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The relationship between volatile organic profiles and emission sources in ozone episode region—a case study in Southern Taiwan
Science of the Total Environment 328 (2004) 131–142
The relationship between volatile organic profiles and emission sources in ozone episode region—a case study in Southern Taiwan Jiun-Horng Tsaia,*, Yi-Chyun Hsub, Jang-Yu Yanga b
a Department of Environmental Engineering, National Cheng-Kung University, Tainan, Taiwan, ROC Department of Environmental Engineering, Kun-Shan University of Technology, Tainan, Taiwan, ROC
Received 31 May 2003; accepted 13 January 2004
Abstract This study investigates the relationship between volatile organic profiles in the atmosphere and emission sources in an ozone non-attainment in region Southern Taiwan. Dynamometer test of vehicles and stack sampling from industrial facilities were conducted to obtain the fingerprints of emissions from on-road mobile sources and stationary sources, respectively. In addition, field sampling of non-methane organic compounds (NMOC) concentration at monitoring stations during episode seasons were also collected by canisters. The influences of different emissions sources on airborne concentrations were estimated by back-trojectory analysis and chemical mass balance model (CMB 8.0) calculation. Field measurement data indicated that the daily average concentration of NMOC ranges between 26.4 and 69.8 ppb at different sites. The mass fraction for paraffins, oleffins and aromatics in airborne samples at these sites were 28–47%, 7–12% and 41–52%, respectively. Toluene was the dominant species among these species, followed by isopentane, n-butane and 1,2,4-trimethylbenzene. The source apportionment of airborne NMOC in the ozone non-attainment region, based on CMB simulation, is passenger cars (28–51%), motorcycles (9– 24%), industrial sources (14–33%), solvent application (13–46%) and biogenic emissions (-1–2.4%), respectively. Both field measurement and model analysis showed that the vehicle exhaust and industrial emission are the dominant contributors of NMOC in the region. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Motorcycle; Dynamometer; Volatile organic compound; Chemical mass balance
1. Introduction Surface ozone has been the subject of numerous studies for the past two decades because of its adverse effects on human health, crops and vegetation (Vukovich, 1997). Despite constant efforts to control the emissions of ozone precursors by *Corresponding author. Tel.: q886-6-275-1084; fax: q8866-208-3152. E-mail address: [email protected] (J.-H. Tsai).
the authorities, ozone levels continue to exceed the air quality standard in most areas of the Southern Taiwan. In fact, Southern Taiwan is facing major air quality challenge because of frequent incidents of high concentration of ozone and particulate matter since the 1990s. Therefore, the Kaoping air basin, located in Southern Taiwan, is classified as a non-attainment region for ozone. In 2000, 10.4% of the air quality data (monitoring site number times monitoring days) were classified
0048-9697/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2004.01.020
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unhealthy as its pollutant standard index (PSI) exceeded 100. The maximum hourly concentrations of ozone ranged between 126 and 203 ppb, which exceeded the maximum hourly standard of 120 ppb, at seven air monitoring stations in Kaoping Air Basin. Approximately two-fifths of the incidents were attributed to ozone episodes especially from October to December (TEPA, 2000). Prior to developing an effective control program for ozone, there is considerable interest for the Taiwan Environmental Protection Agency (TEPA) and local governments to fully understand the inter-relationships among the ozone concentrations, the ozone precursors, and secondary pollutants. Photochemical ozone formation is caused by the gas phase reactions of VOCs with oxides of nitrogen (NOx) in the presence of sunlight. In 1998, the estimated emission inventory of NOx and non-methane hydrocarbon (NHMC) was 423 and 841 thousand metric tons per year and they accounted for 49% of NOx and 33% of NHMC from mobile emissions, respectively (TEPA, 1999). A significant number of motor vehicles (2 950 000) and complex industrial sites (350 000 factories) around the Kaoping air basin contribute to the serious air pollution problem in this region. There are 16 million motor vehicles in Taiwan, with 66% of motorcycles, 32% of gasoline passenger cars, and approximately 1% of diesel trucks and buses (MOT, 1999). Such huge number of vehicles definitely contributes a major portion of air pollutants in the emission inventory. VOCs have different effects on ozone formation because of reaction kinetics and mechanisms (Carter, 1994; Bowman and Seinfeld, 1994). Both anthropogenic and biogenic hydrocarbons are also important in the formation of ozone. Controls on some sources of the VOC emissions may be necessary to reduce photochemical smog formation. However, high concentrations of NMOC were also observed in air mass with episodes of long-range transport and with the highest photochemical age (Derwent, 1996). The chemical mass balance (CMB) model has been widely used to estimate source category contribution to their ambient levels in the atmospheres of various urban areas. The source profiles of the individual NMOC emission are essential
input data for the CMB model. Profiles for NMOC sources have been developed for urban areas in the United States, Canada, Australia, Egypt and Japan (Doskey et al., 1999; Fujita et al., 1995). However, those profiles may not be applicable to all urban areas because the mix of mobile sources and fuel specifications are site-specific. To date, the airborne VOCs have received wide attention in air monitoring programs in Taiwan. This investigation was carried out in the Southern Taiwan in winter during the days with ozone episodes and without ozone episodes. Fifty-one species of VOCs, from anthropogenic and biogenic sources, were quantified by GCyMS and examined at four air monitoring stations in the air basin. Back trajectory simulation of air mass was employed to identify the potential sources and preferred pathways for ozone events. The CMB model was used to calculating the distribution of emission sources. The results of all ambient samples were analyzed by CMB 8.0 model with assistance from fingerprint database of the CMBtwvoc. Relationships of VOCs with ozone concentrations were also examined in an attempt to understand the relative significance of VOCs during the ozone episode. 2. Methods and materials 2.1. Sampling sites Based on air quality data and the trajectory of the air stream, field measurements were conducted at four air quality monitoring stations from Taiwan Air Quality Monitoring Network (TAQMN). The TAQMN consists of 72 air quality monitoring stations, including of 16 stations at the Kaoping air basin. Sampling station A (Samin station) is located in downtown area of Kaohsiung, which is classified as general station and is strongly influenced by emissions from vehicles and various sources. Sampling station B (Shyaukong station) and C (Daliau station) are located within the vicinity of industrial area, which are also strongly influenced both by emissions from vehicles and industrial sources. Sampling station D (Chyouzou station) is located in the downwind rural region with low
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Fig. 1. Locations of sampling site in Southern Taiwan air basin.
