Environ. Sci. Technol. 2000, 34, 2663-2667
Impact of Engine Technology on the Vehicular Emissions in Mexico City I . S C H I F T E R , * L . D IÄ A Z , M . V E R A , M. CASTILLO, F. RAMOS, S. AVALOS, AND E. LOPEZ-SALINAS Instituto Mexicano del Petro´leo, Subdireccio´n de Proteccio´n Ambiental, Eje Central 152, San Bartolo Atepehuacan, Me´xico, D.F., 07730 Mexico
The metropolitan area of Mexico City is the urban area of the country that presents the most critical environmental problem due to fuel consumption. Attempting to reduce the ambient concentration of pollutants, the environmental program adopted consisted of a broad series of strategies designed to improve the air quality. Modification of fuel formulation, which include composition and physical properties, provide a particularly effective way to reduce emissions. The reformulated gasoline in Mexico has reduced aromatics, olefins, and benzene contents as well as a lower Reid vapor pressure. The regulated emissions (carbon monoxide, total hydrocarbons, and nitrogen oxides) were evaluated for each vehicle as well as the speciated hydrocarbons present in the exhaust emission. With these data, we estimated the ozone-forming potential of the fuels and correlate the results with the technology of the vehicles. When these data are compared with those obtained in United States, there is an important difference in the technological devices related to emission control in vehicles of the Mexican market. Moreover, our results strongly suggest that emission standards for new vehicles sold in Mexico must be tighten further in order equal those demanded in other countries.
Experimental Methods
Introduction Over the last centuries, Mexico City has been the political, economic, and cultural activity center of the nation. Today, with over 16.6 million inhabitants (20% of the nation’s population), the Mexico City Metropolitan Area (MAMC) is one of the largest urban center in the world and is believed to have one of the most serious air pollution problems. Eighteen million liters of gasoline is handled daily to supply fuel to over 2.7 million light-duty registered vehicles. Privateowned vehicles cover only 27% of travel per person each day (now estimated at approximately 37 million), which is the transport and average of two persons. This situation creates very slow traffic with an average velocity between 13 and 30 km/h and almost 80% of the air pollution in the area (1, 2). In the mid-1980s, air pollution monitoring revealed severe air quality problems with elevated levels as compared to international standards and a marked increasing trend for practically all contaminant concentrations. Thus, in 1990 the first comprehensive plan to fight air pollution was launched. Several strategies were closely linked to fuel improvements, including (i) stringent emission standards for both new and in-use vehicles; (ii) periodic inspection * Corresponding author e-mail:
[email protected]; fax: (+52)53689226. 10.1021/es991254r CCC: $19.00 Published on Web 05/23/2000
and maintenance of existing vehicles; (iii) fleet modernization and alternative fuel conversions; (iv) stage 0, stage 1, and stage 2 vapor recovery; and (v) restrictions on the use of vehicles (3, 4) among others. Despite the implementation of the emission reduction programs, ozone levels are still very high and often exceed acceptable levels by a factor of 2 or more. Further work has to be done to reduce emissions for virtually all mobile sources. To achieve this objective, the environmental authorities in agreement with the oil company (state-owned) has established a program to improve the air quality with several goals, one of them being to decrease exhaust emissions per kilometer. Fuel modifications, which include changing composition and physical properties, provide a particularly effective way to reduce emissions. This is in contrast to changes in technology that must be phased in over long periods and are subject to deterioration in use. The potential for reformulating gasoline to reduce pollutant emissions has attracted considerable attention worldwide and is the subject of major cooperative research programs between the oil and auto industries (5, 6). However, there is great concern regarding the impact of the reformulated gasoline on the actual and future fleet of vehicles of the MAMC because the optimal fuel could very well be different from that used in other countries. In this work, experimental results on the behavior of representative vehicles of the present vehicular fleet in the MAMC are presented. The potential reactivity of the fuels to generate urban ozone was estimated and correlated with the technology of the vehicles. It is well-known that a small fraction of vehicles make a large contribution to the emissions inventory. Following the results reported by Wenzel and Ross (7) on the investigations of high emitters, the emphasis of this study has been on carburated vehicles and early model fuel-injected vehicles. In this study, we focus in that particular group of vehicles because they represent the major percentage in the MAMC and in many other developing countries. For comparison purposes, we included also an analysis of newer model years sold in Mexico City.
