air7

‚§√ß°“√»÷°…“¡≈¿“«–∑“ßÕ“°“»Õ—π‡π◊ËÕß¡“®“° “√‚æ≈’‰´§≈‘°Õ–‚√¡“µ‘°‰Œ‚¥√§“√å∫Õπ (PAH) „π∫√√¬“°“»‡¢µ°√ÿß‡∑æ¡À“π§√2. °“√°√–®“¬µ—«·≈–√–¥—∫§«“¡‡¢â¡¢âπAirborne Polycyclic Aromatic Hydrocarbon (PAH) In Bangkok Urban AirII. Level and Distribution

Hathairatana Garivait*, Wanna Laowagul*

Phaka Sukasem*, Sunthorn Ngod-Ngam*

Chongrak Polprasert**, Lars Baetz Reutergardh**

∫∑§—¥¬àÕ°“√»÷°…“°“√°√–®“¬µ—«¢Õß “√‚æ≈’‰´§≈‘°Õ–‚√¡“

µ‘°‰Œ‚¥√§“√å∫Õπ (PAH) „π∫√√¬“°“»¢Õß

°√ÿß‡∑æ¡À“π§√ „π√Ÿª∑’Ë‡ªìπ°ä“´·≈–‡ªìπΩÿÉπ≈–ÕÕß

∑”‰¥â ‚¥¬„™â‡§√◊ËÕß¡◊Õ‡°Á∫µ—«Õ¬à“ß∑’Ëª√–°Õ∫¥â«¬

‡§√◊ËÕß‡°Á∫µ—«Õ¬à“ßΩÿÉπ≈–ÕÕß·∫∫·¬°¢π“¥ ·∫∫

Cascade impactor (Andersen çlow volumeé

sampler) ∑’Ë “¡“√∂·¬°¢π“¥¢ÕßΩÿÉπ≈–ÕÕß‰¥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 ‡ªìπ

‡«≈“ §√—Èß≈– 24 ™—Ë«‚¡ß “√ª√–°Õ∫ PAH ∑’Ë∑”°“√

«‘‡§√“–Àå¡’®”π«π 9 ™π‘¥ ∑—Èß∑’Ë‡ªìπ “√°àÕ¡–‡√ÁßRefuse incinerator

*»Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ àß‡ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡ ‡∑§‚π∏“π’ µ.§≈ÕßÀâ“ Õ.§≈ÕßÀ≈«ß ®.ª∑ÿ¡∏“π’ 12120 ‚∑√. 0-2577-1136 ‚∑√ “√. 0-2577-1138Environmental Research and Training Center, Department of Environmental Quality Promotion. Technopolis. Klong 5 Klong Luang, Pathumthani 12120 e-mail: [email protected]**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≈–ÕÕß‡°◊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 °“√»÷°…“¬—ß‰¥âÀ“

§«“¡ —¡æ—π∏å√–À«à“ß§à“ —¡ª√– ‘∑∏‘Ï¢Õß —¥ à«π°“√

°√–®“¬µ—« „π√Ÿª¢ÕßΩÿÉπ≈–ÕÕß·≈–„π√Ÿª¢Õß°ä“´ (Kp)

°—∫§à“§«“¡¥—π‰Õ (p ÌL) ¢Õß “√ª√–°Õ∫ PAH ·µà≈–

µ—« „π‡¢µ°√ÿß‡∑æ¡À“π§√‡æ◊ËÕ‡ªìπµ—«·∑π¢Õß°“√

‡°‘¥°“√°√–®“¬µ—«¢Õß “√¥—ß°≈à“«„π√Ÿª¢ÕßΩÿÉπ·≈–

°ä“´„π∫√√¬“°“»‡¢µ‡¡◊Õß√âÕπ

ABSTRACTThe gas-par ticle par titioning 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 Tri-

methylcholanthrene (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%

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 sub-

cooled liquid vapor pressure (p ÌL) was also

determined to describe the gas-particle

partitioning of those PAH compounds in a

tropical atmosphere.

1. IntroductionPolycyclic 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 impor tance in understanding their

atmospheric transpor t , deposit ion 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

§-58 »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ àß‡ √‘¡§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 recent ly to mit igate 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 relat ionship

between the number of persons suffering of

lung cancer and the concentrat ion of

carcinogenic substances in Thailand atmos-

phere 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. Fur thermore, 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 sub-

stances related to this source.

2. Methods and Materials

2.1 Sampling locationsThe target area of the study encom-

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 bui ldings,

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-

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


were extracted by ultrasonication with

dichloromethane, and dichloromethane :

acetonitrile (1:1, v/v), respectively. The extracts

were transferred into acetonitrile solution and

analyzed by reverse phase high performance

liquid chromatography (HPLC) with fluores-

cence 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 DiscussionAir 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 ÌC, relative humidity,

65-85% and the predominant winds were

from south and southeast wind.

3.1 Mass size distribution of suspen-ded particulate matter (SPM) inBangkok urban air

Lundgren type mass plots26 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-probabi l i ty plots, i t 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.


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

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 i l lustrates 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

Table 1 Average mass size distribution of airborne particles in Bangkok urban and background

areas during the sampling period.

