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‚§√ß°“√»÷°…“¡≈¿“«–∑“ßÕ“°“»Õ—π‡π◊ËÕß¡“®“° “√‚æ≈’‰´§≈‘° Õ–‚√¡“µ‘°‰Œ‚¥√§“√å∫Õπ (PAH) „π∫√√¬“°“»‡¢µ°√ÿ߇∑æ¡À“π§√ 2. °“√°√–®“¬µ—«·≈–√–¥—∫§«“¡‡¢â¡¢âπ Airborne Polycyclic Aromatic Hydrocarbon (PAH) In Bangkok Urban Air II. Level and Distribution Hathairatana Garivait*, Wanna Laowagul* Phaka Sukasem*, Sunthorn Ngod-Ngam* Chongrak Polprasert**, Lars Baetz Reutergardh**

∫∑§—¥¬àÕ

Refuse incinerator

°“√»÷°…“°“√°√–®“¬µ—«¢Õß “√‚æ≈’ ‰´§≈‘°Õ–‚√¡“ µ‘ ° ‰Œ‚¥√§“√å ∫ Õπ (PAH) „π∫√√¬“°“»¢Õß °√ÿ߇∑æ¡À“π§√ „π√Ÿª∑’ˇªìπ°ä“´·≈–‡ªìπΩÿÉπ≈–ÕÕß ∑”‰¥â ‚ ¥¬„™â ‡ §√◊Ë Õ ß¡◊ Õ ‡°Á ∫ µ— « Õ¬à “ ß∑’Ë ª √–°Õ∫¥â « ‡§√◊ËÕ߇°Á∫µ—«Õ¬à“ßΩÿÉπ≈–ÕÕß·∫∫·¬°¢π“¥ ·∫∫ Cascade impactor (Andersen çlow volumeé sampler) ∑’Ë “¡“√∂·¬°¢π“¥¢ÕßΩÿÉ π ≈–ÕÕ߉¥â 8 ™—Èπ µàÕ‡¢â“°—∫À≈Õ¥‡°Á∫°ä“´ (XAD-2 adsorbent tube) ‡¡◊ËÕ°“√‡°Á∫µ—«Õ¬à“ß PAH ∑—Èß„π√Ÿª¢ÕßΩÿÉπ ·¬°¢π“¥·≈–„π√Ÿª¢Õß°ä“´‰¥âÕ¬à“ßµàÕ‡π◊ËÕß ‡ªì𠇫≈“ §√—Èß≈– 24 ™—Ë«‚¡ß “√ª√–°Õ∫ PAH ∑’Ë∑”°“√ «‘‡§√“–Àå¡’®”π«π 9 ™π‘¥ ∑—Èß∑’ˇªìπ “√°àÕ¡–‡√Áß

*»Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡ ‡∑§‚π∏“π’ µ.§≈ÕßÀâ“ Õ.§≈ÕßÀ≈«ß ®.ª∑ÿ¡∏“π’ 12120 ‚∑√. 0-2577-1136 ‚∑√ “√. 0-2577-1138 Environmental Research and Training Center, Department of Environmental Quality Promotion. Technopolis. Klong 5 Klong Luang, Pathumthani 12120 e-mail: hathairat@myrealbox.com **Asian Institute of Technology, Pathumthani, Thailand

(carcinogen) ·≈– “√√à«¡°àÕ¡–‡√Áß (co-carcinogen) ‰¥â · °à Pyrene (PYR), Benz (a) Anthracene (BaA), Benzo (e) Pyrene (BeP), Dibenz (a,e) Anthracene (DbacA), Benzo(k)Fluorauthene (BkF), Benzo(a) Pyrene (BaP), Dibenz (a,h) Anthracene (DbahA), Benzo(ghi)Perylene (BghiP) ·≈– Tri-methyl cholanthrene (3MC) “√ª√–°Õ∫ PAH ∑’Ë¡’πÈ”Àπ—°‚¡‡≈°ÿ ≈ µË” ‡™àπ PYR BeP ·≈– BaA ®–Õ¬Ÿà„π√Ÿª¢Õß°ä“´‡ªìπ à«π„À≠à §◊Õ 80% 40% ·≈– 24% µ“¡≈”¥—∫ „π¢≥–∑’Ë “√ª√–°Õ∫ PAH ∑’Ë¡’πÈ”Àπ—°‚¡‡≈°ÿ≈ ¡“° °«à“®–Õ¬Ÿà „π√Ÿª¢ÕßΩÿÉπ≈–ÕÕ߇°◊Õ∫∑—ÈßÀ¡¥ º≈°“√ »÷°…“æ∫«à“ 30%-60% ‚¥¬πÈ”Àπ—°¢Õß “√ª√–°Õ∫ PAHs ∑’Ë»÷°…“æ∫Õ¬Ÿà„πΩÿÉπ≈–ÕÕß¢π“¥‡≈Á°°«à“ 0.43 ‰¡§√Õπ ·≈–°«à“ 70% æ∫Õ¬Ÿà„πΩÿÉπ≈–ÕÕß¢π“¥‡≈Á° °«à“ 2.1 ‰¡§√Õπ πÕ°®“°π’È °“√»÷°…“¬—߉¥âÀ“ §«“¡ —¡æ—π∏å√–À«à“ß§à“ —¡ª√– ‘∑∏‘Ï¢Õß —¥ à«π°“√ °√–®“¬µ—« „π√Ÿª¢ÕßΩÿπÉ ≈–ÕÕß·≈–„π√Ÿª¢Õß°ä“´ (Kp) °—∫§à“§«“¡¥—π‰Õ (p ÌL) ¢Õß “√ª√–°Õ∫ PAH ·µà≈– µ—« „π‡¢µ°√ÿ߇∑æ¡À“π§√‡æ◊ËÕ‡ªìπµ—«·∑π¢Õß°“√ ‡°‘¥°“√°√–®“¬µ—«¢Õß “√¥—ß°≈à“«„π√Ÿª¢ÕßΩÿÉπ·≈– °ä“´„π∫√√¬“°“»‡¢µ‡¡◊Õß√âÕπ