2000 American Chemical Society
Our facilities were audited by the National Environmental Institute (INE), a Federal regulation agency, prior to the vehicle evaluations. The audit reviewed facilities, procedures, and standards used for certification of all new vehicles sold in the country. The vehicle tests were carried out in duplicate, and a third test sequence was conducted if back-to-back FTP emissions exceeded specified limits (8%). The Federal Test Procedure of the United States, FTP-75 (8), was used throughout all our emission tests (8). For the tests, a Clayton model ECE-50-250 chassis dynamometer with a direct-drive variable-inertia flywheel system was used. The laboratory is equipped with a Venturi constant volume sampler (CVS) and a Horiba drivers aid. The bagged emissions for each phase of the Urban Dynamometer Driving Schedule were measured at the CO, CO2, THC, and NOx analyzers bench, as described elsewhere (8). Three samples of dilute exhaust gas were collected during the FTP corresponding to the cold transient (bag 1) phase, the hot stabilized (bag 2) phase, and the hot transient (bag 3) phase. Quantitative hydrocarbons characterization analysis was performed in three Varian model 3400 gas chromatographs equipped with flame ionization detectors. Tedlar bags containing sample exhaust gases collected during the FTP test for each of the three phases were transferred to the VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2663
FIGURE 1. CO emissions in the FTP-75 test for different vehicles technologies.
TABLE 1. Characteristics of the Vehicle Fleets model year (MY)
no. of vehicles
1984-1990
9
1991-1996
10
1997a
20
1999a
9
emission control technology type carburated with electronic ignition (precatalyst) fuel injection with electronic ignition (with catalytic converterb) adaptive learning for closed loop with TWC adaptive learning for closed loop with TWC
a These vehicle fleets were all brand new. b 1991-1992 vehicles with oxidative catalyst and 1993-1996 with three-way catalytic converter (TWC).
chemical speciation laboratory and analyzed using the three chromatographs. Data collection and peak integration were performed on a PE Nelson model 900 station coupled with a computer program to obtain the mass in milligrams per kilometer for exhaust emissions. The vehicles, whose main characteristics are shown in Table 1, covered a range of model years (MY) from 1984 through 1999. This range was chosen because it represents the most abundant emission control technology found in the MCMA vehicle fleet. Each fleet had similar odometer readings at the start of testing, except the 1997 and 1999 fleets that were all new vehicles. 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. Vehicles ran 1000 km with one gasoline, and then the FTP test was performed. A vehicle fleet before MY 1990 was tested with the regular fuel and a leaded gasoline available in the past for cars without emission control systems. Table 2 shows some relevant physical and chemical properties of the fuels used throughout the study.
Results and Discussion Figures 1-3 show exhaust mass emission rates for carbon monoxide (CO), nitrogen oxides (NOx), and total hydrocarbons (THC) as a function of vehicle technology and the type of gasoline used (Appendix 1 in the Supporting Information shows the vehicle type, MY, and emissions numerical data from which Figures 1-3 were plotted). As a reference, 2664
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 13, 2000
TABLE 2. Fuel Property Data property
leaded
regular
premium
gravity 20/4 °C Reid vapor pressure, psi RON MON (RON + MON)/2 ASTM distillation D-86 (°C) 10% evaporated 50% evaporated 90% evaporated end point sulfur, ppmw lead, g/gal aromatics, vol % olefins, vol % benzene, vol % MTBE, vol %
0.7329 7.5 81
0.7295 6.8 92.7 84.4 88.6
0.7382 7.05 95.8 88.6 92.2
56.6 101.1 167.8 211.6 814 0.11 27.2 7.4 0.92 4.44
55 93.9 163.8 203.7 421 24.6 6.2 0.86 6.79
63.3 108.5 172.8 208.9 382 24.85 4.86 0.5 4.72
Mexican emission standards for certification of vehicles are 2.11 g/km for CO, 0.25 g/km for THC, and 0.62 g/km for NOx. The results suggest some interesting relationships between fuel composition and vehicle hardware that need to be taken into account from the viewpoint of the existing fleet in the MCMA. The MY 1984-1990 fleet represents 38% of all vehicles in the MCMA and can be classified as high emitters. The net gain in pollutant reduction changing from leaded gasoline to regular is perhaps the large reduction in lead emission. Also there is a significant decrease in ozone-forming volatile organic compound emission (VOCs) emissions due to reductions in gasoline vapor pressure and in the olefin content of gasoline. Clearly, a considerable decrease in regulated emissions occurred when changing the fuel formulation and emission control systems in the vehicular fleet of the MCMA. Important reductions were observed for the three pollutants, but a more careful analysis of the results must be made, especially to compare the newest vehicles with others sold in neighboring countries. In Table 3, a comparison of our results using regular gasoline with three U.S. studies reported elsewhere (7), used to define the cutpoints for low emitter vehicles, is shown. In the analysis, three sets of measurements of bag 3 emissions from properly functioning cars at 80 000 km were obtained. From Table 3, the National Cooperative Highway Research Program (NCHRP) results were for MY 1990-1993 determined in
FIGURE 2. THC emissions in the FTP-75 test for different vehicles technologies.
