Measurement of non-methane hydrocarbons in Taipei city and their impact on ozone formation in relation to air quality

Analytica Chimica Acta 576 (2006) 91–99

Measurement of non-methane hydrocarbons in Taipei city and their impact on ozone formation in relation to air quality Ben-Zen Wu b , Chih-Chung Chang c , Usha Sree b , KongHwa Chiu d,∗ , Jiunn-Guang Lo a,∗∗ a

Department of Radiological Technology, Yuan Pei Institute of Science and Technology, Hsinchu, Taiwan 300, ROC b Department of Atomic Science, National Tsing Hua University, Hsinchu, Taiwan 300, ROC c Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan 115, ROC d Department of Natural Science, National Science Council, Taipei, Taiwan 106, ROC Received 20 November 2005; received in revised form 20 February 2006; accepted 2 March 2006 Available online 10 March 2006

Abstract Air pollutants data from semi-continuous measurements at multiple sampling sites in Taipei metropolitan area of Taiwan was obtained by collecting air samples in canisters. The hydrocarbon composition was determined by using GC/MS and GC/FID. The air samples were preconcentrated onto glass beads prior to separation by PLOT and DB-1 columns of GC. The method showed detection limit of <1 ppb and relative standard deviation in the range of 5–30% for different compounds. Aromatic hydrocarbons (toluene, benzene, etc.) and aliphatic hydrocarbons (ethylene, acetylene, propane, etc.) were correlated primarily to determine the source of emission. The estimated hydrocarbons were ranked according to their abundance and photochemical reactivity. The criteria pollutants, ozone and NO2 were measured by UV-differential optical absorption spectroscopy (UV-DOAS), and were utilized to determine the relative importance of non-methane hydrocarbons (NMHC) and significant contribution of NO2 in limiting ozone formation. The obtained results suggest that ozone formation in Taipei city is probably limited by the supply of non-methane hydrocarbons. The concentration profile of targeted pollutants was compared to other metropolitan areas to determine air quality and the pollutant sources. © 2006 Elsevier B.V. All rights reserved. Keywords: NMHC; Benzene; Toluene; Ozone; GC/MS; FID; UV-DOAS

1. Introduction The high ozone concentrations and their long persistence times have now become a new and frightening aspect of life on our planet [1]. Like many urban areas around the world, ground level ozone continues to be a pollution problem in Taipei, the capital of Taiwan, particularly during the long hot summer weather (April–October). Although, Taiwan has experienced a gradual reduction in the number of days of pollution standard index (PSI) due to ozone in recent years, but the number of days due to the increase of ozone concentration in Taipei has increased from 1995–1998. Taipei is located within the northern air quality control basin and the Environmental Protection

∗

Corresponding author. Tel.: +886 2 27377522; fax: +886 2 27377465. Corresponding author. Tel.: +886 3 5715131; fax: +886 3 5718649. E-mail addresses: [email protected] (K. Chiu), [email protected] (J.-G. Lo). ∗∗

0003-2670/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2006.03.009

Agency (EPA) has set up 19 stations in this basin to monitor the air quality. The air quality reports from the 14 ambient airmonitoring stations in and around Taipei city revealed that the primary air pollutants responsible for pollution standard index (PSI) value over 100 in Taipei is ozone. Climatological observations and critical air pollutants monitored in southern Taiwan in the past years established the relationship between atmospheric visibility and major air pollutants and meteorological parameters in an urban area [2]. Taipei basin experiences stagnant weather due to its topography, and this inhibits the dispersion of pollutants thus favoring ozone accumulation near the ground [3]. Because of continued growth in the number of vehicles in the Taipei metropolitan area, automobile emissions account for the main source of air pollution in Taipei city [4]. Automobile emissions are the principal source of benzene to the ambient air. Benzene is not added directly to fuels, but is formed during their manufacture, either through catalytic reforming or steam cracking and is emitted in vehicle exhaust as unburned fuel and as a product of combustion. Toluene is added to gasoline to

