boun.edu.tr (U. İm),

mtayanc@eng.marmara.edu.tr (M. Tayanç), yeniguno@(O. Yenigün).

0169-8095/$ – see front matter © 2008 Elsevier B.V.doi:10.1016/j.atmosres.2008.03.015

a b s t r a c t

Keywords:

Hourly concentration levels of ozone, nitrogen oxide, and hydrocarbon measured between2001 and 2005 in Kadıköy and Saraçhane, two urban districts in the Anatolian and Europeansides of İstanbul, were analyzed. This study focused especially on episodes of 3 days or longer.The highest ozone concentrations were observed in summer periods having sunny days andmaximum temperatures above 25 °C, and the episodes were mainly characterized bysouthwesterly surface winds during the day and northeasterly surface winds during thenight. Considering all high-ozone days, correlations of ozone with nitrogen oxides (NOx) andvolatile organic carbon (VOC) species were calculated to be −0.44 and −0.85 for Kadıköy,respectively, whereas they were estimated to be −0.84 and −0.39 for Saraçhane, respectively.High VOC-/NOx ratios at both stations indicated that NOx-sensitive chemistry was dominant inthe region. However, higher correlations of VOCs in Kadıköy, as compared with those inSaraçhane, indicated that VOCs also make an important contribution to ozone formation. High-ozone days generally showed maximum concentrations at afternoon hours and minimumconcentrations at rush hours due to NOx titration by traffic emissions. In addition, the HybridSingle-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was employed backward forthree long episodes, and the results showed the northerly, northwesterly, and northeasterlytransport of air masses to the city. Since transport was mainly from the Black Sea and from ruralareas north of İstanbul, contributions by advection tomaximum levels of ozone in the city werebelieved to be low.

© 2008 Elsevier B.V. All rights reserved.

OzonePhotochemical precursorsİstanbulPearson correlationHYSPLIT model

1. Introduction

Ozone (O3) levels in the southern European and Medi-terranean regions have been studied by Klemm et al. (1998),Güsten et al. (1997), and Peleg et al. (1997). Kalabokas andBartzis (1998) studied the Aegean region. Photochemicalpollution in İstanbul also has been studied by Anteplioğlu(2000), Kocak et al. (2000), Topçu and İncecik (2002), andTopçu et al. (2003). Topçu and İncecik (2002) studied O3

pollution and its relation with nitrogen oxide (NOx) speciesand some meteorological parameters in İstanbul. They foundthat O3 concentrations generally increased with increasingtemperature during the O3 season. Topçu et al. (2003)

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studied the relationships of high O3 concentrations betweenchemical and meteorological characteristics using themeteorological Systems Applications International MesoscaleModel (SAIMM) and found that high O3 concentrationsgenerally occur during low-wind conditions. Topçu et al.(2005) found that high O3 episodes were characterized bysouthwesterly and west–southwesterly winds and calmconditions and that the diurnal variations of O3, NOx, andnonmethane hydrocarbons (nMHCs) supported buildup of O3

during rush hours. Im et al. (2006) showed that highconcentrations of O3 can be experienced both during after-noon hours and in the late night due to sharp decreases inmixing heights. Daytime, southwesterly winds transportedhigh concentrations of O3 and its precursors to the northduring these episodes. The previous studies also showed thatepisodes of photochemical smog in İstanbul tend to beginduring the strongest phase of the high-pressure system andcontinue as it weakens.

383U. İm et al. / Atmospheric Research 89 (2008) 382–390

Photochemical smog was characterized by high O3 due tocomplex and non-linear chemistry and meteorology. Photo-chemical smog is related to the very complex reactionsbetween NOx and volatile organic compound (VOC) speciesunder solar radiation (Sillman, 1999). In simple terms, O3 isproduced by combining an oxygen molecule with an oxygenatom that is supplied from the photolysis of nitrogen dioxide(NO2) by solar radiation. In addition, hydrocarbons (HCs) areoxidized to form carbon dioxide and water vapor by thehydroxyl radical, which is produced by the photolysis of O3 inthe presence of water vapor. Since these complex reactionsoccur in the atmosphere, measuring O3 levels alone cannothelp in evaluating photochemical conditions. While underNOx-sensitive conditions, O3 formation is almost entirelygoverned by NOx and largely independent of VOCs. UnderVOC-sensitive conditions, O3 formation is related to increasedVOCs and not to NOx changes. It is suggested that VOC–NOx

concentrations higher than 10 indicate NOx-sensitive chem-istry (Pryor, 1998).