ABSTRACT The gas-particle partitioning and particle size distributions of airborne PAH in Bangkok urban air were investigated using an 8 stage size fractionating cascade impactor (Andersen çlow volumeé sampler) and a downstream XAD-2 adsorbent tube for sample collection. Nine PAH classified as carcinogenic and co-carcinogenic compounds - Pyrene (PYR), Benz(a)Anthracene (BaA), Benzo(e)Pyrene (BeP), Dibenz (a,c) Anthracene (DBacA), Benzo(k)Fluoranthene (BkF), Benzo (a) Pyrene (BaP), Dibenz(a,h)Anthracene (DBahA), Benzo(ghi)Perylene (BghiP) and Trimethylcholanthrene (3MC) - were quantified.

The lower molecular weight (MW) PAH such as PYR, BeP and BaA were present mainly in the gaseous phase (80%, 40% and 24%, respectively) while higher MW compounds were present almost totally in the particulate fraction. The results show that 30%-60% §-58

of each PAH by mass were found on particles smaller than 0.43 µm and more than 70% on particles with diameter less than 2.1 µm. In addition, the relationship between the particle/ gas partition coefficient (Kp) and the subcooled liquid vapor pressure (p ÌL) was also determined to describe the gas-particle partitioning of those PAH compounds in a tropical atmosphere.

1. Introduction Polycyclic Aromatic Hydrocarbons (PAH), ubiquitously found in ambient aerosols are usually products of combustion. Extensive experimental data support the emission of PAH during incomplete combustion processes.1-4 PAH are classified as hazardous air pollutants in Title III of the U.S. Clean Air Act Amendments of 1990 due to their carcinogenic properties.5 Once emitted to the atmosphere, PAH partition between the gas and atmospheric aerosol phases according to their volatility. Several observations suggest that PAH are initially generated in the gas phase, then sorbed onto existing particles while undergoing condensation upon cooling of the emission.6-9 Therefore, the atmospheric levels of PAH depend on the distribution of the PAH among the aerosol size fractions and the partition between the gas and particle phases under ambient conditions. Levels of PAH in urban air are of particular importance since they exhibit a predominant occurrence in the respirable fraction particles (< 5 mm), and hence the risk of human exposure to these substances may be high. 7,10,11 The distribution of these substances between gas and particulate phases in the atmosphere is of importance in understanding their atmospheric transport, deposition and transformation mechanisms.12,13 Although there is a lot of data on PAH levels in urban temperate environments,14-19 field data on PAH in tropical urban or rural areas are still »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

scarce, despite noticeable atmospheric pollution in some major cities such as Bangkok, Jarkata and Bombay.20,21 Bangkok is the largest city in Thailand with population of 8 million and a population density of 4,615 inhabitants/ km2. The city now faces serious air pollution problems, particularly of suspended particulate matter (SPM). The major causes of these high SPM levels are construction activity and motor vehicle traffic.22 Although various measures implemented recently to mitigate the suspended particulate matter problem in Bangkok urban air, the incorporation of carcinogenic substances into those SPM was not considered. On the other hand, statistics on cancer in Thailand during 1988-1991 have shown that lung cancer is the most common malignancy in both sexes in area where the air pollution was at a level considered dangerous. 23 However, the relationship between the number of persons suffering of lung cancer and the concentration of carcinogenic substances in Thailand atmosphere is still not documented. The aim of this study is to investigate the levels, phase distribution and particle size dependency of airborne PAH in Bangkok urban air in order to better understand their environmental fate and to estimate the human exposure to these substances. Furthermore, motor vehicle emissions are thought to be a major source of atmospheric PAH in urban area24 and emission from land transport accounts for a very large portion of ground level pollutants in Bangkok urban area.22 The results of this study will also provide useful information about ambient levels of carcinogenic and mutagenic substances related to this source.

