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Interaction patterns of major photochemical pollutants in Istanbul, Turkey
Atmospheric Research 89 (2008) 382–390
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Atmospheric Research j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a t m o s
Interaction patterns of major photochemical pollutants in Istanbul, Turkey Ulaş İm a, Mete Tayanç b, Orhan Yenigün a,⁎ a b
Bogazici University, Institute of Environmental Sciences, İstanbul, Turkey Marmara University, Faculty of Engineering, Department of Environmental Engineering İstanbul, Turkey
a r t i c l e
i n f o
Keywords: Ozone Photochemical precursors İstanbul Pearson correlation HYSPLIT model
a b s t r a c t Hourly concentration levels of ozone, nitrogen oxide, and hydrocarbon measured between 2001 and 2005 in Kadıköy and Saraçhane, two urban districts in the Anatolian and European sides 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 and maximum temperatures above 25 °C, and the episodes were mainly characterized by southwesterly surface winds during the day and northeasterly surface winds during the night. Considering all high-ozone days, correlations of ozone with nitrogen oxides (NOx) and volatile 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 in the region. However, higher correlations of VOCs in Kadıköy, as compared with those in Saraçhane, indicated that VOCs also make an important contribution to ozone formation. Highozone days generally showed maximum concentrations at afternoon hours and minimum concentrations at rush hours due to NOx titration by traffic emissions. In addition, the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was employed backward for three long episodes, and the results showed the northerly, northwesterly, and northeasterly transport of air masses to the city. Since transport was mainly from the Black Sea and from rural areas north of İstanbul, contributions by advection to maximum levels of ozone in the city were believed to be low. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ozone (O3) levels in the southern European and Mediterranean regions have been studied by Klemm et al. (1998), Güsten et al. (1997), and Peleg et al. (1997). Kalabokas and Bartzis (1998) studied the Aegean region. Photochemical pollution in İstanbul also has been studied by Anteplioğlu (2000), Kocak et al. (2000), Topçu and İncecik (2002), and Topçu et al. (2003). Topçu and İncecik (2002) studied O3 pollution and its relation with nitrogen oxide (NOx) species and some meteorological parameters in İstanbul. They found that O3 concentrations generally increased with increasing temperature during the O3 season. Topçu et al. (2003)
⁎ Corresponding author. Tel.: +90 212 359 69 46; fax: +90 212 257 50 33. E-mail addresses: [email protected] (U. İm), [email protected] (M. Tayanç), [email protected] (O. Yenigün). 0169-8095/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2008.03.015
studied the relationships of high O3 concentrations between chemical and meteorological characteristics using the meteorological Systems Applications International Mesoscale Model (SAIMM) and found that high O3 concentrations generally occur during low-wind conditions. Topçu et al. (2005) found that high O3 episodes were characterized by southwesterly and west–southwesterly winds and calm conditions and that the diurnal variations of O3, NOx, and nonmethane hydrocarbons (nMHCs) supported buildup of O3 during rush hours. Im et al. (2006) showed that high concentrations of O3 can be experienced both during afternoon hours and in the late night due to sharp decreases in mixing heights. Daytime, southwesterly winds transported high concentrations of O3 and its precursors to the north during these episodes. The previous studies also showed that episodes of photochemical smog in İstanbul tend to begin during the strongest phase of the high-pressure system and continue as it weakens.
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Photochemical smog was characterized by high O3 due to complex and non-linear chemistry and meteorology. Photochemical smog is related to the very complex reactions between NOx and volatile organic compound (VOC) species under solar radiation (Sillman, 1999). In simple terms, O3 is produced by combining an oxygen molecule with an oxygen atom that is supplied from the photolysis of nitrogen dioxide (NO2) by solar radiation. In addition, hydrocarbons (HCs) are oxidized to form carbon dioxide and water vapor by the hydroxyl radical, which is produced by the photolysis of O3 in the presence of water vapor. Since these complex reactions occur in the atmosphere, measuring O3 levels alone cannot help in evaluating photochemical conditions. While under NOx-sensitive conditions, O3 formation is almost entirely governed by NOx and largely independent of VOCs. Under VOC-sensitive conditions, O3 formation is related to increased VOCs and not to NOx changes. It is suggested that VOC–NOx concentrations higher than 10 indicate NOx-sensitive chemistry (Pryor, 1998). Under NOx-sensitive conditions, low concentrations of NOx and high concentrations of VOC–NOx controls the formation of O3 independent of VOCs (Skov et al., 1997; Seinfeld and Pandis, 1998; Sillman, 1999). Since solar radiation is the key parameter in these reactions, strong photochemical production leads to peak O3 concentrations in the afternoon hours. Different contributions to O3 levels may come from background O3 that originates from stratospheric injection and production far away from local sources—meaning that it involves photo oxidation of CH4 (methane) and CO
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(carbon monoxide), as well as regional photochemical production and depletion of O3 by deposition and loss reactions. In our study, meteorological and air quality observations by the U.S. National Weather Service and İstanbul Greater Municipality, respectively, were analyzed. The diurnal variations of O3, NOx, and VOCs were investigated for selected episodes at two stations, one on each side of İstanbul. The NOx and VOC sensitivities to O3 formation during these days were analyzed by examining the correlations between O3 and its precursors. 2. Materials and methods 2.1. Study area, meteorology, and emissions İstanbul is a large city located at the geographic coordinates of 41°N and 29°E (Fig. 1). Bosporus, a sea water strait extending from the Black Sea to the Marmara Sea, divides the city into European and Anatolian sides. The Black Sea in the north and Marmara Sea in the south produce a differential heating of surfaces, leading to different meteorological conditions that may play a role in the transport of O3. The complex terrain of İstanbul also influences the circulation systems over the city. In the summer months, meteorology in the region is mainly influenced by hot, dry low-pressure systems from the Persian Gulf extending from the southeast of Turkey. This system is the main reason for the dominant northeasterly winds in İstanbul. When the system weakens,
Fig. 1. Map of the study region and locations of the air quality stations.