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raise the octane rating and may then be emitted in automotive exhaust. Toluene is an important contributor to ozone formation as well as aerosol formation. Ozone yield for a volatile organic compound depends significantly on the conditions within polluted atmosphere in which it reacts as VOC to NOx ratio, VOC composition and sunlight intensity. Though benzene contributes to the formation of ozone, toluene reacts at a faster rate with different reaction mechanisms. In urban air, the oxidation of alkanes, alkenes and aromatics, provide a source of reactive species that convert NO to NO2 without destroying ozone, thus building up ozone levels. Organic compounds (NMHCs plus other VOCs) play a major role in producing photochemical air pollution, namely acceleration of the conversion of NO to NO2 and the resulting formation of ozone. Earlier studies on the air quality of Taipei revealed that the photochemical formation of formaldehyde accounted for its ambient level in the atmosphere [5]. Hence, it is important to identify and measure the concentration of hydrocarbons and to assess their contribution towards ozone formation. However, owing to the hot and humid surroundings and heavy traffic in Taipei metropolitan area, there is a need to developing a system for the measurement of NMHCs. Based on this requirement, an automatic system was developed, the system not only reduces the time and labor in the sampling procedure, but also elevates the accuracy and precision of the results. The present study involves near time sampling at 53 locations in Taipei city in the morning and evening for ambient air hydrocarbons, ozone and NO2 . An attempt was made to explain their abundance, ozone diurnal profiles and photochemical reactivity.

FID and a Saturn-2000 MD was used for analysis (Fig. 1). The system contained dual columns with dual detectors, GC (Varian 3800) with PLOT column (50 m, 0.32 mm, 5 ␮m) coupled with FID was for the separation and identification of C2–C4 compounds, and DB-1 column (60 m, 0.32 mm, 1 ␮m) coupled with MS (Saturn 2000) was for the C4–C9 compounds. Among the factors tested biasing the routine performance are effect of humidity (30–90% range), sample storage time and pre-concentration conditions of injection volume and desorption. The schematic step-wise procedure adopted is shown in Fig. 1. The steps adopted are not very different from the procedures described in standard methods.

2. Experimental

2.3. Near time sampling at 53 locations

2.1. Apparatus

Map of Taipei with the sampling sites is shown in Fig. 2. These sites were selected to satisfy the following criteria:

The self-designed canister cleaning system was maintained in an oven at 100 ◦ C. Vacuum in canister attained pressure < 0.05 Torr. Humidified nitrogen gas was added until the pressure reached 1.5 atm maintained for 5 min to reach equilibrium. This procedure was repeated thrice to ensure the required conditions are attained. A Varian CP-3800 GC equipped with a

(1) Hsientien is located outside Taipei city in the south and is having relatively less traffic emissions. Report from Taiwan air quality monitoring network (TAQMN) reveals that this site contributes towards the maximum pollution standard index (PSI).

2.2. Standard gas Three-liter silico canisters (US Restek Corporation) were used for preparing the standard gas. The maximum pressure a canister can withstand is 40 psig. The standard gas (1 ppmv) was purchased from Matheson, US (serial no. SL-15773). The internal standard gas (10.2 ppmv) obtained from spectra (serial no. 149812), in which four standards were contained, there are bromochloromethane, 1,4-diflorobenzene, chloro-benzened5, and 1-bromo-4-fluorobenzene, and their concentrations are 10.2 ppmv, respectively. Gas standards of polar organics and TO-14A compounds were prepared by dilution of 1 ppm standard gas to 2, 5, 10, 25, 50 and 100 ppbv, respectively. 150 ml of sample gas containing both TO-14A compounds and polar compounds was injected at 50 ml min−1 .

Fig. 1. The schematic step-wise procedure including sample pre-treatment, separation and detection.

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Fig. 2. Sampling locations in Taipei city. Monitoring sites are represented as dots and the continuous measuring spots are represented as squares.