emissions. The locations of these air monitoring stations in the air basin are shown in Fig. 1. 2.2. Field measurement Field measurement was conducted from 1997 to 1998. There were a total 14 days of field sampling work, including nine days with ozone episode (8 h ozone conc. )60 ppbv). A total of 138 samples of ambient air were collected at various locations in this region. Ambient air were collected by the active pressurized sampler (Xontech Canister Sampler 910A), with sampling flowrate of 80 mlymin, into prevacuated Summa stainless steel canisters (Scientific Instrumentation Specialists, Moscow, ID). Field samples were taken twice in daytime (0600 to 0900 and 1200 to 1500) and once in nighttime (1800 to 2100). All canisters were cleaned up in a series of pressurizationyevacuation cycles at 105 8C, using humidified ultra-zero air evacuated to less than 0.2 mmHg, and were shipped to the field for sampling. 2.3. Sample analysis Sample air was released from the canister and concentrated by a cold trap (Varian, Inc.) then
purged out and analyzed by a GCyFIDyMS (Varian 3600 GCyFID, Varian Saturn 2000 MS). The temperature of the trap system was cooled down to y160 8C by liquid nitrogen at first. Then the desorber was heated to 160 8C for purging. The gas chromatography was equipped with dual-column capillary. An Al2O3-plot column (6 m, 0.53 mm, Rt-Alumin, TM) to the FID and a fused silica capillary column (60 m, 0.32 mm ID with 1 mm DB-1, J&W) to the MS. A cylinder standard gas (56 Environ-Mat Ozone Precursor, 16183, Matheson Inc., USA) was applied for quality control program. Working standard was prepared by blending the span gas and ultra-high-purity nitrogen. The performance of GCyMS was also evaluated with perfluorotributylamine for quality control. 2.4. Back trajectory simulation Back trajectory simulation of air mass was conducted by taking the meteorological data of monitoring stations during sampling periods into the process. For the maximum hourly ozone concentration estimated at site D, the air mass during the time period of 1200–1400 was usually selected
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Table 1 Profiles of various NMOC sources in CMB-twvoc database, % by volume Compound
Propylene Propane Isobutane n-Butane Trans-2-butene Cis-2-butene Isopentane 1-Pentene n-Pentane Isoprene Trans 2-pentene Cis 2-pentene 2-Methyl-2-butene 2,2-Dimethylbutane Cyclopentene 4-Methyl-1-pentene Cyclopentane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane 2-Mrthyl-1-pentene n-Hexane Trans-2-hexene Cis-2-hexene Methylcyclopentane 2,4-Dimethylpentane Benzene Cyclohexane 2-Methlyhexane 2,3-Dimethylpentane 3-Methylhexane 2,2,4-Trimethylpentane n-Heptane Methylcyclohexane 2,3,4-Trimethylpentane Toluene 2-Methylheptane 3-Methylheptane n-Octane Ethylbenzene m,p-Xylene Styrene o-Xylene n-Nonane Isopropylbenzene n-Propylbenzene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene
Vehicle exhaust Passenger
Motorcycles 2-stroke 4-stroke
Diesel
5.4 – 0.5 1.1 0.5 0.5 6.0 0.4 4.3 0.1 0.6 0.4 0.9 1.2 0.3 0.1 1.4 2.1 7.2 3.5 0.5 2.2 0.1 0.1 2.1 0.2 9.1 0.5 2.0 0.8 2.5 0.4 1.8 0.6 0.3 17.2 1.4 0.9 0.6 2.4 4.4 0.1 2.8 0.2 0.2 0.3 0.6 2.5
0.5 0.0 0.2 0.7 0.3 0.7 15.6 0.3 6.7
3.3 – 0.1 0.5 0.2 0.3 11.6 0.3 5.8
0.6 0.4 1.3 2.0 0.4
0.6 0.4 1.0 0.8 0.4
1.2 2.0 7.3 3.8 0.2 1.8 0.1 – 1.7 3.9 4.4 0.2 3.5 0.9 2.8 1.0 2.3 0.3 0.5 11.7 1.5 1.6 0.7 1.5 6.5 0.2 2.7 0.4 0.3 0.5 1.1 0.5
0.8 1.0 3.1 2.2 0.2 0.9 0.1 0.1 1.2 0.4 3.6 0.1 2.5 0.7 1.8 1.0 1.8 0.4 0.4 16.4 1.3 1.3 0.9 3.6 11.8 0.4 5.6 0.6 0.5 1.1 4.4 2.6
– – – – – – – – – – – – – – – – – – – – – – – – – – 16.3 – – – – – – – – 8.3 – – – 3.5 4.0 – 2.3 6.0 – – 11.6 48.0
Industry
Petroleum refinery
Solvent use
Liquid gasoline
Gasoline vapor
CNG
5.8 2.3 8.7 14.5 0.5 – 9.1 – 5.7 1.1 – – – – – – 3.3 6.0 0.9 0.5 – 6.8 – – 0.7 0.2 4.3 3.4 – – 0.2 0.1 2.5 – – 7.6 – – 1.4 2.4 5.4 3.1 3.3 0.1 – – 0.6 –
10.9 2.7 4.9 1.0 0.9 3.7 – 5.0 0.6 0.3 3.2 2.9 0.7 0.3 – – 3.3 6.0 1.9 – 2.6 – – 0.8 – 13.4 0.6 1.1 0.5 1.8 – 1.8 0.7 – 7.7 0.0 0.3 0.4 2.8 5.5 0.8 2.7 0.4 0.2 0.4 0.4 1.5 –
– – – – – – – – – – – – – – – – – – – – – – – – – – 5.7 – – – – – – – – 49.5 – – – 8.9 23.4 – 12.5 – – – – –
– 0.2 0.4 1.7 0.8 0.9 8.7 0.4 6.1 0.9 1.6 1.1 2.0 1.2 0.9 0.0 3.7 11.0 10.7 5.7 2.1 2.3 0.1 0.2 2.3 0.3 1.3 1.3 1.8 0.6 2.2 0.3 2.0 0.7 0.1 9.8 0.3 0.8 0.8 2.6 2.8 0.4 2.6 0.3 0.4 0.4 1.4 2.4
– 1.1 1.9 10.7 3.1 2.5 24.7 2.2 15.3 2.2 2.3 2.4 0.9 0.3 0.3 – 2.4 4.3 4.3 2.1 0.8 0.8 – – 2.4 – – 0.2 0.8 – 0.7 0.1 0.5 0.5 – 4.9 – – – 1.2 1.1 – 1.3 0.2 0.1 0.2 0.2 0.3
– 52.2 36.5 11.3 – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
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Table 2 Concentration (ppbv) at various NMOC species at different sites in the ozone non-attainment region in Southern Taiwan Sampling sites compounds
A
B
C
D
Propylene Isobutane 1-Butene n-Butane Trans-2-butene Cis-2-butene 3-Methyl-1-butene Isopentane 1-Pentene n-Pentane Isoprene Trans 2-pentene Cis 2-pentene 2-Methyl-2-butene 2,2-Dimethylbutane Cyclopentene 4-Methyl-1-pentene Cyclopentane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane 2-Methyl-1-pentene n-Hexane Trans-2-hexene Cis-2-hexene Methylcyclopentane 2,4-Dimethylpentane Benzene Cyclohexane 2-Methlyhexane 2,3-Dimethylpentane 3-Methylhexane 2,2,4-Trimethylpentane n-Heptane Methylcyclohexane 2,3,4-Trimethylpentane Toluene 2-Methylheptane 3-Methylheptane n-Octane Ethylbenzene m,p-Xylene Styrene o-Xylene n-Nonane Isopropylbenzene n-Propylbenzene 1,3,5-Trimethyl-benzene 1,2,4-Trimethyl-benzene Paraffins Olefins Aromatics Biogenics t-NMOC