2. Methods and Materials 2.1 Sampling locations The target area of the study encom-

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passes an urban area of Bangkok which represents about 18 km from East to West and about 16 km from north to south. There is little change in temperature throughout the year. The annual average temperature is 26 ÌC-32 ÌC, the annual humidity, 70-80% and annual rainfall, over 200 mm, characteristic of a tropical climate with wet and dry seasons.

The prevailing wind direction is Southwest monsoon from February to September and Northeast monsoon during October to January.

Inappropriate city planning makes Bangkok a mix of residential, industrial and commercial areas. Three sampling sites were selected from the National Air Quality Monitoring Network in Bangkok in addition to one background site in suburban Bangkok. The locations are described as follow : (1) The Office of Environmental Policy and Planning (OEPP) is located in an urban residential area and is surrounded by commercial buildings, government offices, houses, roads and an expressway. A few industries are present within 10 km radius of the sampling site. At this site, the sampler was placed on the top of a 7-story building about 20 meters from the ground; (2) The Ratburana Post Office (RATB) is situated in the highly industrialized area of Samut Prakan province located in the southern of Bangkok. There are textiles, iron, food processing, glass and plastic industries and a thermal power plant within 2-5 km south to southwest of the site. The site was downwind from Bangkok during the sampling period, and sampling was done on the roof of the monitoring station 3 meters from the ground; (3) The Ministry of Science Technology and Environment (MOSTE) site was selected because it is located about 3 meters from the busy Rama VI road. The traffic volume is estimated at 55,000 vehicles/day and mainly composed of passenger cars, light duty cars and buses. The sampler was placed 2 meters under expressway ; (4) The Environ§-59

mental Research and Training Center (ERTC), located in a northern suburb about 50 km far from downtown Bangkok. It was selected as a background site to estimate differences in the urban and background levels and distribution of airborne PAH. There are no significant man-made air pollution sources at this location other than a highway that runs about 10 km west. The sampler was placed on the roof of 3-story building (ERTC) which was about 15 meters from the ground. Sampling locations are indicated on Figure 1. Seven sets of 24 hour samples at each sampling site were taken between May 23rd and May 29th, 1996 during the dry season when SPM levels were the highest.

2.2 Experimental Method This study used the sampling and analysis methods with the QA/QC program which have been described in a previous paper.25 Briefly the sampling system consisted of 8 stage size fractionating cascade impactor (Andersen çlow volumeé Sampler) and a down stream XAD-2 adsorbent tube. Air samples were taken at flow rate of 28.3 l/min for 24 hours. The particulate phase was defined as that trapped on the glass coated Teflon fiber filters (T60A20) placed on the different size cuts of the impactor. The corresponding gas phase was defined as that associated with the two in line XAD-2 packed in the adsorbent tube. All particulate phase and gas phase PAH samples

Figure 1 : The map of sampling sites in Bangkok urban area

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were extracted by ultrasonication with dichloromethane, and dichloro- methane : acetonitrile (1:1, v/v), respectively. The extracts were transferred into acetonitrile solution and analyzed by reverse phase high performance liquid chromatography (HPLC) with fluorescence detection. The following nine PAH; Pyrene (PYR), Benz(a)Anthracene (BaA), Benzo (e)Pyrene(BeP), Dibenz(a,c)Anthracene (DBacA), Benzo(k)Fluoranthene (BkF), Benzo (a)Pyrene (BaP), Dibenz(a,h)Anthracene (DBahA), Benzo(ghi)Perylene (BghiP) and Trimethylcholanthrene (3MC) were quanti-tated with the aid of a mixture of authentic standards from Wako Pure Chemical Industries (Osaka, Japan).

3. Results and Discussion Air pollution by airborne PAH in Bangkok urban area was evaluated during the dry season. Typically this season is characterized by low wind (less than 4 m/s at 10 m height) and no precipitation. The average ambient

temperature was 32 Ì C , relative humidity, 65-85% and the predominant winds were from south and southeast wind.

3.1 Mass size distribution of suspended particulate matter (SPM) in Bangkok urban air Lundgren type mass plots 26 were applied to the SPM data by assuming the upper and lower cut off size of Andersen cascade impactor as 30 µm and 0.08 µm, respectively. The SPM mass size distributions at different sampling sites are shown in Figure 2. The results seemed to be distributed in a bimodal form with a peak in the fine particle range (< 2.1 µm) between 0.43-0.65 µm and another one in the coarse particle range (> 2.1 µm) between 4.7-7.0 µm. When the data obtained from this study was presented as log-probability plots, it revealed a characteristic of two additive log-normal distribution. Therefore, the typical parameters such as mass median diameter (MMD) and

Figure 2 : Mass distribution of SPM with respect to aerodynamic particle diameter in Bangkok area.

