Environ. Sci. Technol. 2000, 34, 3606-3610
Estimation of Motor Vehicle Toxic Emissions in the Metropolitan Area of Mexico City I . S C H I F T E R , * L . D IÄ A Z , E. LO Ä PEZ-SALINAS, F. RAMOS, S. AVALOS, G. LO Ä PEZ-VIDAL, AND M. CASTILLO Instituto Mexicano del Petro´leo, Gerencia de Transformacio´n de Energe´ticos, Eje Central La´zaro Ca´rdenas 152, San Bartolo Atepehuacan, 07730 Me´xico, D.F. Mexico
The contribution of gasoline-powered vehicles to air toxic emissions in the Metropolitan Area of Mexico City (MAMC) is not well established. The MAMC has particular geographic conditions such as being in a valley at high altitude (2280 m above sea level) and has a very old and technologically heterogeneous vehicular fleet. Toxic compounds emitted by the exhaust of motor vehicles (e.g., benzene, 1,3-butadiene, formaldehyde, and acetaldehyde) were estimated. In this work, tests using Mexican gasolines in a vehicular fleet representative of the MAMC (ca. 1999) were carried out. As a comparison, some tests were done using an American gasoline (Federal Reformulated Gasoline, RFG-1) in a representative fleet of vehicles operated in Mexico
Introduction Mobile sources are perhaps one category of the most relevant air toxic emission source to outdoor human activities in industrialized societies (1). Concern about mobile source emissions has been expressed almost 50 years ago. Since the mid-1980s, a variety of studies have documented toxic air emissions from mobile sources as a major contributor to overall health risk. However, because of the extreme complexity of estimating mobile source emissions, there still has not been a comprehensive, reliable emission inventory of air toxic pollutants from mobile sources, neither in the United States nor Mexico. The U.S. Clean Air Act Amendments of 1990 specifically targeted reductions in five “toxic air pollutants” from mobile sources: benzene, 1,3-butadiene, formaldehyde, acetaldehyde, and polycyclic aromatics. Nationwide inventories done in other countries have shown that benzene, 1,3-butadiene, and formaldehyde are pollutants whose contribution to the general inventory correlates with mobile sources (2). Many individual volatile organic compounds (VOC) have direct toxic effects on humans, ranging from carcinogenesis to neurotoxicity, among other adverse health effects. Moreover, since most of the main VOC species are photochemically reactive, some of them (e.g., formaldehyde, ethylene, and xylenes) highly so, VOC emissions are a major factor in urban ozone formation. Benzene is present in both exhaust and evaporative emissions from vehicles. Several epidemiology studies on * Corresponding author e-mail:
[email protected]; phone/ fax: +525-5368-9226. 3606
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workers have identified benzene as a carcinogen causing leukemia in humans (3). Roughly 85% of all benzene emissions come from vehicles; the fraction of benzene in the exhaust varies depending on control technology and fuel composition (4). Some reports indicate that nonbenzene aromatics in the fuels can cause about 70-80% of the exhaust benzene formed. Additionally, some benzene also forms from engine combustion of nonaromatic fuel hydrocarbons. In 1993, the U.S. EPA estimated that 1,3-butadiene had the highest upper-bound cancer risk of air toxics emitted by motor vehicles in the United States (5). 