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land–sea breeze circulation prevails, producing southerly and southwesterly winds blowing from the Marmara Sea to İstanbul during the day. Although sea level pressure is not significantly high during those days, the system can be called a “relatively” high-pressure system. During the evening and night, wind shifts to the northerly, northeasterly direction, and this situation leads to the transport of pollutants to the south and southwest. The city experiences a transition climate between Mediterranean and temperate conditions. Strong inversion conditions in the summer sometimes cause O3 concentrations to reach maximum levels in the late afternoon (Topçu and İncecik, 2002). Within the last 40 years, İstanbul has experienced rapid urbanization and industrialization, and statistics show the population is over 10 million. In addition, there are more than 2 million motor vehicles registered in İstanbul (İstanbul Governor's Office, 2005). Liquefied petroleum gas (LPG) has been widely used by taxis from the beginning of 1998. Emissions produced by the transportation sector constitute a large portion of the O3 precursors emitted into the atmosphere. Traffic rush hours clearly create a sink for O3 by the emissions of NOx. There has been a shift from coal to natural gas for domestic heating beginning in the mid-1990s (Tayanç, 2000). According to the 2005 records from the Istanbul Metropolitan Municipality, 3.7 × 109 m3 of natural gas are combusted annually in the city. This region experiences very dense industrial activities—almost the highest in the country. Based on reports from the İstanbul Chamber of Industry, the textile industry comprises 37% of the industrial activities in İstanbul; metal industry, 30%; chemical industry, 21%; food industry, 5%; and other industries, 7%. In addition, air quality in the city is influenced by the emissions from the İzmit Gulf, which is a major source of HC emissions owing to Tüpraş, a giant petroleum refinery. Tüpraş is located in the southeastern part İstanbul, and southeasterly winds can carry these emissions to the study region. High emissions particularly from the textile, petroleum, and chemical industries combined with traffic emissions lead to significant VOC and NOx accumulation, producing favorable conditions for the formation 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 the İstanbul Greater Municipality between 2001 and 2005. The İstanbul Greater Municipality operates 10 air quality stations throughout the city, with only two continuously measuring O3 and its precursors. These stations include the Kadıköy Station on the Anatolian side and the Saraçhane Station on the European side. Both stations demonstrate mixed urban characteristics (residential and commercial). Pollutants measured at the Kadıköy and Saraçhane Stations are CO, NO, NO2, NOx, and O3. Beginning in 2002 in Saraçhane and 2003 in Kadıköy, total hydrocarbons (THC), CH4 and nMHC were added to the analyzed parameters. Nitrogen oxides (NOx) species were monitored using the chemiluminescence method, CO by infrared (IR) adsorption, O3 by ultraviolet (UV) photometry, and HC by gas chromatography. Filtering techniques were developed to scan the raw dataset discarding days containing more than 12 h of missing
Table 1 Maximum O3 concentrations and surface meteorological conditions for selected episodes Date
Station
Maximum concentration (μg m− 3)
Temperature (C)
Pressure (mb)
Wind speed (ms− 1)
14.07.2002 15.07.2002 16.07.2002 23.06.2003 24.06.2003 25.06.2003 26.06.2003 27.06.2003 15.07.2005 16.07.2005 17.07.2005 18.07.2005 19.07.2005 20.07.2005 21.07.2005
Kadıköy Kadıköy Kadıköy Saraçhane Saraçhane Saraçhane Saraçhane Saraçhane Kadıköy Kadıköy Kadıköy Kadıköy Kadıköy Kadıköy Kadıköy
118 140 134 134 187 168 113 141 108 123 132 141 132 123 105
27.6 28.3 29.5 23.1 25.1 26.6 25.3 24.5 23.9 23.5 24.1 25.4 26.2 26.5 26.3
1011.3 1011.9 1010.3 1015.9 1016.4 1011.6 1008.5 1010.2 1012.1 1012.4 1013.1 1013.2 1016.3 1013.8 1011.0
3.6 2.9 2.1 3.0 2.3 3.0 4.7 3.4 6.8 5.5 3.3 1.4 2.7 4.3 6.9
data and months containing more than 15 days of missing data. With these filtering techniques, hourly O3 concentrations exceeding 100, 150, and 240 μg m− 3 also were discarded to identify episodes of photochemical pollution within the period. The Pearson correlation factors were calculated for O3 and its precursors to show the relation among these chemicals. Ratios of VOC to NOx also were calculated to understand the dominant O3 formation regime in high O3 days. Episodes of 3 days or longer were particularly focused on during the study. Three episodes occurring on July 14–16, 2002; June 23–27, 2003; and July 15–21, 2005 were analyzed and are presented in this paper. Selected episodes with maximum O3 concentrations and meteorological factors, such as surface pressure, temperature, and average wind speed, are summarized in Table 1. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to visualize pollutant transport to İstanbul during episodic days. This model is a complete system for computing simple, air parcel trajectories to complex dispersion and deposition simulations. The model default configuration assumes a puff distribution in the horizontal direction and particle dispersion in the vertical direction. In this way, the greater accuracy of the vertical dispersion parameterization of the particle model is combined with the advantage of having an ever-expanding number of particles represent the pollutant distribution. The FNL (final analyses) archive for NCEP (US National Weather Service, National Centers for Environmental Predictions) Global Data Assimilation System (GDAS) analyses were used as input to the HYSPLIT model. The back trajectory simulations were initialized and run 24 h before the first days of the episodic periods. The runs continued throughout the entire episodic period. 3. Results 3.1. 2001–2005 period analysis Daily O3 concentrations for both stations between 2001 and 2005 are presented in Fig. 2. As shown in the figure, high
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Fig. 2. Daily average O3 concentrations in (a) Kadıköy and (b) Saraçhane between 2001 and 2005.
