Mobile sources critical review: 1998 NARSTO assessment

Atmospheric Environment 34 (2000) 2161}2181

Mobile sources critical review: 1998 NARSTO assessment R.F. Sawyer!,*, R.A. Harley", S.H. Cadle#, J.M. Norbeck$, R. Slott%, H.A. Bravo& !Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720-1704, USA "Department of Civil and Environmental Engineering, University of California at Berkeley, Berkeley CA 97420-1710, USA #Health and Environment Department, General Motors R&D, MC480-106-269, 30500 Mound Road, Warren, MI 48090-9055, USA $University of California at Riverside, CE-CERT, Riverside, CA 92521, USA %Consultant, 71 Hawes Avenue, Hyannis, MA 02601, USA &Centro de Ciencias de la Atmosfera, Universidad Nacional Autonoma de Mexico, Seccion de Contaminacion Ambiental, Circuito Exterior, Ciudad Universitaria, Mexico, D.F. 04510, Mexico Received 7 August 1998; accepted 11 September 1999

Abstract Mobile sources of air pollutants encompass a range of vehicle, engine, and fuel combinations. They emit both of the photochemical ozone precursors, hydrocarbons and oxides of nitrogen. The most important source of hydrocarbons and oxides of nitrogen are light- and heavy-duty on-road vehicles and heavy-duty o!-road vehicles, utilizing spark and compression ignition engines burning gasoline and diesel respectively. Fuel consumption data provide a convenient starting point for assessing current and future emissions. Modern light-duty, gasoline vehicles when new have very low emissions. The in-use #eet, due largely to emissions from a small `high emittera fraction, has signi"cantly larger emissions. Hydrocarbons and carbon monoxide are higher than reported in current inventories. Other gasoline powered mobile sources (motorcycles, recreational vehicles, lawn, garden, and utility equipment, and light aircraft) have high emissions on a per quantity of fuel consumed basis, but their contribution to total emissions is small. Additional uncertainties in spatial and temporal distribution of emissions exist. Heavy-duty diesel vehicles are becoming the dominant mobile source of oxides of nitrogen. Oxides of nitrogen emissions may be greater than reported in current inventories, but the evidence for this is mixed. Oxides of nitrogen emissions on a fuel-consumed basis are much greater from diesel mobile sources than from gasoline mobile sources. This is largely the result of stringent control of gasoline vehicle emissions and a lesser (heavy-duty trucks) or no control (construction equipment, locomotives, ships) of heavy-duty mobile sources. The use of alternative fuels, natural gas, propane, alcohols, and oxygenates in motor vehicles is increasing but remains small. Vehicles utilizing these fuels can be but are not necessarily cleaner than their gasoline or diesel counterparts. Historical vehicle kilometers traveled growth rates of about 2% annually in both the United States and Canada will slow somewhat to about 1.5%. Mexican growth rates are expected to be greater. Fuel consumption growth in recent years of about 1.4% annually is projected to continue as slowing gains in fuel economy from #eet turnover are more than o!set by growth and the increasing number of Sport Utility Vehicles. This growth also will erode the emissions reductions resulting from cleaner new vehicles and fuels. Uncertainties in these projections are high and a!ected by economic activity, demographics, and the e!ectiveness of emissions control programs * especially those for reducing in-use emissions. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Motor vehicle emissions; Emissions inventory; Mobile sources; In-use emissions; Emissions uncertainties

* Corresponding author. E-mail addresses: [email protected] (R.F. Sawyer), [email protected] (R.A. Harley), steven.h.cadle@ notes.gmr.com (S.H. Cadle), joe}[email protected] (J.M. Norbeck), [email protected] (R. Slott), rtorres@servidor. unam.mx (H.A. Bravo).

1. Rationale and overview Mobile source emissions come from a range of vehicle, engine, and fuel combinations, listed in Table 1. Those most important to tropospheric ozone, as measured by

1352-2310/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 4 6 3 - X

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Table 1 Mobile source vehicles, engines, and fuels. (listed in approximate order of decreasing use; bold indicates the most important sources of HC and NO x Vehicles

Engines

Fuels

Light-duty on-road Heavy-duty on-road Heavy-duty o4-road Light-duty o!-road Aircraft Ships Locomotives

Spark ignition Compression ignition Gas turbine Electric Steam turbine (marine)

Gasoline Diesel Jet fuel Residual fuel oil Lique"ed petroleum gas Natural gas Electricity Alcohols

their hydrocarbon (HC) and nitrogen oxides (NO ) emisx sions, and of lesser importance, their carbon monoxide (CO) emissions, are shown in bold. This review emphasizes these most important categories with a briefer consideration of the other mobile sources. Historically, the dominant mobile source emissions have been those from on-road vehicles, powered with reciprocating internal combustion engines burning gasoline or diesel fuels. As these sources have come under control, the importance of largely uncontrolled emissions from o!-road motor vehicles, aircraft, ships, and locomotives has increased. 1.1. Fuel based emissions A common approach to estimating road vehicle emissions is to combine estimated average emissions per distance traveled, grouping classes of vehicles, with activity patterns of distance traveled, preferably with temporal and spatial resolution. The emissions estimates derive from direct measurements (light-duty vehicles on dynamometers), indirect measurements (heavy-duty engines on dynamometers combined with vehicle loads), tunnel measurements, and models which combine experimental measurement and physical approximations. Two widely used emissions models are USEPA's MOBILE and the CARB's MVEI. These models are reviewed in a later section. This approach can be extrapolated in some cases to estimate emissions from other transportation modes. Another approach, which is applicable across all modes, is a fuel based emissions model. Emissions are expressed as an `emissions indexa (g pollutant/kg fuel), emission rates determined by combining the emission index with the fuel consumption rate, and total emissions from the product of the emissions index and total fuel consumption. The fuel-based approach allows intermodal comparisons. It also provides some advantages in estimating road vehicle emissions, which are often approximately proportional to fuel consumption over a range of operating conditions. It also provides a means of approaching mobile source emissions in Mexico for

which the vehicle #eet and its emissions are not as well characterized as in the United States and Canada. Fuel consumption data are usually known with greater precision than vehicle usage. Of course, much of the same measured emissions data and many of the same assumptions are common to both approaches. 1.2. Transportation fuels Gasoline and diesel are the dominant on-road and o!-road fuels. Both have `reformulateda forms (RFG and RFD) which reduce emissions and are in use in regions of the United States having the most severe air pollution. Additional areas will use these fuels in the future and additional reformulation will occur with the introduction of the USEPA's Phase II RFG. While national and local policies encourage the use of alternative fuels (natural gas, liquid petroleum gas, methanol, ethanol, and even hydrogen and electricity), they are a minor factor in emissions. The largest use of `alternativea or non-petroleum-based fuels is in the form of oxygenates added to gasoline. Emissions associated with fuel processing appear in the stationary source inventory, but are related to transportation fuel usage. 1.3. Time frames Our review focuses on the current understanding of mobile source emissions estimation, especially the uncertainties. The year 2010 is a date of future interest. It is the current deadline for the most severe air pollution region (the Los Angeles Air Basin) to come into ozone compliance and it is the approximate time for meeting the proposed new ozone air quality standard. Both fuel usage and the nature of the vehicle #eet a!ect future emissions. For the past ten years, fuel consumption has been increasing by about 1.4% per year (EIA, 1994), caused by increasing vehicle kilometers traveled, the growth of the heavy-duty #eet, and a shift from personal passenger cars to personal light-duty trucks, vans, and sport utility

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vehicles. This increase in fuel consumption is expected to continue. In general, the vehicle #eet is getting cleaner as new, lower emitting vehicles with increased durability replace older vehicles. The change in mobile source emissions involves the trade-o! in the increase in fuel consumption and the decrease in the in-use emissions of the vehicle #eet. The uncertainties of the latter are the greater. Future emissions will depend primarily upon lifetime emissions from vehicles, which are now more strongly a!ected by deterioration, maintenance, and the e!ectiveness of inspection and repair programs than by new car emissions standards. Estimates of emissions from the both the current and future #eets contain large uncertainties. 1.4. Fuel consumption data The inventory of transportation fuel consumption (excluding that for pipelines) provides both a good starting point for mobile source emissions estimation, and a basis for extrapolation into the future. `Transportationa and `mobile sourcesa are largely, but not entirely the same. Pipelines are transportation but their associated emissions are assigned to stationary sources. Garden equipment, some o!-road recreational vehicles, railroad refrigeration engines, and other sources may not be considered transportation but generally are included in the mobile source inventory. Fig. 1 summarizes fuel consumption in the mobile source sector for North America, as of the mid-1990s. Data for Canada (StatCan, 1997) and the United States (ORNL, 1996) are readily available. Data on mobile source fuel consumption in Mexico (PetMex, 1997) are limited. Overall, the United States accounts for 87% of North American fuel consumption by mobile sources. Use of fuel by mobile sources is much less in Canada and Mexico: these countries account for 7 and 6%, respectively, of total North American mobile source sector fuel use. 1.5. Motor vehicle yeets The approximately 230 million road vehicles in North America are about 68% passenger cars, 26% light-duty trucks, and 6% heavy-duty trucks and buses, (Table 2). The average age of passenger cars in the United States is 8.6 yr, an increase of one year over the past 10 yr (AAMA, 1997). Thirteen percent of these passenger cars are more than 15 yr old. This aging of the #eet probably results from a combination of increased vehicle durability and increased new vehicle cost. The average truck is 8.3 yr with 19% more than 15 yr old. Compared with passenger cars, the truck #eet contains a higher fraction of both young (because of recent increase in sales of light-duty trucks) and old (because of the longer service life of commercial trucks) vehicles. Vehicle registration in the

Fig. 1. North American mobile source fuel consumption, 1995. Of total fuel consumption: Canada 7%, United States 87%, Mexico 6%.

