Polycyclic Aromatic Hydrocarbons in a Semiaquatic Plant and

of polycyclic aromatic hydrocarbons (PAHs). Sampling sites affected by various levels of traffic intensity were studied. In addition, PAH levels were ...
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Environ. Sci. Technol. 2003, 37, 47-52

Polycyclic Aromatic Hydrocarbons in a Semiaquatic Plant and Semipermeable Membrane Devices Exposed to Air in Thailand HANNA S. SO ¨ DERSTRO ¨ M* AND PER-ANDERS BERGQVIST Environmental Chemistry, Department of Chemistry, Umeå University, SE-901 87 Umea˚, Sweden

Semipermeable membrane devices (SPMDs) were deployed at six sites in the Bangkok region, Thailand, to investigate spatial variations in atmospheric concentrations of polycyclic aromatic hydrocarbons (PAHs). Sampling sites affected by various levels of traffic intensity were studied. In addition, PAH levels were determined in a common human food plant (water spinach) harvested from canals and ponds in the sampling areas. Significant differences in atmospheric PAH concentrations between sites were found, with 10 times higher PAH levels in the urban areas compared to the rural areas. Increasing concentrations of 1-methylphenanthrene relative to phenanthrene were found in the urban air close to the city center, indicating that traffic probably contributed to the higher PAH concentrations detected. Due to SPMD’s passive sampling technique, their long-term operation and high ability to detect spatial differences, they proved to be suitable for semiquantitative field studies of PAHs. The PAH compounds sampled with SPMDs were mainly associated with gaseous PAHs, while both gas phase and particle-bound PAHs were detected in the plant samples. The relative abundance ratios of some PAHs in the plants were not well correlated with the ratios detected in the SPMDs, indicating that gas-phase exposure made low contribution to the PAH concentrations in the plants. However, similarities in the profiles of 3-ring PAHs between the SPMD and plant samples indicate that gas-phase exchange occurs between the atmosphere and the plants.

Introduction Semipermeable membrane devices (SPMDs) are passive samplers often used to measure the dissolved or gaseous concentrations of Persistent Organic Pollutants (POPs) in water (e.g. refs 1-6) and air (7-11), respectively. In spatial studies, passive sampling techniques are favored for their low-cost, long-term operation, and the fact that they need no electricity. For monitoring applications, a low-density polyethylene (LDPE) membrane with 1 mL (0.915 g) of clean triolein can be used. The sampling efficiency depends mainly on the physicochemical properties of the sampled compound, but it is also influenced by the sampler design and environmental variables such as temperature and flow velocityturbulence of the exposure medium. In this study, SPMDs were deployed for three weeks, and integrative sampling was * Corresponding author phone: +46- (0)90-786 9339; fax: +46- (0)90-12 81 33; e-mail: [email protected]. 10.1021/es020127j CCC: $25.00 Published on Web 11/21/2002

 2003 American Chemical Society

completed before most analytes reached steady-state levels. However, some of the lightest gas-phase compounds might have been in the curve-linear uptake phase at this point. In integrative sampling, the sequestered amount of each analyte in the SPMDs, CSPMD, can be used to calculate the concentration in air (Ca) from

Ca ) CSPMD/RS‚t

(1)