O3 levels, generally above a daily average of 80 μg m− 3, were experienced 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 μg m− 3 on July 20, 2001, at Saraçhane Station. The long-term daily average for Kadıköy was calculated to be 21.4 μg m− 3, whereas for Saraçhane, the longterm average was 16.2 μg m− 3. These averages show that the Kadıköy Station was exposed to higher O3 levels as compared to the Saraçhane Station. However, on an hourly basis, the Saraçhane Station measured higher daily average maximum concentrations. Hourly averages for high O3 days were calculated and plotted to establish a generalized diurnal trend for the episodes. As shown in Fig. 3, the generated profile was a typical diurnal trend for O3, reaching maximum concentrations during afternoon hours due to high solar radiation, which triggered the chemistry. Some smaller increases in O3 concentrations were also observed in early morning hours due to a buildup of O3 in the absence, or decreased levels, of NOx. During rush hour traffic between 0700 and 0900 LST (Local Sidereal Time), increasing NOx emissions cause the O3 levels to reach their minimum due to NOx titration, and then O3 begins to build-up due to increasing solar radiation and in turn increasing chemical reaction rate. The dominant wind directions during high O3 days are presented in Fig. 4. The figure agrees well with the general circulation pattern for the region, which is characterized by dominant northeasterly and southwesterly winds. Southwesterly wind plays a particular role in transporting chemicals from regional emissions sources toward the northeast, and northeasterly winds transport pollutants toward the southeast. The overall correlation factors between O3 and its precursors were calculated and are presented in Table 2. As
expected, negative correlations resulted and agreed with the chemistry. Considering all high O3 days, correlations between O3 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 are compared, it would be found that there was a higher negative correlation 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 presented in Fig. 5. It can be seen that the slopes of O3 change (a) against NOx with changes of −0.1952 and −0.6982 for the Kadıköy and Saraçhane Stations, respectively and (b) against VOC with changes of −0.0139 and −0.0005 for the Kadıköy and Saraçhane Stations respectively. Thus, it is possible to say for both Kadıköy and Saraçhane that the change in VOC concentration does not have 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 to Kadıköy (Fig. 5c,d). The VOC–NOx ratios were calculated to be 14.5 for Kadıköy and 13.7 for Saraçhane. Both ratios were higher than 10, showing that the NOx-sensitive chemistry was dominant in the study domain. Fig. 5b and c also confirm that NOx-sensitive chemistry was dominant, meaning O3 formation was affected by changes in NOx levels but considerably less affected by changes in VOC levels. The relation between NO and total NOx species gives clues about the origin of the NOx precursors. As seen in Fig. 6, there is a very strong correlation between NO and NOx, showing that the NOx species are mainly from local sources (Lu and Wang, 2004). Thus, we can say that emissions within the city are an important contributor to the chemistry that takes place
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Fig. 3. Long-term, daily averaged pollutant profiles for (a) Kadıköy and (b) Saraçhane.
in the lower atmosphere. Since the atmospheric lifetime of NOx species (hours) is relatively shorter than the lifetime of O3 (days), we can expect that air masses transported to the region may contain significant levels of O3 together with small amounts of NOx. Moderate wind speeds during some high O3 days point to these conditions. 3.2. Episodic analysis and HYSPLIT trajectories The three episodic days were mostly characterized by northerly and northeasterly winds during the night and
southwesterly winds during the day, which is a characteristic circulation as explained in Im et al. (2006). Temperatures were generally above 25 °C, and there were relatively highpressure systems dominating during the episodes with lowwind speeds (Table 1). Daily maximum O3 concentrations were above 100 μg m− 3 for all days, reaching 140 μg m− 3 during the 2002 and 2005 episodes and 187 μg m− 3 during the 2003 episode. The episodic profiles of O3 and its precursors are given in Fig. 7. In Fig. 7a and b, VOC curves were not presented owing to unreliable hourly data that failed to pass the filtering test. However, NOx and O3 data were high quality. The two episodes were considered to be quite important in presenting episodic characteristics in the city, and therefore,
Table 2 Long-term correlations of O3 with NOx and VOC species and respective levels of significance Kadıköy
NO NO2 NOx CH4 nMHC THC Fig. 4. Frequency distribution of dominant wind directions for high O3 days.
Saraçhane
Correlation
Significance
Correlation
Significance
−0.42 −0.44 −0.44 −0.82 −0.63 −0.85
0.015(⁎⁎) 0.056(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎)
− 0.84 − 0.58 − 0.84 − 0.64 − 0.82 − 0.69
0.2520 0.2250 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎)
⁎Above 95% significance level. ⁎⁎Above 99% significance level.
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Fig. 5. Sensitivity of O3 formation to NOx and VOC changes in a and b, Kadıköy stations, and c and d, Saraçhane Stations.
they will be discussed in this paper. As seen in the figure, maximum O3 concentrations appeared during afternoon hours around 1400–1600 LST, when solar radiation was at the maximum. Minimum concentrations appeared during rush 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) Saraçhane Stations.
Correlation factors and their significance levels were calculated and are presented in Table 3. As was presented for overall high O3 days, higher correlations with NOx species in Saraçhane were calculated for longer episodes than for Kadıköy. This result also was confirmed in Fig. 5a and c where a steeper slope was obtained in Saraçhane than in Kadıköy. The Kadıköy Station generally measured lower maximum daily concentrations of O3, and the reason was believed to be its closer proximity to dense traffic, leading to NOx titration, and to the sea, leading to good mixing of sea– land breezes. HYSPLIT backward trajectory simulation results are presented in Fig. 8. The model was configured to produce the mixing-height profile through each episode in order to understand if the mixing heights contributed to high levels of O3 buildup in the region. As can be seen in Fig. 8a at 500 m altitude, northerly winds were dominant, and on July 14, 2002, northeasterly winds carried the almost clean air of the Black Sea toward İstanbul. On that day, the mixing height was around 250 m, and this low-mixing height was maintained until the morning hours of July 15, 2002, suppressing the pollutants emitted from sources in İstanbul to the surface, increasing their concentrations. The other two days of the 2002 episodic period had typically high-mixing heights during the day and low-mixing heights during the night. The 2003 episode, as seen in Fig. 8b, was characterized by very low-mixing heights during all 5 days of the episode, varying below 1000 m. Especially on June 25 and 26, the daytime 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 direction varying between southwesterly to southeasterly directions during this episode.
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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.
HYSPLIT results for the 2005 episode are presented in Fig. 8c. Relatively low-mixing heights on July 15, 16, 20, and 21, 2005, sometimes lower than 300 m, contributed to elevated levels of O3. Typical properties for all episodes were high-mixing heights reaching 1000 m during the day and very low-mixing heights around 250 m during the night.
Trajectories clearly showed the south, southwest, and southeast to be the main directions of transport for all episodes. When considering transport mainly from the Black Sea and from rural areas in the north, northwest, and northeast, and the variability of mixing heights, it is possible to say that maximum levels measured during certain days in
Table 3 Correlation factors and significance levels of ozone with precursors for selected episodes 14–16.07.2002
NO NO2 NOx CH4 nMHC THC
23–27.06.2003
15–21.07.2005
Correlation
Significance
Correlation
Significance
Correlation
Significance
−0.42 −0.68 −0.53 N/A N/A N/A
0.000(⁎⁎) 0.005(⁎⁎) 0.009(⁎⁎) N/A N/A N/A
−0.56 −0.70 −0.62 N/A N/A N/A
0.001(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎) N/A N/A N/A
−0.34 −0.45 −0.40 −0.53 −0.36 −0.52
0.000(⁎⁎) 0.068 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎)
⁎Above 95% significance level. ⁎⁎Above 99% significance level.