United States over the past 10 yr has grown at an annual rate of about 2%, dominated by a 4.5% annual growth in trucks. Mexico vehicle registrations have increased at an annual rate of 5.5% over the past 10 yr (IMT, 1994; INEG, 1997). The United States #eet includes more than 350,000 alternative fueled vehicles (less than 0.2%), the majority of which operate on propane (EIA, 1996). The Mexican vehicle population is concentrated in highly populated and industrialized areas of Mexico. Mexico City, Monterey, Guadalajara, and the Northern Border of Mexico Zone contain nearly half of Mexico's vehicles.

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Table 2 North American Motor Vehicle Registrations! (millions) Canada

United States

Mexico

Total

Passenger cars Commercial vehicles Light-duty trucks Heavy-duty trucks and buses

13.2 3.5 na na

136.1 65.5 56.8 8.7

8.4 3.8 na na

157.7 72.8 na na

Total

16.7

201.6

12.2

230.5

!AAMA (1997).

2. Gasoline 2.1. Engines and mobile applications Gasoline is the dominant fuel of light-duty road vehicles, medium-duty road vehicles, recreational vehicles, garden equipment, and the general aviation #eet. This fuel}engine combination provides a high energy-density, low-cost engine that probably explains its dominance in these applications. 2.1.1. Light-duty vehicles Light-duty gasoline vehicles use several systems to control emissions. The positive crankcase ventilation (PCV) system was the "rst control placed on vehicles. It directs blowby gases back to the air inlet for reprocessing in the engine. The exhaust gas recirculation (EGR) system directs a small fraction of the exhaust back into the engine to dilute the intake charge and reduce peak combustion temperature, hence reducing the engine-out NO . Exhaust gases are treated by the three way catalyst x system. The term `three-waya refers to the catalyst's ability to oxidize CO and HC to CO while simulta2 neously reducing NO to N . Proper function requires x 2 that the engine operate at the stoichiometric air-to-fuel ratio, the point at which exactly enough air is ingested to oxidize all of the fuel. An oxygen sensor in the exhaust system continuously monitors the air-to-fuel ratio in the exhaust system. Both the catalyst and oxygen sensor must be hot to operate. Thus, they are not functional during cold start operation. Minimizing the resulting cold start emissions is a major focus of current control e!orts. The primary method has been the development of catalysts that can be mounted close to the engine. These catalysts must be able to withstand extreme temperature excursions. The introduction of computer-controlled fuel injection has greatly improved the ability to maintain stoichiometric operation. Fuel injection on most current production vehicles is done in the inlet air system, with the more sophisticated systems using multiple injectors timed to the cylinder intake cycle (sequential multi-port fuel

injection). Development e!orts are currently focusing on direct injection engines. The engine control module (an on-board computer) monitors and controls the engine operation. Current algorithms recognize when engine operating parameters must be changed to optimize emissions performance and store the new parameters for the future. This process is referred to as adaptive learning. New vehicles are now using the second-generation on-board diagnostics system (OBDII) to monitor emissions control system performance. A combination of sensors and algorithms are used by this system to monitor the performance of the engine, the catalyst, and the evaporative emissions control system. If a problem is detected the check engine light is turned on and an error code is stored for diagnostic repair purposes. For some emission system failures engine operation is changed to a `limp-homea operation mode that reduces emissions. The evaporative emission control system is designed to capture HC emissions that occur both during operation and while the vehicle is parked. HC vapors from the fuel are routed to a charcoal canister where they are stored while the vehicle is parked. The canister is rejuvenated via air purging during vehicle operation. Evaporative emissions also are caused by fuel leaks and permeation through hoses and plastic fuel tanks. Current e!orts are focused on improved materials and "ttings to ensure good long-term in-use performance. In addition, a recent regulation requiring that refueling vapors be captured on-board has resulted in the addition of the on-board refueling vapor recovery (ORVR) system as well. Finally, it is best to view the vehicles as a vehicle/fuel system, since the fuel impacts both the exhaust and evaporative emissions. Leaded fuel has been completely phased out in the US and Canada, and is being phased out in Mexico, since lead is toxic and a catalyst poison. More recently, seasonal fuel vapor pressure limits have been mandated. These signi"cantly lower evaporative emissions. Some areas with wintertime exceedances of the CO air quality standards require the addition of oxygenates to the fuel in the winter. This provides bene"ts for older vehicles by essentially &leaning' the mixture,

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which in addition to CO may reduce particulates. However, for modern vehicles, with full closed-loop control and adaptive learning, there is little or no bene"t } in terms of reduced emissions } arising from the addition of oxygenates. Finally, reformulated gasolines have been put into use in all of California and in severe ozone nonattainment areas. One of the features of the California reformulated gasoline is a mandated low sulfur content. Sulfur is a mild catalyst poison, whose e!ects tend to be reversible. A variety of alternative fuels (e.g., natural gas, 85% alcohol/15% gasoline blends, and propane) can be used in vehicles designed for their use. However, the lack of an infrastructure for these fuels together with cost considerations, and the improvements in the emission reductions from new vehicles and gasolines, have severely restricted interest in the use of these alternative fuels. 2.1.2. Medium- and heavy-duty vehicles Emission control technologies for medium- and heavy-duty applications are similar to those used on light-duty vehicles, but their introduction has lagged somewhat. PCV systems were introduced in 1968, evaporative canisters and EGR systems in 1985, and the widespread use of catalysts in 1987. Less stringent oxides of nitrogen standards, compared with light-duty vehicles, allow the continued use of oxidation only catalysts on many vehicles. 2.1.3. Other gasoline engine applications Other applications of gasoline engines include on-road motorcycles, recreational land vehicles, recreational watercraft, lawn, garden and utility equipment, and light aircraft and general aviation. Emissions per kilogram of fuel are high because these applications are either unregulated or only recently regulated. Because fuel consumption in these sectors is low, contributions to total emissions are small compared to on-road emissions from light-, medium-, and heavy-duty road vehicles but will be of increasing importance as the on road sources are controlled. 2.2. Fuels and fuel use 2.2.1. Gasoline Commercial gasoline is a complex mixture of many hydrocarbons and oxygen-containing compounds that are blended to provide combustion characteristics compatible with the engines in which the gasoline is burned. The most important characteristics a!ecting combustion are vapor pressure and octane number. These and other parameters, such as aromatic, ole"n, sulfur, and additive content and distillation temperatures a!ect emissions. In Canada and the United States nearly all gasoline consumed is unleaded, the exception being aviation gasoline. Unleaded, high-octane aviation gasolines are being studied but there is no schedule for phase-out of the

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current fuel. In Mexico, 93% of the gasoline sold is unleaded (IMP, 1997). 2.2.2. Reformulated gasoline The modi"cation of the composition of gasoline to reduce emissions has resulted in a series of `reformulateda gasolines. The addition of an oxygenate in winter reformulated gasoline reduced carbon monoxide levels. Lower vapor pressure, reduced aromatic and ole"n content, addition of oxygenate, reduced sulfur, and a narrowed distillation range (lower ¹ and ¹ and 90 50 higher ¹ ) provide emissions reductions (AOAQIRP, 10 1997). Such reformulated gasolines have been introduced in Canada, the United States, and Mexico as ozone control strategies. The e!ectiveness can vary depending upon local HC/NO ratios. Reformulated gasolines tend x to be more e!ective at reducing hydrocarbons than NO x (AOAQIRP, 1997). The most severely reformulated gasoline is California Phase II, which typically has very low sulfur (31 ppm), a low Reid vapor pressure (51 kPa), a narrow distillation range (¹ "513C, ¹ "1453C), 10 90 and reduced aromatics (25%), ole"ns (4%), and benzene (0.9%), and added MTBE (11%). 2.3. Emissions 2.3.1. Regulations and test methods 2.3.1.1. Light-duty vehicles. National emission standards for vehicles were "rst promulgated in the Clean Air Act published in the Federal Register in 1966. These standards applied to the 1968 model year and were similar to emission standards set in California in 1960. However, the 1970 amendments to the Clean Air Act established the initial comprehensive motor vehicle emissions standards that required a 90% reduction of tailpipe emissions from the then uncontrolled vehicles. These standards applied to 1975 vehicles and initially covered only carbon monoxide and hydrocarbon emissions. This was later expanded in 1977 to include oxides of nitrogen and particulate matter that allowed for diesel vehicles to be included in the regulations also. These Federal standards applied to the 49 States excluding California. California was given the authority to establish their own more stringent standards because of the severity of the air pollution in the Los Angeles area. This situation still exists today although the Federal and California standards and the associated emission control technology have merged including several states adopting the low emission vehicle standards recently established in California. Tailpipe emissions. The evolution of the United States light-duty vehicle emission standards is shown in Fig. 2. The regulations are given in terms of mass of pollutant/distance (g mi~1) and are the accumulated emissions over the Urban Dynamometer Driving Schedule

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Fig. 2. United States light-duty vehicle emissions standards. Increase in NO in 1968 was the result of CO and HC control technology; x NO was "rst controlled in 1976. Crankcase emissions were voluntarily controlled in 1960s. Standards in 2004 are projected default x standards of the Clean Air Act Amendments of 1990.

