Assessing the Contents of Polycyclic Aromatic Hydrocarbons in the

Oct 15, 2002 - Assessing the Contents of Polycyclic Aromatic Hydrocarbons in the Tollbooths of a Highway Toll Station via Direct and Indirect Approach...
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Environ. Sci. Technol. 2002, 36, 4748-4753

Assessing the Contents of Polycyclic Aromatic Hydrocarbons in the Tollbooths of a Highway Toll Station via Direct and Indirect Approaches P E R N G - J Y T S A I , * ,† T U N G - S H E N G S H I H , ‡ HSIAO-LUNG CHEN,† WEN-JHY LEE,§ CHING-HUANG LAI,# AND SAOU-HSING LIOU# Department of Environmental and Occupational Health, Medical College, National Cheng Kung University, 138 Sheng-Li Road, Tainan 704, Taiwan R.O.C., Institute of Occupational Safety and Health, Council of Labor Affairs, Executive Yuan, 99 Lane 407, Heng-Ke Road, Shijr, Taipei, Taiwan R.O.C., Department of Environmental Engineering, National Cheng Kung University, 1 University Road, Tainan 701, Taiwan R.O.C., and Department of Public Health, National Defense Medical Center, P.O. Box 90048-509, Nei-Hu, Taipei, Taiwan, R.O.C.

The present study was set out to assess the contents of polycyclic aromatic hydrocarbons (PAHs) in three types of tollbooths at a highway toll station via direct and indirect approaches. Direct sampling results show that no significant difference could be found in the PAH homologue distributions for samples collected from the car lane/ticketpayment and car lane/cash-payment tollbooths, but both were significantly different from that for the bus/ truck lane tollbooth. The above results could be due to the former two types of tollbooths that were designed for the same type of traffic (i.e., cars and vans), but the latter was designed for a different type of traffic (i.e., buses and trucks). For any given type of tollbooth, the total-PAH content (CTotal-PAHs) found during the day shift () 937015500 ng/m3) were not significantly different from that found during the night shift () 9550-14900 ng/m3), but both were significantly higher than that found during the latenight shift () 5560-11100 ng/m3). During any given work shift, we found CTotal-PAHs for the three types of tollbooths as the following: bus/truck lane () 11100-15500 ng/m3) > car lane/ticket-payment () 7260-13500 ng/m3) > car lane/ cash-payment () 5560-9550 ng/m3). After conducting multivariate regression analyses, we found that none of the three environmental factors (i.e., wind speed, temperature, and relative humidity), except for the vehicle flow rate (QVehicle), had a significant effect on CTotal-PAHs for any given type of tollbooth. Considering directly measuring PAH contents was labor-consuming and costly, and the above * Corresponding author phone: +886-6-2088391; fax: +886-62752484; e-mail: [email protected]. † Department of Environmental and Occupational Health, Medical College, National Cheng Kung University. ‡ Institute of Occupational Safety and Health, Council of Labor Affairs. § Department of Environmental Engineering, National Cheng Kung University. # Department of Public Health, National Defense Medical Center. 4748

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results suggest the possibility of using QVehicle to predict CTotal-PAHs for any given type of tollbooth. After conducting simple linear regression analyses, we found that (1) all resultant regression coefficients were found with positive values indicating that an increase in the QVehicle would lead to an increase in the CTotal-PAHs, (2) from the magnitude of the resultant regression coefficients indicating that an increase in CTotal-PAHs caused by per unit QVehicle for the three types of tollbooths were the following: bus/truck lane > car lane/cash-payment > car lane/ticket-payment, and (3) the resultant R2 values fell to the 0.54-0.75 range indicating that the variations in CTotal-PAHs could be explained well by QVehicle for the three types of tollbooths. It is concluded that measuring QVehicle can be regarded as an effective indirect method for estimating PAH contents in various types of tollbooths.