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Table 1 Average mass size distribution of airborne particles in Bangkok urban and background areas during the sampling period.

Locations 0

1

2

Concentration (µg/m3) Size distribution impactor stage Total Fine* Coarse** a b c 3 4 5 6 7 back up SPM MMD GSD SPM MMD GSD SPM

Urban OEPP RATB MOSTE

57 4.6 11.5 13.5 27.2 129 0.52 2.54 (44) 92 27.9 16.9 23.6 19.1 7.7 16.1 19.1 19.8 36.9 187 0.6 2.46 (49) 129 51.1 17.3 39.2 23.2 11.9 22.1 25.2 26.9 54.7 271 0.51 3.10 (48)

72 6.0 1.75 (56) 95 6.5 1.83 (51) 142 7.0 1.85 (52)

38 9.3 12.5 110 0.55 2.14 (35)

72 6.0 1.90 (65)

12.5 16.4 20.8 15.1

7.6

Background ERTC

11.5 15.5 19.2 14.7 10.9 7.2

9.2

* Fine fraction includes stage 5 to 7 and the back up filter ** Coarse fraction includes stage 0 to 4 a mass median diameter b geometric standard deviation c the unit of SPM is in mg/m3 The number in parenthese represents the percentage of SPM in each fraction

the geometric standard deviation (σg) were calculated to define the size distribution of each of two modes. The results are presented in Table 1. The Lundgren type mass plots, reveal bimodal patterns and these seem to be un-affected by the ambient concentration.

Hence the levels of the particulate matter do not seem to impact the MMD and σg of the fine and coarse modes and these remain relatively independent of the sampling location.

It is apparent that at all urban sites, the SPM levels in ambient air exceed the 24 hour average PM-10 Thai ambient standard of, 120 µg/m3. The average levels of SPM in urban areas varied from 129 µg/m3 at OEPP (residential area) to 271 µg/m3 (curbside area), which are considerably higher than those reported by Baek and co-workers (1991)27 in London, UK (25-90 µg/m3), and the levels of PM-10 found in the metropolitan of Philadelphia area during the summers of 1992 and 1993 (24-33 µg/m3) by Burton and §-62

co-workers.28 It should be noted that the level of SPM in this study are in agreement with the levels obtained during winter seasons in other studies in temperate environments. This high PAH ambient concentrations correlate with fossil fuel combustion used for domestic heating.21,27,29 Since the construction of buildings and express ways around the city of Bangkok is a major contributor to particle levels, the particle size distribution is dominated by the coarse size fraction and this is contrary to the results of the studies in London and Philadelphia.27,28,30 The coarse size fraction at the MOSTE site illustrates the strong influence of wind blown pavement dust (particles larger than 10 µm). This is expected given the low sampling height and the proximity of the sampler to the expressway.

Compared to the urban aerosol size distribution, the distribution of the background site samples tends to show a reduction in the 1.1-2.1 µm size range and enhancement of »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

larger particles, 2.1-3.3 µm. This may be due to the entrainment of larger particles and the lack of fine particle sources as well as the aging process of particles (gas-particles conversion) during transport of the aerosols.31,32

3.2 Levels of airborne PAH The average airborne PAH concentrations and ranges, as well as the corresponding particulate matter concentration through the sample period are summarized in Table 2.

Total PAH concentrations (S of 9 compounds) ranged from 19.48 ng/m3 at OEPP (residential & commercial area) to 42.95 ng/m3 in MOSTE (curbside area) which are about 1.5 to 3 times the concentrations at the background site (ERTC, 14.68 ng/m3). The results show that the variation in PAH levels between different areas in Bangkok are fairly small and seem to correlate to SPM concentration in the air.

The concentration of BaP which is highly carcinogenic, were 1.33 ng/m3 and 2.52 ng/m3 at the urban industrial site (RATB) and curbside site (MOSTE), respectively. This is higher than the guideline value for lung cancer risk given by WHO (World Health Organization) in 1987, 1 ng/m3.33 The maximum

value 2.83 ng/m3 at the MOSTE site was about 3 times higher than the guideline. These levels were, however, lower than the proposed, German Federal Environment Agency guideline limit of 10 ng/m3.34 In this study, none of the urban sites, including curbside area, had a mean near this limit.

It should be noted that in a temperate environment, the amount and range of PAH in urban areas exhibit wide seasonal fluctuations.33,34 In most cases, higher levels are observed in winter compared to summer.

