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1 Overview of Mobile-Source Emissions AN ACCURATE ASSESSMENT of motor-vehicle emissions is essential for an effective air-quality improvement program. The U.S. Environmental Pro- tection Agency's (EPA's) Mobile Source Emissions Factor MOBILE model is the primary tool used by air-quality planners at national, state, and local levels to estimate on-road mobile-source emissions, and hence is key to assessing associated environmental impacts. Because of the model's importance in assessing air-quaTity control programs and because of con- cerns about weaknesses in the accuracy and reliability of the model, Con- gress asked the National Academy of Sciences to review and evaluate the MOBILE model. The National Research CounciT's Committee to Review EPA's Mobile-Source Emissions Factor Model was formed in response to that request. Specifically, the committee was to consider the adequacy of the model's input data, assumptions, structure, and results and recom- mend ways to improve the reliability of its mobile-source emissions esti- mates. This chapter reviews mobile-source emissions within the context of the total air-quality problem in the United States, provides an overview of the categories and estimated levels of mobile-source emissions, introduces the legislative and regulatory initiatives used to control such emissions, de- tails the committee's charge, and describes the structure of this report. 75

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6 M ODE[ING MOB/LE-SOURCE EMISSIONS AIR-QUALITY PROTECTION Air-quality policy in the United States is directed largely towards pro- tecting public health. It also addresses public-welfare issues, such as visi- bility in national parks and wilderness areas and environmental damage due to acid deposition. The National Ambient Air Quality Standards (NAAQS) sets a primary standard for ambient concentrations of criteria pollutants to protect public health with "an adequate margin of safety", and a secondary standard to protect public welfare against environmental and property damage. The attainment of NAAQS for the six criteria pol- lutants, ozone, carbon monoxide (CO), nitrogen dioxide (NO2), lead, sulfur dioxide (SO2), and particulate matter (PM), is one measure of air quality. Another category of pollutants consists of the hazardous air pollutants or "air tonics," including carcinogens, neurotoxins, teratogens, allergens, and other harmful compounds. Control of air tonics is aimed at ensuring that any health risks are less than one in a million from lifetime exposure to a particular toxic. Other air-quality protection programs are directed towards acid-rain and visibility concerns. Acid-deposition regulations are designed to reduce the negative impacts on humans and the environment from the deposition of nitric and sulfuric acids. Visibility regulations are designed to protect the visibility in federal Class I areas, which are primarily national parks and wilderness areas. Pollutants of Interest The pollutants of interest for air quality are both primary and second- ary pollutants. Primary pollutants are those directly emitted to the atmo- sphere and include CO, SO2, and lead. Ambient concentrations of such pollutants are directly related to their sources. Secondary pollutants are those formed by atmospheric processes, including chemical reactions and condensation. Ozone is a secondary pollutant, formed by the action of sun- light and chemical reactions involving volatile organic compounds (VOCs)i iAn organic compound is a compound containing carbon combined with atoms of other elements, commonly hydrogen, oxygen, and nitrogen. Simple carbon-contain- ing compounds such as carbon monoxide and carbon dioxide are usually classified as inorganic compounds. A volatile organic compound is a compound that can exist as a gas under typical atmospheric conditions. This report, unless otherwise noted, will refer to the general class of gaseous organic compounds. as VOCs. Appen- dix B describes the differences among the terms used to refer to gaseous organic compounds.