(2) Zhongcheng is located in downtown Taipei. This site is representative of the downtown area and is located approximately 7 km NW to Hsientien. (3) Hsinyu is an urban center similar to Zhongcheng and lies towards NE to Hsientien. (4) Panchiao situated outside Taipei city 9 km NW of Hsientien. Congested roads slow moving traffic characterize Panchiao. It is a residential area with many industries. (5) Neihu is located N to Hsinyu with fast traffic and relatively less emission compared to Hsinyu. Twenty-four hour measurements were done at five selected places, i.e., Panchiao, Zhongcheng, Hsinyu, Neihu, and Hsientien. Air monitoring was conducted on 30th August 1998 and 53 sites in the morning (between 5:45–6:15 a.m.) and in the evening (5:45–6:15 p.m.) for NMHCs spatially distributed across the Taipei city. Sampling sites were selected to provide a broad spatial coverage of the Taipei area not dominated by local sources. Air samples were collected in 3 l summa polished canister every 2 h at the same position where the UV-DOAS was set up for ozone and NO2 , and NMHCs were analyzed using GC/FID or GC/MS. All canisters were cleaned through a humid zero air certification programs (<0.2 ppbv of targeted NMHCs). The technique adopted for canister sampling was sub-atmospheric pressure sampling where the canister was evacuated to below 0.05 mm Hg and the sampling duration was within 30 s. The samples were analyzed by GC–FID and GC–MS. The method followed was according to TO-14A USEPA guidelines [6]. The benefit of this method is that it can quantify 40 VOCs with the detection limit of 1 ppb. An aliquot of 200 ml air sample was introduced into sample pre-concentration trap (glass beads in stainless tubing) at −150 ◦ C, desorbed from the cryo-trap at 80 ◦ C and injected onto the head of the GC column where the

sample is cryo-focussed at −50 ◦ C. The C2–C4 hydrocarbons were separated by a Chrompack PLOT column and analyzed by FID. The temperature was increased to 0 ◦ C at 10 ◦ C min−1 followed by another increase to 120 ◦ C at 5 ◦ C min−1 and then to 180 ◦ C at 20 ◦ C min−1 for 21.5 min. The data were processed by the operating software Saturn GC/MS workstation version 5.21. Identification of the compounds was based on the retention time of the standard gas (Environ-Mat TM ozone precursor, Matheson, USA) and the fragmentation pattern from NIST mass spectral database. For quantification, a calibration curve was generated using the standard gas and the range was between 20 and 102 ppb. Method detection limit was <1 ppb. The relative standard deviation (R.S.D.) was <5% including toluene and benzene. For the 19 compounds, RSD is <10%. For styrene, 2-butene-2-methyl, 1-pentene, 2-methyl, heptane, hexane, 2methyl, pentane, 2,4, di-methyl, RSD were between 10–15%. For a-pinene and b-pinene, the R.S.D. were >30%. 2.4. UV-DOAS measurements of ozone and NO2 Ozone and NO2 were measured by a UV-DOAS system (OPSIS AB, Sweden) at Panchiao (5/3/99–10/3/99), Zhongcheng (1/4/99–19/4/99), Hsinyu (23/2/99–27/2/99), Neihu (12/3/99–29/3/99) and Hsientien (12/6/99–16/6/99). Different pollutants were measured at different wavelength ranges of the spectrum. The light source, high-pressure xenon lamp of 150 W, was placed at a level of 12 m from the ground on the top of a building. The light detector was placed 150 m from the light source. A clean gas reference spectrum was recorded by using short path length at the site. The gas span calibration was performed by OPSIS in Sweden. From the ambient air measurements, a differential absorption spectrum for the atmospheric gases was produced in two steps. The measurement time includes hundred scans in 3 min. Firstly, the ambient air recorded

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Table 1 Most abundant 18 species and their average concentration in ppb measured from near time and 24 h measurements Hydrocarbon

Average of 53 sites at 6 p.m.