Introduction Polycyclic aromatic hydrocarbons (PAHs) and their derivatives have been recognized as the major culprits causing human lung cancer in urban areas (1-4). Among the various PAH emission sources, the traffic source has been known to be the greatest contributor in many countries (5-7). For example, Benner and his colleagues showed that motor vehicles accounted for ∼36% of the yearly total PAH emissions in the United States (8). In Taiwan, because of its high vehicle density (∼475 vehicles/km2, the highest in the world), PAH emissions from the traffic sources have been intensively investigated. For example, Lee and his colleagues showed that the total PAH contents at the intersection of two main roads in a city area located in Southern Taiwan was ∼5.3 and 8.3 times in magnitude higher than that in the urban and rural atmosphere, respectively (9). Chen and his colleagues found that total PAH content contained in fine particles (Dp < 1.0 µm) at a bus station was ∼2.2 and 9.0 times higher than that for urban and rural areas, respectively (10-12). The above studies clearly suggest the importance on investigating PAH contents in the atmospheres directly associated with traffic emissions. To date, the freeway transportation system has been widely used in many developed and developing countries. In particular, vehicle flow rates for the freeway transportation system have increased dramatically in recent years. For example, the vehicle flow rate in a major freeway in Taiwan increased from 1.39 × 108 vehicles/yr in 1985 to 4.24 × 108 vehicles/yr in 1998. However, to the best of our knowledge, the PAH contents in the ambient air in highway toll stations have never been assessed. This study was conducted on a highway toll station located in Northern Taiwan. Field samples were collected from three types of tollbooths at the selected toll station to assess the contents and the characteristics of PAHs contained in these tollbooths. Considering direct PAH sampling is very labor consuming and costly, and another important objective was to develop an indirect method to predict PAH contents for any type of tollbooth under various traffic and environmental conditions.

Methods and Materials The Selected Highway Toll Station. A highway toll station containing 20 vehicle lanes was selected. Beside each vehicle lane, one tollbooth (L × W × H ) 1.5 m × 1.0 m × 2.1 m) was installed. A total of 20 tollbooths were installed, including 4 booths designed for collecting both cash and prepaid tickets 10.1021/es020721t CCC: $22.00

 2002 American Chemical Society Published on Web 10/15/2002

TABLE 1. Meteorological Conditions for the Selected Highway Toll Station during the Sampling Period meteorological parameter (m/s)a

wind speed air temperature (°C) relative humidity (%)

mean

range

3.6 18.7 89.6

1.8-5.9 17.3-20.6 85.6-94.0

a Wind direction during the sampling period remained the same as ESE.