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OVERVIEW OF MOBI[E-SOURCE EMISSIONS 7 7 and nitrogen oxides (NOX). Airborne PM and air taxies are combinations of primary and secondary pollutants. In urban areas, motor vehicles generally are the dominant emissions sources of VOCs, NOX, and CO and their control is critical for reducing ur- ban antipollution problems caused by these emissions. The adverse effects of ozone and CO are well understood, whereas those of PM and most tonics are less well understood. As more is known about the specific aspects of the toxicity of PM and air tonics, a better understanding of the contribu- tion of motor vehicles to these pollutants becomes important. Mobile-Source Contributions Total mobile source emissions (on-road plus off-road emissions) contrib- ute significantly to overall air pollution in the United States.2 Figure 1-1 shows estimates for 19973 of the contribution of mobile sources (on-road and off-road) to emissions of criteria air pollutants and their precursors. For example, mobile sources in that year contributed over 75% of CO emis- sions (about 67,000 thousand tons), almost half of the NOX (about 11,600 thousand tons), and 40% of the VOCs (about 7,700 thousand tons) (see Figure 1-1) (EPA 1998b). The elimination of lead from gasoline has great- ly reduced mobile-source emissions of this pollutant to just over 500 tons (13% of total lead emissions) in 1997, compared with mobile-source lead emissions in 1970, which were over 180,000 tons (EPA 1998a). According to the EPA (1998b), mobile-source exhaust is a less important source of PM-10 (those particles smaller than 10 mm in diameter) and sulfur diox- ide. Mobile-source contribution to fine particles (those smaller than 2.5 mm in diameter and referred to as PM-2.5) is an area of continuing study. One recent study reported higher than expected PM-2.5 emissions from light-duty vehicles (LDVs) at higher elevations (Cadre et al. 1998~. It should be noted that, for a given location, the fraction of emissions inven- tories contributed by mobile sources varies greatly. 2The principal sources of emissions are stationary sources (fuel combustion by utilities, industry and residential sources), industrial process sources (chemical manufacturing, petroleum refining, solvents and waste disposal), on-road vehicles (light- and heavy-duty gas and diesel-powered vehicles), non-road engines and vehicles (recreational, industrial, and commercial vehicles and engines, trains, marine vessels, and aircraft), and other (biogenic emissions from natural, agricul- tural and forestry sources, and other combustion) (EPA 1998a). 3To the extent that the mobile source emissions estimates are obtained by using the MOBILE model, they are subject to the types of uncertainties discussed throughout this report.

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78 MODEL/NO MOBI[E-SOURCE EMISSIONS CARBON MONOXIDE 956~481 7 e_ I_ f _ 16755- ~ LEAD 503~ _~n .~ 19~: / t. / I:. ~ , ~ i __ ~ "~w I, ..:,, .' 7~6052 , ., , , . : .! . . : : . : . . _ ~ : , : : , , , , i , _ rr , . , _ : . : l , , _ , . . : . , . , . , : , : . : : . . , , , . ~ , , , , , K, , , : . . . _ , _ . , i ~ 50257 ;2897 OXIDES OF NITROGEN 346 7035) , a.;;;;.;; A:.:.::.:.: \' 917 LEGEND VOLATILE ORGANIC CARBON s230~ PARTICULATE MATTER (PM-10) 1101 - 1277,268 new SULFUR DIOXIDE ~-~ :~ - - :) ~17260 STATIONARY INDUSTRIAL ~1 ON-ROAD 1~ NON-ROAD OTHER FIGURE 1-1 Sources of Criteria Air Pollutants. Estimated total annual emissions of criteria pollutants from stationary, industrial process, and mobile (on-road and non-road), and other sources for 1997. Emissions are shown in thousands of tons except for lead, which is shown in tons. Source: EPA l99Sb. Human Health Concerns Total emissions from mobile sources contribute significantly to the det- rimental health effects resulting from exposure to ambient ozone, CO, PM,