Fig. 3. Federal test procedure (FTP) driving cycle. Test is performed on a chassis dynamometer.

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(UDDS) or Federal Test Procedure (FTP) after 80,000 or 160,000 km. Oxides of nitrogen were "rst controlled in 1976; the increase in NO emissions in 1968 was the x result of modi"cations to control CO and HC. Crankcase emission control were "rst applied in the 1960s but only o$cially regulated in 1988. The FTP driving cycle is shown in Fig. 3. This cycle consists of three phases with a total cycle time of 1877 s and an additional 600 s hot soak time. The "rst cycle is 505 s, which includes a cold engine start after soaking the vehicle overnight at an ambient temperature of 183C. The second cycle is 867 s of stabilized driving. Following the 10-min soak with the engine o!, another 505 s transitory phase is driven which includes a hot start. The emissions of each phase are weighted di!erently (0.43 for the cold phase; 1.00 for the stabilized phase; and 0.57 for the warm phase) to account for driving and trip characteristics. Light-duty trucks. In the United States separate emission standards for light-duty trucks (less than 3860 kg GVW) were promulgated in 1975 and, similarly to passenger vehicles, have been continually revised downward. Nearly identical standards were implemented for both Federal and California trucks in 1993}1994. In addition, California has established low emitting truck standards that are to be phased in between 1994 and 2003. There are now three separate truck standards based on vehicle weight. The test methods used for passenger vehicles are also used for the three truck classes. Mexico and Canada also use the US FTP for their emissions certi"cation. Canadian emission standards for light-duty vehicle were less stringent than US standards from 1975 through 1987 but in 1988 became similar to those in the US. Mexico implemented 1981 US Federal emission standards in 1993 and is moving toward standards similar to US Tier I. Evaporative emissions. In addition to tailpipe emissions, the US has established emission standards for evaporative emissions. Evaporative emissions are divided into "ve types: f Diurnal, which are the emissions when the vehicle is at rest which occur due to ambient temperature changes over a typical 24-h period (the portion of emissions at rest driven by the impact of temperature on the vapor pressure above the fuel). f Hot soak, which is driven by residual engine heat once a warmed-up vehicle is parked and the engine is shut o!. f Running loss, which occur when the vehicle is being driven. f Resting loss (the constant at rest evaporative emissions). f Refueling loss (displaced vapors and drippage resulting from refueling). The sealed housing evaporative determination (SHED) method is used throughout North America to measure

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evaporative emissions. The vehicle resides in the SHED during the tests. Emitted hydrocarbons are measured to determine the evaporative emission rate. Procedures for measuring evaporative emissions were revised by USEPA in 1993 and consist of vehicle preconditioning followed by exhaust emission testing, a hot soak test, a running loss test, and a diurnal emissions test. Earlier test methods, which were used to generate much of the data used in emissions inventories, have been recognized as inadequate. A full test now takes "ve days and involves higher temperatures and greater temperature changes than the original procedure. The CARB has developed a similar test, as well as an alternative procedure. 2.3.1.2. Medium- and heavy-duty vehicles. Unlike passenger vehicles and light trucks which are certi"ed and tested as vehicle families, heavy-duty diesel engines are certi"ed and tested on an engine dynamometer, over a transient test procedure, which includes both cold start and warm start operation. The transient test procedure was developed from engine data collected in New York and Los Angeles, during freeway and non-freeway operation. The transient test procedure consists of a prescribed engine speed and load (rpm and torque) schedule, which is speci"c to each engine, and is developed from the maximum torque curve (or maximum torque versus engine speed) for the engine. Cold start and warm start operation are weighted 1/7, and 6/7, respectively. There is a 20-min soak period between cold and warm operation. Emissions data and engine load data (in brake horsepower) are collected over the test procedure, and emission rates of heavy-duty engines are generally reported in g (bhp h)~1. 2.3.1.3. Other gasoline engine emissions regulations. Onroad motorcycles over 50 cm3 displacement were "rst regulated in 1978 by both the USEPA and CARB. Only the CARB regulates emissions from o!-road motorcycles. Emissions controls on some recreational vehicles were "rst implemented in 1996 year in California. The USEPA recently established emissions regulations for some recreational watercraft that will "rst apply in 1998. Both the USEPA and CARB have implemented emissions standards for lawn, garden, and utility equipment under 25 horsepower. 2.3.1.4. Real-world vehicle emissions. Real-world vehicle emissions are di$cult to predict because many factors are involved. When vehicles are new, emissions per kilometer are less than or equal to Federal or California requirements. However, as vehicles age and accumulate mileage, deterioration of fuel system and emission control components can result in much higher emission rates. In some high emitting vehicles HC and CO emissions per kilometer are over two orders of magnitude higher than when those same vehicles were new. These

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high emitting vehicles have been found to have broken fuel system or emission control components which, when repaired, can return the vehicle to near normal emissions levels (Stephens and Cadle, 1991). In the future the amount of time that well-maintained vehicles give high emissions will be greatly reduced. New catalyst technologies are being developed to reduce cold start emissions. USEPA will restrict the amount of fuel rich operation a vehicle can experience with the introduction of a new high power test cycle. In addition, evaporative systems will become much more robust, also in response to a more stringent test procedure. The rate at which high exhaust emitting vehicles occur is decreasing. Signi"cant changes have occurred in vehicle emission control technology since the early 1980s and these changes have reduced the rate of vehicle deterioration with age and mileage accumulation (MSTAC, 1997). The technologies include port fuel injection, more durable catalysts, and more precise fuel controls. It is anticipated that On Board Diagnostic technology (OBD II) on all 1996 and later vehicles will further reduce vehicle emissions deterioration. A series of real world evaporative emissions studies have been completed recently. SHED tests have been made on randomly selected vehicles to determine hot soak emissions (Brooks et al., 1995), diurnal emissions (Haskew, 1997a), and running loss emissions (Haskew, 1997b). Vehicles selected for these tests were solicited so that the acceptance rates were very high to minimize potential selection bias. No repairs or adjustments were made to the evaporative emissions control systems prior to the tests. Many di!erent component failures were observed. Emissions were normally distributed among vehicles except for a few vehicles that had very high emissions, primarily due to fuel leaks. Pierson et al. (1997) have provided a thorough review of evaporative emissions studies. The distribution of in-use emissions among vehicles for each of exhaust HC, CO and NO , and diurnal, hot soak, x running loss HC have been found to be highly skewed. A small number of gross-polluting vehicles, about 10% of the vehicle #eet, are responsible for half of the total emissions for CO, HC, or NO . There is signi"cant x overlap in the subsets of the vehicle #eet that are highemitters of CO and HC. The NO high-emitters comx prise a di!erent, mostly disjoint set of vehicles from the CO and HC gross-polluters. This skewness in the distribution means that a large number of vehicles needs to be tested to ensure that the high emitters are present in statistically signi"cant numbers. The vehicle selection process for testing must be random to avoid a selection bias that would omit high emitting vehicles from the tested sample of the population. A number of techniques have been used in attempting to measure emissions from large numbers of real world vehicles. These include remote sensing of tailpipe ex-

haust, analysis of IM240 results from state inspection/ maintenance programs, random roadside pullover tampering studies, tunnel studies, and ambient speciated hydrocarbon measurements. Remote sensing, and IM240 measurements give information on individual vehicle tailpipe emissions while tunnel tests and analyses based on ambient measurements can give information about both exhaust and evaporative emissions but only on #eets of vehicles. Tunnel and ambient air studies are reviewed in later sections of this paper. Remote sensing of thousands of vehicles at sites where the catalyst would be working if operational (i.e., under light acceleration in a hot stabilized mode) can be used to characterize tailpipe emissions in vehicle #eets measured at the site. Since vehicle license plates can be identi"ed simultaneously, the dependence of tailpipe emissions on age and other vehicle characteristics can be determined. Emissions measured by remote sensing in Denver, Colorado had a vehicle age dependence essentially the same as that observed by IM240 measurements of a similar #eet in the same area. Since the vehicles measured by remote sensing were identi"ed individually, and each could be classi"ed as to whether it had taken a new I/M test, the measurements were used to estimate the e!ectiveness of the new I/M control strategy program (Stedman et al., 1997). Observed emission reductions were less than predicted. The use of IM240 testing in state inspection/maintenance programs o!ers an excellent opportunity to monitor emissions from millions of real world vehicles. Current testing protocols allow for a `quick passa of clean vehicles that limits the comparability of data. Variable waiting times before testing causes some vehicles to have arti"cially high emissions. A few states are collecting data on a random multi-thousand sample of vehicles measured on a full IM240 test. These data would be improved if measurements were made after the vehicles were appropriately pre-conditioned (MSTAC, 1997). Tunnel tests have the advantage that the emissions measured are coming from vehicles passing through the tunnel. The hydrocarbon species present are not subject to photochemical degradation. The hydrocarbon speciation can be used to separately estimate the amount of tailpipe and running loss emissions. Since tailpipe, vapor loss, and liquid loss hydrocarbons contain common species, assignment of the correct source can be di$cult. Ambient measurements of hydrocarbons have been used to estimate the percent contribution of hydrocarbons from mobile sources. Key hydrocarbon species are used to construct source pro"les for di!erent emission sources. Location of the samplers can be used to further clarify emission sources. Uncertainties in these estimates are associated with location of the sampling instruments, comprehensiveness of the source pro"les, accuracy of the source pro"les, and reactivity of some hydrocarbon species.