from buses and trucks (denoted the bus/truck lane tollbooth; lane width ) 318 cm), 12 for collecting prepaid tickets from cars and vans (denoted the car lane/ticket-payment tollbooth; lane width ) 302 cm) and 4 for collecting cash from cars and vans (denoted the car lane/cash-payment tollbooth; lane width ) 302 cm). All tollbooths had the same dimension (L × W × H ) 1.5 m × 1.0 m × 2.1 m). Each of them had one door (W × H ) 75 cm × 190 cm) which opened completely toward the vehicle lane for collecting tolls from vehicles and one window installed on the opposite side (L × W ) 65 cm × 30 cm). The whole study was conducted in winter (Jan/ 3/2001-Jan/12/2001), and we found the window was closed and no ventilation was used at each tollbooth. The toll station ran for three work shifts per day, including a day shift (08:00 AM-16:00 PM), a night shift (16:00 PM-00:00 AM), and a late-night shift (00:00 AM-08:00 AM). Sampling Strategy. PAH samplings were conducted on 2 bus/truck lane tollbooths, 2 car lane/cash-payment tollbooths and 4 car lane/ticket-payment tollbooths during each of the three work shifts on each day during the sampling period of 10 consective days. As a result, a total of 240 PAH samples were obtained, including 120, 60, and 60 samples which were collected from the car lane/ticket-payment, car lane/cash-payment and bus/truck lane booths, respectively. The sampling method adopted in this study was modified from the NIOSH method 5515, which has been used to collect PAH samples from carbon black manufacturing industries (13, 14). The sampling train consisted of a filter cassette (IOM personal sampler, SKC Inc., Eighty-four, PA), followed by a sorbent tube (XAD-2 resin, 3.5 g/0.5 g) for collecting particleand gas-phase PAHs, respectively. The use of the IOM personal sampler (sampler inlet ) 15 mm) was because it would collect more representative samples for assessing booth attendants’ inhalable particle-phase PAH exposures. Each PAH sample was taken at a height of 150 cm inside the tollbooth beside the location of the booth attendant (i.e., the breathing zone) for approximately one work shift (i.e., ∼8 h) with the sampling flow rate specified at 2.0 L/min. Before sampling, both the filter and XAD-2 resin were cleaned and extracted with a solvent solution (mixture of n-hexane and dichloromethane, v:v ) 1:1) for 24 h in a Soxhlet extractor to ensure that it was free from contamination. After sampling, all filters and XAD-2 resin were sent to the lab for PAH analysis. During any given workshift, the traffic for any selected tollbooth was counted by using a pneumatic tube, which was laid across the vehicle lane and was connected to an automatic data logger. This allowed us to calculate vehicle flow rates for each selected tollbooth. Considering the environmental humidity, air temperature, and wind speed might have significant effects on PAH concentrations in tollbooths, the above factors were measured 3 times per shift and were carefully registered (Table 1). Sample Analyses. For PAH analysis, both filter and XAD-2 resin were placed in a solvent solution (the mixture of n-hexane and dichloromethane, v:v ) 500 mL:500 mL) and extracted in a Soxhlet extractor for 24 h. The extract was then concentrated, cleaned-up, and re-concentrated to exactly 1.0 mL or 0.5 mL. PAH contents were then determined using a gas chromatograph (GC) (Hewlett-Packard 5890A) with a

mass selective detector (MSD) (Hewlett-Packard 5972) and computer workstation. GC/MS equipped with a HewlettPackard capillary column (HP Ultra 2∼50 m × 0.32 mm × 0.17 µm) and an HP-7673A automatic sampler was operated under the following conditions: injection volume 1 µL, splitless injection at 310 °C, ion sources temperature at 310 °C, oven from 50 °C to 100 °C at 20 °C/min; 100 °C to 290 °C at 3 °C/min; hold at 290 °C for 40 min. The masses of the primary and secondary PAH ions were determined using the scan mode for pure PAH standards. Qualification of PAHs was performed using the selected ion monitoring (SIM) mode as that used in our previous studies (15-19). The concentrations of 22 PAH species in both filter and XAD-2 resin were determined, including naphthalene (Nap), acenaphthalene (AcPy), acenaphthene (Acp), fluorene (Flu), phenanthrene (PA), anthracene (Ant), fluoranthene (FL), pyrene (Pyr), cyclopenta(c,d)pyrene (CYC), benzo[a]anthracene (BaA), chrysene (CHR), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[e]pyrene (BeP), benzo[a]pyrene (BaP), perylene (PER), indeno(1,2,3,-cd)pyrene (IND), dibenzo(a,h)anthracene (DBA), benzo[b]chrycene (BbC), benzo(ghi)perylene (BghiP), coronene (COR), and dibenzo(a,e)pyrene (DBP). Both gas-phase and particle-phase PAH contents were combined to describe the content of each individual PAH compound contained in the tollbooth ambient air. The total-PAHs content was defined as the sum of the contents of the 22 PAH compounds. To assess the PAH homologue distribution for each collected sample, the PAH compound content with low molecular weights (LM-PAHs, containing 2- to 3-ringed PAHs), middle molecular weights (MM-PAHs, containing 4-ringed PAHs), and high molecular weights (HM-PAHs, containing 5- to 7-ringed PAHs) were also determined. Recovery efficiencies of the 22 PAH compounds and the five internal standards (i.e., NaP-d8, Acp-d10, PA-d10, CHRd12, and PER-d12) were first determined by processing a solution containing known PAH concentrations through the same experimental procedure as that for analyzing field samples. The resultant recovery efficiencies of these five internal standards were then used to correct the original recovery efficiencies of the 22 PAH compounds (i.e., Nap was corrected by Nap-d8; AcPy and Acp were corrected by Acp-d10; Flu, PA, Ant, FL and Pyr were corrected by PA-d10; CYC, BaA, CHR, BbF and BkF were corrected by CHR-d12; BeP, BaP, PER, IND, DBA, BbC, BghiP, COR, and DBP were corrected by PER-d12). We found the recovery efficiencies of the five internal standards were ranging from 80.4 to 99.7%. The corrected recovery efficiencies of the 22 PAH compounds fell to the range 78.7%-102.3% (average ) 84.6%). Analysis of serial dilutions of PAH standards showed the limit of detections (LOD) for the 22 PAH compounds fell to the range 39.0-613 pg. The limit of quantification (LOQ) was defined as the limit of detection divided by the sampling air volume. The LOQ for the 22 PAH compounds fell to the range 40.6639 pg/m3. Blank tests for PAHs were accomplished using the same procedure as the recovery-efficiency tests without adding the known standard solution before extraction. Analyses of field blanks, including the glass fiber filter and XAD-2 cartridge, found no significant contamination (i.e., GC/MS integrated area < detection limit).