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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 transported from other areas. 4. Summary and conclusions İstanbul is experiencing a rapid and uncontrolled growth in urbanization, industrialization, and transportation. The dense and various kinds of industrial activities in the region together with the increasing number of vehicles are causing air quality problems in the city. Air quality data from continuous measurement of O3 and its main precursors at two stations, one on each side of İstanbul, were analyzed to understand the relation among the various chemicals involved in photochemical processes. Results show that high levels of O3 were observed mostly under relatively anticyclonic conditions with considerably low-wind speeds. The dominating wind directions were northeasterly during the night and early morning hours and southly to southwesterly during the day. During the summer, the effect of the Persian Gulf low-pressure system was a key parameter in the meteorology of the region. The system produced northeasterly winds through the Black Sea toward İstanbul. When the system weakened, the so called “relatively” high-pressure system starts to develop over the region exerting its own characteristics of circulation. High O3 days demonstrated a typical diurnal profile with maximum concentrations appearing during afternoon hours and minimum concentrations appearing during rush hours due to NOx titration from traffic emissions. Daily average O3 concentrations were around 100 μg m− 3 whereas individual episodic periods may have had hourly concentrations reaching 180 μg m− 3 in Saraçhane and 140 μg m− 3 in Kadıköy. Lower-daily maximum values at the Kadıköy Station resulted from better ventilation owing to the close location of the station to the sea, and larger titration values of NOx resulted from close proximity to dense traffic. However, VOC–NOx ratios at both stations showed that the chemistry was NOxsensitive, which agrees well with the correlation factors calculated among O3 and its precursors. Higher correlations of O3 with VOCs were calculated in Kadıköy than in Saraçhane, which confirms the scatter diagrams produced for NOx and VOC formation analyses. Volatile organic compounds at Kadıköy were relatively more effective in photochemistry than at Saraçhane. High correlations between NO and NOx were calculated for both stations suggesting that NO x emissions originated locally. For better characterization, episodes of three or more consecutive days were analyzed. These periods were characterized by northeasterly winds during the night and early morning hours and southwesterly winds during the day with relatively high-pressure values. Ozone concentrations tended to increase during afternoon hours around 1400–1600 LST
and decrease during rush hours. Hourly O3 concentrations during afternoon hours were above 100 μg m− 3 and followed a typical diurnal O3 cycle. HYSPLIT modeling results clearly showed that the north, northwest, and northeast were the dominant trajectories of transport to the city. Since transport was mainly from the Black Sea and from rural areas in the north of İstanbul, contribution by advection to the maximum levels of O3 was believed to be low, and the high levels of O3 mainly originated from İstanbul. The results also demonstrated that restricted vertical mixing plays an important role in the elevated levels of pollutants in the İstanbul metropolitan area. The chemistry with restricted mixing conditions can lead to intense episodes of air pollution. References Anteplioğlu, U., 2000. Modeling of surface ozone with UAM: a case study for İstanbul, Ph.D. Thesis, İstanbul Technical University, 147 pp. Güsten, H., Heinrich, G., Monnich, E., et al., 1997. Thessaloniki '91 field measurement campaign — II, ozone formation in the Greater Thessaloniki Area. Atmospheric Environment 37, 1115–1126. Im, U., Tayanç, M., Yenigün, O., 2006. Analysis of major photochemical pollutants with meteorological factors for high ozone days in Istanbul, Turkey. Water, Air, and Soil Pollution 175, 335–359. İstanbul Governor's Office, Director's Office of Environment and Forestry, Environmental Assessment Report of İstanbul, 2005. Kalabokas, P.D., Bartzis, J.G., 1998. Photochemical air pollution characteristics at the station of the NCSR-Demokritos, during the Medcaphot-Trace campaign in Athens, Greece (20 August–20 September 1994). Atmospheric Environment 32, 2123–2139. Klemm, O., Ziomas, I.C., Balis, D., et al., 1998. A summer air-pollution study in Athens, Greece. Atmospheric Environment 32, 2071–2087. Kocak, K., Saylan, L., Sen, O., 2000. Nonlinear time series prediction of O3 concentration in İstanbul. Atmospheric Environment 34, 1267–1271. Lu, W.Z., Wang, X.K., 2004. Interaction of major air pollutants in Hong Kong territory. Science of the Total Environment 324, 247–259. Peleg, M., Luria, M., Sharf, G., et al., 1997. Observational evidence of an ozone episode over the Greater Athens Area. Atmospheric Environment 31, 3969–3983. Pryor, S.C., 1998. A case study of emission changes and ozone responses. Atmospheric Environment 32 (2), 123–131. Seinfeld, J.H., Pandis, S., 1998. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. John Wiley & Sons, Inc., New York. Sillman, S., 1999. The relation between ozone, NOx and hydrocarbons in urban and polluted rural environments. Atmospheric Environment 33, 1821–1845. Skov, H., Egelov, A.H., Granby, K., Nielsen, T., 1997. Relationships between ozone and other photochemical products at L1. Valby, Denmark. Atmospheric Environment 31, 685–691. Tayanç, M., 2000. An assessment of spatial and temporal variation of sulfur dioxide levels over İstanbul, Turkey. Environmental Pollution 107, 61–69. Topçu, S., İncecik, S., 2002. Surface ozone measurements and meteorological influences in the urban atmosphere of İstanbul. International Journal of Environmental Pollution 17, 390–404. Topçu, S., Anteplioğlu, U., İncecik, S., 2003. Surface ozone concentrations and its relation to wind field in İstanbul. Water, Air and Soil Pollution: Focus 3, 53–64. Topçu, S., Kahya, C., İncecik, S., et al., 2005. Review of surface ozone and its precursors in urban atmosphere of Istanbul, Turkey for 2002–2003. In: Topçu, S., Yardım, M.F., Bayram, A., et al. (Eds.), Proceedings of Air Quality Management at Urban, Regional, and Global Scales, vol. 3, pp. 1083–1093.