The reasons are not only the changes in the contributions of the possible sources like residential heating and traffic to the PAH levels but also the result of a higher atmospheric reactivity of PAH in summer.21,35-37 Little is known about the presence or absence of such seasonal variations in Thailand.

Nevertheless, some existing data suggested that the PAH concentrations were high in the dry season and low in the wet season, and that gas phase PAH might be easily decomposed by photochemical reactions under a strong UV irradiation in summer in Thailand. 38 Hence, levels of airborne PAH

Table 2 Mean and range of SPM (µg/m3) and airborne PAH concentrations (Gas & Particulate phases, ng/m3) in Bangkok urban air.

PAH Pyr BaA BeP DbacA BkF BaP DbahA BghiP 3MC Σ PAH

SPM

OEPP

RATB

MOSTE

ERTC*

14.0 (7.78-19.7) 0.66 (0.41-0.81) 1.29 (0.95-1.62) 0.06 (ND-0.01) 0.48 (0.37-0.55) 0.72 (0.46-0.96) 0.05 (ND-0.08) 2.16 (1.36-2.74) 0.06 (ND-0.09) 19.5 129 (76-192)

19.2 (12.5-26.7) 0.89 (0.73-1.00) 2.78 (2.23-3.74) 0.09 (ND-0.01) 0.70 (0.53-0.09) 1.33 (0.85-2.12) 0.11 (0.02-0.12) 3.62 (3.00-4.38) 0.05 (ND-0.08) 28.8 187 (95-452)

26.7 (16.5-36.1) 1.59 (1.20-1.91) 4.34 (3.31-5.80) 0.12 (ND-0.20) 1.13 (0.85-1.36) 2.52 (2.16-2.83) 0.16 (0.05-0.33) 6.32 (5.98-6.59) 0.07 (ND-0.09) 43.0 271 (186-547)

10.9 0.39 1.67 0.10 0.35 0.24 0.07 0.88 0.04 14.7 110

* average of 3 days sampling, May 3-5, 1996 ND = not detected »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

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Table 3 Comparison of total PAH concentration (ng/m3) in different urban areas PAH

Bangkok* (1996)

Pyr BaA BeP DBacA BkF BaP DBahA BghiP 3MC

19.9 (15.0-30.1) 1.04 (0.41-1.92) 2.80 (1.16-5.80) 0.10 (ND-0.20) 0.76 (0.37-1.36) 1.52 (0.46-2.80) 0.10 (ND-0.33) 4.03 (1.36-6.58) 0.06 (ND-0.09)

Birmingham, U.K. (1996)18 winter summer 38.00 5.60 1.20 0.81 0.83 2.00 -

Chicaco(1988)41 winter

3.30 0.34 0.16 0.25 0.07 0.76 -

36.00 21.00 12.00 14.00 11.00 -

London, U.K.42 1991 1992 12.00 0.35 1.80 1.06 5.33 -

6.80 0.33 1.02 0.56 4.40 -

Augsburg, Germany43 1992 2.60 0.44 0.62 0.52 0.16 0.67 -

* The average PAH concentrations in Bangkok were based on 21 samples and the ranges are presented in the parentheses ND = not detected

Table 4 Comparison of particle associated PAH concentration (ng/m3) in different urban areas.

PAH

Bangkok* 1996

Pyr BaA BeP DBacA BkF BaP DBahA BghiP 3MC

2.14 (0.96-3.83) 0.82 (0.28-1.58) 1.73 (0.76-3.26) 0.04 (ND-0.20) 0.74 (0.35-1.36) 1.52 (0.46-2.73) 0.09 (ND-0.33) 4.03 (1.36-6.59) 0.04 (ND-0.09)

Hamilton, Canada39 summer winter

London, U.K.27 summer winter

Po valley, Italy40 summer winter

2.60 0.94 3.20 13.00 -

0.43 0.28 1.14 0.40 0.83 0.07 2.45 -

0.40 0.10 0.10 0.33 -

2.70 1.80 4.20 16.80 -

1.90 1.19 2.72 1.20 2.46 0.26 4.72 -

1.50 0.83 0.90 5.40 -

Massachusetts,USA17 summer 8.10 1.70 1.34 1.17 0.82 -

* The average particle associated PAH concentrations in Bangkok were based on 21 samples and the ranges are presented in the parentheses

strongly depend on the location of the sampling site and the meteorological conditions, which make it very difficult to compare the results from different investigations. Thus, to understand and compare other findings with our results, the studies which used similar sampling techniques (Filter with adsorbent backup and/or size segregated low volume sampler) were selected and wherever possible, distinctions were made between summer and winter studies. The total PAH concentration (gaseous and particulate phases), and particle phase concentrations in different urban areas are shown in Table 3 and 4, §-64