Results and Discussion Distributions of PAH Homologues. Tables 2 to 4 show the contents of the 22 PAH compounds and total-PAHs, and the PAH homologue fractions for LM-PAHs, MM-PAHs, and HMPAHs for samples collected during the three work shifts from the three types of tollbooths, respectively. Results show that no significant difference could be found in the PAH homologue distributions for samples collected during the three work shifts for any given type of lane booth (p > 0.05, ChiVOL. 36, NO. 22, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Contents of the 22 PAH Compounds and Total-PAHs and the Fractions of PAH Homologues of LM-PAHs, MM-PAHs, and HM-PAHs for Samples Collected from the Car Lane/Ticket-Payment Booth during the Three Different Work Shifts day shift (n ) 40) compound

mean

night shift (n ) 40) SD

mean

late-night shift (n ) 40)

SD

mean

SD

1460 149 33.9 67.1 97.7 30.9 19.2 38.0 15.9 16.0 17.5 29.7 36.3 60.7 49.4 47.6 16.0 17.9 98.2 18.1 96.1 24.7 1960

5897 217 128 152 171 68.1 51.3 63.2 5.71 2.83 4.29 31.9 47.6 62.9 49.3 78.9 21.1 33.8 77.3 11.6 61.3 22.7 7260

1240 87.5 44.8 55.2 74.5 31.5 21.2 30.3 10.1 8.6 10.3 19.6 32.0 29.9 70.4 66.0 26.9 13.4 69.7 18.5 70.9 39.6 1620

(ng/m3)

Nap AcPy Acp Flu PA Ant FL Pyr CYC BaA CHR BbF BkF BeP BaP PER IND DBA BbC Bghip COR DbP total-PAH

11000 288 92.8 184 304 167 86.1 93.1 10.9 7.91 19.0 76.1 60.1 72.2 81.2 101 52.8 62.1 147 66 370 87.7 13500

LM-PAHs MM-PAHs HM-PAHs

89.7 1.53 8.81

1800 85.4 61.0 61.7 121 43.8 32.2 36.5 18.8 24.8 14.9 37.4 27.4 52.8 51.8 62.9 20.2 28.5 110.5 41.1 129 22.9 1710