3.9"4.1 1.9"1.7 3.6"1.9 4.4"4.2 0.5"0.4 0.6"0.7 0.1"0.2 4.9"4.0 0.2"0.2 4.7"4.1 0.3"0.2 0.6"0.4 0.3"0.3 1.2"0.7 0.7"0.6 0.4"0.4 0.1"0.2 0.9"0.7 0.8"0.7 3.0"2.5 2.4"2.2 0.1"0.1 1.7"1.8 0.03"0.05 0.1"0.1 1.8"1.6 0.1"0.2 2.4"2.0 0.6"0.7 0.7"0.6 0.4"0.4 1.0"0.8 0.1"0.1 0.8"0.6 0.3"0.4 0.1"0.1 13.0"7.1 0.4"0.3 0.3"0.2 0.3"0.2 1.6"1.1 2.0"1.9 0.4"0.7 1.3"1.2 0.3"0.2 0.1"0.2 0.3"0.4 0.7"0.6 3.2"2.5 32.6"11.5 11.6"6.5 25.3"11.1 0.3"0.2 69.8
4.7"3.5 2.2"1.4 3.4"1.6 5.1"2.9 0.5"0.4 0.7"0.6 0.1"0.1 5.3"3.4 0.3"0.2 4.6"2.8 0.5"0.3 0.5"0.4 0.5"0.3 0.8"0.4 0.7"0.5 0.3"0.2 0.1"0.1 0.8"0.5 0.7"0.5 2.6"1.8 2.0"1.3 0.2"0.1 1.3"0.8 0.04"0.03 0.1"0.1 1.5"0.9 0.1"0.1 2.7"1.4 1.0"1.2 0.5"0.4 0.3"0.3 0.8"0.4 0.1"0.1 0.7"0.4 0.3"0.2 0.1"0.03 11.9"3.8 0.3"0.3 0.3"0.3 0.4"0.3 1.7"0.9 1.7"1.1 0.7"0.5 1.1"0.7 0.3"0.2 0.1"0.1 0.2"0.1 0.7"0.3 2.6"1.7 32.0"9.8 12.1"5.9 23.4"6.8 0.5"0.3 68.0
2.7"2.6 1.5"1.8 3.2"2.4 3.8"4.1 0.2"0.4 0.3"0.5 0.04"0.06 3.7"4.2 0.1"0.2 3.1"3.7 0.4"0.3 0.2"0.3 0.1"0.2 0.5"0.8 0.4"0.5 0.1"0.2 0.03"0.06 0.5"0.5 0.4"0.6 1.5"1.7 1.2"1.5 0.2"0.2 1.0"0.8 0.1"0.2 0.1"0.2 1.0"1.4 0.1"0.2 2.1"1.9 0.6"1.1 0.4"0.7 0.2"0.4 0.5"0.5 0.1"0.2 0.5"0.8 0.2"0.3 0.1"0.1 18.9"16.5 0.2"0.2 0.2"0.3 0.3"0.3 1.6"1.5 1.6"1.2 0.4"0.5 1.2"1.6 0.2"0.3 0.1"0.2 0.2"0.2 0.5"0.6 1.4"1.5 21.6"16.1 7.8"7.2 28.1"17.3 0.4"0.3 57.9
1.5"0.6 0.8"0.5 2.5"1.2 1.8"1.3 0.11"0.04 0.1"0.1 0.04"0.02 1.7"1.2 0.1"0.1 1.4"1.1 0.4"0.2 0.1"0.1 0.1"0.1 0.5"0.3 0.3"0.1 0.1"0.1 0.04"0.02 0.3"0.1 0.2"0.1 0.9"0.5 0.7"0.3 0.03"0.02 0.7"0.4 0.03"0.04 0.1"0.1 0.5"0.3 0.04"0.02 1.5"1.0 0.2"0.1 0.2"0.2 0.1"0.1 0.3"0.2 0.03"0.03 0.3"0.2 0.1"0.1 0.03"0.04 5.7"4.1 0.1"0.1 0.08"0.03 0.1"0.1 0.6"0.4 0.5"0.4 0.1"0.1 0.6"0.6 0.1"0.1 0.1"0.2 0.1"0.1 0.2"0.1 0.6"0.5 11.0"3.8 5.1"2.3 9.9"3.5 0.4"0.2 26.4
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Fig. 2. Source apportionment of airborne NMOC in Southern Taiwan and other cities.
for daily back trajectory simulation. Air mass back trajectories were obtained from the Taiwan Atmospheric Data Center for specific ozone pollution episode. Based on the back trajectory model, resident time of the air mass and conditional residence probability were employed to identify the potential sources and preferred pathways for each ozone episodes. The back trajectory simulation was based on the grid system with 1 km. A sensitivity test to check the simulation result.
2.5. CMB simulation Chemical mass balance (CMB) model was used to evaluate the contribution of airborne VOC from various emissions sources. The results of all ambient samples were analyzed by the CMB 8.0 model and coordinated the fingerprint database with local source profiles. The database of CMB-twvoc is the source profiles for NMOC in Southern Taiwan (Tsai et al., 2000, 2003a,b; Hsu et al., 2000, 2001)
Fig. 3. Variation of NMOC concentration during various sampling periods.
J.-H. Tsai et al. / Science of the Total Environment 328 (2004) 131–142
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Fig. 4. Profile of the airborne NMOC various sampling sites.
NMOC profiles of various vehicles (passenger car, 2-stroke motorcycle, 4-stroke motorcycle, diesel truck), oil refinery, liquefied gas (LPG) and natural gas, synthetic organic chemical manufacturing processes, and industrial coating factories (solvent use). Emission profiles of various NMOC sources in CMB-twvoc database are shown in Table 1. 3. Results and discussion 3.1. Concentrations of NMOC and the speciations Table 2 presents the mean values of airborne organics species at various sampling sites in the air basin. Higher value of airborne NMOC concentrations were observed at site A (69.8 ppb), B (68.0 ppb) and C (57.9 ppb), which were located in downtown area or nearby the industrial complex. The airborne NMOC concentration at sampling D (26.4 ppb) was clearly lower than those of other three sites, but the ozone concentration at this site is always higher than the others. The paraffin and aromatics, which were the major groups of NMOC, accounted for 37–47% and 34– 49% of total NMOC concentrations, respectively. The fraction of olefin compounds was in the range of 14–19%. Field measurement data shows that the biogenic source-related components were less than 2% of total NMOCs. All airborne NMOCs species are categorized into four groups as paraffin,
olefins, aromatic and biogenic organics, as shown in Table 2. Fig. 2 compares the NMOC groups in this study with the others. Paraffins, olefins, and aromatics groups measured in both Boston and Houston, account for 60–67%, 6.6 –16% and 20–28%, respectively. The paraffins, a major group of the NMOC, measured in the Southern Taiwan air basin was apparently lower than the values in other cities. However, the olefins and aromatics in Southern Taiwan were higher than the other cities with a value of 3–15%. Average concentration of the major groups of NMOC during different sampling periods is shown in Fig. 3. At sampling sites A, B and C, higher concentrations of these groups were observed in the morning (06.00 to 09.00), especially at the Table 3 The characteristic ratios of major NMOC components at various sites Ratios_Site
A
B
C
D
Isopentaneybenzene n-Pentaneybenzene 2-Methylpentaneybenzene 3-Methylpentaneybenzene Tolueneybenzene m,p-Xyleneybenzene o-Xyleneybenzene 1,2,4-Trimethyl-benzeneybenzene
2.04 1.91 1.21 0.98 5.37 0.82 0.55 1.31
1.95 1.71 0.98 0.75 4.40 0.62 0.41 0.98
1.72 1.45 0.69 0.58 8.85 0.75 0.55 0.66
1.13 0.97 0.58 0.51 3.86 0.37 0.39 0.41
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Fig. 5. Trajectory of air parcels during the sampling periods.