where t is the exposure time in days and RS (m3‚day-1) is the precalibrated sampling rate. The RS values have been calibrated for a wide range of compounds at different temperatures in water (2, 5, 6, 12, 13) and air (8). The mechanism of uptake in SPMDs mimics accumulation through the cell membrane into the lipid layer of an organism. Thus, compounds accumulated in SPMDs may also be accumulated in organisms. The uptake of organic compounds in SPMDs has therefore been compared with the uptake in different biota (3, 4, 14, 15). For example, Echols et al. compared the polychlorinated biphenyl (PCB) concentrations and patterns in sediments, caged fish, and SPMDs, finding that there was a greater relative abundance of lowerchlorinated polychlorinated biphenyls (PCBs) in the SPMDs compared to fish and sediments (3). However, no studies prior to this have analyzed the concentrations of polycyclic aromatic hydrocarbons (PAHs) in SPMDs and plants at the same locations. Plants play an important role in the regional and global distribution of POPs, and many studies have found that POPs accumulate in plants (e.g. refs 16-18). The main uptake route for lipophilic POPs in terrestrial plants has been shown to be from the atmosphere (17). For plants living at the watersurface, uptake may occur from both the water and the air. The plant uptake from the atmosphere occurs mainly through gas deposition or wet/dry particle deposition. In water, the plant uptake occurs via the dissolved or the particle-bound fraction. PAHs in the dissolved fraction are generally adsorbed to the plant surface and/or accumulated via the stomata in the cuticle of the plants, whereas particle-associated PAHs are mainly retained on the plant surface (19). The deposition on the vegetation will be influenced by the different air concentrations, the properties of the plant such as architecture and morphology of the plant species and the environmental conditions (e.g. temperature and wind-speed/ hydrodynamic) (20). In this study, the uptake of lipophilic POPs into water spinach (Ipomoea aquatica) was investigated. Water spinach is a perennial, semiaquatic plant that grows on the water surface or near the water (21). It occurs, both wild and cultivated, in the southern Asia and other subtropics areas. The plant has a long, thin stem that generally grows floating with horizontally oriented leaves that are mostly held slightly above the water surface. The roots are produced from the nodes and either grow freely in the water or penetrate in the wet soil. Water spinach grows very rapidly: up to three cm‚day-1 or even more. The young shoot, including leaves and stem of the upper 30 cm of the plant, is a highly regarded food and is frequently consumed by the inhabitants of the Bangkok region, Thailand. Water spinach is often cultivated in rivers and canals in polluted areas, making it a potential source of human exposure to pollutants. Polycyclic aromatic hydrocarbons (PAHs) are a class of POPs that are widely distributed in the environment. Some of them have reported carcinogenic effects. PAHs are mainly found in emissions from combustion and diverse industrial processes. In the atmosphere they display a wide range of gas-particle partitioning characteristics. The gas-particle VOL. 37, NO. 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Description of the SPMD and Plant Sampling Sitesa site

description

A B C

>1 km from rural area, background air rural area, background/rural air semiurban area < 5 m from moderate traffic urban area < 50 m from heavy traffic urban area < 5 m from heavy traffic urban area < 5 m from heavy traffic

D E F

T (°C) SPMD plant 29.2 29.3 29.3

Y Y Y

N Y Y

29.0 31.9 30.9

Y Y Y

Y N N

a Y - sampling. N - no sampling. T - average temperature (°C) during sampling period.

partitioning is determined by a compound’s subcooled liquid vapor pressure (Pl) and/or its octanol-air partition coefficient (log Koa). For PAHs, the subcooled liquid vapor pressure (Pl) ranges from 101 to 10-6 and the log Koa values range between 6 and 12. PAHs with three or four rings, which have relatively low log Koa values, are mainly associated with the vapor phase, while PAHs with higher log Koa values, such as 5- and 6-ring PAHs, are mainly bound to particles (7). For a given compound the gas-particle partitioning depends on the environmental conditions, such as ambient temperature, the nature of the ambient aerosols, and interactions between the compound and the aerosol (22). In this study, the concentrations of the gas-phase PAHs in the atmosphere were investigated in the Bangkok region of Thailand using SPMDs. The atmospheric concentrations of PAHs in the Bangkok region are elevated. Possible sources of the high atmospheric concentrations of PAHs in this area are fossil fuel combustion for power generation and food preparation, together with a massive increase in the number of motor vehicles in the area (from 600 000 in 1980 to 3 900 000 in 1997 (23)). The sampling sites were chosen to cover a wide range of traffic intensity. Thus, this study also evaluated the contribution of traffic to atmospheric concentrations of PAHs. In addition, water spinach (Ipomoea aquatica) was collected. These plants were studied in parallel with a larger study on this species in Bangkok (21).

Experimental Section Sampling Sites. Six locations in the Bangkok region were used as sampling sites. SPMD samples were collected at all the sites, while plants were collected at three of them (see descriptions in Table 1). The aim was to study sites with a wide range of traffic intensity, ranging from remote areas with background air quality to urban areas with heavy traffic. Therefore special attention was taken to avoid other potential sources of PAHs in the area, although there is always a risk of other unknown sources of PAHs affecting the reported results. Average temperatures during the sampling period ranged between 29.0 and 31.9 °C (see Table 1). SPMD Sampling. At each sampling site, two standard SPMDs (obtained from ExposMeter AB, Umeå, Sweden) were carefully placed on separate steel spiders. The SPMDs were deployed inside a metal umbrella, and the sampling devices were hung up on metal racks at one to three m heights. The metal umbrella was designed to protect the SPMDs from sunlight, rain, wind, and direct particle deposition. Air was still able to pass freely under and around the SPMDs. The SPMDs were exposed to air for three weeks in March and April 2000. Before and after sampling, the SPMDs were stored at -18 °C, in sealed solvent-cleaned tin cans. To provide controls for the exposure during deployment in the field, two single SPMDs were exposed to air during deployment and retrieval in the same manner as the SPMD samples. The two SPMDs from each sampling site were dialyzed as a combined single sample. Our experiences with SPMDs, and also other studies (7, 9), have shown that their sequestration 48