Under NOx-sensitive conditions, low concentrations ofNOx and high concentrations of VOC–NOx controls theformation of O3 independent of VOCs (Skov et al., 1997;Seinfeld and Pandis, 1998; Sillman, 1999). Since solar radia-tion is the key parameter in these reactions, strong photo-chemical production leads to peak O3 concentrations in theafternoon hours. Different contributions to O3 levels maycome from background O3 that originates from stratosphericinjection and production far away from local sources—mean-ing that it involves photo oxidation of CH4 (methane) and CO

Fig. 1. Map of the study region and loc

(carbon monoxide), as well as regional photochemicalproduction and depletion of O3 by deposition and lossreactions.

In our study, meteorological and air quality observationsby the U.S. National Weather Service and İstanbul GreaterMunicipality, respectively, were analyzed. The diurnal varia-tions of O3, NOx, and VOCs were investigated for selectedepisodes at two stations, one on each side of İstanbul. The NOx

and VOC sensitivities to O3 formation during these days wereanalyzed by examining the correlations between O3 and itsprecursors.

2. Materials and methods

2.1. Study area, meteorology, and emissions

İstanbul is a large city located at the geographic coordi-nates of 41°N and 29°E (Fig. 1). Bosporus, a sea water straitextending from the Black Sea to the Marmara Sea, divides thecity into European and Anatolian sides. The Black Sea in thenorth and Marmara Sea in the south produce a differentialheating of surfaces, leading to different meteorologicalconditions that may play a role in the transport of O3. Thecomplex terrain of İstanbul also influences the circulationsystems over the city. In the summer months, meteorology inthe region is mainly influenced by hot, dry low-pressuresystems from the Persian Gulf extending from the southeastof Turkey. This system is the main reason for the dominantnortheasterly winds in İstanbul. When the system weakens,

ations of the air quality stations.

Table 1Maximum O3 concentrations and surface meteorological conditions forselected episodes

Date Station Maximumconcentration(μg m−3)

Temperature(C)

Pressure(mb)

Windspeed(ms−1)

14.07.2002 Kadıköy 118 27.6 1011.3 3.615.07.2002 Kadıköy 140 28.3 1011.9 2.916.07.2002 Kadıköy 134 29.5 1010.3 2.123.06.2003 Saraçhane 134 23.1 1015.9 3.024.06.2003 Saraçhane 187 25.1 1016.4 2.325.06.2003 Saraçhane 168 26.6 1011.6 3.026.06.2003 Saraçhane 113 25.3 1008.5 4.727.06.2003 Saraçhane 141 24.5 1010.2 3.415.07.2005 Kadıköy 108 23.9 1012.1 6.816.07.2005 Kadıköy 123 23.5 1012.4 5.517.07.2005 Kadıköy 132 24.1 1013.1 3.318.07.2005 Kadıköy 141 25.4 1013.2 1.419.07.2005 Kadıköy 132 26.2 1016.3 2.720.07.2005 Kadıköy 123 26.5 1013.8 4.321.07.2005 Kadıköy 105 26.3 1011.0 6.9

384 U. İm et al. / Atmospheric Research 89 (2008) 382–390

land–sea breeze circulation prevails, producing southerly andsouthwesterly winds blowing from the Marmara Sea toİstanbul during the day. Although sea level pressure is notsignificantly high during those days, the system can be calleda “relatively” high-pressure system. During the evening andnight, wind shifts to the northerly, northeasterly direction,and this situation leads to the transport of pollutants to thesouth and southwest. The city experiences a transitionclimate between Mediterranean and temperate conditions.Strong inversion conditions in the summer sometimes causeO3 concentrations to reach maximum levels in the lateafternoon (Topçu and İncecik, 2002).