1,3-Butadiene appears in vehicle’s exhaust due to the incomplete combustion of the fuel. Butadiene is not present in vehicle evaporative and refueling emissions and appears to increase roughly in proportion to total hydrocarbon emissions (5). Because of its reactivity, it is estimated to have a short atmospheric lifetime, which depends on the conditions at the time of release. Formaldehyde is the most prevalent aldehyde in vehicle exhaust and comes from incomplete combustion of the fuel. This compound is of particular concern both because of its role in ozone formation and its suspected carcinogenicity. Formaldehyde can also be a short-term respiratory and skin irritant, particularly for sensitive individuals. On a per carbon basis, formaldehyde is the most important VOC precursor of smog in urban atmosphere (5). Acetaldehyde is another air toxic compound that appears in vehicle exhaust formed because of incomplete combustion of the fuel. In the atmosphere, acetaldehyde decomposes through photooxidation and oxidation by hydroxyl radicals. Whereas formaldehyde produces CO upon reaction of photolysis, acetaldehyde produces organic radicals that ultimately form peroxyacetyl nitrate and formaldehyde (5). Turnover rate of the Mexican vehicle fleet is considerably slower than in the United States. Furthemore, the incorporation of emission controls in new vehicles was delayed in Mexico as compared to those in the United States. Thus, older, less fuel-efficient and more polluting vehicles comprise a larger percentage of the current vehicular fleet in the MAMC. Approximately 60% of the vehicular fleet is not equipped with a catalytic converter since the introduction of these emission control systems did not occur until the model year (MY) 1991 for new vehicles (6). Unleaded gasoline, introduced in the MAMC in 1990, was oxygenated in 1994 to decrease CO emissions. The elimination of leaded gasoline occurred in stages: by January 1998, it disappeared from all gas stations in the Mexico. In 1996, the unleaded gasoline was again reformulated (less aromatics and olefins and limited concentration of benzene) and a premium, high-octane type, was introduced in major cities in Mexico. In the present study, we have measured tailpipe toxic emissions (benzene, 1,3-butadiene, formaldehyde, and acetaldehyde) from representative vehicles of the MAMC in order to estimate the total amount emitted per year. The effects of the different emission control technology, the type of gasoline (regular or premium), and the distance traveled on the emissions factors are also discussed.
Experimental Procedures Our facilities are regularly audited by the National Environmental Institute (INE), a Federal regulation agency. The audit reviews facilities, procedures, and standards used for certification of all new vehicles sold in Mexico. The evaluation is done following the Mexican Official Procedure, which is similar to the Federal Test Procedure, FTP-75, of the Code 10.1021/es9913784 CCC: $19.00
2000 American Chemical Society Published on Web 08/06/2000
TABLE 1. Model Year, Number of Vehicles, and Emission Control Technology of Vehicular Fleet Evaluated in This Study model year
no. of vehicles
1984-1990 fleet 1984 1986 1988 1989 1990 1991-1996 fleet 1991-1992 1993 1994 1995 1996 1997-1999 fleet
a
4 4 4 4 2 4 4 2 2 3 17
TABLE 2. Relevant Properties of the Gasolines Employed in This Study Mexican
characteristics carburator electronic ignition no catalytic converter
electronic light-on fuel injectors 3-way catalytic converter electronic light-on adaptive programming OBDa system
United States
property
regular
premium
regular
premium
gravity, 20/4 °C vapor pressure, lb/in2 distillation, ASTM D-86 10% evaporated 50% evaporated 90% evaporated end point sulfur, ppm aromatics, vol % olefins, vol % MTBE, vol % TAME, vol % benzene, vol % (RON+MON)/2
0.7336 6.8
0.7382 7.05
0.7413 6.65
0.7363 6.7
55 93.9 163.8 203.7 324 26.9 5.7 6.5 2.4 1.2 87.9
63.3 108.5 172.8 208.9 196 29.2 3.6 8.0 0.0 1.8 93.2
59.8 90.6 160.2 203.8 218 24.5 10.9 14.4 0.0 1.3 87.9
58.5 89.2 170.4 211.8 168 28.5 8.1 10.3 0.0 0.8 93.6
On board diagnostic.