Hsinyu

Neihu

Alkanes Ethane Propane Isopentane n-Butane Pentane Pentane, 2-methyl Hexane

6.03 5.65 10.30 4.26 5.38 5.06 2.98

14.10 12.60 10.00 9.23 6.41 5.62 3.53

8.05 16.70 10.50 9.20 5.13 4.82 3.15

6.13 5.34 5.92 4.15 2.98 2.98 1.54

5.26 3.53 3.07 2.54 1.48 1.62 1.20

3.85 3.36 3.20 3.46 1.60 1.36 0.81

Alkenes Ethylene Propene 1-Propene, 2-methyl Isobutene

13.80 3.92 4.10 2.66

13.50 4.60 3.98 5.81

14.60 4.10 3.07 3.35

6.75 1.42 1.94 2.87

4.32 0.91 1.08 1.55

3.15 1.53 1.24 1.58

Alkynes Acetylene

12.80

14.62

13.10

7.31

2.61

3.85

Aromatics Benzene Toluene m,p-Xylene o-Xylene Ethylbenzene 1,2,4 TMB

3.22 23.50 8.87 4.45 2.84 4.86

3.21 26.21 9.16 4.62 3.17 5.45

2.61 20.10 7.51 3.70 2.41 4.42

1.59 11.10 3.14 2.01 1.51 2.01

1.01 10.10 5.31 2.11 2.43 1.61

1.22 5.88 2.13 0.94 0.67 1.04

spectrum was divided by a reference spectrum i.e. a spectrum that is pre-recorded without any gas absorption. The result was a differential absorption spectrum, which originated from the gaseous substances distributing between the lamp and the detector. Secondly, for each gaseous substance, a wavelength range was selected in order to minimize the interferences from other components. The concentration of the different gaseous substances was calculated by fitting pre-stored differential spectra to the measured spectra by a least mean square fit. The three minutes data recorded by UV-DOAS were averaged for 1 h. 3. Results and discussion

Zhongcheng

Panchiao

Hsientien

8 a.m. Ethane, propane and butane occurred in higher percentage. All NMHCs except ethane and propane showed good correlation (R2 > 0.9) among each other in the morning and evening data suggesting most of them have a common source. Poor correlation of propane and ethane indicated the different sources of emissions. High ambient levels of light hydrocarbons have been reported in Taipei city due to the use of liquefied petroleum gas and natural gas [7]. The aromatic composition at Panchiao is significantly higher than at the other four sites. Toluene is the most abundant NMHC across Taipei city. Benzene was not present on the top 15 lists at any of the sites including the near time measurements. At Neihu, higher emission of propane and butane were measured as the sampling site was near to LPG gas sta-

3.1. Results from neartime measurements Table 1 lists the 18 most abundant hydrocarbons measured from the five sites including that from the near time measurements. The average composition of NMHCs measured in the evening is shown in Fig. 3. Alkanes comprise 38.60% (w/w of NMHC), 15.97% alkenes, 12.85% acetylene and 36.6% aromatics. The alkanes are the most abundant species group at all sites except Panchiao. Among the alkanes, the species groups decreased in the following order: C5 > C6 > C4 > C2 > C3 > C7 > C8 > C9. The most prevalent species were toluene, ethylene, acetylene, isopentane, ethane, propane, pentane, m,p-xylenes and 1,2,4-trimethyl benzene. The highly reactive biogenic isoprene was below detection limit at many sites. The monoterpenes were not detected. Percentage of aromatics (31.8), alkenes (12.5) and acetylene (5) were significantly reduced in the morning measurements. The reduction may presumably be due to the dilution of NMHCs as the prevailing wind and the morning rush hour starts around

Fig. 3. Percent of concentration of hydrocarbons at different locations in the evening hours (at 6 p.m.).

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Fig. 5. Diurnal variation of ozone at different locations (×): Hsien Tien; (): Zhong Cheng; (
): Pan Chiao; (): Nei Hu; (䊉): Hsin Yu.

Fig. 4. Percent of reactivity of hydrocarbons at different locations.