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Atmospheric Research j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a t m o s
Interaction patterns of major photochemical pollutants in Istanbul, Turkey Ulaş İm a, Mete Tayanç b, Orhan Yenigün a,⁎ a b
Bogazici University, Institute of Environmental Sciences, İstanbul, Turkey Marmara University, Faculty of Engineering, Department of Environmental Engineering İstanbul, Turkey
a r t i c l e
i n f o
Keywords: Ozone Photochemical precursors İstanbul Pearson correlation HYSPLIT model
a b s t r a c t Hourly concentration levels of ozone, nitrogen oxide, and hydrocarbon measured between 2001 and 2005 in Kadıköy and Saraçhane, two urban districts in the Anatolian and European sides 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 and maximum temperatures above 25 °C, and the episodes were mainly characterized by southwesterly surface winds during the day and northeasterly surface winds during the night. Considering all high-ozone days, correlations of ozone with nitrogen oxides (NOx) and volatile 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 in the region. However, higher correlations of VOCs in Kadıköy, as compared with those in Saraçhane, indicated that VOCs also make an important contribution to ozone formation. Highozone days generally showed maximum concentrations at afternoon hours and minimum concentrations at rush hours due to NOx titration by traffic emissions. In addition, the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was employed backward for three long episodes, and the results showed the northerly, northwesterly, and northeasterly transport of air masses to the city. Since transport was mainly from the Black Sea and from rural areas north of İstanbul, contributions by advection to maximum levels of ozone in the city were believed to be low. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ozone (O3) levels in the southern European and Mediterranean regions have been studied by Klemm et al. (1998), Güsten et al. (1997), and Peleg et al. (1997). Kalabokas and Bartzis (1998) studied the Aegean region. Photochemical pollution in İstanbul also has been studied by Anteplioğlu (2000), Kocak et al. (2000), Topçu and İncecik (2002), and Topçu et al. (2003). Topçu and İncecik (2002) studied O3 pollution and its relation with nitrogen oxide (NOx) species and some meteorological parameters in İstanbul. They found that O3 concentrations generally increased with increasing temperature during the O3 season. Topçu et al. (2003)
⁎ Corresponding author. Tel.: +90 212 359 69 46; fax: +90 212 257 50 33. E-mail addresses: [email protected] (U. İm), [email protected] (M. Tayanç), [email protected] (O. Yenigün). 0169-8095/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2008.03.015
studied the relationships of high O3 concentrations between chemical and meteorological characteristics using the meteorological Systems Applications International Mesoscale Model (SAIMM) and found that high O3 concentrations generally occur during low-wind conditions. Topçu et al. (2005) found that high O3 episodes were characterized by southwesterly and west–southwesterly winds and calm conditions and that the diurnal variations of O3, NOx, and nonmethane hydrocarbons (nMHCs) supported buildup of O3 during rush hours. Im et al. (2006) showed that high concentrations of O3 can be experienced both during afternoon hours and in the late night due to sharp decreases in mixing heights. Daytime, southwesterly winds transported high concentrations of O3 and its precursors to the north during these episodes. The previous studies also showed that episodes of photochemical smog in İstanbul tend to begin during the strongest phase of the high-pressure system and continue as it weakens.
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Photochemical smog was characterized by high O3 due to complex and non-linear chemistry and meteorology. Photochemical smog is related to the very complex reactions between NOx and volatile organic compound (VOC) species under solar radiation (Sillman, 1999). In simple terms, O3 is produced by combining an oxygen molecule with an oxygen atom that is supplied from the photolysis of nitrogen dioxide (NO2) by solar radiation. In addition, hydrocarbons (HCs) are oxidized to form carbon dioxide and water vapor by the hydroxyl radical, which is produced by the photolysis of O3 in the presence of water vapor. Since these complex reactions occur in the atmosphere, measuring O3 levels alone cannot help in evaluating photochemical conditions. While under NOx-sensitive conditions, O3 formation is almost entirely governed by NOx and largely independent of VOCs. Under VOC-sensitive conditions, O3 formation is related to increased VOCs and not to NOx changes. It is suggested that VOC–NOx concentrations higher than 10 indicate NOx-sensitive chemistry (Pryor, 1998). Under NOx-sensitive conditions, low concentrations of NOx and high concentrations of VOC–NOx controls the formation of O3 independent of VOCs (Skov et al., 1997; Seinfeld and Pandis, 1998; Sillman, 1999). Since solar radiation is the key parameter in these reactions, strong photochemical production leads to peak O3 concentrations in the afternoon hours. Different contributions to O3 levels may come from background O3 that originates from stratospheric injection and production far away from local sources—meaning that it involves photo oxidation of CH4 (methane) and CO
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(carbon monoxide), as well as regional photochemical production and depletion of O3 by deposition and loss reactions. In our study, meteorological and air quality observations by the U.S. National Weather Service and İstanbul Greater Municipality, respectively, were analyzed. The diurnal variations of O3, NOx, and VOCs were investigated for selected episodes at two stations, one on each side of İstanbul. The NOx and VOC sensitivities to O3 formation during these days were analyzed by examining the correlations between O3 and its precursors. 2. Materials and methods 2.1. Study area, meteorology, and emissions İstanbul is a large city located at the geographic coordinates of 41°N and 29°E (Fig. 1). Bosporus, a sea water strait extending from the Black Sea to the Marmara Sea, divides the city into European and Anatolian sides. The Black Sea in the north and Marmara Sea in the south produce a differential heating of surfaces, leading to different meteorological conditions that may play a role in the transport of O3. The complex terrain of İstanbul also influences the circulation systems over the city. In the summer months, meteorology in the region is mainly influenced by hot, dry low-pressure systems from the Persian Gulf extending from the southeast of Turkey. This system is the main reason for the dominant northeasterly winds in İstanbul. When the system weakens,
Fig. 1. Map of the study region and locations of the air quality stations.