respectively. The level in Bangkok urban air raged from average the residential area (OEPP) to highest in the roadside area (MOSTE). A comparative study of levels of individual PAH shows that our results appear to be of the same magnitude as those observed in other urban areas in Europe and U.S. and comparable with those found in the winter time of those countries when domestic heating were used.17,18,27,39-43 With the phase-out of organo-lead octane enhancers, PAH have been suggested as alternative automobile tracers. 44 Many studies have recommended that some »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

specific PAH or ratios between PAHs may be used for source identification. Miguel and co-workers45 reported that diesel trucks were the major source of light PAH such as Fluoranthene, PYR and BaA, whereas light-duty gasoline vehicles were the dominant source of higher molecular weight such as BaP, BghiP and DBahA. Nielsen1 reported that the ratio of BghiP to BeP were 1.82 and 1.15 for street and car park samples, respectively. Li and Kamens46 proposed that the ratio of BaA to BaP were 0.5 and 1.0 for gasoline and diesel exhaust, respectively. Greenberg and co-workers47 suggested that Coronene and BghiP are good indicators of automobile traffic.

The high level of BghiP, BaP, BeP and PYR found in this study, ratio of BaA to BaP (0.63 to 0.92) and the ratio of BghiP to BeP (1.30 to 1.68) let us expect automobile traffic to be the major source of airborne PAH in this city.

3.3 Distribution of PAH in Bangkok urban air 3.3.1 Gas-particle distributions of PAH The relative proportion of airborne PAH in the gas and particulate phases are shown in Figure 3. The variation in gas-particle distributions of the prevailing PAH with respect to their molecular weights (MW) has been clearly observed at all sites. PYR was found to be > 80 % in gas phase while BaA was found to be < 30 %. Conversely, the compounds with MW > 252 were primarily associated with the particles on the filter (> 95 % of BkF, BaP, and BghiP were present in the particle phase). BeP was distributed approximately equally between the two phases (~ 40 % was found in gas phase) at the MOSTE and RATB sites, contrary to the OEPP and ERTC sites where only 20-30 % was found in gas phase. These results are in reasonable

Figure 3 : Relative proportion of airborne PAH in gas and particulate phases at each sampling site.

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§-65

agreement with other studies27,42 with the exception of BaA and BeP. The low gas phase BaA and BeP concentrations compared to other studies may be due to the long time scale required to re-establish equilibrium after gas phase photolysis and reaction with ozone, hydroxyl radicals and nitrate.48 In fact, the distribution of PAH in the atmosphere between the gas and particulate phases is determined by several factors such as the vapour pressure, the amount of particulate matters in terms of the surface area available for adsorption, the ambient temperature, the affinity of the compound for the particlesû organic matrix, and the reactivity and stability of the compound.49-52 As a consequence, the partitioning ratio has an important influence on the fate, transport, transformation and the physical removal (i.e., wet and dry deposition) of the compounds in the urban atmosphere.

A number of studies on distribution factors, defined as the ratios of particulate to gas phase concentrations, for PAH have been reported; Yamasaki and co-workers52 have used Langmuirian adsorption/desorption theory to describe the state of gas-particle partitioning of urban PAH as a function of temperature and aerosol concentration. By assuming that the surface coverage of PAH on the particles was low, they introduced a temperature-dependent partition coefficient of the form; K =

A F / TSP

(1)

where A and F are the concentrations of PAH in gas phase and associated particulate phase in the atmosphere (ng/m3), respectively, and TSP is the concentration of particulate matter in the atmosphere (mg/ m3). Pankow53 has proposed the inverse definition of partition coefficient. It is considered to be more convenient and intuitive to define the partition coefficient Kp as §-66

Kp = F / TSP = K-1 A

(2)

where increasing Kp implies increasing partitioning to the particle phase. It has been suggested that the sub-cooled liquid vapour pressure of semivolatile organic compounds is the dominating factor governing both adsorption and absorption processes.54 log Kp = mr log poL + br

(3)

Pankow and Bidleman49 have demonstrated that equation (3) can be effectively used for determining of gas-particle partitioning of organic compounds in actual field samples. Therefore, for comparative reasons, equation (3) was used in this study to illustrate the gas-particle partitioning of PAH compounds in Bangkok urban air. For the purpose, the temperature dependent subcooled liquid vapor pressures (p Ì L) were obtained as follows. Yamasaki and co-workers55 have reported p ÌL (mmHg) and the heat of vaporization (DH, kcal/mol) for a number of PAH compounds at 25 ÌC. Assuming that the DH for a given PAH remains constant over a small temperature range, the data were used to calculate the p ÌL at 30 ÌC of each of the individual PAH collected in this study by referring to Clausius-Clapeyron approximations; H 1 (4) +b R T r where R is the gas constant in kcal -1 -1 K mol , T is the average ambient temperature in K, and b is constant related to the entropy of vaporization. Kp (m3/mg) was plotted against p Ì L (mmHg) on a log-log scale for each sampling site. The relationships between partition coefficient, Kp and saturation liquid phase vapor pressure, p Ì L at each sampling site in this study is shown in Figure 4.