tion. Elevated concentrations of toluene, m,p-xylene and ethyl benzene were measured at Panchiao. 3.2. Diurnal variation of NMHCs Diurnal variation of toluene, ethylene, acetylene and isopentane based on their abundance and justified for reactivity towards ozone formation (using Carters MIRs) for Hsinyu, Neihu, Panchiao, Hsientien, and Zhongcheng was assessed. The pattern suggested that the dominant effect in the early morning and late evening was the increase of traffic density, and thus resulted in the increase of exhaust from vehicles. The pattern obtained at Panchiao was not similar to those found in other places due to solvent emissions. 3.3. Reactivity of NMHCs towards ozone formation The principle behind ozone-forming potential (OFP) or reactivity is, in addition to the amount of a specific VOC species emitted into a given airshed, the difference in the chemistry of each of the VOCs needs to be considered when assessing the impact of those species on ozone formation. MIR (maximum incremental reactivity) is a significant model to estimate the OFP from NMHC. The reason that MIR was chosen in this study is due to the sensitivity of MIR to different airsheds being medium. In addition, this model is appropriate while applied in the metropolitan area. The relative contribution of the NMHCs to ozone formation is estimated using the maximum incremental reactivity (MIR) scale developed by Carter, 1994 [8]. The MIR scale provides an estimate of moles ozone formed per mole carbon of each hydrocarbon measured. The reactivity of each species towards ozone formation is estimated by multiplication of its concentration by its MIR factor (Fig. 4) [9,10]. The OFP can be expressed by the following equation: ozone yield =

53
i=1

R i Yi

where R = maximum ozone forming coefficient (reference?); Y = concentration of the ozone precursor (g); i = different VOCs species. Aromatics (40.5%) and alkenes (43%) have contributed almost equally to the ozone formation. The low reactivity of alkanes (15%) has resulted in rather low ozone formation potential. Contributions from aromatics were higher at Panchiao (57%). The following seven compounds are important for production of ozone in Taipei: ethylene—22%, m,p-xylenes—12%, toluene—10.5%, 1,2,4 trimethyl benzene—8.5%, 2-methyl-1propene—8%, propene—7.8%, and o-xylene—6%. Ethylene ranks first towards the formation of ozone. Among the other alkenes, the reactivity follows the order (in decreasing order of importance): 2-methyl 2-butene, trans-2-pentene, 2-butene, cis-2-pentene and 1-pentene. The most abundant alkane, isopentane is ranked 8 and the other alkanes important towards ozone formation in decreasing order are 2-methyl pentane, butane, isobutene and n-pentane. Among the other aromatics, the reactivity towards ozone formation decreased in the following order: 1,3,5 trimethyl benzene, ethyl benzene, propyl benzene and benzene (Table 2). 3.4. Diurnal variation of ozone Diurnal variation of ozone is shown in Fig. 5. At Hsientien, the concentration continues to rise about 12:00 p.m. followed by a sharp decrease for 1 h due to fresh NO and levels off till 4:00 p.m. After 4:00 p.m., the concentration at Hsientien declines sharply. For the remaining nighttime (after 10:00 p.m.), the concentration at Hsientien is less than 10 ppb. The diurnal variation remains the same for all the four other places except for Hsientien. At Zhongcheng, Panchiao, Hsinyu, and Neihu around 10:00 a.m., the concentration rises rapidly, reaches the maximum around 12:00 p.m. and about 2:00 p.m., the concentration declines sharply. The rush hour traffic provides fresh NO to quickly titrate the ozone concentration after the peak is reached around 2:00 p.m. As is typical of urban area, the daily maximum ozone concentrations at Taipei are related to the daily maximum temperature with the highest ozone formed on the warmest days. The temperatures were 25 and 20.5 ◦ C, respectively, at Zhongcheng and a corresponding decrease in ozone concentrations were observed on these days. Among all the five sites, as expected, Hsientien has the least emission. Nevertheless, ozone levels were the highest at Hsi-

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Table 2 Concentration of NMHC related to the estimation of the maximum ozone forming amount in Taipei metropolitan NMHC concentration (ppbv), 1998/08/15 (5:00) Alkane 1. Ethane 2. Propane 3. n-Butane 4. Isobutane 5. n-Pentane 6. Hexane 7. Heptane 8. Octane 9. Nonane 10. Butane, 2,2-dimethyl 11. Butane, 2,3-dimethyl 12. Butane, 2-methyl 13. Pentane, 2-methyl 14. Pentane, 2,3-dimethyl 15. Pentane, 2,4-dimethyl 16. Pentane, 3-methyl 17. Hexane, 2-methyl 18. Hexane, 2,2-dimethyl 19. Hexane, 3-methyl 20. Cyclohexane 21. Cyclopentane, methyl 22. Cyclohexane, methyl 23. Heptane, 2-methyl 24. Heptane, 3-methyl Alkene 25. Ethylene 26. Propene 27. 1-Butene 28. 2-Butene 29. cis-2-Butene 30. 1-Pentene 31. cis-2-Pentene 32. trans-2-Pentene 33. 2-Hexene 34. 3-Hexene (cis) 35. 1-Pentene, 2-methyl 36. 2-Butene-2-methyl 37. 1,3-Butadiene, 2-methyl 38. Cyclopentene 39. 1-Propene, 2-methyl Aromatic compounds 40. Benzene 41. Benzene, 1,2-dimethyl 42. Benzene, 1-methyl-1-ethyl 43. Benzene, propyl 44. Benzene, 1,2,4-trimethyl 45. Benzene, 1,3,5-trimethyl 46. Benzene, 1,3-dimethyl (p-xylene) 47. Ethylbenzene 48. Toluene 49. Styrene (1,3,5,7-cyclooctatetraene) Alkyne 50. Ethyne 51. Propyne 52. 1-Butyne 53. 2-Butyne