urban stations (site A and B). Retative low concentrations were always observed in the noon-time (12.00 to 15.00) samples. Diurnal variations of NMOC groups in urban areas should be influenced by the vehicular exhaust during rush hours. However, the trend was not clearly at site C. As the biogenic NMOC species are highly reactive, the amient concentrations of these species are always very low in arban area. The contribution of biogenic NMOC compounds in photochemical reactions in the air basin should be further investigated. Field measurement results also indicated that toluene has the highest average concentration (5.7–18.9 ppb) among these NMOC species. Besides, methylpentane (1.6–5.4 ppb), isopentane(1.7–5.3 ppb), n-butane (1.8–5.1 ppb), propylene(1.5–4.7 ppb), n-pentane(1.4–4.7 ppb),
n-butane (2.5–3.6 ppb), xylene (1.1–3.3 ppb) and benzene (1.5–2.7 ppb) also show their higher concentrations than other species. Toluene, isopentane, propylene and n-pentane, benzene, m,pxylene are known as the major NMOC components in gasoline engine exhaust (Gertler et al., 1993). Studies of ambient NMOC concentrations from Europe have indicated that toluene (8.2–14.3 ppb), iso-pentane (6.4–11.7 ppb), n-butane (2.1–7.8 ppb) and n-pentane (4.1–4.9 ppb) are the major compounds (Chameides, 1992; Moschonas and Glavas 1996). The concentrations of toluene, m,pxylene, iso-pentane, n-pentane, and benzene measured in Osaka, Japan are 13.9 ppb, 12.1 ppb, 11.7 ppb, 4.2 ppb and 2.3 ppb, respectively. Field measurement data indicated the airborne NMOC profile in Southern Taiwan air basin is similar to
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Osaka, Japan and is higher than the concentration in other cities. In addition, the percentage of major component in NMOC of the samples at these four sites are shown in Fig. 4. The percentage of toluene in the atmosphere of site C is (32.6% of t-NMOC) apparently higher than the values in the exhausts of passenger cars and 2-stroke motorcycles (11.7– 17.2%). Local emissions probably caused the high value of airborne toluene at station C. The characteristic ratio of various components was shown in Table 3. For some specific species, such as m,pxylene and o-xylene, the characteristic rations of m,p -xyleneybenzene and o-xyleneybenzene at these four stations were 0.37–0.85 and 0.39–0.55, which are similar to mobile emissions (0.48–1.48 and 0.31–0.61). The ratio of m,p-xyleneybenzene and o-xyleneybenzene at site D were 0.37 and 0.39, which were lower than those of other sites (0.62–0.82 and 0.41–0.55). Field data show that the airborne profiles of NMOC components at these four sites are similar to the profiles of mobile emissions. It means the airborne concentration of NMOC in the air basin are strongly influenced by the mobile source emissions. 3.2. Back trajectory of air mass Results of back trajectory simulation of air mass during ozone episodes (ozone concentration at site D was higher than 120 ppb) is shown in Fig. 5. The air pollution concentrations at site D had been influenced apparently by the emission sources in the upwind urban area. Approximate 85% of air mass traveled through the urban area during 15 ozone episodes in cold season of 1997. The air mass shifted in direction around southeast coastal area after disembarkation and passed through the industrial metropolitan area with significant emissions of NMOC and NOx in the Southern Taiwan air basin (TEPA, 2000). After a time lag of 4–6 h, high concentration of photochemical pollutant was observed at monitoring station D. The trajectories of air mass during the ozone episodes in autumn were the synoptic cases in Southern Taiwan air basin. Ozone episodes then were observed frequently at site D in cold seasons.
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Table 4 Source apportionment(%) of airborne NMOC in Southern Taiwan air basin Site Hour
PCAR
MOT
INDL
BIO
SOV
VHL
A
6–9 12–15 18–21
35.7 33.9 41.5
27.4 23.3 24.0
26.9 17.4 17.6
0.2 0.7 0.2
12.5 29.3 18.8
63.1 57.2 65.5
B
6–9 12–15 18–21
48.2 33.7 51.3
14.0 9.0 15.0
26.0 25.9 18.9
0.1 2.3 0.1
16.4 38.1 20.8
62.2 42.7 66.3
C
6–9 12–15 18–21
38.0 36.6 33.7
11.1 11.8 9.4
32.9 27.3 13.5
0.3 1.0 0.1
27.2 32.2 46.2
49.1 48.3 43.1
D
6–9 12–15 18–21
35.8 27.6 32.8
20.0 19.7 26.0
26.1 31.2 23.8
1.1 2.5 0.5
22.7 24.1 21.2
55.8 47.3 58.8
PCARspassenger car; MOTsstroke motorcycle; INDLs industrial source; BIOsbiogenic; SOVssolvent; VHLs PCARq2-MOTq4-MOT.
3.3. CMB simulation The quantitative contributions of airborne NMOC by different sources at various sampling stations were shown in Table 4 and Fig. 6. The airborne NMOC in the regions was dominantly contributed by mobile sources and stationary emissions. For the urban and suburban monitoring sites, the gasoline-powered vehicles contributed more than half of airborne NMOC. The contributions from vehicle exhaust were between 40 and 70% based on CMB simulation. At sites A and B, the airborne NMOC were strongly influenced by the mobile sources during rush hours. The contribution of airborne NMOC from vehicles in this study was among the values of CMB simulation in Chicago (47–62%), Boston (50–69%), Los Angeles (38– 83%) and San Francisco (56–83%), respectively (O’Shea and Scheff, 1988; Fujita, 1995b, Fujita et al., 1997). The NMOC profile of vehicle emissions in Taiwan was unique because of the great percentage of motorcycles. CMB simulation indicated that approximate 43–66% NMOC was contributed by mobile sources. Industrial sources and solvent consumption were also another important contributors, which account for 15–33% and 13–45%,
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Fig. 6. Source apportionment of airborne NMOC in the Southern Taiwan air basin.
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respectively. The emission inventory indicates that the vehicle exhaust and industrial emission account for 33% and 35% of NMOC emission in the air basin, respectively (TEPA, 1999). The simulation of vehicle emission contribution by CMB was slightly overestimated. The results of CMB simulation also indicate that the biogenic emissions was insignificant (0.1– 2.4%) to the airborne concentration in the air basin. However, the exact proportion of biogenic contribution is always underestimated because of their high photochemical reactivity and short residence time in the atmosphere. The source contributions of various NMOC species at different sampling sites were presented in Fig. 6. Benzene was emitted dominantly both from industrial and mobile sources. Gasoline vapor was an important contributor to the paraffins, such as n-butane, isobutane, n-pentane, isopentane, and n-hexane. At site C and D, high contributions of gasoline vapor in C4 –C5 paraffins were also observed. Besides, toluene, xylene and ethylbenzene were found to be associated with industrial solvent application at site C, which was close to the industrial area. However, high contribution from vehicle exhaust was also observed in site A and B which were located in urban. 4. Conclusions The airborne NMOC concentrations and species profiles at various sites in the ozone non-attainment region of Southern Taiwan air basin had been investigated. Field measurement data indicated that low concentrations of airborne NMOC were observed in the downwind region but with high concentration of ozone. The aromatics and paraffins were the dominant species of NMOC, which contributed significantly to the ozone formation. At sampling site in urban and suburban area, the vehicle exhaust was the major contributor of airborne NMOC. The data derived by CMB simulation indicated that mobile emission sources account for more than half of airborne NMOCs in the airbasin. Control strategy for the abatement of mobile sources emissions should be developed with high priority.
141
Acknowledgments The authors gratefully acknowledge the National Science Council, Republic of China for supporting the study (NSC 90-2211-E-006-124 & NSC-892211-E-006-019). References Bowman FH, Seinfeld JH. Ozone productivity of atmospheric organics. Geophys Res 1994;99:5309 –5321. Carter WPL. Development of ozone reactivity scales for volatile organic compounds. J Air Waste Manage Assoc 1994;44:881 –889. Chameides WL. Ozone precursor relationships in the ambient atmosphere. Geophys Res 1992;97:6037 –6043. Derwent RG. Photochemical ozone creation potentials for a large number of reactive hydrocarbons under European condition. Atmos Environ 1996;25:1661 –1678. Doskey PV, Fukui Y, Sultan M. Source profiles for nonmethane organic compounds in the atmosphere of Cairo, Egypt. J Air Waste Manage Assoc 1999;49:814 –822. Fujita EM, Watson JG, Chow JC, Magliano KL. Receptor model and emissions inventory source apportionment of non-methane organic gases in California San Joaquin Valley and San Francisco Bay Area. Atmos Environ 1995;29:3019 – 3035. Fujita EM, Lu Z, Sheetz L, Harshfield G, Hayes T, Zielinska B. Hydrocarbon source apportionment in Western Washington. Prepared for State of Washington Department of Ecology. Lacy, WA, Desert Research Institute, Reno, NV, 1997. Gertler AW, Fujita EM, Pierson WR, Wittorff DN. Apportionment of VOC tailpipe vs. non-tailpipe emissions in the Fort McHenry and Tuscarora tunnels regional photochemical measurement and modeling studies specialty. Conference on Air and Waste Management Association San Diego, California, 1993. Hsu YC, Tsai JH, Chen HW, Lin WY. Tunnel study of onroad vehicle emissions and the photochemical potential in Taiwan. Chemosphere 2001;41(11):151 –158. Hsu YC, Tsai JH, Lin TC. Metal speciations on particulate matter and volatile organic compounds from external oil combustion boilers. J Environ Sci Health: A 2000;35(6):929 –939. Ministry of Transportation (MOT). Monthly report of traffic affairs, Taiwan, ROC, 1999. Moschonas N, Glavas S. C3–C10 Hydrocarbons in the atmosphere of Athens, Greece. Atmos Environ 1996;30:2769 – 2772. O’Shea, W.J., Scheff, P.A. A chemical mass balance for volatile organics in Chicago. JAPCA 1988, 38, pp: 1020–1026. Taiwan Environmental Protection Administration (TEPA), 2000. Air pollutant emission abatement and emission cap program for Kaoping and Yuen-chia-nan Air Basins. EPA89-FA12-03-020, 2.11-2.26, Taiwan, 2000.