Contents 10200 386 119 234 263 93.4 73.9 89.8 19.4 13.5 22.1 53.4 77.9 111 78.0 67.8 47.4 49.2 171 33.1 236 15.5 12500

Fractions of PAH Homologues (%) 4.67 90.6 0.86 1.61 3.29 7.84

square test). The above results clearly indicate that the same lane booth type might have a very similar PAH emission source considering that each type of tollbooth was designed for collecting tolls from the same type of vehicles, which suggests that the above inference could be theoretically plausible. However, it should be noted that both the car lane/ ticket-payment and car lane/cash-payment tollbooths were specified for collecting tolls from the same type vehicles (i.e., cars and vans), and thereby it is not so surprising to see that no significant differences could be found in their PAH homologue distributions (p > 0.05; Mann-Whitney U test). However, it should be noted that the type of vehicles specified for the bus/truck lane tollbooth (i.e., buses and trucks) was different from those for the above two types of car lane tollbooths. Therefore, it is not so surprising to see that the PAH homologue distribution of the bus/truck lane booth was significantly different from the above two types of tollbooths (p < 0.05; Mann-Whitney U test). Total-PAH Contents for the Three Types of Tollbooths. We found that the total-PAH contents for the day shift were not significantly different from those for the night shift at the car lane/ticket payment booths (p > 0.05; Mann-Whitney U test), but both were significantly higher than that for the late-night shift (p < 0.05; Mann-Whitney U test) (Table 2). A similar pattern could also be found for both the car lane/ cash-payment (Table 3) and bus/truck lane booth (Table 4). However, it should be noted that the PAH emission sources for any given type of tollbooth during the three work shifts could be very similar. Based on this, it was expected that the above differences in total-PAH contents for the three work shifts could be mainly due to their intrinsic vehicle flow rate differences. Table 5 shows vehicle flow rates for the three types of tollbooth during the three work shifts. For any given type of tollbooth, the vehicle flow rates for both the day shift and the night shift were not significantly different (p > 0.05; Mann-Whitney U test), but both were significantly higher than those for the late-night shift (p < 0.05; Mann-Whitney 4750

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5.52 0.94 3.44

90.7 1.77 7.49

5.46 0.96 3.04

U test). Obviously, the above results were consistent with the pattern found on total-PAH contents. The above results suggest that a higher vehicle flow rate would lead to higher total-PAH contents for any given type of tollbooth. However, it should be noted that significant differences were found on the vehicle flow rates for the three types of tollbooths during any given work shift (p < 0.05; Chi-square test) with the values in the following sequence: car lane/ ticket-payment > car lane/cash-payment > bus/truck lane (Table 5). But as we examine the total-PAH contents for the three types of tollbooths during any given work shift, this study yielded a totally different pattern: bus/truck lane > car lane/ticket-payment > car lane/cash-payment (see Table 2 to Table 4). The above results suggest that the unit totalPAH emission rates (the amount of total-PAH emissions caused by per unit of vehicle flow rate) for the three types of tollbooths could be different. The above results also implied that, to predict the total-PAH contents, it was necessarily to relate the total-PAH contents (CTotal-PAH; ng/m3) to the corresponding vehicle flow rates (QVehicle; vehicles/shift) for each of the three types of tollbooths separately. To date, the occupational exposure limit for total-PAHs has not been established yet because of the complexity of PAHs in their chemical composition. Nevertheless, the U.S. Occupational Safety and Health Administration (OSHA) has set the time-weighted-average permissible exposure limits (PELs-TWA) for several individual PAH compounds (such as: Ant ) 0.2 mg/m3; BaP ) 0.2 mg/m3; CHR ) 0.2 mg/m3; etc.). As compared these PELs-TWA to the corresponding PAH concentrations found in the three type of tollbooth (all less than 1 µg/m3) indicating that booth attendants’ PAH exposures were not significant. However, it should be noted that vehicle exhaust has been known associated with the emission of other hazardous compounds, such as volatile organic carbons (VOCs). Considering VOC contents in these three types of tollbooth were not investigated which warrants the need for further investigation in the future.