In poL =

»Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

Figure 4 : The relationships between the partition coefficient, Kp, and the saturation liquid phase vapor pressure, p ÌL, at each sampling station.

The results from the regression gave an r2 = 0.84 and show that Kp is well correlated with p ÌL for the residential area (OEPP) samples.

This is within the range of values obtained from other urban areas, r2 = 0.80-0.99. (41, 53, 56-58) More scatter was observed at the other sampling sites; r2 = 0.65, 0.60 and 0.64 for the RATB, MOSTE and ERTC samples, respectively. RATB and MOSTE are classified as industrial and roadside area, respectively.

Since Bangkok is an area of heavy traffic and the sampling at these two sites was carried out almost at ground level (3 m from the ground), the fluctuations in PAH and SPM levels during sampling may be high due to poor homogeneity of the pollutants, causing scatter in the Kp-p ÌL relationship observed »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

at these two sites. In the case of ERTC (the rural site), the possibility of PAH depletion during transport in the atmosphere is probably the cause of the poor correlation. During the sampling period, the ERTC was downwind of Bangkok urban. As air masses are dispersed from source areas to remote regions, the volatile PAH which favor the gas phase are depleted by photochemical reaction. 12,59 Moreover, the existence of photo-chemical reaction in Pathumthani area and in the Bangkok area as a whole has been suggested.60 Field investigations of gas-particle partitioning are often complicated by artifacts caused by changing variables during sample collection. In the absence of artifacts, §-67

equilibrium conditions for compounds of the same class can be estimated by equation (3).

The expected value for the slope (mr) is -1 and the intercept (br) is related to the aerosol properties. In this study, the slope of equation (3) at all sampling sites was greater than -1.

The slopes and intercepts of the log Kp vs log p ÌL lines were not significantly different (two standard errors) within the urban sites (mr = -0.77 to -0.74; br = -6.65 to -6.55), whereas the shallowest slope and the most positive intercept was observed at the rural site (mr = -0.52 ; br = -4.83). The slopes values in Bangkok urban were more or less consistent with those found in other urban air studies, m r = -0.38 to -1.04,41,61 even though the intercepts values did not compare very well. Several factors contribute to the deviation in values of mr from -1 such as effects of temperature and humidity, atmospheric concentration of contaminants, SPM and percent of non-exchangeable material, and blow-off losses from or adsorption gains to particles on the filter.20,62,63 The intercept term (br) depends on the assumed mechanism of gas-particle interaction. If gas to solid adsorption is assumed, br is related to the specific surface area of the particulate matter and the heat of desorption.49 If an absorption model, in which organic compounds are assumed to partition into a liquid-like film on the aerosol is considered, br depends on the fraction of organic matter in the particle that is involved in the partitioning processes and the activity coefficient of the compound in the organic film.48,64 Moreover, factors that change mr can also lead to changes in br.54 In our case, the ambient temperature and relative humidity remained relatively constant over the sampling period at all sites.

As clean air (probably at night time) passed over previously collected particle PAH, a çblow-off artifacté could become important.

This would lead to shallow slopes because the §-68

most volatile PAH would be in the direction of the gas phase and this is consistent with our observations that slopes tend to be greater than -1. On the other hand, it is likely that urban air contains some non-exchangeable PAH, bound to highly active sites or trapped within the particles during their formation.49,51 These PAH will not equilibrate with its vapor phase in the atmosphere but will be extracted with solvent during analysis and operationally counted along with the exchangeable PAH. This phenomenon would also lead to shallow slopes and we feel it is probably consistent across most of our samples.

Another possibility is that of liquid aerosols, the activity coefficient (g) of the different PAHs, dissolved in different kinds of ambient particles changes depending on the chemical make of the particles. Jang et al. have observed that if this is not taken into account, log K p vs -log p Ì L tend to have shallow slopes.65 This will promote the particulate PAH in Kp and hence leading to shallower slopes. Therefore, we speculate that shallow slopes are most probably related to blow-off artifact by clean air and activity coefficient effects.

3.3.2 Particle size dependency of PAH The particle size dependency of PAH in Bangkok urban was using Lundgren type distributions. The PAH concentrations at each sampling site as a function of aerodynamic particle size are shown in Figures 5,6,7 and 8.