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Taiwan Environmental Protection Administration (TEPA). State of the environment. Taiwan, ROC, 1999. Tsai JH, Hsu YC, Weng HC, Lin WY, Jeng FT. Air pollutant emissions from new and in-use motorcycles tested on dynamometer. Atmos Environ 2000;34:4747 –4754. Tsai JH, Chiang HL, Liu YY, Yang CY. Volatile organic profiles and photochemical Potentials from motorcycles engine exhaust. A&WMA 2003a;53:516 –522.
Tsai JH, Chiang HL, Hsu YC, Weng HC, Yang CY. The speciation of volatile organic compounds(VOCs) from motorcycle engine exhaust at different driving modes. Atmos Environ 2003b;37:2485 –2496. Vukovich FM. Time scales of surface ozone variations in the regional, non-urban environment. Atmos Environ 1997;31:1513 –1530.
The relationship between volatile organic profiles and emission sources in ozone episode region—a case study in Southern Taiwan Jiun-Horng Tsaia,*, Yi-Chyun Hsub, Jang-Yu Yanga b
a Department of Environmental Engineering, National Cheng-Kung University, Tainan, Taiwan, ROC Department of Environmental Engineering, Kun-Shan University of Technology, Tainan, Taiwan, ROC
Received 31 May 2003; accepted 13 January 2004
Abstract This study investigates the relationship between volatile organic profiles in the atmosphere and emission sources in an ozone non-attainment in region Southern Taiwan. Dynamometer test of vehicles and stack sampling from industrial facilities were conducted to obtain the fingerprints of emissions from on-road mobile sources and stationary sources, respectively. In addition, field sampling of non-methane organic compounds (NMOC) concentration at monitoring stations during episode seasons were also collected by canisters. The influences of different emissions sources on airborne concentrations were estimated by back-trojectory analysis and chemical mass balance model (CMB 8.0) calculation. Field measurement data indicated that the daily average concentration of NMOC ranges between 26.4 and 69.8 ppb at different sites. The mass fraction for paraffins, oleffins and aromatics in airborne samples at these sites were 28–47%, 7–12% and 41–52%, respectively. Toluene was the dominant species among these species, followed by isopentane, n-butane and 1,2,4-trimethylbenzene. The source apportionment of airborne NMOC in the ozone non-attainment region, based on CMB simulation, is passenger cars (28–51%), motorcycles (9– 24%), industrial sources (14–33%), solvent application (13–46%) and biogenic emissions (-1–2.4%), respectively. Both field measurement and model analysis showed that the vehicle exhaust and industrial emission are the dominant contributors of NMOC in the region. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Motorcycle; Dynamometer; Volatile organic compound; Chemical mass balance
1. Introduction Surface ozone has been the subject of numerous studies for the past two decades because of its adverse effects on human health, crops and vegetation (Vukovich, 1997). Despite constant efforts to control the emissions of ozone precursors by *Corresponding author. Tel.: q886-6-275-1084; fax: q8866-208-3152. E-mail address: [email protected] (J.-H. Tsai).
the authorities, ozone levels continue to exceed the air quality standard in most areas of the Southern Taiwan. In fact, Southern Taiwan is facing major air quality challenge because of frequent incidents of high concentration of ozone and particulate matter since the 1990s. Therefore, the Kaoping air basin, located in Southern Taiwan, is classified as a non-attainment region for ozone. In 2000, 10.4% of the air quality data (monitoring site number times monitoring days) were classified
0048-9697/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2004.01.020
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unhealthy as its pollutant standard index (PSI) exceeded 100. The maximum hourly concentrations of ozone ranged between 126 and 203 ppb, which exceeded the maximum hourly standard of 120 ppb, at seven air monitoring stations in Kaoping Air Basin. Approximately two-fifths of the incidents were attributed to ozone episodes especially from October to December (TEPA, 2000). Prior to developing an effective control program for ozone, there is considerable interest for the Taiwan Environmental Protection Agency (TEPA) and local governments to fully understand the inter-relationships among the ozone concentrations, the ozone precursors, and secondary pollutants. Photochemical ozone formation is caused by the gas phase reactions of VOCs with oxides of nitrogen (NOx) in the presence of sunlight. In 1998, the estimated emission inventory of NOx and non-methane hydrocarbon (NHMC) was 423 and 841 thousand metric tons per year and they accounted for 49% of NOx and 33% of NHMC from mobile emissions, respectively (TEPA, 1999). A significant number of motor vehicles (2 950 000) and complex industrial sites (350 000 factories) around the Kaoping air basin contribute to the serious air pollution problem in this region. There are 16 million motor vehicles in Taiwan, with 66% of motorcycles, 32% of gasoline passenger cars, and approximately 1% of diesel trucks and buses (MOT, 1999). Such huge number of vehicles definitely contributes a major portion of air pollutants in the emission inventory. VOCs have different effects on ozone formation because of reaction kinetics and mechanisms (Carter, 1994; Bowman and Seinfeld, 1994). Both anthropogenic and biogenic hydrocarbons are also important in the formation of ozone. Controls on some sources of the VOC emissions may be necessary to reduce photochemical smog formation. However, high concentrations of NMOC were also observed in air mass with episodes of long-range transport and with the highest photochemical age (Derwent, 1996). The chemical mass balance (CMB) model has been widely used to estimate source category contribution to their ambient levels in the atmospheres of various urban areas. The source profiles of the individual NMOC emission are essential
input data for the CMB model. Profiles for NMOC sources have been developed for urban areas in the United States, Canada, Australia, Egypt and Japan (Doskey et al., 1999; Fujita et al., 1995). However, those profiles may not be applicable to all urban areas because the mix of mobile sources and fuel specifications are site-specific. To date, the airborne VOCs have received wide attention in air monitoring programs in Taiwan. This investigation was carried out in the Southern Taiwan in winter during the days with ozone episodes and without ozone episodes. Fifty-one species of VOCs, from anthropogenic and biogenic sources, were quantified by GCyMS and examined at four air monitoring stations in the air basin. Back trajectory simulation of air mass was employed to identify the potential sources and preferred pathways for ozone events. The CMB model was used to calculating the distribution of emission sources. The results of all ambient samples were analyzed by CMB 8.0 model with assistance from fingerprint database of the CMBtwvoc. Relationships of VOCs with ozone concentrations were also examined in an attempt to understand the relative significance of VOCs during the ozone episode. 2. Methods and materials 2.1. Sampling sites Based on air quality data and the trajectory of the air stream, field measurements were conducted at four air quality monitoring stations from Taiwan Air Quality Monitoring Network (TAQMN). The TAQMN consists of 72 air quality monitoring stations, including of 16 stations at the Kaoping air basin. Sampling station A (Samin station) is located in downtown area of Kaohsiung, which is classified as general station and is strongly influenced by emissions from vehicles and various sources. Sampling station B (Shyaukong station) and C (Daliau station) are located within the vicinity of industrial area, which are also strongly influenced both by emissions from vehicles and industrial sources. Sampling station D (Chyouzou station) is located in the downwind rural region with low
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Fig. 1. Locations of sampling site in Southern Taiwan air basin.