TABLE 3. Contents of the 22 PAH Compounds and Total-PAHs and the Fractions of PAH Homologues of LM-PAHs, MM-PAHs, and HM-PAHs for Samples Collected from the Car Lane/Cash-Payment Booth during the Three Different Work Shifts day shift (n ) 20) compound

mean

night shift (n ) 20)

SD

mean

late-night shift (n ) 20)

SD

mean

SD

1309 106 41.9 56.8 65.8 13.6 13.2 11.3 10.1 6.41 12.3 21.3 31.3 39.6 74.2 64.0 1.33 1.30 31.5 17.9 156 41.1 1620

4240 230 151 116 184 59.9 41.1 58.1 14.4 11.5 22.2 36.8 61.2 56.1 58.4 54.3 4.56 5.89 39.1 15.9 56.1 41.1 5560

958 121 39.8 86.0 109 25.3 24.2 20.9 12.9 16.2 18.5 27.2 63.3 182 86.4 19.5 13.8 14.9 309 20.5 146 20.4 1420

Fractions of PAH Homologues (%) 5.79 89.6 6.37 0.97 2.39 1.69 3.26 7.98 5.02

90.0 2.37 7.61

(ng/m3)

Nap AcPy Acp Flu PA Ant FL Pyr CYC BaA CHR BbF BkF BeP BaP PER IND DBA BbC Bghip COR DbP total-PAH LM-PAHs MM-PAHs HM-PAHs

7440 276 100 216 267 109 58.6 54.0 23.7 19.8 45.2 44.7 61.8 141 99.8 98.3 10.3 11.0 91.3 27.9 195 61.8 9370

1180 125 38.8 82.1 121.0 30.6 51.8 27.6 33.4 23.0 19.3 124 254 110 53.8 136 70.7 36.9 95.9 50.6 153 224 1280

89.0 2.88 8.17

Contents 7500 250 114 201 270 99.8 68.2 86.5 23.1 26.3 37.5 74.3 54.1 107.0 106 92.4 5.82 6.32 73.5 31.3 265 62.9 9550

8.46 1.84 5.60

TABLE 4. Contents of the 22 PAH Compounds and Total-PAHs and the Fractions of PAH Homologues of LM-PAHs, MM-PAHs, and HM-PAHs for Samples Collected from the Bus/Truck Lane Booth during the Three Different Work Shifts day shift (n ) 20)

night shift (n ) 20)

late-night shift (n ) 20)

compound

mean

SD

mean

SD

mean

SD

Nap AcPy Acp Flu PA Ant FL Pyr CYC BaA CHR BbF BkF BeP BaP PER IND DBA BbC Bghip COR DbP total-PAH

11800 295 241 255 343 133 195 173 57.7 74.3 67.3 208 250 123 161 29.0 32.4 145 192 62.2 358 242 15500

1260 87.5 62.2 53.3 76.1 31.8 69.3 47.2 48.1 35.3 42.2 198 128 89.8 76.9 34.0 39.2 73.5 43.8 46.1 121 199 1410

Contents (ng/m3) 11300 221 278 172 310 77.9 185 228 62.1 85.5 118 186 202 183 167.3 119 61.8 146 73.6 67 247 219 14900

1370 87.5 62.2 53.3 40.8 31.8 64.3 57.2 38.1 39.3 40.2 98.3 109 90.8 86.2 54.0 19.2 33.5 63.3 33.1 97.7 180 1310

8380 199 170 75.3 229 110 89.9 136 47.7 60.2 73.9 314 134 129 123 381 38.8 109 113 27.3 124 125 11100

1570 91.1 50.4 27.0 72.7 42.7 48.1 28.1 34.8 11.1 32.6 80.3 94.2 61.1 60.9 42.1 21.5 35.6 48.9 21.5 83.7 187 1590