FIGURE 3. NOx emissions in the FTP-75 test for different vehicles technologies. 1996-1997 (9). The FTP Revision Project measurements were carried out on new MY 1991-1994 vehicles with 80 000 km laboratory-aged catalyst (10). The American Automobile Manufacturers data refers to MY 1991-1992 measured in 1995-1996 (11). Exhaust emissions in the NCHRP project (7) defined low and high cutpoints values (g/km) for low emitting vehicles. The high and (low) cutpoint values were 3.75 (1.88), 0.31 (0.12), and 0.63 (0.25) for CO, THC, and NOx, respectively. The 1991-1996 fleet showed values similar to those of the high cutpoint level, except for NOx emissions, which were even larger than the high cutpoint value, i.e., the fleet can be classified as high emitters. The results of the 1997 and 1999 fleet are similar but still higher in NOx than the much older American vehicles with 80 000 km odometer reading.
TABLE 3. Comparison between Emissions from FTP Bag 3 in Mexican and American Vehicles FTP cycle hot transient (bag 3) data set
MY
CO (g/km)
THC (g/km)
NOx (g/km)
NCHRP FTP-RP AAMA in use
1990-93 1991-94 1991-92 1991-96 1997 1999
1.69 0.94 1.56 3.18 1.16 1.00
0.14 0.10 0.13 0.23 0.08 0.06
0.22 0.21 0.14 0.85 0.36 0.34
this study
Improper signal from the oxygen sensor or improper functioning of the electronic engine control or transiently VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2665
FIGURE 4. Comparison of THC emissions (NMOG) and ozone-forming potential (COPK) between American and Mexican vehicles.
FIGURE 5. Specific reactivity of reformulated gasoline in the United States and Mexico for different vehicle technologies.
TABLE 4. Comparison of Vehicular Emission Regulations in Mexico and the United States in 1999 regulated compounds (g/km) country
regulation code
THC
NMOG
CO
NOx
Mexico U.S.
NOM-042 (tier 0) tier 1
0.25 0.25
0.156
2.11 2.11
0.62 0.25
bad catalyst performance could be possible physical failure mechanisms leading to that type of emissions. A comparison of present U.S. and Mexican environmental regulations is shown in Table 4. It is clear that Mexican regulations are still behind the U.S. ones, although the companies that produce and import vehicles in Mexico and the United States are mostly the same. Two aspects are worth mentioning when comparing U.S. and Mexican vehicular 2666
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 13, 2000
emissions regulations. In Mexico, a minimum mileage guarantee within which new cars can meet emissions regulations is not demanded (80 000-120 000 mi are mandatory in the United States). Until 1999, the Mexican regulation allowed 2.5 times more NOx emissions (see Table 4) than that in the United States. From the year 2001, tier 1 regulations will be enforced in Mexico, representing a new challenge for the vehicles industry. Carter has published factors useful to estimate the ozoneforming reactivity of a particular hydrocarbon (12). Using this approach, a maximum incremental reactivity (CMIR) value can be assigned to individual exhaust constituents. CMIR represents the predicted impact of the respective constituent on urban atmospheric ozone formation, expressed as grams of ozone per gram of the constituent. A comparison of the ozone-forming potential on a gram per kilometer (COPK) or mile basis for each fuel is achieved by
multiplying the CMIR for each hydrocarbon by the emission rate of that hydrocarbon. The summation of these products yields the ozone-forming potential for a specific fuel-vehicle. A comparison of ozone-forming potential and specific reactivity (SR) using regular gasoline in American (19841990) and Mexican vehicles (1991-1999) is shown in Figure 4. The U.S. data come from three sets of studies performed by Chevron, Arco, and the Society of Automotive Engineers using reformulated gasoline (13-15). In these studies and ours, we correlate results according to the type of emission control that vehicles had using the mass of the exhausted non-methane organic gases (NMOG) and their ozoneforming potential (COPM) both in grams per mile. The results of this comparison suggest that the COPM in the Mexican vehicles with adaptive learning have similar behavior to that of American vehicles that are 7-9 times older. Figure 4 also indicates that there is a decrease in NMOG in the Mexican newest fleet but that exhaust emission is much more reactive for ozone formation. To easily understand the emissions data, SR is used similarly to integrate changes in complex species profiles and serves to compare potential ozone-forming effects of emissions from different fuels and vehicle technology on a unit mass basis. It is expressed as