Estimated maximum ozone forming amount (ppbv), 1998/08/15 (5:00)

NMHC concentration (ppbv), 1998/08/15 (18:00)

Estimated maximum ozone forming amount (ppbv), 1998/08/15 (18:00)

11.74 7.25 6.56 2.69 3.88 1.34 0.73 0.47 0.19 0.19 0.00 4.49 2.01 0.21 0.08 0.79 1.63 0.00 0.55 0.48 0.68 0.07 0.18 0.14

1.14 1.16 2.16 1.05 1.28 0.42 0.19 0.09 0.03 0.05 0.00 2.47 0.97 0.09 0.04 0.38 0.56 0.00 0.24 0.20 0.62 0.04 0.05 0.04

9.24 6.52 7.22 3.34 4.82 3.43 2.41 1.83 0.43 0.77 0.08 10.10 4.10 0.69 0.33 2.16 4.29 0.02 1.81 0.78 1.92 0.81 0.60 0.73

0.90 1.04 2.38 1.30 1.59 1.06 0.63 0.35 0.07 0.20 0.03 5.56 1.97 0.28 0.16 1.04 1.46 0.01 0.80 0.32 1.73 0.48 0.18 0.22

7.41 2.09 0.21 0.12 0.00 0.10 0.09 0.25 0.04 0.02 0.00 0.35 0.01 0.05 0.75

17.78 6.28 0.59 0.35 0.00 0.19 0.25 0.69 0.09 0.04 0.00 0.53 0.05 0.11 2.17

17.57 5.50 0.40 0.25 0.00 0.40 0.15 0.42 0.01 0.04 0.01 0.62 0.01 0.11 1.75

42.16 16.49 1.17 0.71 0.00 0.81 0.42 1.19 0.03 0.09 0.02 0.93 0.04 0.25 5.08

2.20 2.03 0.15

0.30 4.26 0.10

4.76 5.42 0.43

0.64 11.37 0.29

1.13 0.72

0.77 2.02

2.77 1.82

1.88 5.09

8.87

28.40

19.54

62.53

2.27

4.77

5.01

10.52

1.64 11.55 0.58

1.41 10.17 0.41

3.38 29.27 1.74

2.91 25.76 1.23

4.68 0.00 0.00 0.00

0.75 0.00 0.00 0.00 95.72

15.05 0.00 0.00 0.00

2.41 0.00 0.00 0.00 217.81

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Fig. 6. Diurnal profile of (a) benzene and (b) toluene at different sampling sites.