LM-PAHs MM-PAHs HM-PAHs

84.8 3.29 11. 9

Fractions of PAH Homologues (%) 5.86 84.0 1.07 4.26 3.70 12.7

Predicting Total-PAH Contents for the Three Types of Tollbooth. As mentioned in the above section, the vehicle flow rate might have a significant effect on PAH contents in any given type of tollbooth. However, it should be noted that

6.06 1.35 4.10

82.7 3.25 14.1

6.36 1.81 4.32

the PAH contents might also be affected by the involved environmental conditions. Therefore, we first conducted the multivariate regression analyses to relate the total-PAH contents to the corresponding vehicle flow rate, wind speed, VOL. 36, NO. 22, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 5. Vehicle Flow Rates for Each of the Three Types of Tollbooths during the Three Different Work Shifts during a 10 Consecutive Day Sampling Period unit: vehicles/shift day shift (n ) 40)

night shift (n ) 40)

late-night shift (n ) 40)

type of tollbooth

mean

SD

mean

SD

mean

SD

car lane/ticket-payment (n ) 40) car lane/cash-payment (n ) 20) bus/truck lane (n ) 20)

6691 3778 2832

628 285 628

6585 3751 2744

697 273 522

2829 1918 1484

804 316 380

temperature, and relative humidity for each of the three types of tollbooth. Results show that none of the three environmental factors (i.e., wind speed, temperature, and relative humidity) (all with p-values > 0.05), except for the vehicle flow rate (p-value , 0.05), had a significant effect on PAH contents for any given type of tollbooth. Considering this study was conducted on Jan/3/2001-Jan/12/2001, the above result could be partly due to the small variations in the above three environmental factors (Table 1). However, it should be noted that the involved toll-collecting procedure at the tollbooths might also play an important role. During samplings, we found that each vehicle was requested to stop beside the tollbooth to pay the toll. This led to the fact that the following vehicle might block the vehicle lane while waiting behind, and thereby might eliminate the effects of the above three environment factors on PAHs transportation. Based on this, it is not so surprising to see that none of the above three environmental factors was significant. It is known that the three types of tollbooth were designed for different types of traffic. Therefore, in this study the relationship between the total-PAH contents (CTotal-PAH; ng/ m3) and the corresponding vehicle flow rates (QVehicle; vehicles/ shift) for each of the three types of tollbooths was determined separately. Figure 1 shows the linear regression results as follows:

car lane/ticket-payment: CTotal-PAH ) 1.43QVehicle + 3230 (R2 ) 0.75, n ) 60) (1) car lane/cash-payment: CTotal-PAH ) 2.16QVehicle + 1440 (R2 ) 0.62, n ) 60) (2) bus/truck lane: CTotal-PAH ) 2.71QVehicle + 7660 (R2 ) 0.54, n ) 120) (3) All of the resultant regression coefficients (i.e., the slopes of the above regression equations) were found to have positive values indicating that an increase in the vehicle flow rate would lead to an increase in the total-PAH content. Moreover, significant differences were found in the magnitude of the above regression coefficients, further confirming the plausibility of our previous finding (i.e., the unit total-PAH emission rates for the three types of tollbooths were significantly different). We found the magnitudes of the above regression coefficients shown in sequence as the following: bus/truck lane () 2.71) > car lane/cash-payment () 2.16) > car lane/ticket-payment (1.43). These results warrant the need for further discussion. In our previous studies we have found that, under idling conditions, the total-PAH content exhausted from a heavy-duty diesel engine () 1500 µg/m3) (4) was significantly higher than that exhausted from a gasolinepowered engine () 337 µg/m3, using 95-leadfree gasoline) (16). In addition, we found the time for a vehicle spent at the bus/truck lane booth, car lane/cash-payment, and car lane/ cash-payment for paying the toll were ∼7.6, 6.3, and 3.8 s, respectively. Based on these, it is not surprising to see that 4752