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land–sea breeze circulation prevails, producing southerly and southwesterly winds blowing from the Marmara Sea to İstanbul during the day. Although sea level pressure is not significantly high during those days, the system can be called a “relatively” high-pressure system. During the evening and night, wind shifts to the northerly, northeasterly direction, and this situation leads to the transport of pollutants to the south and southwest. The city experiences a transition climate between Mediterranean and temperate conditions. Strong inversion conditions in the summer sometimes cause O3 concentrations to reach maximum levels in the late afternoon (Topçu and İncecik, 2002). Within the last 40 years, İstanbul has experienced rapid urbanization and industrialization, and statistics show the population is over 10 million. In addition, there are more than 2 million motor vehicles registered in İstanbul (İstanbul Governor's Office, 2005). Liquefied petroleum gas (LPG) has been widely used by taxis from the beginning of 1998. Emissions produced by the transportation sector constitute a large portion of the O3 precursors emitted into the atmosphere. Traffic rush hours clearly create a sink for O3 by the emissions of NOx. There has been a shift from coal to natural gas for domestic heating beginning in the mid-1990s (Tayanç, 2000). According to the 2005 records from the Istanbul Metropolitan Municipality, 3.7 × 109 m3 of natural gas are combusted annually in the city. This region experiences very dense industrial activities—almost the highest in the country. Based on reports from the İstanbul Chamber of Industry, the textile industry comprises 37% of the industrial activities in İstanbul; metal industry, 30%; chemical industry, 21%; food industry, 5%; and other industries, 7%. In addition, air quality in the city is influenced by the emissions from the İzmit Gulf, which is a major source of HC emissions owing to Tüpraş, a giant petroleum refinery. Tüpraş is located in the southeastern part İstanbul, and southeasterly winds can carry these emissions to the study region. High emissions particularly from the textile, petroleum, and chemical industries combined with traffic emissions lead to significant VOC and NOx accumulation, producing favorable conditions for the formation 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 the İstanbul Greater Municipality between 2001 and 2005. The İstanbul Greater Municipality operates 10 air quality stations throughout the city, with only two continuously measuring O3 and its precursors. These stations include the Kadıköy Station on the Anatolian side and the Saraçhane Station on the European side. Both stations demonstrate mixed urban characteristics (residential and commercial). Pollutants measured at the Kadıköy and Saraçhane Stations are CO, NO, NO2, NOx, and O3. Beginning in 2002 in Saraçhane and 2003 in Kadıköy, total hydrocarbons (THC), CH4 and nMHC were added to the analyzed parameters. Nitrogen oxides (NOx) species were monitored using the chemiluminescence method, CO by infrared (IR) adsorption, O3 by ultraviolet (UV) photometry, and HC by gas chromatography. Filtering techniques were developed to scan the raw dataset discarding days containing more than 12 h of missing
Table 1 Maximum O3 concentrations and surface meteorological conditions for selected episodes Date
Station
Maximum concentration (μg m− 3)
Temperature (C)
Pressure (mb)
Wind speed (ms− 1)
14.07.2002 15.07.2002 16.07.2002 23.06.2003 24.06.2003 25.06.2003 26.06.2003 27.06.2003 15.07.2005 16.07.2005 17.07.2005 18.07.2005 19.07.2005 20.07.2005 21.07.2005
Kadıköy Kadıköy Kadıköy Saraçhane Saraçhane Saraçhane Saraçhane Saraçhane Kadıköy Kadıköy Kadıköy Kadıköy Kadıköy Kadıköy Kadıköy
118 140 134 134 187 168 113 141 108 123 132 141 132 123 105
27.6 28.3 29.5 23.1 25.1 26.6 25.3 24.5 23.9 23.5 24.1 25.4 26.2 26.5 26.3
1011.3 1011.9 1010.3 1015.9 1016.4 1011.6 1008.5 1010.2 1012.1 1012.4 1013.1 1013.2 1016.3 1013.8 1011.0
3.6 2.9 2.1 3.0 2.3 3.0 4.7 3.4 6.8 5.5 3.3 1.4 2.7 4.3 6.9
data and months containing more than 15 days of missing data. With these filtering techniques, hourly O3 concentrations exceeding 100, 150, and 240 μg m− 3 also were discarded to identify episodes of photochemical pollution within the period. The Pearson correlation factors were calculated for O3 and its precursors to show the relation among these chemicals. Ratios of VOC to NOx also were calculated to understand the dominant O3 formation regime in high O3 days. Episodes of 3 days or longer were particularly focused on during the study. Three episodes occurring on July 14–16, 2002; June 23–27, 2003; and July 15–21, 2005 were analyzed and are presented in this paper. Selected episodes with maximum O3 concentrations and meteorological factors, such as surface pressure, temperature, and average wind speed, are summarized in Table 1. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to visualize pollutant transport to İstanbul during episodic days. This model is a complete system for computing simple, air parcel trajectories to complex dispersion and deposition simulations. The model default configuration assumes a puff distribution in the horizontal direction and particle dispersion in the vertical direction. In this way, the greater accuracy of the vertical dispersion parameterization of the particle model is combined with the advantage of having an ever-expanding number of particles represent the pollutant distribution. The FNL (final analyses) archive for NCEP (US National Weather Service, National Centers for Environmental Predictions) Global Data Assimilation System (GDAS) analyses were used as input to the HYSPLIT model. The back trajectory simulations were initialized and run 24 h before the first days of the episodic periods. The runs continued throughout the entire episodic period. 3. Results 3.1. 2001–2005 period analysis Daily O3 concentrations for both stations between 2001 and 2005 are presented in Fig. 2. As shown in the figure, high
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Fig. 2. Daily average O3 concentrations in (a) Kadıköy and (b) Saraçhane between 2001 and 2005.