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2.3.1.5. Inspection and maintenance. An e!ective inspection and maintenance program can reduce in-use emissions, especially those from the high emitter fraction of the #eet. These programs are really maintenance, inspection, and repair with inspection serving three purposes, (1) promotion of better maintenance, (2) identi"cation of high emissions and their cause leading to repair, scrappage, or sale out of the area, and (3) veri"cation of the e!ectiveness of repair. The e!ectiveness of many current programs is in question and the resulting emissions reductions uncertain (Pierson, 1996a). E!ective inspection and maintenance programs, however, are an essential part of dealing with in-use emissions from the total motor vehicle #eet, especially, but not exclusively, the oldest vehicles. Both the regulators and industry anticipate that on-board diagnostics (OBD) will improve inspection and maintenance e!ectiveness. In the absence of an e!ective program, #eet in-use emissions will remain a large multiple of new car emission standards. This area is critical to limiting emissions from the in-use highway #eet, both light and heavy duty. The assessment of the current or future e!ectiveness of inspection and maintenance programs and, therefore, the reliable projection of future emissions, is di$cult. An important secondary bene"t from inspection and maintenance programs is the collection of emissions data for millions of vehicles representative of the in-use #eet. These data, especially those coming from programs that measure emissions in gm/km, can reveal the distribution of emissions among vehicles by age, model, and technology. In future years these data will provide a record of reductions in emissions associated not only with the inspection and maintenance program but also with the introduction of both new technology and fuels.

3. Diesel 3.1. Engines, mobile applications, and emissions regulations 3.1.1. Light-duty vehicles Light-duty diesel engines have seen very limited use in North America. Their primary advantage is better fuel economy than gasoline engines. However, this is not enough of an incentive to overcome the higher cost of the engines and a reputation for odor, noise, poor acceleration, and hard starting. The modern diesel engine is now a high-speed, direct injection engine that can be turbocharged. Meeting the NO and PM standards is likely to x require improved exhaust aftertreatment. Light-duty diesel vehicles must meet emissions standards that generally are the same as those for light-duty gasoline vehicles. For some periods the diesels had less stringent NO emission standards. Diesel vehicles are x required to meet particulate matter (PM) standards

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whereas gasoline vehicles are generally exempt (because they have inherently low levels of PM emissions). 3.1.2. Medium and heavy-duty vehicles The challenge for heavy-duty diesel engines is also NO and PM control. Current engines have improved x combustion chamber design, operate at lower engine speeds, use high injection pressures, use injection rate shaping, electronic control for improved timing, and use turbochargers with waste gates. Oxidation catalysts are used on many vehicles for control of the organic carbon fraction of the particulate. Neither EGR nor particulate traps are currently used although EGR is viewed as the most promising technology for meeting future stringent NO emission requirements. More strinx gent standards will require continued improvements. As with the light-duty diesel, new aftertreatment systems likely will be required. Alternative fuels are seeing greater use in heavy-duty engines than light-duty, since some vehicle #eets can be refueled at a central location. 3.1.3. Ow-road vehicles and equipment Both direct and indirect injection diesel engines are used in o!-road vehicles and equipment. Over the past 20 yr, direct injection engines have been gradually replacing indirect injection engines. At the present time, indirect injection engines power only about 2% of equipment sales (USEPA, 1994). Use of turbochargers has also increased over time and currently represents about 35% of sales. Currently, about half of turbocharged engines sold are also equipped with water-jacket after-coolers. Air-to-air aftercoolers are limited to very high-power output applications. Both CARB and USEPA have implemented emission standards for engines used to power new o!-road vehicles and equipment that are rated at or above 37.3 kW (USFR, 1994). The onset of Tier 1 USEPA and CARB regulations in 1996}2000 will have a relatively minor impact on diesel engine technology. Injection timing on most engines will be retarded to reduce NO emissions. x Also manufacturers may have to modify injector nozzle design, increase fuel injection pressure or turbocharger boost, add turbochargers and aftercoolers to a small fraction of sales, and add waste gates and smoke limiting devices to most turbocharged engines. The onset of the Tier 2 CARB NO and PM standards x in 1999 and beyond are expected to lead to even more use of the techniques just mentioned above with regard to the Tier 1. Implementation of the proposed USEPA Tier 2 standards in 2001}2006 (depending on engine power rating) are expected to cause an increase in the use of electronic engine controls and high-pressure injection systems. To harmonize o!-road engine regulations in the United States it is likely that CARB would rescind its Tier 2 standards if USEPA follows through with the promulgation of the national Tier 2 standards.

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Finally, the implementation of the proposed USEPA Tier 3 standards in 2006}2008 are expected to lead to the widespread use of electronic engine controls and high pressure injection systems, as well as the use of exhaust gas recirculation. Canada currently has no requirements or emission standards for o!-highway gasoline or diesel equipment. The government is, however, redrafting existing legislation to include the authority for regulating heavy-duty o!-road equipment (primarily agriculture and construction equipment), utility engines (used in hand-held and non-hand-held equipment), and the recreational marine category (EnvCan, 1997). Mexico has yet to regulate o!-road mobile sources. 3.1.4. Military Military equipment is often powered by diesel engines or distillate-fueled turbines due to the low #ammability potential of these fuels relative to gasoline. Diesel engines used in combat or tactical vehicles and equipment are designed primarily for performance, reliability and the ability to use fuel of sub-standard quality. These vehicles and equipment have been exempted from both California and Federal emission regulation. Thus, their engines incorporate little if any features designed to control emissions. Non-combat military vehicles and equipment are subject to Federal emission standards, unless they receive a national security exemption. The engines powering non-combat vehicles and engines would generally be the same as those used to power other on- and o!-road diesel vehicles. 3.1.5. Locomotives Locomotive diesel engines have long lifetimes (40 yr or more), have been unregulated, and are very high emitters of NO . Locomotives account for about 10% of mobile x source NO (EPA, 1997a). Locomotives in line haul x service emit about 80 g kg~1 fuel; those in switching service, about 110 g kg~1 (EPA, 1997b). Initial Tier 0 emission standards apply at remanufacturing to engines built through 2001 and require a 20}25% reduction in NO from average uncontrolled levels. Tier 2 reducx tions of about 55}60% apply to locomotive manufactured or remanufactured in 2005 and later. 3.1.6. Ships Diesel-powered ships often can use either diesel or bunker fuels, in some cases switching to cleaner diesel fuel for in-port operations. NO emissions levels are x high, about 70 g kg~1 of bunker fuel and 50 g kg~1 of diesel fuel (BAQMD, 1997). There are no emissions regulations on ships. Bunker fuels contain high sulfur levels, making ships the only important mobile source of SO 2 emissions. Ship emissions directly impact urban areas with ports. Their o!shore emissions can impact upwind boundary conditions.

3.2. Fuels and fuel use Diesel fuel has low volatility and thus evaporative emissions are not considered a problem. Thus, there is no evaporative control system. Diesel fuels have also been reformulated. Currently, federal on-road diesel fuel can not exceed 0.05 wt% sulfur. In addition, diesel ignition fuel quality is controlled either directly via 40 minimum cetane number or indirectly by requiring a maximum of 35% volume aromatics. California has a separate regulation. It also requires a maximum of 0.05% weight sulfur, but, in addition, imposes a maximum of 10% volume of aromatics or certi"cation via an engine test to demonstrate that an alternate formulation has equivalent of better emissions performance than a 10% maximum aromatics certi"cation fuel. Nationwide, typical on-road diesel fuels now have approximately the same sulfur content as the average gasoline pool in the US. The sulfur reductions are especially bene"cial for engine out particulate emissions by mitigating against excessive generation of sulfate particulate over oxidation catalysts. The European Auto/Oil study (EPEFE) has demonstrated the e!ects of diesel fuel reformulations for both light-duty and heavy-duty diesel engines (Hublin et al., 1996; Signer et al., 1996). 3.2.1. In-use emissions In-use heavy-duty vehicle emissions have been measured in tunnel studies. As of 1995, typical NO emission x factors for the in-use heavy-duty diesel #eet ranged from 30 to 50 gm per kg of fuel burned. When normalized to work output instead of fuel input, this emissions factor corresponds to the uncontrolled emission level for diesel engines of about 10 gm NO per brake horsepower-hour. x Concerns exist about the e!ectiveness of post-1988 exhaust emissions standards for heavy-duty engines because some engine manufacturers may be optimizing for low NO emissions on the transient engine dynamometer x certi"cation test, and re-optimizing for maximum fuel economy under in-use driving conditions (McCracken, 1998). Heavy-duty emissions factors are signi"cantly higher than light-duty vehicle values of 5 to 15 gm NO per kg x of fuel. Use of control devices such as three-way catalytic converters have lowered light-duty vehicle NO emisx sions, whereas only oxidation mode catalysts have been applied to heavy-duty diesel engines to control hydrocarbon emissions and odor problems. Application of threeway catalysts to heavy-duty diesel engines has not been possible to date because the excess air used during diesel fuel combustion prevents reduction of NO in the exhaust. x 4. Alternative fuels During the last two decades there has been a considerable e!ort in the United States to develop alternatives to