of Federal Regulations of the United States (7). In this study, vehicle tests were carried out in duplicate, and a third test sequence was conducted if differences in back-to-back FTP emissions exceeded specified limits (8%). The FTP is an emission certification test procedure used for light-duty vehicles. A Clayton model ECE-50-250 chassis dynamometer with a direct-drive, variable-inertia flywheel system was used for the entire testing. Our emissions laboratory is equipped with a Venturi constant volume sampler (CVS) and a Horiba Driver’s Aid. The FTP incorporates a cold start, stable running, and hot start emissions (referred to as bag 1-3 emissions, respectively). The FTP composite emission rate is a weighted average of the three measured bags to represent two trips. The first trip is a cold start trip after a 12-h soak, and the other is a hot start trip after a 10-min soak. The hydrocarbons and aldehydes were speciated and quantified. Benzene and 1,3-butadiene hydrocarbons were quantified in a Varian 3400 gas chromatograph with a flame ionization detector. The aldehydes were quantified in an HP 1100 high-performance liquid chromatograph (HPLC) equipped with an ultraviolet-visible detector. The aldehydes were collected in silica gel cartridges with 2,4-dinitrophenylhydrazine (DPNH) as the derivative reagent, extracted with acetonitrile, and later analyzed by HPLC. In each test, the fleet was composed of representative vehicles used as private cars, taxis, and light-duty trucks. The vehicles used represent the most common emissions control technologies available in the MAMC fleet; the selection was established considering the Permanent License Program from the local authorities. The vehicles covered a range of MY from 1984 through 1999 (see Table 1). This range captures the most prevalent emission control technology found in the MAMC’s vehicle fleet. Eighteen vehicles were MY 1984-1990, carburated with electronic ignition and no catalytic converters. Fifteen vehicles were MY 1991-1996, with fuel injection and electronic light on. In Mexico, catalytic converters were of the oxidative type in MY 1991-1993, while the MY 1994-1996 were furnished with a three-way catalytic converters (TWC). Seventeen vehicles were MY 1997-1999, with adaptive programming for closed loop and TWC. Each fleet had similar odometer readings at the start of testing, except the 1997 and 1999 fleets that were all new vehicles (see Table 1). Subsequently, randomized duplicate emission tests were conducted using the fuel. Whenever fuels or the vehicle configuration was changed, an extended vehicle preparation cycle was run to minimize adaptive learning. Table 2 shows some relevant physical and chemical properties of the fuels used throughout the study. The American gasoline
TABLE 3. Toxic Emissions from the MY 1984-1990 Fleet Using a Regular Gasoline compound
exhaust emissions (mg/km)
1,3-butadiene benzene formaldehyde acetaldehyde
26.7 53.3 14.7 6.2
total
100.8
TABLE 4. Toxic Emissions from the MY 1991-1996 Fleet Using Premium and Regular Gasoline exhaust emissions (mg/km) compound
regular gasoline
premium gasoline
1,3-butadiene benzene formaldehyde acetaldehyde
1.5 14.4 1.7 0.6
1.3 11.2 1.8 0.4
total
18.2
14.6
samples were purchased in Fort Bend County, Houston, TX.
Results and Discussion Toxic emission factors of the MY 1984-1990 fleet with regular gasoline are shown in Table 3. The emissions of this fleet are the most important since it represents about 60% of the total light-duty vehicles in the MAMC. From Table 3, benzene is by far the largest toxic compound emitted, followed by 1,3butadiene and formaldehyde. Table 4 shows the emission factors of the MY 1991-1996 fleet where most of the values decreased considerably (73, 94, and 88% reductions for benzene, 1,3-butadiene, and formaldehyde, respectively, for the regular gasoline case) in comparison with those in the 1984-1990 fleet. Obviously this generalized toxic emission decrease is directly associated with the emission control technology included in this fleet (see Table 1). The use of the premium gasoline in the MY 1991-1996 fleet did not decreased significantly the toxic emissions except for benzene, which decreased 22% in comparison with the regular gasoline case. Noteworthy of mention, the reduction in benzene emission in the premium gasoline case occurred, although this gasoline contains more benzene than the regular gasoline (see Table 2). The emission factors of toxic pollutants in the most recent MY 1997-1999 fleet with premium and regular gasolines as VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 5. Evolution of Air Toxics from Regular Gasoline as a Function of the Traveled Distance in the MY 1997-1999 Fleeta traveled distance (km)
1,3-C42) (mg/km)
Bz (mg/km)
FrmAld (mg/km)
AcAld (mg/km)
1000 4000 8000 12000 16000 ZKL DET/10-5
0.14 0.10 0.24 0.24 0.29 0.10 1.16
2.97 2.62 2.87 3.55 3.56 2.64 5.75
1.31 1.23 1.40 1.33 1.50 1.25 1.24
0.83 0.89 0.86 0.72 0.79 0.86 0.00
TABLE 8. 1,3-Butadiene Emissions from the FTP-75 Test in the MY 1997-1999 Fleet as a Function of the Traveled Distance 1,3-butadiene traveled distance (km)
a AcAld, acetaldehyde; FrmAld, formaldehyde; Bz, benzene; 1,3-C 2), 4 1,3-butadiene; ZKL, zero-kilometer coefficient; DET, deterioration coefficient.