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of lipophilic POPs from the atmosphere is highly reproducible. In addition, several labeled compounds were used as internal standards for the analysis, and the PAH amounts found in the SPMDs were corrected for recovery of these internal standards. Plant Sampling. The 20-25 cm of the upper part of each plant was cut off, wrapped in aluminum foil and transported to the Asian Institute of Technology (AIT), Bangkok, Thailand. This young part of the plant had an age ranging from three to 10 days and had mostly been growing above the water surface. Thus, the main uptake routes for this part of the plant are from the atmosphere. However, minor uptake could also occur through water. In the laboratory, the plant samples were rinsed in tap water followed by distilled water on the day of collection. These procedures simulate the household preparation of the plants and are supposed to reduce the abundance of particles on the plant surface. Shoots from 10 plants were pooled to provide one sample, and for each sampling site triplicate samples were obtained. The samples were weighed (wet weight), wrapped in aluminum foil, and stored at -18 °C. Extraction and Cleanup. Frozen SPMD and plant samples were delivered to the Environmental Chemistry, Department of Chemistry, Umeå University, Sweden, for analysis. SPMD. The SPMDs were mechanically brushed in water, shaken in n-hexane followed by cleaned hydrochloride acid (1 M), and dried with Kleenex tissues. The SPMDs were then dialyzed for 24 h in 100 mL of 95:5 (v:v) cyclopentane: dichloromethane followed by dialysis for another 24 h in 180 mL of the same solvent mixture. All solvents used in the study were of glass-distilled quality (Burdick & Jackson, Neuulm, Germany). After extraction, six 2H-labeled PAH standards (Promochem, Kungsbacka, Sweden) were added as internal standards for the cleanup. The spiked extracts were fractionated by high-resolution gel permeation chromatography (HRGPC) as detailed in Bergqvist et al. (1). However, different columns were used in this investigation from those in the cited study, namely a 300 × 22.5 mm Envirosep-ABC HRGPC column and a 75 × 22.5 mm Envirosep-ABC guard column (both from Phenomenex, St. Torrance, USA). The GPC-fractions were further cleaned on a mixed silica gel column (10 mm i.d.) consisting of 1 g of deactivated silica (Merck, Darmstadt, Germany), 5 g of potassium sodium silica, and 1 g of sodium sulfate (Na2SO4). The samples were eluted with 60 mL of 1:1 (v:v) n-hexane:dichloromethane, and the volume of the sample was reduced with rotary evaporation. Toluene was added to the sample, the volume was further reduced under a stream of nitrogen, and the solvent was changed to toluene. 2H-labeled dibenzofuran (Promochem, Kungsbacka, Sweden) was added as a recovery standard for the GC/MS-analysis. The spiked samples were transferred to GC-vials for analysis. Plant. Each sample was homogenized and 5 g (wet weight) of the homogenate was mixed with 50 g of Na2SO4. The mixture was transferred to an open glass column, a portion of the solution of 2H-labeled PAH standards used for the SPMD analyses was added, and the lipophilic phase was extracted with 100 mL of 1:2 (v:v) n-hexane:acetone, 70 mL of 9:1 (v:v) n-hexane:ether, and 50 mL of 9:1 (v:v) n-hexane: ether. 50 mL of ethanol (99.5%) was added to the lipid extract and the extract was dried by rotary evaporation. The extract was dialyzed using a low-density polyethylene membrane for 2 × 24 h in 2 × 50 mL of n-hexane. The resulting dialyzate was further cleaned with the GPC-system and the mixed silica gel column as described above. Analysis. The plant and SPMD samples were analyzed for the 16 EPA PAHs and one methyl-PAH listed in Table 2, with the high-resolution gas chromatography (HRGC)/lowresolution mass spectrometry (LRMS)-system described by Bergqvist et al. (1). In the mass spectrometry, the most