emissions. The locations of these air monitoring stations in the air basin are shown in Fig. 1. 2.2. Field measurement Field measurement was conducted from 1997 to 1998. There were a total 14 days of field sampling work, including nine days with ozone episode (8 h ozone conc. )60 ppbv). A total of 138 samples of ambient air were collected at various locations in this region. Ambient air were collected by the active pressurized sampler (Xontech Canister Sampler 910A), with sampling flowrate of 80 mlymin, into prevacuated Summa stainless steel canisters (Scientific Instrumentation Specialists, Moscow, ID). Field samples were taken twice in daytime (0600 to 0900 and 1200 to 1500) and once in nighttime (1800 to 2100). All canisters were cleaned up in a series of pressurizationyevacuation cycles at 105 8C, using humidified ultra-zero air evacuated to less than 0.2 mmHg, and were shipped to the field for sampling. 2.3. Sample analysis Sample air was released from the canister and concentrated by a cold trap (Varian, Inc.) then
purged out and analyzed by a GCyFIDyMS (Varian 3600 GCyFID, Varian Saturn 2000 MS). The temperature of the trap system was cooled down to y160 8C by liquid nitrogen at first. Then the desorber was heated to 160 8C for purging. The gas chromatography was equipped with dual-column capillary. An Al2O3-plot column (6 m, 0.53 mm, Rt-Alumin, TM) to the FID and a fused silica capillary column (60 m, 0.32 mm ID with 1 mm DB-1, J&W) to the MS. A cylinder standard gas (56 Environ-Mat Ozone Precursor, 16183, Matheson Inc., USA) was applied for quality control program. Working standard was prepared by blending the span gas and ultra-high-purity nitrogen. The performance of GCyMS was also evaluated with perfluorotributylamine for quality control. 2.4. Back trajectory simulation Back trajectory simulation of air mass was conducted by taking the meteorological data of monitoring stations during sampling periods into the process. For the maximum hourly ozone concentration estimated at site D, the air mass during the time period of 1200–1400 was usually selected
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Table 1 Profiles of various NMOC sources in CMB-twvoc database, % by volume Compound
Propylene Propane Isobutane n-Butane Trans-2-butene Cis-2-butene Isopentane 1-Pentene n-Pentane Isoprene Trans 2-pentene Cis 2-pentene 2-Methyl-2-butene 2,2-Dimethylbutane Cyclopentene 4-Methyl-1-pentene Cyclopentane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane 2-Mrthyl-1-pentene n-Hexane Trans-2-hexene Cis-2-hexene Methylcyclopentane 2,4-Dimethylpentane Benzene Cyclohexane 2-Methlyhexane 2,3-Dimethylpentane 3-Methylhexane 2,2,4-Trimethylpentane n-Heptane Methylcyclohexane 2,3,4-Trimethylpentane Toluene 2-Methylheptane 3-Methylheptane n-Octane Ethylbenzene m,p-Xylene Styrene o-Xylene n-Nonane Isopropylbenzene n-Propylbenzene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene
Vehicle exhaust Passenger
Motorcycles 2-stroke 4-stroke
Diesel
5.4 – 0.5 1.1 0.5 0.5 6.0 0.4 4.3 0.1 0.6 0.4 0.9 1.2 0.3 0.1 1.4 2.1 7.2 3.5 0.5 2.2 0.1 0.1 2.1 0.2 9.1 0.5 2.0 0.8 2.5 0.4 1.8 0.6 0.3 17.2 1.4 0.9 0.6 2.4 4.4 0.1 2.8 0.2 0.2 0.3 0.6 2.5
0.5 0.0 0.2 0.7 0.3 0.7 15.6 0.3 6.7
3.3 – 0.1 0.5 0.2 0.3 11.6 0.3 5.8
0.6 0.4 1.3 2.0 0.4
0.6 0.4 1.0 0.8 0.4
1.2 2.0 7.3 3.8 0.2 1.8 0.1 – 1.7 3.9 4.4 0.2 3.5 0.9 2.8 1.0 2.3 0.3 0.5 11.7 1.5 1.6 0.7 1.5 6.5 0.2 2.7 0.4 0.3 0.5 1.1 0.5
0.8 1.0 3.1 2.2 0.2 0.9 0.1 0.1 1.2 0.4 3.6 0.1 2.5 0.7 1.8 1.0 1.8 0.4 0.4 16.4 1.3 1.3 0.9 3.6 11.8 0.4 5.6 0.6 0.5 1.1 4.4 2.6
– – – – – – – – – – – – – – – – – – – – – – – – – – 16.3 – – – – – – – – 8.3 – – – 3.5 4.0 – 2.3 6.0 – – 11.6 48.0
Industry
Petroleum refinery
Solvent use
Liquid gasoline
Gasoline vapor
CNG
5.8 2.3 8.7 14.5 0.5 – 9.1 – 5.7 1.1 – – – – – – 3.3 6.0 0.9 0.5 – 6.8 – – 0.7 0.2 4.3 3.4 – – 0.2 0.1 2.5 – – 7.6 – – 1.4 2.4 5.4 3.1 3.3 0.1 – – 0.6 –
10.9 2.7 4.9 1.0 0.9 3.7 – 5.0 0.6 0.3 3.2 2.9 0.7 0.3 – – 3.3 6.0 1.9 – 2.6 – – 0.8 – 13.4 0.6 1.1 0.5 1.8 – 1.8 0.7 – 7.7 0.0 0.3 0.4 2.8 5.5 0.8 2.7 0.4 0.2 0.4 0.4 1.5 –
– – – – – – – – – – – – – – – – – – – – – – – – – – 5.7 – – – – – – – – 49.5 – – – 8.9 23.4 – 12.5 – – – – –
– 0.2 0.4 1.7 0.8 0.9 8.7 0.4 6.1 0.9 1.6 1.1 2.0 1.2 0.9 0.0 3.7 11.0 10.7 5.7 2.1 2.3 0.1 0.2 2.3 0.3 1.3 1.3 1.8 0.6 2.2 0.3 2.0 0.7 0.1 9.8 0.3 0.8 0.8 2.6 2.8 0.4 2.6 0.3 0.4 0.4 1.4 2.4
– 1.1 1.9 10.7 3.1 2.5 24.7 2.2 15.3 2.2 2.3 2.4 0.9 0.3 0.3 – 2.4 4.3 4.3 2.1 0.8 0.8 – – 2.4 – – 0.2 0.8 – 0.7 0.1 0.5 0.5 – 4.9 – – – 1.2 1.1 – 1.3 0.2 0.1 0.2 0.2 0.3
– 52.2 36.5 11.3 – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
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Table 2 Concentration (ppbv) at various NMOC species at different sites in the ozone non-attainment region in Southern Taiwan Sampling sites compounds
A
B
C
D
Propylene Isobutane 1-Butene n-Butane Trans-2-butene Cis-2-butene 3-Methyl-1-butene Isopentane 1-Pentene n-Pentane Isoprene Trans 2-pentene Cis 2-pentene 2-Methyl-2-butene 2,2-Dimethylbutane Cyclopentene 4-Methyl-1-pentene Cyclopentane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane 2-Methyl-1-pentene n-Hexane Trans-2-hexene Cis-2-hexene Methylcyclopentane 2,4-Dimethylpentane Benzene Cyclohexane 2-Methlyhexane 2,3-Dimethylpentane 3-Methylhexane 2,2,4-Trimethylpentane n-Heptane Methylcyclohexane 2,3,4-Trimethylpentane Toluene 2-Methylheptane 3-Methylheptane n-Octane Ethylbenzene m,p-Xylene Styrene o-Xylene n-Nonane Isopropylbenzene n-Propylbenzene 1,3,5-Trimethyl-benzene 1,2,4-Trimethyl-benzene Paraffins Olefins Aromatics Biogenics t-NMOC