TABLE 6. Evolution of Toxic Emissions from Premium Gasoline as a Function of the Traveled Distance in the MY 1997-1999 Fleeta traveled distance (km)
1,3-C42) (mg/km)
Bz (mg/km)
FrmAld (mg/km)
AcAld (mg/km)
1000 4000 8000 12000 16000 ZKL DET/10-5
0.32 0.42 0.24 0.24 0.29 0.38 0.00
3.57 3.56 2.87 3.77 3.83 3.52 1.93
1.51 1.49 1.53 0.93 1.61 1.48 0.75
0.89 0.99 0.91 0.95 0.98 0.91 0.37
a AcAld, acetaldehyde; FrmAld, formaldehyde; Bz, benzene; 1,3-C 2), 4 1,3-butadiene; ZKL, zero-kilometer coefficient; DET, deterioriation coefficient.
TABLE 7. Benzene Emissions from the FTP-75 Test in the MY 1997-1999 Fleet as a Function of the Traveled Distance benzene traveled distance (km)
bag 1 (mg/km)
bag 2 (mg/km)
bag 3 (mg/km)
1 000 4 000 8 000 12 000 16 000
Regular 12.34 14.28 12.66 13.77 13.70
2.39 1.43 1.33 1.63 1.93
1.13 1.26 0.75 0.24 0.32
1 000 4 000 8 000 12 000 16 000
Premium 11.37 11.84 12.37 13.92 11.57
0.64 0.60 0.28 1.08 1.35
0.64 0.78 0.84 0.81 0.77
a function of the traveled distance are shown in Tables 5 and 6. A plot (not shown) of the emission factors as a function of the traveled distance, adjusted by least-squares linear regression, allowed us to calculate the deterioration coefficient (DET, the slope value) and the zero-kilometer coefficient (ZKL, the intercept). The deterioration coefficient values indicate that, in the case of the regular gasoline (Table 5), benzene emission increases more rapidly than that of the other toxic compounds as the distance traveled increases. However, when using a premium gasoline, the deterioration coefficient for benzene is 66% lower in comparison with the regular gasoline. A difference in the deterioration of the other toxic emissions is not evident when using either regular or premium gasolines. It should be noted in Tables 7 and 8 that most benzene and 1,3-butadiene emissions occurred during cold starts, 3608
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a
bag 1 (mg/km)
bag 2 (mg/km)
bag 3 (mg/km)
1 000 4 000 8 000 12 000 16 000
Regular 1.20 2.24 1.71 2.37 1.89
nda nd 0.32 nd nd
nd nd nd nd 0.92
1 000 4 000 8 000 12 000 16 000
Premium 0.69 0.59 1.12 0.94 1.51
nd nd nd nd nd
nd nd nd nd nd
nd, not detected.