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OVERVIEW OF MOBIIE-SOURCE EMISSIONS 7 9 and air tonics. Urban ozone has been one of the most persistent health con- cerns. The current 1-fur primary NAAQS for ozone is 0.12 parts per million (ppm), which is the daily maximum not to be exceeded more than once per year on average. (This is averaged over 3 years, so the fourth highest 1-fur value over 3 years is the one that is used to compare with the standard.) Health effects associated with exposures above this standard are well doc- umented and are summarized by Lippmann (1989~. They range from short-term consequences such as chest pain, decreased lung function, and increased susceptibility to respiratory infection, to possible long-term con- sequences, such as premature lung aging and chronic respiratory illnesses. In 1997, approximately 48 million U.S. residents in 77 counties, primarily urban and suburban regions, lived in areas where the second highest daily maximum concentration exceeded 0.12 ppm (EPA 1998b).Due to concerns about prolonged exposures to lower levels of ozone, EPA adopted an 8-fur standard of 0.08 ppm, which would put a much greater area and popula- tion into nonattainment with the standard (Wolff 1996; Chameides et al. 1997~. Based on the period from 1993 to 1995, EPA estimated that 248 counties with a population of 83 million people would have violated the 8- hr ozone standard (EPA 1997a). Prospects for the implementation of the new standards are uncertain because a U.S. Court of Appeals in May 1999 remanded them for further consideration by EPA. Attaining CO standards has been far more successful. There are two primary standards for CO, a 1-fur average of 35 ppm and an 8-fur average of 9 ppm. The health effects from exposures to concentrations exceeding these standards are also documented. CO enters the blood stream and links to hemoglobin, reducing the amount of oxygen the blood can carry and causing mental and physical impairment. In 1997, approximately 9 million people in three urban counties lived in areas that exceeded these standards (EPA 1998b). This number has been declining over time. Standards for PM and air tonics are currently changing due to increased knowledge about their health effects. Current regulations for ambient con- centrations of PM focus on particles less than or equal to 10 mm. New regulations to control smaller particles (less than or equal to 2.5 mm), in- tended to address some preliminary findings that these particles have the greatest impact on human health (Dockery et al.1993), were remanded for consideration back to EPA by a U.S. Court of Appeals in May 1999. Air tonics include a wide class of emissions and effects. Few of these sub- stances have been studied to examine possible health effects. In 1998, the California Air Resources Board (CARB) identified diesel PM as a toxic air contaminant. An issue unique to mobile sources is the proximity of sources to recep- tors. Air vents on cars can scoop up exhaust from a vehicle just ahead so

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20 MODELING MOBILE-SOURCE EMISSIONS that exposures in the cab of a car or truck are higher than at the roadside. A jogger at a roadside, breathing deeply and rapidly, might inhale much larger amounts of contaminants than a sedentary individual sitting out- doors away from the road. Urban canyons can retain, and thus concen- trate, contaminants emitted by motor vehicles. Figure 1-2 shows how ex- posure to CO varied for one individual during typical commutes. Because health effects are a function of what is actually inhaled, this variation in exposure is a critical feature of mobile-source pollutants. Environmental Concerns Mobile sources contribute significantly to the risk of detrimental envi- ronmental effects of ozone, acidic deposition, PM, and air tonics. However, environmental impacts have received much less attention than health ef- fects. For most criteria pollutants, the secondary NAAQS, set to protect the environment and susceptible outdoor structures, is the same as the primary standard set to protect human health. Ozone concentrations can damage crops and other vegetation (Heck et al. 1982; EPA 1986), exhaust emissions are toxic to roadside flora and fauna (particularly evident when lead was used (EPA 1998b), acid can damage human artifacts (e.g., stain- ing of buildings and corrosion of outdoor statuary) (NAPAP 1990), and p articulate s contribute to haze and poor visibility (NRC 1993~. Mobile sources also contribute to greenhouse gas emissions. Approxi- mately one-third of the total U.S. anthropogenic emissions of CO2 comes from the transportation sector (Energy Information Administration 1997), including about one-quarter of the total from light-duty vehicles and heavy-duty vehicles (Heinz Center 1998~. Also, mobile-source emissions of chlorofluorocarbons and hydrochiorofluorocarbons from air-conditioning systems contribute to stratospheric ozone depletion. Although chlorofluoro- carbons are now banned from use in the United States, existing systems containing these compounds will continue to leak them to the atmosphere (Holmes and Ellis 1997), and significant illegal use has been reported (Vallette 1995~. ESTIMATING EMISSIONS FROM MOBILE SOURCES Importance of Source Identification and Quantification An effective air-quality improvement program requires the identifica- tion, inventory, and control of emissions sources, including mobile sources.