R.F. Sawyer et al. / Atmospheric Environment 34 (2000) 2161}2181

the use of gasoline and conventional diesel fuel for transportation. The primary motives for this e!ort have been two-fold: energy security and improvement in air quality, most notably ozone. The anticipated improvement in air quality is associated with a decrease in the atmospheric reactivity, and sometimes a decrease in the mass emission rate of the organic gas emissions from vehicles using alternative fuels when compared to conventional transportation fuels. The most common alternative transportation fuels are methanol, ethanol, compressed natural gas (CNG), lique"ed petroleum gas (LPG), reformulated gasoline, and electricity. There are a limited number of alternative fuels that can be used in compression ignition (diesel) engines (both light and heavy duty). Vegetable oil esters can be used either neat or blended with diesel fuel in essentially unmodi"ed engines though care is needed to assure compatibility with sealing materials and lubricants. However, the emissions bene"ts are at best limited and selective. Dimethyl ether (DME), which o!ers more attractive emission bene"ts, can also be used directly in compression ignition engines, but signi"cant modi"cation to the fuel delivery system is required to handle this gaseous fuel. Other fuels have been used in diesel engines, namely natural gas, propane, and alcohols. All require signi"cant engine modi"cation, essentially converting the diesel compression ignition engine into a spark ignition engine. Overall, there are emissions advantages to some of the alternative fuels, but cost and availability considerations have limited their use. Legislation in the United States, most notably the Alternative Motor Fuels Act of 1988 (AMFA) has accelerated the implementation and emission testing of alternative fueled vehicles (AFVs) nationwide. CARB has established a reactivity-based HC emission standard for alternative fuels to coincide with the future LEV emission standards discussed previously. Each vehicle/fuel combination will have a reactivity adjustment factor based on a quantitative scale related to the amount of ozone generated per unit mass of fuel. This requires the determination of the emission rates for all of the non-methane organic species and the associated incremental reactivity factor. The emissions-weighted average of the individual incremental reactivity factors provides an overall reactivity adjustment factor (RAF) for each fuel/vehicle combination.

5. Jet fuel Traditionally only that fraction of aircraft operations thought to impact urban air pollution, meaning landings, take-o!s, and ground operations, has been included in emissions inventories. The altitude range considered is roughly within one kilometer of the ground, or less. The possible impact of emissions from fuel burned between

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1 and 15 km is not clear but there is a possible contribution of long range NO transport from these operations x on tropospheric ozone, although it is probably of secondary importance. About 9% of fuel used by mobile sources in the United States goes to jet aircraft. Some regions have a higher fraction. In California, 17% of transportation energy use is jet fuel (Brownstone and Lave, 1992). Most of this, nearly all of commercial operations, is Jet-A. Military operations account for about 10% of jet fuel consumption in the United States. Over the past 30 yr turbofan engines with higher ef"ciencies have replaced turbojet engines. Aircraft fuel e$ciency (seat km) has doubled in the past 30 yr. Turboprop engines account for a small fraction of jet fuel usage, less than 2%. Modi"ed ground operations of aircraft can be instituted to reduce emissions during idle (reduced idle time) and taxiing (using fewer engines). An important part of airport emissions are those from ground support equipment, which should be included in o!-road vehicle emissions. Jet engines are high emitters of NO , especially during x high power operations (take-o! ), where emissions range from 30 to 45 g NO per kg fuel (Baughcum, x 1996), which is 10}20 times that of a modern automobile. Cruise emissions are about a factor of ten lower. Total North American emissions of NO from jet aircraft (all x altitudes operation) is estimated at 0.2 MT, or less than 1% of total NO emissions so that this source is not x a major contributor. Hydrocarbon and carbon monoxide emissions are highest at low power (taxiing) operations, 0.7 to 25 gm HC per kg fuel and 10 to 40 g CO per kg fuel. The newer engines are the lower emitting and these levels are lower than those for modern automobiles. Therefore jet aircraft are not a major source of HC or CO.

6. Bunker fuels Residual, or bunker, fuel is the very heavy, high sulfur content (3}5%) distillate left over after the re"ning process. It is used in ships and power plants. Since ships in international trade can carry large quantities of the fuel, relating where it is purchased to where it is used is di$cult and often left out of fuel inventories. Most of the residual fuel used in ships is used in large low or medium speed diesel engines. The remainder is used to "re boilers for steam turbine power. Ships also often burn residual fuel in boilers for heating purposes. Oxides of nitrogen levels are about 70 g per kg~1 fuel in diesel applications and 15 g kg~1 fuel in boiler applications (BAQMD, 1997). As with airports, ships contribute only a part of the total facility emissions. Much of the port inventory

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will be associated with o!-road operations, including heavy-duty trucks that do not leave the facility.

combining these with modal emissions data. Several models are being developed to integrate transportation dynamics and emissions and are described elsewhere (Barth et al., 1996; Barth and Norbeck, 1994, 1993).

7. Other issues 7.2. Emissions models The preceding sections dealt with the nature of emissions from the mobile sector, including methods to obtain emissions factors and the relative importance of di!erent mobile sources as can be deduced from fuel consumption and fuel based emissions data. Under `other issuesa we treat the methodology for quantitative estimation of motor vehicle emissions. Speci"cally the progression from vehicle activity, to mobile source emissions models, to mobile source inventories. How this information is used with additional information on spatial and temporal variations as the input to ozone estimation models is reviewed. Then we examine how the validity of the mobile source emissions estimates and trends may be checked through `real worlda observations using remote sensing, tunnel measurements, and atmospheric measurements. A preliminary assessment of the intersection between ozone and particulate mobile source issues provides a glimpse to the problem of dealing with mobile sources and particulates. 7.1. Vehicle activity The common modeling approach used to produce a mobile source emission inventory is based on two processing steps: (1) determining a set of emission factors which speci"es the rate at which emissions are generated (tailpipe, evaporative, or running loss emissions); and (2) determining an estimate of vehicle activity as a function of vehicle class, time of day, location, speed, and density. The emission inventory is then calculated by multiplying the results of these two steps. From this process one can obtain a time and spatially resolved emission inventory. A review of this process has been published elsewhere (Maldonado, 1991; Barth et al., 1996; Markey, 1993). This is usually based on laboratory measurements of predetermined driving conditions. There have been several e!orts to develop a more comprehensive approach to emission modeling based on vehicle operating parameters such as engine load, vehicle acceleration, vehicle speed, etc (BDM Int., 1991; Barth and Norbeck, 1994,1993). However, this approach has yet to be implemented into emission models at this time. Few transportation models have been combined with vehicle emission pro"les, and those that do simply predict vehicle density and speed as a function of link and time to be integrated with current speed-emissions data. Although this is a step in the right direction, much better emission estimates can be achieved using a transportation model that can predict dynamic vehicle operating characteristics such as acceleration and deceleration, and

There are two regulatory emissions models, MOBILE, which is the Federal model, and MVEI, which is the California model. The MVEI is often referred to as EMFAC, the emission factor component of the model. Both models are structured similarly. In their simplest form they can be expressed as: Emissions(g)"(activitydata[mi]) (emission rates(g mi~1). The models are structured to use the emissions data generated by the certi"cation test procedures for exhaust and evaporative emissions. Thus, the starting point for exhaust emissions is the FTP-UDDS. This test has three portions: cold start, hot-stabilized driving, and hot start. The model assigns model-year speci"c, zero-kilometer emission rates to each class of vehicles. For post-1981 light-duty gasoline vehicles the rate is adjusted by technology group, i.e. how many vehicles used carburetors, throttle-body fuel injectors, or multi-port fuel injectors? This is not done for other vehicle classes. A deterioration rate is then applied to the emissions as a function of mileage. The rate increases signi"cantly after 80,000 km in the current, MOBILE5B model, although that is expected to change in the forthcoming model update. To account for the fact that some vehicles experience malfunctions or tampering that increase emissions, there are additional emitter categories, which for MOBILE5B are labeled high, very high, and super emitters. The population of vehicles in the emitter categories is increased with vehicle age. Of course, the FTP represents only one driving condition, an average speed of 19.6 mph at approximately 75 3F at a prescribed relative humidity and no road grade. A variety of algorithms have been developed to account for variations in vehicle speed and load, ambient temperature and relative humidity, altitude, and the impact of fuels. E!ects of road grade are not included in the model. Credits are also given for programs such as oxygenated fuels and inspection/maintenance. Evaporative HC emissions modeling is also based largely on data from the certi"cation test procedures. Thus, the models estimate and track hot soak, diurnal, running loss and resting loss emissions. The emissions are adjusted for ambient temperature, fuel vapor pressure, technology type (pre-1981, post-1981 carbureted, TBI, etc.), and I/M performance. Diurnal emissions are adjusted for the number of partial, full-day and multi-day diurnals. Running losses are also corrected for vehicle