potential ozone formed per gram of NMOG. Both reactivity-weighted emissions and SR are to be interpreted in a relative sense. They are not meant to predict actual levels of ozone formation. Such predictions require detailed atmospheric photochemical models, which take into account all other reactive emissions in the atmosphere. The California Air Resources Board has determined that for a representative group of vehicles that meet Transition Low Emission Values, the average potential gram of ozone per gram of NMOG is 3.42 (16). Moreover, Figure 5 shows a comparison of the SR for the American and Mexican vehicle fleets. As mentioned before, in this manner it is possible to analyze exclusively the reactivity of emissions associated to the vehicle/gasoline system. In that sense, one must say that fuel reformulation in the MAMC has decreased the reactivities of emissions, and the most advanced vehicles from the viewpoint of control emissions (the 1997-1999 fleet) give an average of 3.1 g of O3/g of NMOG. Accordingly, Siegl et al. (17) reported SR ) 3.57 for a MY 1990 vehicle fleet with accumulated mileage of 3920 km (6273 mi) at the start of the testing. The obtained value indicates an important technological gap in emission control between the same MY vehicles in the United States with those sold in Mexico. While emission standards for new vehicles have resulted in significant emission reductions, there is a need to bring these standards up to date to attain and maintain air quality goals in the most affected areas. The result would be the introduction of lower emission vehicles with state-of-the-art on-board diagnostics, which is needed to assist drivers and maintain vehicles properly. There are neither durability standards for new vehicle emission systems nor a requirement for a
durability guarantee. Thus, it is the owner’s responsibility to make corrections on the vehicle should the vehicle fail to pass the periodic I/M test, even if the malfunction was due to a manufacturing defect.
Supporting Information Available One table showing the average FTP-75 emissions data (1 page). This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Faiz, A.; Weaver, C. S.; Walsh, M Air pollution from motor vehicles. standards and technologies for controlling emissions; World Bank: Washington, DC, 1996. (2) Margulis, S. Report of the World Bank on Back of the Envelope Estimates of Environmental Damage Costs in Mexico; WP S824; World Bank: Washington, DC, 1992. (3) MAQRI (Mexico Air Quality Research Initiative). Report IMP/ LA-12699; Mexican Petroleum Institute and Los Alamos National Laboratory, IMP: Mexico, 1994. (4) Eximbank-World Bank Report on the Environmental Impact Study of Pemex Gasoline Project; Briefing to the Japan Eximbank/ World Bank Mission, Mexico, 1997. (5) AQIRP (Air Quality Improvement Research Program). Mass Exhaust Emissions of Toxic Air Pollutants Using Reformulated Gasolines; Technical Bulletin 4; Auto/Oil Coordinating Research Council Inc.: Atlanta, GA, 1991. (6) AQIRP (Air Quality Improvement Research Program). Report on the Auto/Oil Quality Improvement Research Program Phase I. Final Report; Auto /Oil Coordinating Research Council Inc.: Atlanta, GA, 1993. (7) Wenzel, T.; Ross, M. SAE Tech. Pap. Ser. 1998, No. 981414. (8) Control of air pollution from new motor vehicles and new motor vehicle engines: Certification and Test Procedures; 40 CFR 86; U.S. Government Printing Office: Washington, DC, July 1, 1990 (revised). (9) Feng, A.; Barth, M.; Norbeck, J.; Ross, M. Transp. Res. Rec. 1997, No. 1587, 52. (10) Haskew, H.; Cullen, K.; Liberty, T. F.; Langhorst, W. SAE Tech. Pap. Ser. 1994, No. 94CO16. (11) Haskew, H.; Berens, D.; Orteca, R. Proceedings of the 7th CRC On-Road Vehicle Emissions Workshop; Coordinating Research Council: Atlanta, GA, 1997. (12) Carter, W. P. J. Air Waste Manage. Assoc. 1994, 44, 881. (13) Schoonveld, G.; Marshall, W. F. SAE Tech. Pap. Ser. 1991, No. 9103380. (14) Boekhaus, K.; Cohu, L. K.; Rapp, L. A.; Segal, J. S. Report on clean fuel. No. 91-02; Arco Products Company: 1991. (15) Hoekman, S. K. Environ. Sci. Technol. 1992, 26, 19. (16) Report on the initial statement of proposed rulemaking reactivity adjustment factors for transitional low-emission vehicles and staffs suggested changes to the proposal. California Air Resources Board: Sacramento, CA, 1991. (17) Siegl, W. O.; Korniski, J.; Richert, J. F.; Chladek, E.; Weir, J. E.; Jensen, T. E. SAE Tech. Pap. Ser. 1996, No. 961904.
Received for review November 8, 1999. Revised manuscript received March 9, 2000. Accepted March 27, 2000. ES991254R
VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2667