entien. This is a downwind site, guarded by mountain ranges towards the south of Taipei and with significant emissions centers towards the northwest. The high level of ozone is likely to be due to the geographical position and on the prevailing meteorology at Hsientien. The occurrence of multiple peak ozone concentrations within a 24 h period indicating the presence of ozone transported to the site from elsewhere. The prevailing wind was from the southeast in the nighttime and was from the northwest in the daytime. The southeast side of Hsientien is surrounded by mountain ranges and nighttime wind brings clean air into Hsientien. Daytime northwest wind blowing into Hsientien is already polluted with emissions from the upwind regions. Ozone concentrations were below 10 ppb on 13th June due to precipitation on that day which can reduce temperature and solar radiation and resulting in decreased ozone concentrations. The ratio of NMHC to NO2 that characterizes the efficiency of ozone formation was below 5 for all the five sites. 3.5. Benzene and toluene The concentration of benzene and toluene measured at all sampling sites (as that in Fig. 2) in the morning and evening is shown in Fig. 6a and b. The ratio of abundance measured at morning 6 a.m. was found to be significantly low and might be due to the dilution of NMHCs because of the high speed of the prevailing wind in Taipei and moreover, the morning rush hour starts only around 8 a.m. Scatter plot of benzene with toluene showed very good correlation (R2 = 0.94) indicating that both are primarily emitted by mobile source. The slope is much low compared to the typical urban benzene to toluene ratio of 0.4 in US cities. The low ratio in Taipei simply indicates the characteristics of the fuel composition. The typical daily patterns of hourly benzene–toluene (B/T) concentration data exhibited well-defined diurnal cycles. The diurnal pattern of B/T is similar to the other VOCs (isopentane, xylenes, etc.) with emissions generally dominated by automobile related sources. The peak value around 8 a.m. and 10 p.m. corresponds to the heavy traffic hours in Taipei. The morning maximum is associated with high emissions; the midday minimum is associated with decreased mobile source emissions, increased mixing due to high wind speed and increased reaction

rates due to higher temperatures and maximum solar radiation. The late evening maximum is due to increased mobile source emissions. The lowest concentration occurs between 9 a.m. and 6 p.m. and the levels rise again to a second peak by late evening. The B/T ratios at Zhongcheng and Neihu are also consistent with a predominant, continuous influence of fresh, on-site, automobile emissions. The diurnal profile observed at Panchiao gave a good correlation while daytime concentration of toluene deviated significantly. Excess concentration of toluene must have resulted from industrial sources located at Panchiao. In Taipei, ambient air measured 14–16 ppbv% of toluene of the total hydrocarbon. Benzene averaged 2% of total hydrocarbons. The low percentage at Neihu is due to higher concentrations of propane and butane thus increasing the average NMHC concentration. Higher concentration of propane and butane may have resulted from a LPG gas station operating near to the sampling site. Though there can be other sources of benzene and toluene in ambient air besides motor vehicle emissions, the consistency in the percentage of the individual hydrocarbon points to a single major source except for Panchiao. The emission characteristics of benzene and toluene are different and toluene emits a larger percentage of unburned fuel. Most of the benzene results from exhaust emission and a smaller percentage from evaporative emissions. Benzene emissions are related to the benzene and total aromatic content of the fuel as well as engine and catalyst characteristics of the vehicle. Low concentrations of benzene emission may have resulted from reduced amount of benzene in the fuel. 3.6. Emissions in tunnels The major aromatics hydrocarbons measured in tunnels of different cities in Taiwan are summarized in Table 3. In Taiwan, the major parts of vehicles are the great number of motorcycles (ranking no. 1) and followed by cars (bus and sedan). In recent years, increases in traffic volume, old vehicles, different fuel composition, and two-stroke motorcycles are accounted for increased emissions [11]. In Taiwan, benzene content in gasoline averages 2.8% but varies slightly based on the location. The vehicles in Taiwan with catalytic converters were minimal among cars and motorcycles because only cars that manufactured after

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Table 3 Hydrocarbon emissions in tunnels Hydrocarbon

Benzene Toluene Ethylbenzene m,p-Xylene o-Xylene a b c d

Taichung (1998)

Kaohsiung (1997)

Taipei (1998)

Motorcyclea

Cara

Taipei (1992) Motorcycleb

Carb

Totalc

Morningd

Eveningd

145 442 74 109 89

135 371 213 90 76

160 367 77 149 95

248 599 112 230 161

6–102 23–176 10–15 18–206 25–35

0.5–18 1–100 0.5–10 0.6–34 0.3–15

1–32 2.4–106 0.7–14 8.8–51 0.5–44

Kuo et al. [13]. Chan et al. [12]. Hsu et al. [15]. Hwa et al. [14].