O3 levels, generally above a daily average of 80 μg m− 3, were experienced 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 μg m− 3 on July 20, 2001, at Saraçhane Station. The long-term daily average for Kadıköy was calculated to be 21.4 μg m− 3, whereas for Saraçhane, the longterm average was 16.2 μg m− 3. These averages show that the Kadıköy Station was exposed to higher O3 levels as compared to the Saraçhane Station. However, on an hourly basis, the Saraçhane Station measured higher daily average maximum concentrations. Hourly averages for high O3 days were calculated and plotted to establish a generalized diurnal trend for the episodes. As shown in Fig. 3, the generated profile was a typical diurnal trend for O3, reaching maximum concentrations during afternoon hours due to high solar radiation, which triggered the chemistry. Some smaller increases in O3 concentrations were also observed in early morning hours due to a buildup of O3 in the absence, or decreased levels, of NOx. During rush hour traffic between 0700 and 0900 LST (Local Sidereal Time), increasing NOx emissions cause the O3 levels to reach their minimum due to NOx titration, and then O3 begins to build-up due to increasing solar radiation and in turn increasing chemical reaction rate. The dominant wind directions during high O3 days are presented in Fig. 4. The figure agrees well with the general circulation pattern for the region, which is characterized by dominant northeasterly and southwesterly winds. Southwesterly wind plays a particular role in transporting chemicals from regional emissions sources toward the northeast, and northeasterly winds transport pollutants toward the southeast. The overall correlation factors between O3 and its precursors were calculated and are presented in Table 2. As
expected, negative correlations resulted and agreed with the chemistry. Considering all high O3 days, correlations between O3 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 are compared, it would be found that there was a higher negative correlation 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 presented in Fig. 5. It can be seen that the slopes of O3 change (a) against NOx with changes of −0.1952 and −0.6982 for the Kadıköy and Saraçhane Stations, respectively and (b) against VOC with changes of −0.0139 and −0.0005 for the Kadıköy and Saraçhane Stations respectively. Thus, it is possible to say for both Kadıköy and Saraçhane that the change in VOC concentration does not have 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 to Kadıköy (Fig. 5c,d). The VOC–NOx ratios were calculated to be 14.5 for Kadıköy and 13.7 for Saraçhane. Both ratios were higher than 10, showing that the NOx-sensitive chemistry was dominant in the study domain. Fig. 5b and c also confirm that NOx-sensitive chemistry was dominant, meaning O3 formation was affected by changes in NOx levels but considerably less affected by changes in VOC levels. The relation between NO and total NOx species gives clues about the origin of the NOx precursors. As seen in Fig. 6, there is a very strong correlation between NO and NOx, showing that the NOx species are mainly from local sources (Lu and Wang, 2004). Thus, we can say that emissions within the city are an important contributor to the chemistry that takes place
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Fig. 3. Long-term, daily averaged pollutant profiles for (a) Kadıköy and (b) Saraçhane.
in the lower atmosphere. Since the atmospheric lifetime of NOx species (hours) is relatively shorter than the lifetime of O3 (days), we can expect that air masses transported to the region may contain significant levels of O3 together with small amounts of NOx. Moderate wind speeds during some high O3 days point to these conditions. 3.2. Episodic analysis and HYSPLIT trajectories The three episodic days were mostly characterized by northerly and northeasterly winds during the night and
southwesterly winds during the day, which is a characteristic circulation as explained in Im et al. (2006). Temperatures were generally above 25 °C, and there were relatively highpressure systems dominating during the episodes with lowwind speeds (Table 1). Daily maximum O3 concentrations were above 100 μg m− 3 for all days, reaching 140 μg m− 3 during the 2002 and 2005 episodes and 187 μg m− 3 during the 2003 episode. The episodic profiles of O3 and its precursors are given in Fig. 7. In Fig. 7a and b, VOC curves were not presented owing to unreliable hourly data that failed to pass the filtering test. However, NOx and O3 data were high quality. The two episodes were considered to be quite important in presenting episodic characteristics in the city, and therefore,
Table 2 Long-term correlations of O3 with NOx and VOC species and respective levels of significance Kadıköy
NO NO2 NOx CH4 nMHC THC Fig. 4. Frequency distribution of dominant wind directions for high O3 days.
Saraçhane
Correlation
Significance
Correlation
Significance
−0.42 −0.44 −0.44 −0.82 −0.63 −0.85
0.015(⁎⁎) 0.056(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎)
− 0.84 − 0.58 − 0.84 − 0.64 − 0.82 − 0.69
0.2520 0.2250 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎)
⁎Above 95% significance level. ⁎⁎Above 99% significance level.
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Fig. 5. Sensitivity of O3 formation to NOx and VOC changes in a and b, Kadıköy stations, and c and d, Saraçhane Stations.
they will be discussed in this paper. As seen in the figure, maximum O3 concentrations appeared during afternoon hours around 1400–1600 LST, when solar radiation was at the maximum. Minimum concentrations appeared during rush 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) Saraçhane Stations.
Correlation factors and their significance levels were calculated and are presented in Table 3. As was presented for overall high O3 days, higher correlations with NOx species in Saraçhane were calculated for longer episodes than for Kadıköy. This result also was confirmed in Fig. 5a and c where a steeper slope was obtained in Saraçhane than in Kadıköy. The Kadıköy Station generally measured lower maximum daily concentrations of O3, and the reason was believed to be its closer proximity to dense traffic, leading to NOx titration, and to the sea, leading to good mixing of sea– land breezes. HYSPLIT backward trajectory simulation results are presented in Fig. 8. The model was configured to produce the mixing-height profile through each episode in order to understand if the mixing heights contributed to high levels of O3 buildup in the region. As can be seen in Fig. 8a at 500 m altitude, northerly winds were dominant, and on July 14, 2002, northeasterly winds carried the almost clean air of the Black Sea toward İstanbul. On that day, the mixing height was around 250 m, and this low-mixing height was maintained until the morning hours of July 15, 2002, suppressing the pollutants emitted from sources in İstanbul to the surface, increasing their concentrations. The other two days of the 2002 episodic period had typically high-mixing heights during the day and low-mixing heights during the night. The 2003 episode, as seen in Fig. 8b, was characterized by very low-mixing heights during all 5 days of the episode, varying below 1000 m. Especially on June 25 and 26, the daytime 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 direction varying between southwesterly to southeasterly directions during this episode.
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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.
HYSPLIT results for the 2005 episode are presented in Fig. 8c. Relatively low-mixing heights on July 15, 16, 20, and 21, 2005, sometimes lower than 300 m, contributed to elevated levels of O3. Typical properties for all episodes were high-mixing heights reaching 1000 m during the day and very low-mixing heights around 250 m during the night.
Trajectories clearly showed the south, southwest, and southeast to be the main directions of transport for all episodes. When considering transport mainly from the Black Sea and from rural areas in the north, northwest, and northeast, and the variability of mixing heights, it is possible to say that maximum levels measured during certain days in
Table 3 Correlation factors and significance levels of ozone with precursors for selected episodes 14–16.07.2002
NO NO2 NOx CH4 nMHC THC
23–27.06.2003
15–21.07.2005
Correlation
Significance
Correlation
Significance
Correlation
Significance
−0.42 −0.68 −0.53 N/A N/A N/A
0.000(⁎⁎) 0.005(⁎⁎) 0.009(⁎⁎) N/A N/A N/A
−0.56 −0.70 −0.62 N/A N/A N/A
0.001(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎) N/A N/A N/A
−0.34 −0.45 −0.40 −0.53 −0.36 −0.52
0.000(⁎⁎) 0.068 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎) 0.000(⁎⁎)
⁎Above 95% significance level. ⁎⁎Above 99% significance level.