3.9"4.1 1.9"1.7 3.6"1.9 4.4"4.2 0.5"0.4 0.6"0.7 0.1"0.2 4.9"4.0 0.2"0.2 4.7"4.1 0.3"0.2 0.6"0.4 0.3"0.3 1.2"0.7 0.7"0.6 0.4"0.4 0.1"0.2 0.9"0.7 0.8"0.7 3.0"2.5 2.4"2.2 0.1"0.1 1.7"1.8 0.03"0.05 0.1"0.1 1.8"1.6 0.1"0.2 2.4"2.0 0.6"0.7 0.7"0.6 0.4"0.4 1.0"0.8 0.1"0.1 0.8"0.6 0.3"0.4 0.1"0.1 13.0"7.1 0.4"0.3 0.3"0.2 0.3"0.2 1.6"1.1 2.0"1.9 0.4"0.7 1.3"1.2 0.3"0.2 0.1"0.2 0.3"0.4 0.7"0.6 3.2"2.5 32.6"11.5 11.6"6.5 25.3"11.1 0.3"0.2 69.8
4.7"3.5 2.2"1.4 3.4"1.6 5.1"2.9 0.5"0.4 0.7"0.6 0.1"0.1 5.3"3.4 0.3"0.2 4.6"2.8 0.5"0.3 0.5"0.4 0.5"0.3 0.8"0.4 0.7"0.5 0.3"0.2 0.1"0.1 0.8"0.5 0.7"0.5 2.6"1.8 2.0"1.3 0.2"0.1 1.3"0.8 0.04"0.03 0.1"0.1 1.5"0.9 0.1"0.1 2.7"1.4 1.0"1.2 0.5"0.4 0.3"0.3 0.8"0.4 0.1"0.1 0.7"0.4 0.3"0.2 0.1"0.03 11.9"3.8 0.3"0.3 0.3"0.3 0.4"0.3 1.7"0.9 1.7"1.1 0.7"0.5 1.1"0.7 0.3"0.2 0.1"0.1 0.2"0.1 0.7"0.3 2.6"1.7 32.0"9.8 12.1"5.9 23.4"6.8 0.5"0.3 68.0
2.7"2.6 1.5"1.8 3.2"2.4 3.8"4.1 0.2"0.4 0.3"0.5 0.04"0.06 3.7"4.2 0.1"0.2 3.1"3.7 0.4"0.3 0.2"0.3 0.1"0.2 0.5"0.8 0.4"0.5 0.1"0.2 0.03"0.06 0.5"0.5 0.4"0.6 1.5"1.7 1.2"1.5 0.2"0.2 1.0"0.8 0.1"0.2 0.1"0.2 1.0"1.4 0.1"0.2 2.1"1.9 0.6"1.1 0.4"0.7 0.2"0.4 0.5"0.5 0.1"0.2 0.5"0.8 0.2"0.3 0.1"0.1 18.9"16.5 0.2"0.2 0.2"0.3 0.3"0.3 1.6"1.5 1.6"1.2 0.4"0.5 1.2"1.6 0.2"0.3 0.1"0.2 0.2"0.2 0.5"0.6 1.4"1.5 21.6"16.1 7.8"7.2 28.1"17.3 0.4"0.3 57.9
1.5"0.6 0.8"0.5 2.5"1.2 1.8"1.3 0.11"0.04 0.1"0.1 0.04"0.02 1.7"1.2 0.1"0.1 1.4"1.1 0.4"0.2 0.1"0.1 0.1"0.1 0.5"0.3 0.3"0.1 0.1"0.1 0.04"0.02 0.3"0.1 0.2"0.1 0.9"0.5 0.7"0.3 0.03"0.02 0.7"0.4 0.03"0.04 0.1"0.1 0.5"0.3 0.04"0.02 1.5"1.0 0.2"0.1 0.2"0.2 0.1"0.1 0.3"0.2 0.03"0.03 0.3"0.2 0.1"0.1 0.03"0.04 5.7"4.1 0.1"0.1 0.08"0.03 0.1"0.1 0.6"0.4 0.5"0.4 0.1"0.1 0.6"0.6 0.1"0.1 0.1"0.2 0.1"0.1 0.2"0.1 0.6"0.5 11.0"3.8 5.1"2.3 9.9"3.5 0.4"0.2 26.4
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Fig. 2. Source apportionment of airborne NMOC in Southern Taiwan and other cities.
for daily back trajectory simulation. Air mass back trajectories were obtained from the Taiwan Atmospheric Data Center for specific ozone pollution episode. Based on the back trajectory model, resident time of the air mass and conditional residence probability were employed to identify the potential sources and preferred pathways for each ozone episodes. The back trajectory simulation was based on the grid system with 1 km. A sensitivity test to check the simulation result.
2.5. CMB simulation Chemical mass balance (CMB) model was used to evaluate the contribution of airborne VOC from various emissions sources. The results of all ambient samples were analyzed by the CMB 8.0 model and coordinated the fingerprint database with local source profiles. The database of CMB-twvoc is the source profiles for NMOC in Southern Taiwan (Tsai et al., 2000, 2003a,b; Hsu et al., 2000, 2001)
Fig. 3. Variation of NMOC concentration during various sampling periods.
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Fig. 4. Profile of the airborne NMOC various sampling sites.
NMOC profiles of various vehicles (passenger car, 2-stroke motorcycle, 4-stroke motorcycle, diesel truck), oil refinery, liquefied gas (LPG) and natural gas, synthetic organic chemical manufacturing processes, and industrial coating factories (solvent use). Emission profiles of various NMOC sources in CMB-twvoc database are shown in Table 1. 3. Results and discussion 3.1. Concentrations of NMOC and the speciations Table 2 presents the mean values of airborne organics species at various sampling sites in the air basin. Higher value of airborne NMOC concentrations were observed at site A (69.8 ppb), B (68.0 ppb) and C (57.9 ppb), which were located in downtown area or nearby the industrial complex. The airborne NMOC concentration at sampling D (26.4 ppb) was clearly lower than those of other three sites, but the ozone concentration at this site is always higher than the others. The paraffin and aromatics, which were the major groups of NMOC, accounted for 37–47% and 34– 49% of total NMOC concentrations, respectively. The fraction of olefin compounds was in the range of 14–19%. Field measurement data shows that the biogenic source-related components were less than 2% of total NMOCs. All airborne NMOCs species are categorized into four groups as paraffin,
olefins, aromatic and biogenic organics, as shown in Table 2. Fig. 2 compares the NMOC groups in this study with the others. Paraffins, olefins, and aromatics groups measured in both Boston and Houston, account for 60–67%, 6.6 –16% and 20–28%, respectively. The paraffins, a major group of the NMOC, measured in the Southern Taiwan air basin was apparently lower than the values in other cities. However, the olefins and aromatics in Southern Taiwan were higher than the other cities with a value of 3–15%. Average concentration of the major groups of NMOC during different sampling periods is shown in Fig. 3. At sampling sites A, B and C, higher concentrations of these groups were observed in the morning (06.00 to 09.00), especially at the Table 3 The characteristic ratios of major NMOC components at various sites Ratios_Site
A
B
C
D
Isopentaneybenzene n-Pentaneybenzene 2-Methylpentaneybenzene 3-Methylpentaneybenzene Tolueneybenzene m,p-Xyleneybenzene o-Xyleneybenzene 1,2,4-Trimethyl-benzeneybenzene
2.04 1.91 1.21 0.98 5.37 0.82 0.55 1.31
1.95 1.71 0.98 0.75 4.40 0.62 0.41 0.98
1.72 1.45 0.69 0.58 8.85 0.75 0.55 0.66
1.13 0.97 0.58 0.51 3.86 0.37 0.39 0.41
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Fig. 5. Trajectory of air parcels during the sampling periods.