Table 4 Aromatics measured in other cities Hydrocarbons

Kaohsiung (7–9 p.m., 2000)

Taipei (6 p.m., 1997)

Benzene Toluene m,p-Xylene Ethylbenzene

1–7 4–22 1–17 4–35

0.6–6 3–46 1–5 0.5–3

1st July 1990 were required to meet emission standards that were equivalent to those in the United States in 1983 [12]. Taichung’s exceedingly high concentration of benzene might be due to the relatively high concentrations of aromatic components (including benzene) in unleaded gasoline that became widely used in Taiwan in 1991 and the low proportion of emission controlled vehicles in Taiwan [13]. In the previous studies, the top three abundant VOCs measured in different tunnels of Taipei are toluene, ethene and 1,2,4-TMB [14]. The estimated averaged emission factors for them are 29, 26 and 14 mg km−1 veh−1 , respectively. The characteristic of VOCs emissions may directly reflect the specific formula of gasoline or diesel fuel provided by local refinery industry. Average traffic emission of total VOCs from Taipei tunnel experiment produces 1.02 g ozone per vehicle per kilometer traveled. Among these, ethene, 1,2,4-TMB and propene contribute over 100 mg ozone per vehicle per kilometer traveled. In Kaohsiung city, toluene, benzene and m,p-xylene were higher in the tunnels [15]. To be concise, hydrocarbon emissions on roadside are diluted due to meteorological conditions. However, while in the tunnels, being enclosed structures, building up of hydrocarbons resulted in very high values. Comparing the ambient air aromatics composition obtained during rush hours, Kaohsiung being a highly industrialized city than Taipei, emissions were higher (Table 4). Also the topography and meteorological conditions vary widely [16]. 4. Conclusions To investigate the impact of ozone on environment, the factors such as mechanism of ozone forming, representative sampling sites and reliable separation/detection method are essential and critical. The analytical approach developed in this study, canister sampling by pre-concentration trap at low tempera-

ture, separation and detection for different VOCs by PLOT/FID and DB-1/MSD, has successfully measured the concentration of the precursors of ozone in a special and complex geological/meteorological metropolitan, such as the basin topography, humid weather, wind speed and direction, intensity of sunlight. In addition, the generation of ozone is mainly due to the existence and reactivity of VOCs and NO2 under the effect of meteorological and geological condition. From the concentration of both VOCs and ozone at 53 sampling sites, we conclude that VOCs and ozone is closed related according to MIR model. The model is appropriate while applied in the complex metropolitan area. The 53 sampling sites selected in this study constitute a net of measurement of NMHCs in Taipei City. The net encompasses densely populated area, industrial section, high traffic road, and closed (tunnel) environment. Based on this study, the sources of VOCs, such as from LPG or vehicular and industrial emissions, can be checked and thus to enforce the environmental protection. Acknowledgements The Taiwan Environmental protection agency (EPA-88FA32-03-2214) and the Taiwan, National Science Council (NSC-88-2113-M-007-007) financially supported this work. The authors would like to thank all the graduate students for their assistance with the fieldwork. References [1] T. Cvitaˇs, L. Klasinc, N. Kezele, S.P. McGlynn, W.A. Pryor, Atmos. Environ. 39 (2005) 4607. [2] I.Y. Tsai, Atmos. Environ. 39 (2005) 5555. [3] J.L. Wang, W.L. Chen, G.R. Her, C.C. Chan, Atmos. Environ. 36 (2002) 3041. [4] C.C. Hsieh, K.H. Chang, Y.H. Kao, Chemosphere 39 (2001) 1433. [5] L. Mathew, W.R. Tai, J.G. Lo, J. Air Waste Manage. Assoc. 51 (2001) 174. [6] US EPA. Compendium method TO-14A, the determination of volatile organic compounds (VOCs) in ambient air using specially prepared canisters with subsequent analysis by gas chromatography, 1999 (http://www.epa.gov/ttnamtil/files/ambient/airtox/to-14a.pdf). [7] J.L. Wang, W.H. Ding, T.Y. Chen, Chemosphere 2 (2000) 11. [8] W.P.L. Carter, J. Air Waste Manage. Assoc. 44 (1994) 881. [9] S.K. Hoekman, Environ. Sci. Technol. 26 (1992) 1206.

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