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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 transported from other areas. 4. Summary and conclusions İstanbul is experiencing a rapid and uncontrolled growth in urbanization, industrialization, and transportation. The dense and various kinds of industrial activities in the region together with the increasing number of vehicles are causing air quality problems in the city. Air quality data from continuous measurement of O3 and its main precursors at two stations, one on each side of İstanbul, were analyzed to understand the relation among the various chemicals involved in photochemical processes. Results show that high levels of O3 were observed mostly under relatively anticyclonic conditions with considerably low-wind speeds. The dominating wind directions were northeasterly during the night and early morning hours and southly to southwesterly during the day. During the summer, the effect of the Persian Gulf low-pressure system was a key parameter in the meteorology of the region. The system produced northeasterly winds through the Black Sea toward İstanbul. When the system weakened, the so called “relatively” high-pressure system starts to develop over the region exerting its own characteristics of circulation. High O3 days demonstrated a typical diurnal profile with maximum concentrations appearing during afternoon hours and minimum concentrations appearing during rush hours due to NOx titration from traffic emissions. Daily average O3 concentrations were around 100 μg m− 3 whereas individual episodic periods may have had hourly concentrations reaching 180 μg m− 3 in Saraçhane and 140 μg m− 3 in Kadıköy. Lower-daily maximum values at the Kadıköy Station resulted from better ventilation owing to the close location of the station to the sea, and larger titration values of NOx resulted from close proximity to dense traffic. However, VOC–NOx ratios at both stations showed that the chemistry was NOxsensitive, which agrees well with the correlation factors calculated among O3 and its precursors. Higher correlations of O3 with VOCs were calculated in Kadıköy than in Saraçhane, which confirms the scatter diagrams produced for NOx and VOC formation analyses. Volatile organic compounds at Kadıköy were relatively more effective in photochemistry than at Saraçhane. High correlations between NO and NOx were calculated for both stations suggesting that NO x emissions originated locally. For better characterization, episodes of three or more consecutive days were analyzed. These periods were characterized by northeasterly winds during the night and early morning hours and southwesterly winds during the day with relatively high-pressure values. Ozone concentrations tended to increase during afternoon hours around 1400–1600 LST
and decrease during rush hours. Hourly O3 concentrations during afternoon hours were above 100 μg m− 3 and followed a typical diurnal O3 cycle. HYSPLIT modeling results clearly showed that the north, northwest, and northeast were the dominant trajectories of transport to the city. Since transport was mainly from the Black Sea and from rural areas in the north of İstanbul, contribution by advection to the maximum levels of O3 was believed to be low, and the high levels of O3 mainly originated from İstanbul. The results also demonstrated that restricted vertical mixing plays an important role in the elevated levels of pollutants in the İstanbul metropolitan area. The chemistry with restricted mixing conditions can lead to intense episodes of air pollution. References Anteplioğlu, U., 2000. Modeling of surface ozone with UAM: a case study for İstanbul, Ph.D. Thesis, İstanbul Technical University, 147 pp. Güsten, H., Heinrich, G., Monnich, E., et al., 1997. Thessaloniki '91 field measurement campaign — II, ozone formation in the Greater Thessaloniki Area. Atmospheric Environment 37, 1115–1126. Im, U., Tayanç, M., Yenigün, O., 2006. Analysis of major photochemical pollutants with meteorological factors for high ozone days in Istanbul, Turkey. Water, Air, and Soil Pollution 175, 335–359. İstanbul Governor's Office, Director's Office of Environment and Forestry, Environmental Assessment Report of İstanbul, 2005. Kalabokas, P.D., Bartzis, J.G., 1998. Photochemical air pollution characteristics at the station of the NCSR-Demokritos, during the Medcaphot-Trace campaign in Athens, Greece (20 August–20 September 1994). Atmospheric Environment 32, 2123–2139. Klemm, O., Ziomas, I.C., Balis, D., et al., 1998. A summer air-pollution study in Athens, Greece. Atmospheric Environment 32, 2071–2087. Kocak, K., Saylan, L., Sen, O., 2000. Nonlinear time series prediction of O3 concentration in İstanbul. Atmospheric Environment 34, 1267–1271. Lu, W.Z., Wang, X.K., 2004. Interaction of major air pollutants in Hong Kong territory. Science of the Total Environment 324, 247–259. Peleg, M., Luria, M., Sharf, G., et al., 1997. Observational evidence of an ozone episode over the Greater Athens Area. Atmospheric Environment 31, 3969–3983. Pryor, S.C., 1998. A case study of emission changes and ozone responses. Atmospheric Environment 32 (2), 123–131. Seinfeld, J.H., Pandis, S., 1998. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. John Wiley & Sons, Inc., New York. Sillman, S., 1999. The relation between ozone, NOx and hydrocarbons in urban and polluted rural environments. Atmospheric Environment 33, 1821–1845. Skov, H., Egelov, A.H., Granby, K., Nielsen, T., 1997. Relationships between ozone and other photochemical products at L1. Valby, Denmark. Atmospheric Environment 31, 685–691. Tayanç, M., 2000. An assessment of spatial and temporal variation of sulfur dioxide levels over İstanbul, Turkey. Environmental Pollution 107, 61–69. Topçu, S., İncecik, S., 2002. Surface ozone measurements and meteorological influences in the urban atmosphere of İstanbul. International Journal of Environmental Pollution 17, 390–404. Topçu, S., Anteplioğlu, U., İncecik, S., 2003. Surface ozone concentrations and its relation to wind field in İstanbul. Water, Air and Soil Pollution: Focus 3, 53–64. Topçu, S., Kahya, C., İncecik, S., et al., 2005. Review of surface ozone and its precursors in urban atmosphere of Istanbul, Turkey for 2002–2003. In: Topçu, S., Yardım, M.F., Bayram, A., et al. (Eds.), Proceedings of Air Quality Management at Urban, Regional, and Global Scales, vol. 3, pp. 1083–1093.