Concentration (µg/m3) Size distribution

Locations impactor stage Total Fine* Coarse**

0 1 2 3 4 5 6 7 back up SPM MMDa GSDb SPMc MMD GSD SPM

Urban57 72

OEPP 12.5 16.4 20.8 15.1 7.6 4.6 11.5 13.5 27.2 129 0.52 2.54 (44) 6.0 1.75 (56)92 95

RATB 27.9 16.9 23.6 19.1 7.7 16.1 19.1 19.8 36.9 187 0.6 2.46 (49) 6.5 1.83 (51)129 142

MOSTE 51.1 17.3 39.2 23.2 11.9 22.1 25.2 26.9 54.7 271 0.51 3.10 (48) 7.0 1.85 (52)

Background38 72

ERTC 11.5 15.5 19.2 14.7 10.9 7.2 9.2 9.3 12.5 110 0.55 2.14 (35) 6.0 1.90 (65)

* Fine fraction includes stage 5 to 7 and the back up filter** Coarse fraction includes stage 0 to 4a mass median diameterb geometric standard deviationc the unit of SPM is in mg/m3

The number in parenthese represents the percentage of SPM in each fraction


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 PAHThe average airborne PAH concentra-

tions 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 atmos-

pheric 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 easi ly

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 OEPP RATB MOSTE ERTC*

Pyr

BaA

BeP

DbacA

BkF

BaP

DbahA

BghiP

3MC

ΣΣΣΣΣ PAH

SPM

* average of 3 days sampling, May 3-5, 1996

ND = not detected

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


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

PAH Bangkok* Birmingham, U.K. (1996)18 Chicaco(1988)41 London, U.K.42 Augsburg, Germany43

(1996) winter summer winter 1991 1992 1992

Pyr 19.9 (15.0-30.1) 38.00 3.30 36.00 12.00 6.80 2.60

BaA 1.04 (0.41-1.92) 5.60 0.34 21.00 - - 0.44

BeP 2.80 (1.16-5.80) - - 12.00 - - 0.62

DBacA 0.10 (ND-0.20) - - - 0.35 0.33 -

BkF 0.76 (0.37-1.36) 1.20 0.16 - 1.80 1.02 -

BaP 1.52 (0.46-2.80) 0.81 0.25 14.00 1.06 0.56 0.52

DBahA 0.10 (ND-0.33) 0.83 0.07 - - - 0.16

BghiP 4.03 (1.36-6.58) 2.00 0.76 11.00 5.33 4.40 0.67

3MC 0.06 (ND-0.09) - - - - - -

* 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* Hamilton, Canada39 London, U.K.27 Po valley, Italy40 Massachusetts,USA17

1996 summer winter summer winter summer winter summer

Pyr 2.14 (0.96-3.83) - - 0.43 1.90 0.40 1.50 8.10

BaA 0.82 (0.28-1.58) - - 0.28 1.19 - - 1.70

BeP 1.73 (0.76-3.26) 2.60 2.70 1.14 2.72 - - 1.34

DBacA 0.04 (ND-0.20) - - - - - - -

BkF 0.74 (0.35-1.36) 0.94 1.80 0.40 1.20 0.10 0.83 -

BaP 1.52 (0.46-2.73) 3.20 4.20 0.83 2.46 0.10 0.90 1.17

DBahA 0.09 (ND-0.33) - - 0.07 0.26 0.33 5.40 -

BghiP 4.03 (1.36-6.59) 13.00 16.80 2.45 4.72 - - 0.82

3MC 0.04 (ND-0.09) - - - - - - -

* 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

sampl ing site and the meteorological

conditions, which make it very difficult to

compare the results from different investiga-

tions. 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,

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


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 Bangkokurban 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 com-

pounds with MW > 252 were primarily associ-

ated with the particles on the filter (> 95 % of

BkF, BaP, and BghiP were present in the

particle phase). BeP was distributed approxi-

mately 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.


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;

(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

definit ion of par t it ion coef f icient. It is

considered to be more convenient and intuitive

to define the partition coefficient Kp as

(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

(3)

Pankow and Bidleman49 have demon-

strated that equation (3) can be effectively

used for determining of gas-particle parti-

tioning 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 sub-

cooled liquid vapor pressures (p ÌL) 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 approxi-

mations;

(4)

where R is the gas constant in kcal

K-1 mol-1, T is the average ambient tempe-

rature in K, and b is constant related to

the entropy of vaporization. Kp (m3/mg) was

plotted against p ÌL (mmHg) on a log-log

scale for each sampling site. The relationships

between partition coefficient, Kp and saturation

liquid phase vapor pressure, p ÌL at each

sampling site in this study is shown in Figure 4.

K =A

F / TSP

log Kp

= mr log po

L + b

r

Kp

=F / TSP

A= K-1

In poL

= + br

H

R

1

T


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 Ì

L relationship observed

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 ar tifacts,

Figure 4 : The relationships between the partition coefficient, Kp, and the saturation

liquid phase vapor pressure, p ÌL, at each sampling station.


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; b

r = -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, mr = -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 adsorp-

tion 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 b

r.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

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 Kp 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


Figure 5 : Mass distribution of Pyr, BaA, BeP, BkF, BaP and BghiP with

respect to aerodynamic particle diameter at ERTC.


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

sites appeared between 0.08-0.65 µm with

a generally high contribution from the sub-

micron 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


Figure 7 : Mass distribution of Pyr, BaA, BeP, BkF, BaP and BghiP with respect to

aerodynamic particle diameter at RATB.


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

processes that the aerosols undergo during

transpor t . 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


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).

Fur thermore, 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 par ticle 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.

4. ConclusionsThis 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

explored and based on Kp vs log p Ì

L slopes

less than -1, non-equilibrium conditions are

suggested. However, correction for non-ideality

of gas-l iquid absorption and sampling

artifacts, may prove this not to be the case.68

The Kp-p Ì

L 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.

5. AcknowledgmentsThis 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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