Within the last 40 years, İstanbul has experienced rapidurbanization and industrialization, and statistics show thepopulation is over 10 million. In addition, there are more than2 million motor vehicles registered in İstanbul (İstanbulGovernor's Office, 2005). Liquefied petroleum gas (LPG) hasbeen widely used by taxis from the beginning of 1998.Emissions produced by the transportation sector constitute alarge portion of the O3 precursors emitted into the atmo-sphere. Traffic rush hours clearly create a sink for O3 by theemissions of NOx. There has been a shift from coal to naturalgas for domestic heating beginning in the mid-1990s (Tayanç,2000). According to the 2005 records from the IstanbulMetropolitan Municipality, 3.7×109 m3 of natural gas arecombusted annually in the city. This region experiences verydense industrial activities—almost the highest in the country.Based on reports from the İstanbul Chamber of Industry, thetextile industry comprises 37% of the industrial activities inİstanbul; metal industry, 30%; chemical industry, 21%; foodindustry, 5%; and other industries, 7%. In addition, air qualityin the city is influenced by the emissions from the İzmit Gulf,which is a major source of HC emissions owing to Tüpraş, agiant petroleum refinery. Tüpraş is located in the south-eastern part İstanbul, and southeasterly winds can carry theseemissions to the study region. High emissions particularlyfrom the textile, petroleum, and chemical industries com-bined with traffic emissions lead to significant VOC and NOx

accumulation, producing favorable conditions for the forma-tion of dense photochemical smog due to complex land-use,topography, and atmospheric circulation systems.

2.2. Data, analysis, and HYSPLIT back trajectory model

Air quality data used in the study were obtained from theİstanbul Greater Municipality between 2001 and 2005. Theİstanbul Greater Municipality operates 10 air quality stationsthroughout the city, with only two continuously measuringO3 and its precursors. These stations include the KadıköyStation on the Anatolian side and the Saraçhane Station on theEuropean side. Both stations demonstrate mixed urbancharacteristics (residential and commercial). Pollutants mea-sured at the Kadıköy and Saraçhane Stations are CO, NO, NO2,NOx, and O3. Beginning in 2002 in Saraçhane and 2003 inKadıköy, total hydrocarbons (THC), CH4 and nMHC wereadded to the analyzed parameters. Nitrogen oxides (NOx)species were monitored using the chemiluminescencemethod, CO by infrared (IR) adsorption, O3 by ultraviolet(UV) photometry, and HC by gas chromatography.

Filtering techniques were developed to scan the rawdataset discarding days containing more than 12 h of missing

data and months containing more than 15 days of missingdata. With these filtering techniques, hourly O3 concentra-tions exceeding 100, 150, and 240 μg m−3 also were discardedto identify episodes of photochemical pollution within theperiod. The Pearson correlation factors were calculated for O3

and its precursors to show the relation among thesechemicals. Ratios of VOC to NOx also were calculated tounderstand the dominant O3 formation regime in high O3

days. Episodes of 3 days or longer were particularly focusedon during the study. Three episodes occurring on July 14–16,2002; June 23–27, 2003; and July 15–21, 2005 were analyzedand are presented in this paper. Selected episodes withmaximumO3 concentrations andmeteorological factors, suchas surface pressure, temperature, and averagewind speed, aresummarized in Table 1.

The Hybrid Single-Particle Lagrangian Integrated Trajec-tory (HYSPLIT) model was used to visualize pollutanttransport to İstanbul during episodic days. This model is acomplete system for computing simple, air parcel trajectoriesto complex dispersion and deposition simulations. The modeldefault configuration assumes a puff distribution in thehorizontal direction and particle dispersion in the verticaldirection. In this way, the greater accuracy of the verticaldispersion parameterization of the particle model is com-bined with the advantage of having an ever-expandingnumber of particles represent the pollutant distribution. TheFNL (final analyses) archive for NCEP (US National WeatherService, National Centers for Environmental Predictions)Global Data Assimilation System (GDAS) analyses were usedas input to the HYSPLIT model. The back trajectory simula-tions were initialized and run 24 h before the first days of theepisodic periods. The runs continued throughout the entireepisodic period.

3. Results

3.1. 2001–2005 period analysis

Daily O3 concentrations for both stations between 2001and 2005 are presented in Fig. 2. As shown in the figure, high

Fig. 2. Daily average O3 concentrations in (a) Kadıköy and (b) Saraçhane between 2001 and 2005.