R.F. Sawyer et al. / Atmospheric Environment 34 (2000) 2161}2181

speed and trip duration. Finally, resting losses di!er between open and closed bottom canisters, but not by technology type. Refueling emissions are also included in the model. Developing adequate data on which to base the model is extremely di$cult. The emission tests are expensive and time consuming. In addition, it has been recognized that the FTP test procedures have not adequately characterized in-use emissions. Recently, changes in both the exhaust and evaporative emissions tests have been made, but the resources are not available to retest the existing #eet. These models are designed to provide area wide emissions inventory information. They are not designed to provide a high degree of temporal or spatial emissions information. Nor are they designed to address issues such as the impact of tra$c control measures, or to examine the emissions in local situations such as intersections. An entirely di!erent modeling approach is needed to address these questions. Currently several groups are working on the development of modal emissions models. The approach taken in these models is to develop emission rates on the basis of vehicle operating modes, then to aggregate the modes to "t the modeling domain. This requires that emissions data be collected on a s-by-s basis along with a variety of vehicle operating parameters. E!orts are still underway to determine how many modes are required to properly characterize emissions and to build the models. A third approach is fuel-based modeling. This approach, which is examined in some detail in this paper, uses emissions per volume of fuel used rather than emissions per vehicle kilometer traveled. This has several advantages. First, fuel consumption data are readily available on an area wide basis. Thus, if the emission rates can be determined, inventories can be easily calculated. Second, it has been shown that pollutant emission rates are much more constant on a fuel basis than on a distance basis. For example, emission rates per kilometer traveled determined in underwater vehicle tunnels varied by a factor of two depending on whether tra$c was going downhill or uphill (Pierson et al., 1996b). On-road measurements show similar results (Cicero-Fernandez et al., 1997). On a fuel basis, however, emission rates were approximately constant. Third, remote-sensing data are directly applicable to fuel base modeling, as would be roadside studies that measure pollutant ratios to CO . The drawbacks to this approach 2 are that start, diurnal, hot soak, and resting loss emissions are not only related to the quantity of fuel used but also depend on how the vehicle is operated. Furthermore, fuel sales data can not be used to provide the spatial or temporal data needed for some studies. 7.2.1. Comments on MOBILE6 and EMFAC99 Both the USEPA and CARB are working on the next versions of their models. Changes discussed to date in-

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clude: addition of credits for remote sensing and other in-use control programs, adding bene"ts of the Federal OBD-II requirements, using 24-h diurnal evaporative emissions data to replace the current data based on accelerated 1 h tests, inclusion of a new emissions category to handle gross liquid leakers, updated heavy-duty emission factors, revised e!ects of fuel oxygen on exhaust CO, inclusion of the e!ect of sulfur on exhaust emissions, addition of the e!ect of air conditioning on emissions, inclusion of facility speci"c o!-cycle emissions, and revised deterioration rates for running exhaust emissions. Some of the current de"ciencies, such as the failure to treat grades, will probably remain. 7.3. Emission inventories National mobile source emission estimates were developed as part of this review, using fuel sales data for each country, and representative emission factors for NO and NMOC measured in tunnel studies (see Table x 3). Typical values of 10 and 40 g NO per kg of fuel x burned were used for gasoline and diesel fuel, respectively. Fuel energy content was converted to mass units using lower heating values of 44 and 43 MJ kg~1 for gasoline and diesel fuel, respectively (Heywood, 1998). For Canada, total annual mobile source NO emissions x were estimated to be &1 Tg, with roughly equal contributions from on-road diesel, o!-road diesel, and gasoline engines. The o!-road contribution in Canada is somewhat higher when NO emissions due to combustion of x jet fuel by aircraft and residual fuel oil in marine engines are included. For the US, total annual mobile source NO emissions were &10 Tg, again roughly equally x divided among gasoline engine sources, on-road diesel, and heavy-duty o!-road sources (diesel, jet, and marine engines). In both the US and Canada, gasoline engines are responsible for '90% of mobile source NMOC emissions. This conclusion is supported by the NMOC emission indices shown in Table 3, and strengthened by noting that additional NMOC emissions not re#ected in Table 3 occur due to gasoline evaporation. Furthermore, the NMOC emission factors shown in Table 3 for HD vehicles are upper bound values when applied to diesel engines, because these measurements included contributions from HD gasoline engines. Suitable emission factors for Mexican mobile sources are not yet available. Emission inventories of VOC and NO were examined x in the United States to determine the contributions of on-highway and o!-highway to the total mobile source inventory for VOC and NO . These inventories are x shown in Table 4 below. The source of the United States inventories is USEPA's 1995 Emission Trends Report (USEPA, 1996). The United States inventories are for calendar year 1995, and the California inventories are for 1997. The United States inventories are estimated under annual average, rather than ozone season conditions.

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Table 3 Measured on-road emissions from light- and heavy-duty vehicles (mass of pollutant emitted per unit mass of fuel burned) Tunnel

Year sampled

Tuscarora, Pa# Fort McHenry$ (Baltimore, MD) uphill tra$c downhill tra$c Caldecott% (Oakland, CA) Cassiar& (Vancouver, BC) Callahan',) (Boston, MA) Lincoln (New York, NY) Deck Park (summer) (Phoenix, AZ) Sepulveda (Los Angeles, CA) Sherman Way (Van Nuys, CA)

1992 1992

Tunnel length (m)

1623 2174

Roadway grade

Level

LD! emission index (g kg)~1

HD" Emission index (g kg)~1

CO

NO x

CO

NMOC

NO x

NMOC

65

3.9

5.2

12

1.4

39

75 64 104 90

6.6 6.8 5.0 7.2

10.5 7.5 10.1 16.2

24 24 na 17

4.3 3.4 na na

37 34 na 48

1994 1995

1100 730

#3.3% !1.8% #4.2% Level

1995 1995 1995

1545 2440 804

Up#down* Up#down* Level

61 53 61

6.1 7.1 8.3

12.6 14.8 11.4

(3.7% HD)* (13.3% HD)+ (5.5% HD)+

1995

582

Level

76

7.1

9.9

(2.4% HD)+

1995

222

&Level

123

9.2

10.2

(2.8% HD)+

!Light-duty vehicles, nearly all gasoline-powered. "Heavy-duty vehicles, '70% diesel-powered. #Pierson et al. (1996b). $Pierson et al. (1996b). %Kirchstetter et al. (1996). &Rogak et al. (1997a). 'Sagebiel et al. (1996). )Gertler et al. (1997a). *Underwater tunnel; includes both downhill and uphill driving (grade ranges from !3.8 to #3.5%). +Data from Gertler et al. (1997a) for LD and HD vehicles combined (the fraction of total vehicles that were HD is shown for each tunnel). The HD contribution of NO is signi"cant, especially in the Lincoln tunnel. x

Table 4 Mobile source VOC and NO inventories in the United States (1995) x Source

Fuel

Vehicle categories

VOC (tpd)

NO (tpd) x

On-road

Gasoline

Cars and all gasoline trucks Motorcycles Diesel trucks and buses

15,800 101 840 16,700

15,300 36 5540 20,900

4790 690

360 4720

600 6080

400 5480

22,800 60,000

26,400 62,500

Diesel Total on-road O!-road

Gasoline Diesel Aviation Fuel

Recreational vehicles, utility equipment Agricultural, construction, commercial boats, trains, and ships Aircraft

Total o!-road Total on-road#o!-road Total, all sources (also includes area and point sources, excludes biogenic)

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7.4. Tying emissions to ozone 7.4.1. Spatial and temporal distribution of emissions Mobile source emissions must be inventoried on the spatial and temporal scales over which tropospheric ozone pollution problems are observed. This requires that inventories be resolved within urban and regional spatial scales, by hour of the day, for summer days when conditions are conducive to ozone formation in Canada and the United States, and year-round in Mexico City. Annual fuel use data for mobile sources are readily available at the national level in Canada, Mexico, and the United States (see Fig. 1). Usually, fuel use can be resolved to the state or provincial level and by month as well. Market surveys in which gasoline sales are measured directly at service stations are conducted routinely in the United States and Canada; such data can be used to apportion gasoline sales to subregions within a state or province. It is more di$cult to apportion spatially the diesel fuel consumption, because the point of sale can be widely separated from most of a long-haul diesel truck trip (large diesel trucks can travel up to 1600 km between refueling. For on-road vehicles, the in#uence on emissions of monthly variations in fuel sales is small compared to the more important seasonal factors such as changes in temperature and variations in gasoline properties such as vapor pressure. Fuel use by o!-road mobile sources is not well known, because o!-road mobile sources are rarely subject to registration or licensing requirements. Seasonal variations are expected in activity levels of o!-road mobile sources, for example in the agriculture sector. Determining fuel use and vehicle emissions on "ne spatial scales is a challenging task. It is common practice to use travel demand models to predict motor vehicle activity within the urban scale (Harvey and Deakin, 1993; TRB, 1995). Travel demand models make use of socioeconomic data such as population, employment, automobile ownership, and household income, combined with information about travel times between points, available modes of transportation, and a description of the roadway network. Such models predict spatially and temporally resolved vehicle activity, which can be combined with emission factor model predictions (discussed below) to develop the overall emission inventory. Travel demand models can be used to predict future tra$c volumes based on forecasts of socioeconomic conditions and land use. The goal of travel demand modeling has been to estimate total tra$c volumes, which are dominated by light-duty passenger vehicles. Travel demand models do not describe heavy-duty truck travel explicitly; often truck travel is estimated as a constant fraction of total tra$c volumes. However, diesel truck activity does not follow the same pattern as passenger vehicle travel. On