PYR, BaA and BeP, predominant compounds in the gas phase, exhibit a bimodal distribution with peaks between 0.43 to 1.1 µm in fine particle range (< 2.1 µm) and 3.3 to 7.0 µm for coarse particle range (> 2.1 µm) in both urban and rural areas. The results showed the maxima of the particle size distribution of these PAH coincides with the maxima of the ambient aerosol suggesting that the gaseous PAH are adsorbed, after the production, onto preexisting particles. The existences of low »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

Figure 5 : Mass distribution of Pyr, BaA, BeP, BkF, BaP and BghiP with respect to aerodynamic particle diameter at ERTC.

Figure 6 : Mass distribution of Pyr, BaA, BeP, BkF, BaP and BghiP with respect to aerodynamic particle diameter at OEPP. molecular weight PAH in the coarse particle fractions suggests a migration from the gas phase to these larger particle sizes. This observation was in good agreement with those found in other urban areas.13,17,27 In contrast, the distribution of BkF, BaP and BghiP was uni-modal. The peak of distribution in urban »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

sites appeared between 0.08-0.65 µm with a generally high contribution from the submicron range particles (< 0.43 µm) collected on the backup filters. This is also consistent with the observations of other studies17 and is consistent with the particle size of mobile-source particulate emissions in Bangkok city which §-69

Figure 7 : Mass distribution of Pyr, BaA, BeP, BkF, BaP and BghiP with respect to aerodynamic particle diameter at RATB.

Figure 8 : Mass distribution of Pyr, BaA, BeP, BkF, BaP and BghiP with respect to aerodynamic particle diameter at MOSTE. fall almost entirely within the size ≤ 2.5 µm.22 Despite orders of magnitude differences in the actual concentrations, similar distributions were observed at rural site with the peak shifted to 0.43-1.1 µm. A depletion in PAH concentrations in the sub-micron range indicated the probable existence of the aging §-70

processes that the aerosols undergo during transport. 66 The preponderance of the contribution of respirable particles to the total concentration of each PAH was evaluated by calculating cumulative percentage concentrations associated with fine and coarse fractions.

The results revealed that PAH extracted from »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

ambient aerosols, as expected, were almost exclusively found in the respirable size range.

For the compounds predominantly in gaseous phase, PYR, BaA and BeP, about 55 to 75% of them appeared to be associated with aerosols of diameter less than 2.1 µm, while higher relative contributions in the same particle size range were found, 80 to 95%, for the compounds which exist almost totally in the particulate phase (BaP, BkF and BghiP).

Furthermore, the PAH present in low concentrations in this area (DBacA, DBahA and 3MC), which are considered to be moderately to strongly carcinogenic, were exclusively found on particles smaller than 0.43 µm. The results obtained here are consistent with the partitioning of PAH in the atmosphere. It is generally accepted that PAH are incorporated, by adsorption and condensation processes, into particulate matter with a marked preference for the sub-micron size particles due to its large specific surface area. 3,27,67 This would be greatest for the least volatile PAH (MW ≥ 252) since particle size dependency of PAH in Bangkok urban atmosphere revealed a decrease in the fraction of PAH associated with larger aerosols as the molecular weight increased.

explored and based on Kp vs log p LÌ slopes less than -1, non-equilibrium conditions are suggested. However, correction for non-ideality of gas-liquid absorption and sampling artifacts, may prove this not to be the case.68 The K p-p ÌL relationship also may suggest the presence of non-exchangeable PAH and that significant time scales are required to re-establish equilibrium after photochemical reaction of the gas phase in the tropical atmosphere. The magnitude of the effect, however, could not be precisely determined.

These results are consistent with a recent study on çParticulate Matter (PM) Abatement Strategy for the Bangkok Metropolitan Areaé. 22 The size-classified particulate matter measurements show a substantial fraction of the ambient PM10 being PM2.5, with more than half of the PM2.5 fraction falling into the 0.43 µm and smaller range. Furthermore, it appears likely that mobile sources are the main source of ambient PM2.5 in Bangkok. As a consequence, the results of this study provide useful information on the phase and particle size distribution of PAH. This should be taken into account as new ambient air quality standards for particulate matter less than 2.5 µm will be deliberated in Thailand.

4. Conclusions

5. Acknowledgments

This study is the first comprehensive field study of levels and distributions of airborne PAH in Bangkok. The results showed high level of some specific PAH and the predominance of particulate PAH in the fine fraction of respirable particles (< 2.1 µm) and the evaluation suggested that automobile traffic was the major source of airborne PAH in Bangkok city. The relationship between the particle/gas partition coefficient (Kp) and the sub-cooled liquid vapor pressure (p ÌL) was

This research was supported by the Environmental Research and Training Center, Thailand. We deeply appreciate Pollution Control Department, Thailand for their assistance in providing sampling sites and air quality data in Bangkok. We are extremely grateful to Professor Richard Kamens and Bharadaj Chandramouli, University of North Carolina at Chapel Hill USA, for their very helpful comments.

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§-71

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