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O3 levels, generally above a daily average of 80 μg m−3, wereexperienced in summer months. The highest daily average O3

concentration was calculated to be 94 μg m−3 on August 6,2005, at the Kadıköy Station and 82 μgm−3 on July 20, 2001, atSaraçhane Station. The long-term daily average for Kadıköywas calculated tobe21.4 μgm−3,whereas for Saraçhane, the long-term average was 16.2 μg m−3. These averages show that theKadıköy Station was exposed to higher O3 levels as compared totheSaraçhaneStation.However, onanhourlybasis, theSaraçhaneStationmeasured higher daily averagemaximumconcentrations.

Hourlyaverages forhighO3dayswere calculated andplottedto establish a generalized diurnal trend for the episodes. Asshown in Fig. 3, the generated profilewas a typical diurnal trendfor O3, reaching maximum concentrations during afternoonhoursdue tohigh solar radiation,which triggered thechemistry.Somesmaller increases inO3 concentrationswere also observedin early morning hours due to a buildup of O3 in the absence, ordecreased levels, of NOx. During rush hour traffic between 0700and 0900 LST (Local Sidereal Time), increasing NOx emissionscause theO3 levels to reach theirminimumdue to NOx titration,and then O3 begins to build-up due to increasing solar radiationand in turn increasing chemical reaction rate.

The dominant wind directions during high O3 days arepresented in Fig. 4. The figure agrees well with the generalcirculation pattern for the region, which is characterized bydominant northeasterly and southwesterly winds. Southwes-terlywindplays a particular role in transporting chemicals fromregional emissions sources toward the northeast, and north-easterly winds transport pollutants toward the southeast.

The overall correlation factors between O3 and itsprecursors were calculated and are presented in Table 2. As

expected, negative correlations resulted and agreed with thechemistry. Considering all high O3 days, correlations betweenO3 and NOx and VOC species were calculated to be −0.44 and−0.85, respectively, for Kadıköy, whereas they were to be−0.84 and −0.39, respectively, for Saraçhane. If the values arecompared, it would be found that there was a higher negativecorrelation with THC species for Kadıköy than for Saraçhane.

To get a clearer understanding of the chemistry, VOC–NOx

ratios were calculated, and the scatter diagrams are presentedin Fig. 5. It can be seen that the slopes of O3 change (a) againstNOx with changes of −0.1952 and −0.6982 for the Kadıköy andSaraçhane Stations, respectively and (b) against VOC withchanges of −0.0139 and −0.0005 for the Kadıköy and SaraçhaneStations respectively. Thus, it is possible to say for both Kadıköyand Saraçhane that the change in VOC concentration does nothave as much effect on O3 formation as the change in NOx

concentration has (Fig. 5a,b). However, the effect of VOCs on O3

formation in Saraçhane has much less effect as compared toKadıköy (Fig. 5c,d). The VOC–NOx ratios were calculated to be14.5 for Kadıköyand 13.7 for Saraçhane. Both ratioswere higherthan 10, showing that the NOx-sensitive chemistry wasdominant in the study domain. Fig. 5b and c also confirm thatNOx-sensitive chemistry was dominant, meaning O3 formationwas affected by changes in NOx levels but considerably lessaffected by changes in VOC levels.

The relation between NO and total NOx species gives cluesabout the origin of the NOx precursors. As seen in Fig. 6, thereis a very strong correlation between NO and NOx, showingthat the NOx species are mainly from local sources (Lu andWang, 2004). Thus, we can say that emissions within the cityare an important contributor to the chemistry that takes place

Fig. 3. Long-term, daily averaged pollutant profiles for (a) Kadıköy and (b) Saraçhane.

386 U. İm et al. / Atmospheric Research 89 (2008) 382–390

in the lower atmosphere. Since the atmospheric lifetime ofNOx species (hours) is relatively shorter than the lifetime of O3

(days), we can expect that air masses transported to theregion may contain significant levels of O3 together withsmall amounts of NOx. Moderate wind speeds during somehigh O3 days point to these conditions.