the plant concentrations had not reached steady state due to the high grow rate in the sampled part of the plant. Total amounts of the 15 remaining EPA PAHs determined ranged between 17 and 134 ng‚SPMD-1‚day-1. Phenanthrene was the most abundant individual compound measured, for which the sequestered amounts in the SPMDs ranged between 9.8 and 64 ng‚SPMD-1‚day-1. Dibenz[a,h]anthracene was not detected in the samples, and indeno[1,2,3-c,d]pyrene and benzo[g,h,i]perylene were only detected in the SPMD samples at sites C and F. Analysis of laboratory and field blanks showed no significant levels of PAHs.

TABLE 2. Physicochemical Properties of Analyzed PAHs naphthalene acenaphthene fluorene phenanthrene 1-methylphenanthrene anthracene fluoranthene pyrene benzo[a]anthracene chrysene benzo[b]fluoranthene benzo[k]fluoranthene benzo[a]pyrene benzo[e]pyrene dibenz[a,h]anthracene indeno[1,2,3-c,d]pyrene benzo[g,h,i]perylene

abr

MWa

log KOWb

Rs c

Na Ace Fl Phe 1-mPhe Ant Fluo Py BaA Chr BbF BkF BaP BeP DahA IcdP BghiP

128.3 (2) 154.2 (3) 166.2 (3) 178.2 (3) 192.2 (3) 178.2 (3) 202.3 (4) 202.3 (4) 228.3 (4) 228.3 (4) 252.3 (5) 252.3 (5) 252.3 (5) 252.3 (5) 278.4 (5) 267.0 (6) 276.3 (6)

3.45 4.22 4.38 4.46

0.5 2.4 2.8 5.0

4.54 5.20 5.30 5.91 5.61 5.78 6.2 6.35

4.6 6.8 7.6 4.7 7.6 3.3 5.5 5.4

6.75 6.51 6.9

3.4 4.7 2.4

The analyzed amounts in the SPMDs can be used for calculating the air concentration (ng/m3) from eq 1 and precalibrated RS values. However, calibration data for PAHs in air were not available, so in this study the RS values of PAHs in water calibrated by Huckins et al. (13) were used for preliminary calculation of the PAH concentrations in air. No adjustments were made for the differences in density between water (1.0 kg‚L -1) and air (1.2 kg‚m-3). Thus, the units of the RS values in water (L‚day-1) were translated to m3‚day-1 units for the RS values in air. The total PAH concentrations in the atmospheric gas-phase varied between 3.2 and 31 ng/m3. The air concentrations of the major compound, phenanthrene, ranged between 2.0 and 13 ng/m3. These levels of phenanthrene are similar to the sequestered amounts detected by SPMDs in the northwest of England by Lohmann et al. (7). In 1996-1997 the concentrations of PAHs bound to atmospheric particles were measured about 40 km from sampling site D and were found to range from not detected to 4.5 ng/m3 for individual compounds (24). However, the SPMDs sampled mainly the gas-phase and collected particlebound PAHs only to a small degree, so these values are not comparable.

a Molecular weight and number of benzene rings in the molecular structure. b Preferred or selected octanol-water partition coefficient from Mackay et al. (30). c SPMD sampling rate (L‚d-1) in water at 26°C from Huckins et al. (13).

abundant ion of the native compounds and the 2H-labeled PAH standards were monitored in the SIR mode. When interfering peaks were present in the chromatogram, the maximum, uncorrected amounts of the affected compounds detected were calculated, noting that they were “not quantified” (nq; see Table 3). In cases where the calculated levels in the laboratory blank samples were higher than in the plant samples, the laboratory blank values were reported (in µg/ sample), and the compounds were listed as laboratory blank values (lb; see Table 3). Undetected compounds were listed as “not detected” (nd), and the criterion threshold for detection (three times the noise levels in the chromatogram) were reported (see Table 3).