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OVERVIEW OF MOB`LE-SOURCE EM/SS!ONS 2 7 Sol 40 - E At 0 30 z us 8 20 10 Morning commute PASS "S Lo ROUTE 66] RourE 215 S5AS,. IVA _D Evening commute WALK - IN EPA GARAGE DRIVE- I N GAR A GE COLLEC TION 800TH SW FREEWAY I 495 I ROUTE 66 IN OFFICE ~ HARASSES, VA ~ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 5 6 7 a 9 10 al 12 13 14 15 16 17 18 19 20 HOUR OF toy FIGURE 1-2 Daytime exposure to carbon monoxide during peak-hour commutes. Source: Ott 1985. This requires not only a broad understanding of which pollutants are de- rived from which sources, but also details about their spatial and temporal variation, the contributions of subsets of sources, the chemical and physi- cal characteristics that determine their propensity to form secondary pol- lutants, their levels of exposure and toxicity, and the actual effectiveness of strategies to control emissions. The large number of individual sources, the large variability of emissions characteristics among these sources, and the need for emissions estimation methods to fulfill many applications cre- ates daunting challenges. To aid in identifying, estimating, and reducing risks of motor-vehicle emissions, EPA has developed a series of models referred to as the MOBILE model. The current (MOBILE5b) and upcom- ing (MOBILES) versions of MOBILE provide some but not all of the required information. MOBILE estimates emissions factors from broad vehicle classes using average vehicle speed to represent highway condi- tions. However, MOBILE provides only limited information on the physi- cal and chemical characteristics of emissions, and it does not simulate emissions related to dynamic traffic-Row conditions. The MOBILE model deals only with the on-road vehicle emissions of CO, VOCs, and NOx. Related models cover other mobile-source emissions (e.g., NONROAD for off-road emissions, PARTS for PM, and MOBTOX for air tonics). These related models are often discussed in the same context as MOBILE because they are needed to represent the full suite of emis- sions from mobile sources and often use the same input data as MOBILE.

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22 MODELING MOBI[E-SOURCE EMISSIONS On-Road Vehicle Emissions Categories Regulations and the MOBILE model divide on-road vehicles into two broad categories, light-duty (LDVs) and heavy-duty (HDVs) vehicles. The size boundary between LDVs and HDVs historically has been 8,500 pounds, gross vehicle weight (the weight of the vehicle plus the weight of the rated load-hauling capacity). LDVs are fueled primarily by gasoline. HDVs are fueled by diesel and gasoline with the heavy HDVs (those with gross vehicle weights greater than 26,000 pounds) fueled with diesel. These heavy HDVs are important because, though they represent a small fraction of the total number of vehicles, they represent a significant frac- tion of total vehicle miles traveled, fuel consumption, and emissions (Davis 1999; EPA 1998a). Figure 1-3 displays the separation of emissions by fuel use. Light-Duty Vehicles Historically, LDVs were sub-divided into two main categories4: passen- ger cars and light-duty trucks (LDTs), with the latter used primarily for commercial purposes. Two different sets of exhaust emissions standards applied. Differences between the two categories of vehicles have dimin- ished and the regulatory distinction will disappear in 2007. Tailpipe emis- sions standards were applied in 1968 and now call for a reduction of more than 90% from 1968 levels for all pollutants. Passenger cars, as the name implies, refer to personal vehicles used pri- marily to transport people. A single set of emissions standards applies to all passenger cars, regardless of size, passenger occupancy, or use. Emis- sions are regulated on the basis of grams of pollutant per mile (g/mi) and vehicles are certified on a chassis dynamometer test.5 Table 1-1 shows historical categories of standards for passenger cars for up to 50,000 miles or from 50,001 to 100,000 miles.6 4Motor cycles also form a category of on-road vehicles. However, their emissions tend to be small relative to passenger cars and light-duty trucks. Thus, they are not discussed here. sin a chassis dynamometer test, the whole vehicle is mounted on a dynamometer for testing. In contrast, in an engine dynamometer test only the engine, rather than the whole vehicle, is mounted on a dynamometer. Manufacturers are allowed to certify compliance using low mileage cars and an agreed-upon deterioration assumption. They are not required to recall and test in- service vehicles. (Recalls may be required if emissions control systems are shown to be faulty.)