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weekdays within urban areas, diesel truck travel peaks around midday, and falls o! before the afternoon commuter peak period (Schlappi et al., 1993). The e!ect on ozone of greater heavy-duty diesel NO emissions during x the middle of the day has not been assessed. Day-of-week di!erences in ozone concentrations have been observed: in some urban areas, average ozone concentrations are higher on weekends (Altshuler et al., 1995). Di!erences in mobile source emissions are likely to contribute to this phenomenon. Diesel truck activity and emissions within urban areas decrease dramatically on weekends (Dreher and Harley, 1998). In contrast, the total amount of passenger vehicle travel is similar on weekends and weekdays. Related observations showed a small hourly variation in long-haul diesel operation but a large hourly variation in passenger car operation. Travel demand models have a strong employment-driven component to their predictions, and so require modi"cations before they can be used to assess weekend travel. An alternate approach to de"ning vehicle activity involves direct observations of tra$c volumes by automated counters. Many highways are now heavily instrumented, and it is possible to extract hourly tra$c counts and speed distributions on each highway segment. Automated tra$c counters also are being installed on urban arterial roadways. Weigh-in-motion sensors provide information on heavy-duty truck travel. Where possible, direct measurement of vehicle activity, especially for heavy-duty trucks and for all vehicles on weekends, appears preferable to relying on travel demand model predictions. 7.4.2. VOC speciation and reactivity Roadway tunnel measurements have been used to de"ne the detailed chemical composition of mobile source VOC emissions (Lonneman et al., 1986; Zielinska and Fung, 1994; Kirchstetter et al., 1996; Sagebiel et al., 1996; Rogak et al., 1997a). Numerous dynamometer studies of exhaust VOC composition also have been reported (Sigsby et al., 1987; Hoekman, 1992; Hochhauser et al., 1992). Analyses of both dynamometer and tunnel data have concluded that unburned gasoline constitutes 50% or more of total VOC exhaust emissions (Leppard et al., 1992; McLaren et al., 1996). Weight fractions of combustion-derived compounds such as acetylene and ethene in exhaust have been declining over time, while the ratio of ethene to acetylene has been increasing (comparisons of engine-out and tailpipe-out emissions con"rm that catalytic converters are especially e!ective in removing acetylene). Relative to United States studies, Canadian vehicle VOC emissions measured in the Cassiar Connector (Gertler et al., 1997b; Rogak et al., 1997a) show much higher propane, and lower aromatic fractions. Likely factors contributing to these di!erences include the presence of more LPG-powered vehicles, and less use of aromatics in gasoline because methylcyclopentadienyl

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manganese tricarbonyl (MMT) is allowed as an octaneimprover in Canada. Carter (1994) has described reactivity scales that can be used to compare the reactivity of di!erent VOC with respect to ozone formation. The maximum incremental reactivity (MIR) scale has been widely used for this purpose. Speci"c or normalized reactivity is computed as a weighted average using weight fractions and MIR values for all individual VOC present in an emissions sample; the result is reported in units of g O per 3 g NMOC. For any given speciation pro"le, the absolute assessment of reactivity is subject to numerous assumptions and modeling uncertainties. Conclusions about reactivity are more robust when they are made in a relative sense (e.g., comparing the reactivity of exhaust or evaporative emissions using two di!erent gasoline formulations). Speci"c reactivity of light-duty vehicle exhaust emissions calculated using the MIR scale ranges from 2 to 3 g O per g NMOC emitted in dynamometer studies 3 (Hoekman, 1992; Hochhauser et al., 1992). On-road determinations of exhaust reactivity in tunnels suggest somewhat higher values of &4 g O per g NMOC 3 (Kirchstetter et al., 1996; Sagebiel et al., 1996). Calculations for the Canadian light-duty vehicle #eet using speciation pro"les presented by Rogak et al. (1997a) suggest lower speci"c reactivity of &3 g O per g 3 NMOC, possibly due to lower aromatic hydrocarbon levels in Canadian gasoline. 7.5. Remote sensing Much has been learned about in-use motor vehicle emissions through use of spectroscopic remote sensing techniques that measure emissions from individual vehicles as they drive by roadside sensors (Stephens and Cadle, 1991; Bishop et al., 1989; Guenther et al., 1995; Zhang et al., 1996, 1993). Vehicle emissions have been measured using remote sensors in numerous United States cities, as well as in Canada, Mexico (Beaton et al., 1992; Bishop et al., 1997), and other locations around the world (Zhang et al., 1995). Among the advantages of remote sensing techniques are: the ability to measure emissions from large numbers of vehicles, the ability to monitor vehicles in their normal in-use operating condition, and reduced sampling bias because drivers are not noti"ed of the testing or given the option of not participating. The sample is usually restricted to those vehicles with tailpipes near roadway level, although limited measurements of diesel truck emissions have been made with a remote sensor elevated &5 m above roadway height (Bishop et al., 1996). While some long-pathlength spectroscopic measurements of aircraft emissions have been made across airport runways, in general there has been little use of remote sensing techniques to monitor o!-road mobile source emissions.

Since the light beam passes through only a small portion of the vehicle exhaust plume, it is not possible to obtain an absolute emission rate with this technique. Instead, a large number of simultaneous concentration measurements are made of both CO and the species 2 of interest within a one-second time interval. The measurements are used to calculate the ratio between the pollutant and CO . This approach has two advantages. 2 First, if the ratio is not constant during the measurement period, the data are suspect and can be rejected. Second, if both CO and CO are monitored, then almost all of the 2 carbon that was present in the combusted gasoline has been measured. By using an average gasoline C : H : O ratio and density, the emission rate of the pollutant can be calculated in either units of grams-per-liter of gasoline consumed or percent concentration in the raw exhaust. Thus, the data can be directly used in fuel-based inventory modeling. Vehicle emissions can be highly variable on both a short and a long-term basis. Short term variability is caused by changes in operating mode (i.e. throttle dither), enrichment due to high-load events, rapid changes in catalyst e$ciency during light-o!, and erratic emission control due to emission control component failures or tampering. Long-term variability is caused by vehicle deterioration, changes in fuel, and failure mode operations. A failure mode that results in rich operation can easily result in a 100-fold increase in a vehicle's HC or CO emission rate. Large di!erences also exist between individual vehicles of di!erent ages due to improvements in the emission control technology. Understanding this emission variability is the key to properly using and interpreting remote sensing. First, the high HC and CO emission rates associated with cold start operation and high-load events can be avoided by careful selection of the remote sensing location. Second, one must recognize that while emissions during hot-stabilized operation from properly functioning vehicles vary greatly s-by-s, their emission rates rarely approach those that are typical for a high emission vehicle. Third, some high emission vehicles can experience very wide changes in emission rate, such that they appear to be low emission vehicles one second and high emission vehicles shortly thereafter. Overall, this variability makes it di$cult to use remote sensing as a highly accurate tool for identifying high emitting vehicles. Two approaches are under investigation to minimize errors in separating high and low emitters. One is to identify high emitters based on multiple remote sensing measurements. The second is to identify only low emitters, also based on multiple remote sensing measurements. For inventory purposes, the issue of emissions variability is moot. A single remote sensor can measure the emissions from thousands of vehicles per day. By averaging the emission rates from large numbers of vehicles, the s-to-s variability issue is removed, although one must still be careful of biasing the data by

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selecting a site with atypical vehicle operation or an atypical #eet. Simultaneous recording of license plate information combined with automated license plate readers, makes it practical to identify most vehicles by model year or even make and engine type through state registration data. A large number of remote sensing studies has now been conducted in the United States and elsewhere. In the United States, these studies show that a small percentage of the in-use vehicle #eet contributes the majority of the exhaust emissions. Typically, 10% of the #eet is responsible for 50% of the emissions. Nine to 11 year old vehicles typically contribute the most emissions to the inventory. The impact of older vehicles is decreased due to scrappage and decreased VKT. 7.6. Tunnel studies Roadway tunnels are being used to measure on-road vehicle emissions. Measured emissions in tunnels have been compared to emission factor model predictions for tunnels in Van Nuys, CA (Pierson et al., 1990), Tuscarora, PA and Baltimore, MD (Pierson et al., 1996b; Robinson et al., 1996), and Vancouver, Canada (Gertler et al., 1997b; Rogak et al., 1997a). Due to uncertainties in tunnel air #ow measurements and in the vertical pro"le of pollutant concentrations at tunnel inlets (Rogak et al., 1997b), it is common practice to consider ratios of pollutant emission rates (e.g., CO/NO ) which are less sensix tive to measurement and modeling uncertainties. It is especially useful to normalize measured pollutant concentrations to total exhaust carbon (i.e., the sum of CO , CO, and VOC). Table 3 summarizes on-road 2 measurements of emission rates for CO, NMOC, and NO , expressed per unit mass of fuel burned based on the x normalization to total carbon. Light-duty (LD) vehicles are almost entirely gasoline-powered, whereas heavyduty (HD) vehicles are '70% diesel-powered. Based on the emission indices shown in Table 3, and knowing that gasoline sales are several times greater than diesel fuel sales by mass, it is clear that HD vehicles are a minor source of CO and NMOC, and a major source of NO x emissions. LD vehicle emissions in Canada, as measured in the Cassiar Connector in Vancouver, are within the range of values seen in United States tunnels, with the exception of NO , which is higher. Rogak et al. (1997a) x report that di!erences in new-vehicle emission standards are unlikely to explain the higher NO emissions obx served for Canadian LD vehicles. Further study of this issue is needed. For LD vehicles, both tailpipe and some evaporative emissions are found in tunnels. Evaporative emissions are estimated to account for 10}15% of total NMOC emissions in both United States and Canadian tunnel studies (McLaren et al., 1996; Gertler et al., 1996). Evaporative emissions are thought to be negligible for diesel vehicles, because of the low volatility of diesel fuel.