SPMD Measurements at Different Levels of Traffic Intensity. The data showed significant differences in the atmospheric concentration of PAHs between sampling sites (Table 3). Furthermore, the differences were correlated with the assumed degree of pollution at the sampling sites according to the traffic intensity (Figure 1). For example, SPMDs at the urban sites E and F sequestered about 10 times more PAHs than SPMDs at site A (rural area). The ability of SPMDs to detect differences in atmospheric concentrations and profiles of PAHs between sampling sites has also been

Results and Discussion SPMD Amounts and Air Concentrations. Amounts of PAHs sequestered in the SPMDs exposed to the air are given in Table 3. The naphthalene concentrations are not included in this discussion, as steady-state concentrations were reached in the SPMDs, whereas it was possible that some of

TABLE 3. Amounts of PAHs in SPMDs Exposed to Air and in Water Spinach (Ipomoea aquatica) at Sampling Sites A-F in Bangkok Region, Thailandc SPMD (ng‚SPMD-1‚day-1)

acenaphthene fluorene phenanthrene 1-methylphenanthrene anthracene fluoranthene pyrene benzo[a]antracene chrysene benzo[b+k]fluoranthenea benzo[a]pyrene benzo[e]pyrene dibenz[a,h]anthracene indeno[1.2.3-c,d]pyrene benzo[g,h,i]perylene Σ PAHsb

plant (µg kg-1 ww) (n ) 3)

A

B

C

D

E

F

B

C

D

0.12 0.57 9.8 1.3 0.14 4.0 1.4 0.030 0.54 0.19 0.035 0.13 nq (0.15) nq (0.071) nq (0.10) 17

0.81 3.4 23 3.0 0.79 11 5.6 0.34 2.0 0.57 0.19 0.035 nd (0.014) nq (0.051) nq (0.033) 47

1.23 4.68 26 6.4 2.0 14 9.5 0.91 2.8 0.62 0.16 0.31 nd (0.025) 0.086 0.092 62

2.7 8.6 46 8.2 2.8 17 13 1.0 4.1 0.68 0.11 0.32 nd (0.020) nq (0.077) nq (0.079) 96

0.54 4.7 64 29 4.8 34 15 1.7 4.7 0.70 0.10 0.28 nd (0.029) nd (0.019) nd (0.021) 131

3.2 9.1 62 55 5.3 29 17 1.5 4.8 0.67 0.13 0.29 nd (0.0060) 0.31 0.18 134

lb (0.0021) 0.32 2.4 2.4 0.073 0.53 2.8 lb (0.0041) lb (0.0030) 0.088 nd (0.019) nd (0.016) nd (0.016) nd (0.019) nd (0.018) 4.5

lb (0.0021) 0.33 3.3 5.3 0.18 0.51 1.3 lb (0.0041) lb (0.0030) 0.11 nd (0.038) nd (0.022) nd (0.036) nd (0.038) nd (0.034) 4.2

lb (0.0021) 0.83 2.5 0.24 0.12 0.68 6.0 lb (0.0041) 0.13 0.26 nd (0.080) 0.22 nd (0.047) nd (0.23) nd (0.26) 9.6

a Σ of benzo[b]fluoranthene and benzo[k]fluoranthene. b Σ of 15 EPA PAHs excluding 1-methylphenanthrene. nd - not detected in sample (figures in the parentheses show 3‚the noise level). nq, not quantified due to interfering peaks (figures in the parentheses show the maximum, calculated concentrations of the affected compounds). lb - level in sample < level in laboratory blanks (figures in the parentheses show the laboratory blank values reported in µg/sample)). c The SPMDs were exposed for 21 days, and the results were presented in ng SPMD-1‚day-1.