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OVERVIEW OF MOB/LE-SOURCE EMISSIONS 23 OXIDES OF NITROGEN 1 933 ~ / . 2::~ :,:,.:::, - .:.:::.:::.: ::-:::,.::.:.: :..\ :-:i-::-i:-: ::.- :ii:-:-:-\ ::-::-::-:: ::::- .~ :.-::.-:.-.:-: ,~,~ . - , -.i. ,- . ~ ::.:-.-.: .:- .;: .:.i: :-: -:-: : -: -: ii: :-:: -:-:-:.i: :-: :-:-. ~~ :- -,-. 03 VOLATILE ORGANIC CARBON 238~ ^:..N \ '- i.~ ".2 .-i-.- . ,2 ;-: :-:-:-::: i:-:-:-i:;-i:-i :~ :.-:.-:.-.-:.-::.-.:~ . : . ~ _ ~ , : : - 2.i. 'i,..i.i. i.,..,i..,i.,.i...-... :- i: ::-i-: - -i:--:- -- :-:-i:- ~ . ' .i -:.-.i .- ~ '~ it- .2 .2.- ii.~ ii. i- i. .- :. i. ~ A non ~ ~: ~ ~ -~0~ PARTICULATE MATTER (PM-10) 162 CARBON MONOXIDE 84- SULFUR DIOXIDE 48749 PARTICULATE MATTER (PM-2.5) A. . .:-.:-:. .- ..:.. .:. ..-.:...~105 :: :.-.:.:: ::::: :.:::::~:::: .:.:-..:.:.-.:....,:.::....:.:.:..-: :-:-:-:-i:-:-:---:-:~ :-:-:-:-:- 1 : -: :-- -.::: . 2.:~ : :-.: 1 I.. :. 1 \ ~ LEGEND I ~ .. .. GAS _ D I ESEL FIGURE 1-3 Estimated mobile-source emissions by fuel type. MOBILES and PARTS estimates of 1997 emissions from the on-road motor-vehicle fleet. It is likely that MOBILES underestimates gasoline VOC and diesel NOx emissions. Emissions are shown in thousands of tons. Source: EPA l99Sa.

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24 MODEL/NO MOBILE-SOURCE EMISSIONS TABLE 1-1 Passenger-Car Exhaust Gaseous Em 50,000 miles issions Standards (g/mi) 100,000 miles NOX NMHCa CO NOx THC (NMHC)a CO Mode! Year Pre-control 10.6 84.0 4.1 1968-71 4.1 34.0 3/4 3/4 3/4 3/4 1972-74 3 0 28.0 3.1 3/4 3/4 3/4 1975-76 1.5 15.0 3.1 3/4 3/4 3/4 1977-79 1.5 15.0 2.0 3/4 3/4 3/4 1980 0.41 7.0 2.0 3/4 3/4 3/4 Category Tier 0 (1981-93) 0.41 3.4 1 0 3/4 3/4 3/4 Tier 1 (beginning 0.41 (0.25) 3.4 0.4 0.31 4.2 0.6 with model year 1994- ~ NLEV (beginning 3/4 3/4 3/4 0.09 4.2 0.3 with model year 1999- ) Tier 2 - Default set 3/4 3/4 3/4 0.125 1.7 0.2 in CAAA9O (beginning with model year 2004- ) Tier 2- Current 3/4 3/4 3/4 >o.o9b >4.2b 0.07 proposed standards (beginning with model year 2004- ) Note: Emissions standards were originally written for total hydrocarbons (THC) and later for nonmehtane hydrocarbons (NMHCs). Appendix B describes the differences among the terms used to refer to gaseous organic compounds. This report, unless otherwise noted, will refer to the general class of gaseous organic compounds as VOCs. b Note: The proposed Tier 2 standards are a corporate average standard with a focus on NOX emissions. This allows NMHCs and CO emissions standards to "float", in that fleet emissions rates depend on the mix of vehicles used to meet the NOx standard. The emissions standards shown for NMHCs and CO are those that would result given the mix assumed in the Notice of Final Rulemaking (EPA 1999a) to meet the NOx standard. Sources: EPA 1998c,d, 1999a; Davis 1997; Chrysler Corp. 1998.