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7.7. Atmospheric measurements and source reconciliation Concentrations of CO and NO are measured roux tinely at North American air monitoring sites. Ambient VOC concentrations also are measured, though the spatial and temporal coverage of the monitoring is much less complete than for CO and NO . Continuous monitoring x of total non-methane organic compound (NMOC) concentrations should be an integral part of air monitoring networks. This information can be augmented by occasional measurements of full NMOC speciation. Longterm records of ambient concentration data are useful in assessing emission trends. Ambient concentration data also provide a useful independent check on mobile source emission inventories. A widely cited top-down (starting with ambient concentrations) analysis of the mobile source emission inventory has been presented by Fujita et al. (1992) for southern California. Fujita et al. assumed NO emissions were x estimated correctly, and then determined from measured ambient CO/NO and NMOC/NO ratios that CO and x x NMOC emissions in 1987 were understated in mobile source inventories by factors of 1.5 and 2.0}2.5, respectively. A critical review by Yarwood et al. (1994) notes that the acceptability and accuracy of the NO data is the x cornerstone of the study by Fujita et al.; potential problems with the NO ambient and emissions data are the x weak link in the conclusions about the CO and NMOC inventories. Harley et al. (1997) used NO /CO and x NMOC/CO ambient ratios, combined with a fuel-based estimate of CO emissions (Kirchstetter et al., 1996; Singer and Harley, 1996), to estimate absolute NMOC and NO x emissions in southern California. While MVEI 7G model predictions for NO were consistent with top-down estix mates made by Harley et al., NMOC emissions remained underestimated by a factor of 2.4. Clearly, large uncertainties remain in NMOC emissions estimates for historical ozone episodes; top-down assessments of mobile source emission inventories are needed urgently for present-day conditions. 7.8. Intersection of ozone and PM mobile source issues Mobile sources contribute to the atmospheric burden of particulate matter (PM) through three mechanisms, primary PM emissions, secondary PM formation, and fugitive emissions. Primary particles are those directly emitted by vehicles. They are emitted in the vehicle exhaust, and also are emitted via brake and tire wear. The PART5 model is the United States USEPA's o$cial model for projecting PM emissions from on-road motor vehicles. This model uses the same vehicle classes and activity data as the MOBILE5 model, and estimates emissions from both light- and heavy-duty vehicles (Rykowski et al., 1996). However, the algorithms for PM are much simpler than those used for other emissions.

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For example, there is no separation of cold start from other operating modes and no deterioration of emission factors. This is largely due to a lack of data. Several studies have recently, or are currently, examining lightduty gasoline and heavy-duty diesel in-use PM exhaust emissions. The results will be used to improve current model estimates. Both tire and brake wear PM emissions factors are out of date and need to be addressed. The greatest changes have probably occurred with brake wear PM emissions, since brake pad materials have been changed since the earlier studies. Secondary PM formation comes via emissions of HC, NO , and SO which then react in the atmosphere to x 2 give organic, nitrate and sulfate PM, respectively. Roughly 30}50% of all PM is estimated to be due to secondary inorganic PM and ca. 5% due to organic secondary PM. The formation routes of secondary organic PMs are not well quanti"ed. However, it is believed that C7#HCs react in the atmosphere (if it contains ozone) to form less volatile organic species that condense into the particle phase, thereby contributing to the carbonaceous particle burden (Odum et al., 1997; Grosjean, 1992; Wang et al., 1995; Turpin and Huntzicker, 1995; Schauer et al., 1996). The rate of secondary organic PM formation from aromatic compounds is believed to be roughly twice as fast as those from alkanes and alkenes. NO oxidizes to nitric acid, which reacts with ammonia x to form ammonium nitrate. The amount of ammonia in the atmosphere can be the controlling factor in the amount of particulate nitrate formed. SO is oxidized to 2 sulfuric acid, which exists primarily in the particle phase. Mobile sources are minor contributors to the SO in 2 most areas but are often major contributors to NO and x C7#HCs. Since ozone is formed via atmospheric reactions involving HC and NO , improving our underx standing of the mobile source HC and NO inventory x will increase our knowledge of the ozone and PM mobile source issues and how they interact.

8. Projecting the future The uncertainties in assessing current mobile source emissions are compounded when estimating future year emissions. Because of the slow turnover of the motor vehicle #eet, especially heavy-duty vehicles, the impact of new technology is felt slowly. Fuel changes, if they occur and apply to current vehicles, have a more immediate e!ect. Advances in technology and fuels tend to be o!set by growth in population, fuel consumption, numbers of vehicles, and vehicle kilometers driven. Attempts to constrain or reduce private vehicle use through transportation control measures (TCMs) have met resistance and produced minimal results. Projecting future in-use #eet average emissions is di$cult under the best of situations. Doing so with models

that have di$culty projecting current emissions adds to the uncertainty. As discussed, current emissions are heavily in#uenced by the emissions of high emitters that make up a relatively small fraction of the #eet. Since the emissions from properly functioning new vehicles are continuing to decrease, the relative importance of malfunctioning vehicles is likely to grow. O!-setting this trend are programs designed to identify and repair high emitters. Thus, future emissions estimates must account for changes in vehicle durability, changes in emissions related tampering, e!ectiveness of inspection/maintenance programs, the e!ectiveness of the OBDII system, and human behavior in terms of the willingness of people to maintain their vehicles. In addition, estimates have to be made of the in-use e!ectiveness of new emissions regulations, and socio-economic issues such as the rate of #eet turnover. In the long term, 20}50 yr, the introduction of new technologies and fuels could have a substantial impact on motor vehicle emissions. A new generation of combustion engine/electric hybrid vehicles with greatly increased fuel economy (2}3 times) and reduced emissions (at the ULEV level or lower) is under development with the "rst production and sales already begun in Japan. A later version could be a fuel-cell hybrid with near zero emissions depending upon the fuel. A continued role for petroleum based motor fuels seems likely as long as supplies remain abundant and prices remain low. Transition to a truly clean fuel, such as pure electric or hydrogen, may not occur until petroleum fuels become in short supply. 9. Critical issues Overall, it is concluded that the uncertainties in current mobile source emission inventories compromise the con"dent development of tropospheric ozone control strategies. Improvement of these inventories is critical as they form the foundation of all control strategies. Speci"cally: f At present, large and signi"cant uncertainties exist in the estimates of the mobile source emissions inventory. These uncertainties exist for all vehicle types and classes throughout North America. f The mobile source inventory is dynamic. Large changes in the in-use vehicle #eet will continue to occur between now and the year 2010 due to changes in vehicle technology, fuel composition, and vehicle activity. f There is currently no routine collection of in-use emissions data from all mobile source types. A process needs to be established to correct this de"ciency. For example, validated I/M data should be collected centrally and should be available for use in emission model revisions.

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f The emission inventory needs to include adequate temporal and spatial resolution for ozone modeling. Information on chemical composition of the hydrocarbon emissions also needs to be included. Current models are inadequate in these areas. f Accurate activity data (i.e. number of cold starts, hot starts, trip length, soak periods, tra$c congestion, day-of-week variations, etc.) is as important as accurate emission rate data. More activity data are required to account for geographic and both short- and long-term temporal changes in driving patterns. f A sensitivity analysis of the uncertainties associated with mobile source emissions needs to be included in the emission factor models. f Policy makers need to be aware of di!erences in e!ectiveness of mobile source control strategies for HC and NO . For example, both on- and o!-road HD diesel x emissions are important NO sources, but are minor x sources of HC. Reformulated gasolines are, generally, more e!ective in reducing HC than NO . x f Mobile sources are responsible for about one-half of NO emissions, more in some areas. These are roughly x divided equally among heavy-duty on-road, heavyduty o!-road, and light-duty vehicles. Ozone control strategies that require NO reduction must pay x greater attention to reducing NO from heavy-duty x sources. f Pollutant #uxes at the boundaries of modeling domains are important. Rural emissions from on-road and o!-road vehicles as well as ships are currently underestimated. f Validation studies based on direct measurement of the in-use #eet need to be performed to assess the accuracy of the emissions models. Con"dence in the inventory will remain low until agreement is obtained between top-down and bottom-up validation approaches.

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