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FIGURE 1. Total amounts of 15 EPA PAHs (excluding naphthalene) in SPMDs exposed to air (sites A-F) and in water spinach (Ipomoea aquatica) (sites B-D) (n ) 3, error bars show the standard deviation). reported by Lohmann et al. (7). Since SPMD sampling is based on physical partitioning, it reflects differences in exposure with good precision and thus has a high ability to describe spatial differences. PAH Concentrations in Plants/Human Diet. The total average plant concentrations (n ) 3) of the 15 EPA PAH compounds, shown in Table 3, ranged from 4.2 to 9.6 µg‚kg-1 wet weight (ww). Pyrene was the most abundant measured compound, with concentrations between 1.3 and 6.0 µg‚kg-1 ww. Five of the 5- and 6-ring PAHs were not detected in the samples. Detected PAH concentrations in the laboratory blanks were subtracted from reported results. This resulted in levels of acenaphthene, benzo[a]anthracene, and chrysene being classed as “undetected” and instead the laboratory blank values (lb) were reported. Similar PAH concentrations could be detected for plants sampled at sites B and C (Figure 1). However, the PAH concentrations in plants at site D were two times higher than those at sites B and C. To compare these results with data obtained in other studies, the dry weight (dw) of the plant was estimated to be 10% of the wet weight (ww). Thus, the average plant concentrations of the 15 EPA PAHs ranged from 42 to 96 µg‚kg-1 dw. These values are in the same range as concentrations detected for the 16 EPA PAHs in vegetables growing in an industrial area of northern Greece (16, 18). In a study by Wallin et al. covering 393 households, the daily intake of water spinach (Ipomoea aquatica) per person in the Bangkok region was estimated (25). These households were divided into three groups with consumption ranging from low to high. The results from the cited study were used to estimate the daily intake (ng‚day-1) of PAHs, according to the levels found in water spinach sampled at sites B, C, and D. The daily intake of PAHs ranged from 9 to 21, 26-60, and 152-348 ng‚day-1 for the low, moderate, and high consumption group, respectively. For the high consumption group the daily intake was comparable to the daily intake of the 16 EPA PAHs via lettuce grown in an industrial area of Greece (18). The daily intake for low and moderate consumption of water spinach was similar to the daily intake via carrot and leek in the same Greek study. PAH Profiles in SPMDs. The compounds phenanthrene, 1-methylphenanthrene, fluoranthene, and pyrene were the most abundant PAHs in all samples. Phenanthrene and pyrene have been shown to be generally predominant not only in SPMD samples (7) but also in air samples measured with high volume (HiVol) samplers in an urban location of Birmingham, U.K. (26). The proportion of phenanthrene in the samples ranged between 32 and 54% of the total amount of PAHs. The fluoranthene/pyrene ratio ranged between 1.3 50

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FIGURE 2. 1-Methylphenanthrene/phenanthrene ratio versus the sum of 15 EPA PAHs (excluding naphthalene) in SPMDs at sampling sites A-F (found amounts after the second cleanup with GPC). and 2.8 (mean 1.9), with no correlation with the assumed degree of traffic intensity. The ratio between fluoranthene and pyrene levels has been suggested as a useful measure for detecting contributions to PAHs in air from vehicle emissions. Other comparisons that have been used to estimate PAH contributions originating from traffic are to compare levels of the PAH compounds benzo[g,h,i]perylene with benzo[e]pyrene and coronene with benzo[e]pyrene (27, 28). Many of the profiles suggested earlier for evaluating traffic emissions of PAHs require combined gas and particle sampling. However, SPMDs are mainly based on diffusive sampling of the gas phase, so these comparisons cannot be used. The Potential of 1-Methylphenanthrene/Phenanthrene Ratio To Estimate Traffic PAH Emissions. High concentrations of alkyl-PAHs compared to unsubstituted PAHs have been suggested to indicate the presence of petrogenic components (27-29). Since the RS values of SPMDs are probably similar for PAHs and alkyl-PAHs, the ratio between these compounds provides a possible approach to detect the contribution of traffic to PAH concentrations in air sampled with SPMDs. This study found that the 1-methylphenanthrene/phenanthrene ratio tended to increase with increasing total PAH amounts in the SPMDs (Figure 2). For sites A-E the ratio increased with increasing PAH sequestration in the SPMDs, indicating that vehicle emissions contributed to the higher PAH concentrations detected. In an urban location of Birmingham, UK, road traffic was detected as by far the major source of PAHs in the atmosphere (26). Since the Bangkok region is an area with high traffic intensity, traffic emissions should be considered as one of the main source of PAHs and alkyl-PAHs in this area. SPMDs at sites E and F had comparable levels of the total amounts of the 15 EPA PAHs, but the 1-methylphenanthrene/ phenanthrene ratios were 0.46 and 0.88, respectively, at these sites. Rogge et al. suggested that traffic congestion could increase the alkyl-PAH emissions compared to the unsubstituted PAH emissions, due to the slower movement of the vehicles and lower engine temperatures that occur under such conditions (29). Another factor that might cause concentrations of methylated PAHs to increase compared to unsubstituted PAHs is increasing use of fossil fuel for home heating, but the PAH emissions attributable to this source are negligible in Thailand. The higher 1-methylphenanthrene/phenanthrene ratio at site F compared to site E could therefore be due to the traffic congestion that frequently occurred at site F. However, other unknown sources such as new-laid asphalt could also have contributed to the observed differences between sites E and F. When comparing the ratios of individual compounds between different samples, it is highly important to ensure