3.2. Episodic analysis and HYSPLIT trajectories

The three episodic days were mostly characterized bynortherly and northeasterly winds during the night and

Fig. 4. Frequency distribution of dominant wind directions for high O3 days

Table 2Long-term correlations of O3 with NOx and VOC species and respective levelsof significance

Kadıköy Saraçhane

Correlation Significance Correlation Significance

NO −0.42 0.015(⁎⁎) −0.84 0.2520NO2 −0.44 0.056(⁎⁎) −0.58 0.2250NOx −0.44 0.000(⁎⁎) −0.84 0.000(⁎⁎)CH4 −0.82 0.000(⁎⁎) −0.64 0.000(⁎⁎)nMHC −0.63 0.000(⁎⁎) −0.82 0.000(⁎⁎)THC −0.85 0.000(⁎⁎) −0.69 0.000(⁎⁎)

⁎Above 95% significance level.

.

southwesterly winds during the day, which is a characteristiccirculation as explained in Im et al. (2006). Temperatureswere generally above 25 °C, and there were relatively high-pressure systems dominating during the episodes with low-wind speeds (Table 1). Daily maximum O3 concentrationswere above 100 μg m−3 for all days, reaching 140 μg m−3

during the 2002 and 2005 episodes and 187 μgm−3 during the2003 episode. The episodic profiles of O3 and its precursorsare given in Fig. 7. In Fig. 7a and b, VOC curves were notpresented owing to unreliable hourly data that failed to passthe filtering test. However, NOx and O3 datawere high quality.The two episodes were considered to be quite important inpresenting episodic characteristics in the city, and therefore,

⁎⁎Above 99% significance level.

Fig. 5. Sensitivity of O3 formation to NOx and VOC changes in a and b, Kadıköy stations, and c and d, Saraçhane Stations.

387U. İm et al. / Atmospheric Research 89 (2008) 382–390

they will be discussed in this paper. As seen in the figure,maximum O3 concentrations appeared during afternoonhours around 1400–1600 LST, when solar radiation was atthe maximum. Minimum concentrations appeared duringrush hours when NOx emissions were at their maximum,leading to NOx titration of O3.

Fig. 6. Relations between NOx and NO levels in (a) Kadıköy and (b) SaraçhaneStations.

Correlation factors and their significance levels werecalculated and are presented in Table 3. As was presented foroverall high O3 days, higher correlations with NOx species inSaraçhane were calculated for longer episodes than forKadıköy. This result also was confirmed in Fig. 5a and cwhere a steeper slope was obtained in Saraçhane than inKadıköy. The Kadıköy Station generally measured lowermaximum daily concentrations of O3, and the reason wasbelieved to be its closer proximity to dense traffic, leading toNOx titration, and to the sea, leading to good mixing of sea–land breezes.

HYSPLIT backward trajectory simulation results are pre-sented in Fig. 8. The model was configured to produce themixing-height profile through each episode in order tounderstand if the mixing heights contributed to high levelsof O3 buildup in the region. As can be seen in Fig. 8a at 500 maltitude, northerly winds were dominant, and on July 14,2002, northeasterly winds carried the almost clean air of theBlack Sea toward İstanbul. On that day, the mixing height wasaround 250 m, and this low-mixing height was maintaineduntil the morning hours of July 15, 2002, suppressing thepollutants emitted from sources in İstanbul to the surface,increasing their concentrations. The other two days of the2002 episodic period had typically high-mixing heightsduring the day and low-mixing heights during the night.

The 2003 episode, as seen in Fig. 8b, was characterized byvery low-mixing heights during all 5 days of the episode,varying below 1000 m. Especially on June 25 and 26, thedaytime mixing heights were around 500 m or below,restricting vertical mixing. Together with low-wind speeds,restricted vertical mixing trapped pollutants in the surface,leading to high O3 values reaching 187 μg m−3 in Saraçhane.Generally, the 500-m transport had a southerly directionvarying between southwesterly to southeasterly directionsduring this episode.

Fig. 7. Ozone, NOx, and VOC profiles for the (a) July 14–16, 2002, episode; (b) July 23–27, 2003, episode; and (c) July 15–21, 2005, episode.

388 U. İm et al. / Atmospheric Research 89 (2008) 382–390

HYSPLIT results for the 2005 episode are presented inFig. 8c. Relatively low-mixing heights on July 15, 16, 20, and21, 2005, sometimes lower than 300 m, contributed toelevated levels of O3. Typical properties for all episodes werehigh-mixing heights reaching 1000 m during the day andvery low-mixing heights around 250 m during the night.