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OVERVIEW OF M OBILE-SOURCE EMISSIONS 25 The LDT category originally described vehicles designed for Toad-haul- ing rather than for passenger transportation, and was further divided into weight categories. Some of these LDTs, and some passenger cars, have evolved into vans and sport-utility vehicles. As a result, differences in size and function among passenger cars, LDTs, vans, and sport-utility vehicles have diminished. As a consequence EPA in its Final Rule for Tier 2 emis- signs standards (EPA 1999a) mandates the same emissions standards to passenger cars and LDTs.7 As with passenger cars, pollutant emissions limits for LDTs are expressed in grams of pollutant per mile and vehicles are certified on a chassis dynamometer. The MOBILE model retains the historical distinction between passenger cars and LDTs because current emissions standards and control technologies differ among vehicle classes. Heavy-Duty Vehicles Different exhaust emissions standards apply to the two broad categories of HDVs: trucks and buses, with further differentiation made between gas- oline and diesel engines. Emissions are regulated on grams of pollutant per brake-horsepower-hour (BHP-hr) because of the difficulty of devising reasonable grams per mile limits for the broad range of vehicles covered and the difficulty in developing a practical chassis dynamometer test. En- gines are certified on an engine dynamometer. Emissions standards were applied later than for LDVs and are less stringent than for LDVs. Because certification is for engines rather than vehicles, measurements are of engine emissions that are then used to estimate vehicle emissions. To obtain emissions in grams of pollutant per mile, which is what MO- BILE calculates, vehicle efficiency (BHP-hr/mile) must be estimated. The problem of converting engine dynamometer emissions to on-road emissions is the same for both buses and trucks. Engines used in buses must meet more stringent PM standards than engines used in trucks. Evaporative Emissions Evaporative emissions, including those resulting from leaks of liquid fuel, can be classified into five categories for modeling purposes: diurnal, 7EPA has also introduced a new category of vehicle in the Tier 2 proposal, the medium-duty passenger vehicle for passenger vans and sport-utility trucks between 8,500 and 10,000 pounds gross vehicle weight. This category of vehicle will also be required to meet the Tier 2 emission standard.

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26 M ODE[`NG MOB![E-SOURCE EMISSIONS hot soak, running loss, resting loss, and refueling toss. Originally, evapo- rative emissions were regulated for a combined sum of hot soak and diur- nal emissions. In more recent years, a separate limit was placed on run- ning loss evaporative emissions. The first on-board refueling-loss standard began with a 3-year phase-in period on passenger cars in 1998. An on- board refueling vapor canister controls these emissions. Another method of reducing refueling emissions, Stage II refueling controls, alters the de- sign of gasoline service station nozzles to allow the collection of vapors dis- placed during refueling. Large uncertainties are associated with the abil- ity to accurately characterize evaporative emissions. Of particular concern are large liquid leakers, which have been shown to result in extremely high VOC emissions for such vehicles (Haskew et al. 1999~. Off-Road Emissions Categories Non-road, or off-road, vehicles have been less regulated, and have come to contribute a greater fraction of pollutants as emissions from on-road vehicles have been controlled. These emissions are estimated separately from the MOBILE model. Their contribution is becoming increasingly im- portant in estimating total (on-road plus off-road) mobile-source emissions. Figure 1-4 shows an increasing percentage of NOX emissions coming from off-road sources between 1970 and 1997. The wide variety of sources in this category include: Construction, logging, mining, and farm equipment This equipment uses engines similar to those in heavy-duty trucks, primarily diesel, but operating characteristics can vary. Lawn and garden equipment This category includes small de- vices such as lawn mowers, chain saws, and leaf blowers. Both two-stroke and four-stroke gasoline engines are used. Recreational vehiclesThis category includes land and watercraft that use diesel and both two- and four-stroke gasoline engines. Ind ustrial, light commercial, and airport servicesThese vehi- cles are similar to those used for on-road and specialty applications, and use both gasoline and diesel engines in a variety of applications. Locomotive, marine, and aircraft engines Very large diesel and gas turbine engines dominate these applications. Emissions sources that do not impact air pollution are usually excluded, such as emissions from offshore ships and aircraft operations at high altitudes. Marine emissions associated with offshore oil operations are classified as stationary source . . emissions.