that the extracts are clean and selective analytical methods are used. In this study, the samples were analyzed by HRGC/ LRMS in SIR mode, detecting the most abundant ion of each analyte. After the first analysis of the samples, only one sample displayed contamination that interfered with the ions used to identify phenanthrene and 1-methylphenanthrene. However, the remaining samples showed contamination that interfered with ions used to detect pyrene and fluoranthene, so all samples were further cleaned with a second GPC-step. Detected ratios of 1-methylphenanthrene/phenanthrene in the first and second GPC analyses displayed a linear relationship (R2 ) 0.996), but they were twice as high (slope ) 2.14) in the second compared to the first GPC analyses. Since deuterated anthracene was added as an internal standard to detect analytical variations in phenanthrene, anthracene, and 1-methylphenanthrene between samples and treatments, these results show that the ratios between individual compounds are not necessarily constant and demonstrate the importance of using selective analytical methods when relying on ratios to identify sources of emissions. PAH Profiles in Plants. In the plant samples the detected PAH profiles were dominated by five compounds: fluorene, phenanthrene, 1-methylphenanthrene, fluoranthene, and pyrene. The major compounds were phenanthrene and pyrene, accounting for from 22 to 30% and 12-58% of the total PAHs, respectively. Since the uptake routes of particles in plants are uncertain, traditional comparisons used to detect traffic contributions to the PAH concentrations in plants are difficult to use. However, the uptake rates of alkyl-PAHs and PAHs in plants are probably similar, and, thus, comparison of their levels could provide possible means to evaluate the contribution of petrogenic emissions to the PAH concentrations in plants. This study found that 1-methylphenanthrene/ phenanthrene ratios in plants from sites B, C, and D ranged between 0.10 and 1.6 and that the ratios were not correlated with the total concentration of the 15 EPA PAHs in the plants. The higher traffic intensity at sampling site D had, therefore, a low effect on the PAH concentrations in water spinach, and consequently the contribution of traffic to the PAH concentrations in the plants was low. PAH Profiles in SPMDs versus Plants. In both SPMD and plant samples the major PAHs were fluorene, phenanthrene, fluoranthene, and pyrene (Figure 3). However, some differences in the PAH profiles were detected between the SPMD and plant samples (Figure 3). In the SPMD samples, PAHs with three rings (MW < 202) were predominant, while the profiles in plant samples were generally dominated by PAHs with four rings (MW < 202). In a study by Lohmann et al. in which the gas-particulate partitioning of PAHs was studied, > 75% of the PAHs with MW < 202 were found to be associated with the gas phase while PAHs with MW > 252 were mainly bound to particles (22). Thus, the PAHs detected in SPMDs were mainly lighter, gas-phase PAHs, while both the gas and the particle-bound PAHs were accumulated in the plants. Figure 3 presents the individual PAH concentrations found in the air and the plants, showing that the concentrations of pyrene in the plant were not correlated with those detected in the gas phase (SPMDs). Since pyrene, with a MW > 202, can be found both in the gas phase and bound to particles, our results indicate that PAHs bound to particles could be taken up by the plant but not sequestered by the SPMDs. Another explanation to the differences in the profiles between SPMDs and plants could be uptake from the water into the plant. However, possible PAH exposure from the water to the plants was not investigated in this study. Furthermore, the concentration of pyrene in the plants was not correlated with the traffic intensity at the sampling site. Therefore, sources other than traffic, as well as exposure routes other than gas exchange, could have contributed to

FIGURE 3. Individual PAH concentrations in a) the atmosphere and b) the plants at sampling sites B (b), C (2), and D (9). the concentration of pyrene in the plants. However, the similarities in the atmospheric and plant profiles of PAHs with three rings indicate that gas exchange occurred between the plants and the atmosphere.

Acknowledgments We are grateful to SIDA (the Swedish program for International Development Assistance) and EU project ERBIC 15 CT98 0339 for the financial support of this study and to Prof. Bengt-Erik Bengtsson for introducing us to the issue of pollutants in water spinach. We are also grateful to Prof. Kim Oanh, Aree Rattanasule, and Eva Knekta for technical assistance in Bangkok and Prof. Mats Tysklind for reviewing the manuscript.

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Received for review July 3, 2002. Revised manuscript received October 21, 2002. Accepted October 23, 2002. ES020127J