Table 3Correlation factors and significance levels of ozone with precursors for selected epi

14–16.07.2002 23–27.06.20

Correlation Significance Correlation

NO −0.42 0.000(⁎⁎) −0.56NO2 −0.68 0.005(⁎⁎) −0.70NOx −0.53 0.009(⁎⁎) −0.62CH4 N/A N/A N/AnMHC N/A N/A N/ATHC N/A N/A N/A

⁎Above 95% significance level.⁎⁎Above 99% significance level.

Trajectories clearly showed the south, southwest, andsoutheast to be the main directions of transport for allepisodes. When considering transport mainly from the BlackSea and from rural areas in the north, northwest, andnortheast, and the variability of mixing heights, it is possibleto say that maximum levels measured during certain days in

sodes

03 15–21.07.2005

Significance Correlation Significance

0.001(⁎⁎) −0.34 0.000(⁎⁎)0.000(⁎⁎) −0.45 0.0680.000(⁎⁎) −0.40 0.000(⁎⁎)N/A −0.53 0.000(⁎⁎)N/A −0.36 0.000(⁎⁎)N/A −0.52 0.000(⁎⁎)

Fig. 8. HYSPLIT trajectory and mixing-height results for (a) July 14–16, 2002, episode; (b) July 23–27, 2003, episode; and (c) July 15–21, 2005, episode.

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İstanbul most likely originated locally than being trans-ported from other areas.

4. Summary and conclusions

İstanbul is experiencing a rapid and uncontrolled growthin urbanization, industrialization, and transportation. Thedense and various kinds of industrial activities in the regiontogether with the increasing number of vehicles are causingair quality problems in the city. Air quality data fromcontinuous measurement of O3 and its main precursors attwo stations, one on each side of İstanbul, were analyzed tounderstand the relation among the various chemicalsinvolved in photochemical processes. Results show that highlevels of O3 were observed mostly under relatively antic-yclonic conditions with considerably low-wind speeds. Thedominating wind directions were northeasterly during thenight and early morning hours and southly to southwesterlyduring the day. During the summer, the effect of the PersianGulf low-pressure system was a key parameter in themeteorology of the region. The system produced north-easterly winds through the Black Sea toward İstanbul. Whenthe systemweakened, the so called “relatively” high-pressuresystem starts to develop over the region exerting its owncharacteristics of circulation.

High O3 days demonstrated a typical diurnal profile withmaximum concentrations appearing during afternoon hoursand minimum concentrations appearing during rush hoursdue to NOx titration from traffic emissions. Daily average O3

concentrations were around 100 μg m−3 whereas individualepisodic periods may have had hourly concentrations reach-ing 180 μg m−3 in Saraçhane and 140 μg m−3 in Kadıköy.Lower-daily maximum values at the Kadıköy Station resultedfrom better ventilation owing to the close location of thestation to the sea, and larger titration values of NOx resultedfrom close proximity to dense traffic. However, VOC–NOx

ratios at both stations showed that the chemistry was NOx-sensitive, which agrees well with the correlation factorscalculated among O3 and its precursors. Higher correlations ofO3 with VOCs were calculated in Kadıköy than in Saraçhane,which confirms the scatter diagrams produced for NOx andVOC formation analyses. Volatile organic compounds atKadıköy were relatively more effective in photochemistrythan at Saraçhane. High correlations between NO and NOx

were calculated for both stations suggesting that NOx

emissions originated locally.For better characterization, episodes of three or more

consecutive days were analyzed. These periods were char-acterized by northeasterly winds during the night and earlymorning hours and southwesterly winds during the day withrelatively high-pressure values. Ozone concentrations tendedto increase during afternoon hours around 1400–1600 LST

and decrease during rush hours. Hourly O3 concentrationsduring afternoon hours were above 100 μg m−3 and followeda typical diurnal O3 cycle.

HYSPLIT modeling results clearly showed that the north,northwest, and northeast were the dominant trajectories oftransport to the city. Since transport was mainly from theBlack Sea and from rural areas in the north of İstanbul,contribution by advection to the maximum levels of O3 wasbelieved to be low, and the high levels of O3 mainly originatedfrom İstanbul. The results also demonstrated that restrictedvertical mixing plays an important role in the elevated levelsof pollutants in the İstanbul metropolitan area. The chemistrywith restrictedmixing conditions can lead to intense episodesof air pollution.

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