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OVERVIEW OF MOBILE-SOURCE EMISSIONS 27 in o ._ cn _` co in t ~ O a _ co x O ~ ~ O _ o . - z 8000 6000 ~ 1 20000 o ._ cn .m o o 80000 ._ x o ~ ~ o 40000 o 20000 C~ ~:5 ~,> 16000 o Q O ~n ~ S O C (D .0 _ U) Ct .m O > a 12000 1 0000 4000 - 2000 - O - 14nno o 1970 1975 1980 1985 1990 1995 Year o 1970 1975 1980 1985 1990 1995 Year 1970 1975 1980 1985 1990 1995 Year | Non-road 1~1 On-road FIGURE 1-4 On-road and non-road emissions for 1970-1997. Linear in- terpolation of data between 1970-1975, 1975-1980, 1980-1985, and 1985- 1987. Source: EPA 1998a.

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28 MODELING MOB/IE-SOURCE EMISSIONS Mobile-Source Emissions Using MOBILE and Related Models MOBILE is but one of many models used for the estimation of motor- vehicle emissions. Details and limitations of MOBILE are discussed in Chapter 3, and alternative approaches are discussed in Chapter 5. The MOBILE model is used to estimate vehicle emissions factors that are com- bined with estimates of vehicle mix and activities to estimate emissions inventories. The emissions factors can be tailored to specific fuel composi- tions, vehicle technology types (i.e., noncatalyst, carbureted three-way cat- alyst, and port fuel injection), and operating modes (i.e., cold start and hot stabilized). MOBILE also adjusts emissions factors for parameters such as ambient temperature, altitude, average speed, and trip characteristics (i.e., numbers of cold starts and length of diurnal soaks). This information is developed for a default or specified fleet composition. The end result is an estimate of the average emissions per mile for a vehicle type and the vehicle fleet. Vehicle activity is specified in terms of miles traveled by ve- hicle type within some defined area. This information is combined with MO- BILE's estimated emissions rate per mile to produce an emissions inven- tory for the area. Other related models use this information to estimate particulate and air-toxics emissions (PARTS and MOBTOX, respectively), the impacts of fuel composition (COMPLEX), and hydrocarbon speciation (SPECIES) profiles for air-quality modeling. Non-road mobile-source emissions are estimated in the NONROAD model by combining activity estimates for these sources with aggregate emissions factors. This model- ing approach is much simpler than the approach used in MOBILE. This report discusses several areas of concern about the use of MO- BILE. The MOBILE model is now used for purposes for which it was not originally intended, such as providing detailed air-quality modeling inputs, selecting control strategies based on prospective emissions reductions, and demonstrating conformity of transportation projects with the Clean Air Act. The General Accounting Office (GAO) has pointed to problems arising from use of the MOBILE model by multiple users for multiple purposes (GAO 1997~. Many of these uses are dictated by legislative and regulatory initiatives, discussed in the next section. Limited validation and the fail- ure of the model to address uncertainty create additional problems. Some observed shortcomings of the MOBILE model are summarized in Table 1- 2, and discussed at greather length later. LEGISLATIVE AN D REGULATORY ~ NITIATIVES Legislative Requirements and Compliance Attainment Plans The Clean Air Act and its amendments require that areas that have not met the NAAQS for ozone, carbon monoxide, nitrogen dioxide, sulfur diox-