urban stations (site A and B). Retative low concentrations were always observed in the noon-time (12.00 to 15.00) samples. Diurnal variations of NMOC groups in urban areas should be influenced by the vehicular exhaust during rush hours. However, the trend was not clearly at site C. As the biogenic NMOC species are highly reactive, the amient concentrations of these species are always very low in arban area. The contribution of biogenic NMOC compounds in photochemical reactions in the air basin should be further investigated. Field measurement results also indicated that toluene has the highest average concentration (5.7–18.9 ppb) among these NMOC species. Besides, methylpentane (1.6–5.4 ppb), isopentane(1.7–5.3 ppb), n-butane (1.8–5.1 ppb), propylene(1.5–4.7 ppb), n-pentane(1.4–4.7 ppb),
n-butane (2.5–3.6 ppb), xylene (1.1–3.3 ppb) and benzene (1.5–2.7 ppb) also show their higher concentrations than other species. Toluene, isopentane, propylene and n-pentane, benzene, m,pxylene are known as the major NMOC components in gasoline engine exhaust (Gertler et al., 1993). Studies of ambient NMOC concentrations from Europe have indicated that toluene (8.2–14.3 ppb), iso-pentane (6.4–11.7 ppb), n-butane (2.1–7.8 ppb) and n-pentane (4.1–4.9 ppb) are the major compounds (Chameides, 1992; Moschonas and Glavas 1996). The concentrations of toluene, m,pxylene, iso-pentane, n-pentane, and benzene measured in Osaka, Japan are 13.9 ppb, 12.1 ppb, 11.7 ppb, 4.2 ppb and 2.3 ppb, respectively. Field measurement data indicated the airborne NMOC profile in Southern Taiwan air basin is similar to
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Osaka, Japan and is higher than the concentration in other cities. In addition, the percentage of major component in NMOC of the samples at these four sites are shown in Fig. 4. The percentage of toluene in the atmosphere of site C is (32.6% of t-NMOC) apparently higher than the values in the exhausts of passenger cars and 2-stroke motorcycles (11.7– 17.2%). Local emissions probably caused the high value of airborne toluene at station C. The characteristic ratio of various components was shown in Table 3. For some specific species, such as m,pxylene and o-xylene, the characteristic rations of m,p -xyleneybenzene and o-xyleneybenzene at these four stations were 0.37–0.85 and 0.39–0.55, which are similar to mobile emissions (0.48–1.48 and 0.31–0.61). The ratio of m,p-xyleneybenzene and o-xyleneybenzene at site D were 0.37 and 0.39, which were lower than those of other sites (0.62–0.82 and 0.41–0.55). Field data show that the airborne profiles of NMOC components at these four sites are similar to the profiles of mobile emissions. It means the airborne concentration of NMOC in the air basin are strongly influenced by the mobile source emissions. 3.2. Back trajectory of air mass Results of back trajectory simulation of air mass during ozone episodes (ozone concentration at site D was higher than 120 ppb) is shown in Fig. 5. The air pollution concentrations at site D had been influenced apparently by the emission sources in the upwind urban area. Approximate 85% of air mass traveled through the urban area during 15 ozone episodes in cold season of 1997. The air mass shifted in direction around southeast coastal area after disembarkation and passed through the industrial metropolitan area with significant emissions of NMOC and NOx in the Southern Taiwan air basin (TEPA, 2000). After a time lag of 4–6 h, high concentration of photochemical pollutant was observed at monitoring station D. The trajectories of air mass during the ozone episodes in autumn were the synoptic cases in Southern Taiwan air basin. Ozone episodes then were observed frequently at site D in cold seasons.
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Table 4 Source apportionment(%) of airborne NMOC in Southern Taiwan air basin Site Hour
PCAR
MOT
INDL
BIO
SOV
VHL
A
6–9 12–15 18–21
35.7 33.9 41.5
27.4 23.3 24.0
26.9 17.4 17.6
0.2 0.7 0.2
12.5 29.3 18.8
63.1 57.2 65.5
B
6–9 12–15 18–21
48.2 33.7 51.3
14.0 9.0 15.0
26.0 25.9 18.9
0.1 2.3 0.1
16.4 38.1 20.8
62.2 42.7 66.3
C
6–9 12–15 18–21
38.0 36.6 33.7
11.1 11.8 9.4
32.9 27.3 13.5
0.3 1.0 0.1
27.2 32.2 46.2
49.1 48.3 43.1
D
6–9 12–15 18–21
35.8 27.6 32.8
20.0 19.7 26.0
26.1 31.2 23.8
1.1 2.5 0.5
22.7 24.1 21.2
55.8 47.3 58.8
PCARspassenger car; MOTsstroke motorcycle; INDLs industrial source; BIOsbiogenic; SOVssolvent; VHLs PCARq2-MOTq4-MOT.
3.3. CMB simulation The quantitative contributions of airborne NMOC by different sources at various sampling stations were shown in Table 4 and Fig. 6. The airborne NMOC in the regions was dominantly contributed by mobile sources and stationary emissions. For the urban and suburban monitoring sites, the gasoline-powered vehicles contributed more than half of airborne NMOC. The contributions from vehicle exhaust were between 40 and 70% based on CMB simulation. At sites A and B, the airborne NMOC were strongly influenced by the mobile sources during rush hours. The contribution of airborne NMOC from vehicles in this study was among the values of CMB simulation in Chicago (47–62%), Boston (50–69%), Los Angeles (38– 83%) and San Francisco (56–83%), respectively (O’Shea and Scheff, 1988; Fujita, 1995b, Fujita et al., 1997). The NMOC profile of vehicle emissions in Taiwan was unique because of the great percentage of motorcycles. CMB simulation indicated that approximate 43–66% NMOC was contributed by mobile sources. Industrial sources and solvent consumption were also another important contributors, which account for 15–33% and 13–45%,
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Fig. 6. Source apportionment of airborne NMOC in the Southern Taiwan air basin.
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respectively. The emission inventory indicates that the vehicle exhaust and industrial emission account for 33% and 35% of NMOC emission in the air basin, respectively (TEPA, 1999). The simulation of vehicle emission contribution by CMB was slightly overestimated. The results of CMB simulation also indicate that the biogenic emissions was insignificant (0.1– 2.4%) to the airborne concentration in the air basin. However, the exact proportion of biogenic contribution is always underestimated because of their high photochemical reactivity and short residence time in the atmosphere. The source contributions of various NMOC species at different sampling sites were presented in Fig. 6. Benzene was emitted dominantly both from industrial and mobile sources. Gasoline vapor was an important contributor to the paraffins, such as n-butane, isobutane, n-pentane, isopentane, and n-hexane. At site C and D, high contributions of gasoline vapor in C4 –C5 paraffins were also observed. Besides, toluene, xylene and ethylbenzene were found to be associated with industrial solvent application at site C, which was close to the industrial area. However, high contribution from vehicle exhaust was also observed in site A and B which were located in urban. 4. Conclusions The airborne NMOC concentrations and species profiles at various sites in the ozone non-attainment region of Southern Taiwan air basin had been investigated. Field measurement data indicated that low concentrations of airborne NMOC were observed in the downwind region but with high concentration of ozone. The aromatics and paraffins were the dominant species of NMOC, which contributed significantly to the ozone formation. At sampling site in urban and suburban area, the vehicle exhaust was the major contributor of airborne NMOC. The data derived by CMB simulation indicated that mobile emission sources account for more than half of airborne NMOCs in the airbasin. Control strategy for the abatement of mobile sources emissions should be developed with high priority.
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