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the bus/truck lane booth (assuming most vehicles with diesel engines) had a higher unit total-PAH emission rate (due to the vehicle emitted a higher PAH concentration and spent a longer time at the tollbooth) than the other two types of car lane tollbooth (assuming most vehicles with gasoline powered engines). Because both the car lane/cash-payment tollbooth and car lane/ticket-payment tollbooth had similar PAH emission sources, the higher unit total-PAH emission rate was found for the former than the latter obviously could be due to their intrinsic differences in the toll collection time. In principle, the theoretical unit total-PAH emission rate for a vehicle at any given type of tollbooth could be determined as the following: the emitted total-PAH concentration × the time spent at the tollbooth. According to the above assumption, we found the ratio on the theoretical unit total-PAH emission rate for the three types of tollbooth (i.e., bus/truck lane:car lane/cash-payment:car lane/ticket-payment ) 1:0.19: 0.11) was not consistent with the ratio on the unit total-PAH emission rate found in this study () 1:0.96:0.53). This could be partly due to the fact that the idling condition was not able to fully explain the real driving condition for a vehicle at the tollbooth (i.e., from the low speed to the idling, then to the low speed conditions). We found the intercepts of the three regression lines in Figure 1 were not consistent () 3230, 1440, and 7660 for the car lane/ticket-payment, car lane/cash-payment, and bus/ truck lane, respectively) indicating that the original background PAH concentrations in the three types of tollbooth were not the same. The highest background concentration found in the bus/truck lane tollbooth might be because the involved vehicles had larger body sizes (and thereby was able to block the tollbooth door and led to the highest PAHs accumulation inside the tollbooth) than the other two types of tollbooth. On the other hand, considering both the car lane/ticket-payment and car lane/cash-payment were designed for the same type of traffic, it is expected that the involved vehicles for both types of tollbooth could have very similar body sizes. But it should be noted that the vehicle flow rates for the car lane/ticket-payment were higher than that for the car lane/cash-payment. Considering that the higher vehicle flow rate would result in the longer time for vehicles blocking the tollbooth, it is not so surprising to see that the background PAH concentration for the former was higher than that for the latter. Finally, we found that the coefficient of determination (i.e., R2) for the three types of tollbooths fell to the range 0.54-0.75. Subjected to the limitation of this study, the traffic densities were available only for the 8 selected tollbooths (i.e., no data was available for the other 12 tollbooths). Whether the error of predictions for the three regression lines was due to the effects of emissions from vehicles in the neighboring lanes warrants the need for further investigation. Nevertheless, the results obtained from this study do indicate that QVehicle was able to explain 54%-75% variations in CTotal-PAH for the three types of tollbooth. Considering measuring QVehicle is much less labor consuming and costly than direct measuring PAHs, the results obtained from this study can be regarded, at least, as a useful indirect method

countries could be different from that of in Taiwan (bus: truck:car ) 1:36:197). However, it should be noted that the type of traffic designed for a given type of tollbooth in other countries could be quite similar to that in Taiwan. This indicates that the results obtained from this study could still be applicable in other countries. But it should be noted that the selected toll station is located in the plain rural area (∼45 km away from the Taipei city). Therefore, the results obtained from this study should be used with caution at other toll stations with different surroundings and topographical features.

Acknowledgments The authors wish to thank the Institute of Occupational Safety and Health (IOSH) of the Council of Labor Affairs in Taiwan for funding this research project.

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FIGURE 1. Relationships between total-PAH contents and vehicle flow rates for samples collected during the three work shifts from the three types of tollbooth.

Received for review May 3, 2002. Revised manuscript received August 7, 2002. Accepted September 4, 2002.

for estimating the PAH content in the three types of tollbooth. Yet, it might be true that the composition of traffic in other

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