when the TWC has not yet reached its optimal operation temperature. Therefore, most of the contribution of these toxic pollutants in the averaged three bag emission factors takes place upon cold starts. The use of heated catalysts in future years will reduce these cold start emissions. Bag 1 (cold start) emissions are generally higher than composite emissions for all species (8). A comparison of any significant differences in toxic emissions when using representative American and Mexican gasolines, both tested in the MAMC with same Mexican vehicles (see Experimental Procedures), was attempted. Tables 9 and 10 show the toxic pollutants emission factors using regular and premium gasolines, respectively. In general, almost all the toxic compounds showed a similar emission factors. The most significant difference between the Mexican and American regular gasolines comes from the formaldehyde emissions, which were significantly higher in the U.S. gasoline, probably because of the higher MTBE content (see Table 2). On the other hand, when comparing toxic emissions derived from the use of premium gasolines (see Table 10), the most significant difference is the lower benzene emission factors for MY 1988-1990 fleet when using American premium gasoline. Table 11 shows an estimation of the total toxic emissions in ton per year in the MAMC. These calculations were carried out taking into consideration the average daily traveled distance, the emission factors shown in previous tables, and the total vehicles inventory in the MAMC. Siegl et al. made a comparison of the emissions from a vehicle in both normal and three malfunctioning operation modes: disabled heated exhaust gas oxygen sensor (HEGO), inactive catalytic converter, and uncontrolled misfire (8). Emission of air toxic substances was significantly affected. With HEGO sensor disabled, formaldehyde and acetaldehyde emissions increased 2 times and 1,3-butadiene increased 7 times. On the other hand, benzene emissions increased 14 times (under high fuel conditions, hydrodealkylation of alkylaromatics over the catalysts produces benzene). For the three operation modes with active catalyst, benzene is the major contributor to the total toxic emissions. However, formaldehyde was the dominant toxic with the inactive catalysts. The catalyst very efficiently converts the formaldehyde after the light off. If the four air toxic compounds are broken down by phases of the FTP cycle, the pattern of acetaldehyde and 1,3-butadiene emissions were similar to the formaldehyde; however, with inactive catalyst, emission increased from approximately 10-100 times. Acetaldehyde emissions increased relative to formaldehyde under fuelrich conditions.
TABLE 9. Comparison of Toxic Emissions between Mexican and American Regular Gasolinesa Mexican regular (mg/km)
a
United States regular (mg/km)
MY
1,3-C42)
Bz
FrmAld
AcAld
1,3-C42)
Bz
FrmAld
AcAld
1988-1990 1994-1995 1997-1999
11.1 1.2 0.5
57.1 12.9 4.9
1.6 0.9 1.4
2.7 0.8 0.8
14.3 0.6 0.7
55.0 13.8 3.9
5.8 1.2 1.7
3.4 0.7 0.8
AcAld, acetaldehyde; FrmAld, formaldehyde; Bz, benzene; 1,3-C42), 1,3-butadiene.
TABLE 10. Comparison of Toxic Emissions between Mexican and American Premium Gasolinesa Mexican premium (mg/km)
United States premium (mg/km)
MY
1,3-C42)
Bz
FrmAld
AcAld
1,3-C42)
Bz
FrmAld
AcAld
1988-1990 1994-1995 1997-1999
11.0 0.8 0.3
65.4 12.2 3.1
9.0 0.6 1.5
3.3 0.6 0.9
10.9 0.7 0.6
44.0 10.1 3.3
7.9 2.0 1.7
3.3 0.6 0.5
a
AcAld, acetaldehyde; FrmAld, formaldehyde; Bz, benzene; 1,3-C42), 1,3-butadiene.
TABLE 11. Estimation of Toxic Emissions per Year in the MAMCa fleet type vehicle taxis minibus
fleet MY
no. of vehicles
distance traveled (km/day)
total distance/106 (km/yr)
AcAld (ton/yr)
FrmAld (ton/yr)
Bz (ton/yr)
1,3-C42) (ton/yr)
1984-1990 1991-1996 1997-1999 1984-1990