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OVERVIEW OF MOBILE-SOURCE EMISSIONS 29 TABLE 1-2 Issues Relating to the Use of MOBILE Discussed in This Report Multiple uses and users need more than MOBILE can Chapter 2 provide. EPA, DOT, state and local agencies, and the automobile and oil industries have differing needs. Higher resolution of spatial and temporal scales are needed to evaluate the effectiveness of control strategies; that is, to provide estimates by time of day, day of week, and sublocations. Chapter 2 Models usually estimate emissions for only a few Chapter 3 pollutants, specifically CO, VOCs, NOX, PM (from the PART model), and air tonics (from the MOBTOX model). The impacts of these emissions vary with the chemical composition of the species and the size of the particles. Modeled effectiveness of emissions control programs, Chapters 3 such as motor vehicle inspection and maintenance, and 4 reformulated gasoline, and oxygenated fuels, are not directly linked to the measured effectiveness of such programs. Estimated emissions do not correspond to field observations. Observations of the in-use fleet include remote sensing and tunnel measurements. ice, and inhalable particulate material develop plans, known as State Im- plementation Plans or SIPs, describing how they will attain compliance. The widespread ozone and PM problems in urban areas have driven much of the clean air planning and regulation nationwide. The Clean Air Act as amended in 1990 (CAAA90) requires a comprehensive attainment SIP from every ozone nonattainment area classified as serious, severe, or ex- treme. The Act prescribes certain minimum control measures for each ozone nonattainment area, based on the severity of the problem. The Act also prescribes technical criteria; for example, each plan must contain a current emissions inventory, adequate ambient air-quality data, and an analysis of future air quality based on photochemical grid modeling. Out- side of California, the MOBILE model is used to estimate emissions from on-road mobile sources as part of the SIP. To ensure a minimum rate of progress, each ozone SIP must specify emissions targets for identified milestone years. The milestone years which emissions targets must be es- tablished include the attainment year and every third year of progress to- ward attainment.

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30 MODEI/NG MOBILE-SOURCE EMISSIONS Legislative Requirements Conformity Plans The CAAA90 prohibits regionally significant transportation activities, regardless of funding source, from impeding a region's progress toward attainment of the NAAQS. The conformity requirement is intended to en- sure that emissions associated with transportation improvements are com- pletely accounted for in the SIP. It is also to ensure that these improve- ments will not cause or increase the frequency of air-quality violations or delay attainment. The U.S. Department of Transportation (DOT) and met- ropolitan planning organizations (MPOs) determine whether projects pass the Conformity Demonstration. The procedures for Conformity Demon- stration have been amended several times since the final conformity rule was issued by EPA (EPA 1993a). As with attainment plans, the MOBILE model is matched with travel activity (e.g., trips, vehicle miles traveled, and speed profiles) to quantify the emissions of either a regionally significant project or a regional trans- portation plan. Regulatory Initiatives EPA has the authority to adopt standards for a broad range of mobile sources to assist states in achieving the NAAQS. For example, EPA used the CAAA90 to set tailpipe emissions standards for cars and light trucks beginning with the 1994 model year. These amendments require EPA to determine whether further emissions reductions from these vehicles are necessary. In 1998, EPA concluded that more stringent vehicle stand- ards- known as Tier 2 standards are needed to meet the NAAQS for ozone, and that the technology to meet these vehicle emissions standards is available and cost-effective. Along with the Tier 2 vehicle standards, EPA is lowering the allowable levels of sulfur in gasoline (EPA 1999a). Automobile manufacturers successfully argued that large reductions in gasoline sulfur content were needed to enable emissions control equipment to attain the Tier 2 emissions standards. EPA has also set emissions stan- dards for medium- and heavy-duty gasoline vehicles, and is considering increasing the stringency of some of these standards in the near future. MOBILE is the tool used to estimate the emissions benefits from all of these regulatory initiatives. EPA has also focused efforts on controlling emissions from heavy-duty diesel trucks and buses. Control of these emissions is important because diesel-fueled engines emit a complex mixture of gases, vapors, and parti- cles. These include NOx, PM, and many toxic air contaminants, including benzene, aldehydes, nickel, and polycyclic aromatic hydrocarbons. The fine particles produced by diesel engines are small enough to be inhaled

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OVERVIEW OF MOBILE-SOURCE EMISSIONS 3 7 and lodge deep in the lungs. Lower emissions standards for new heavy- duty diesel engines will be effective for the 2004 model year (EPA 1997b), and EPA is considering more stringent requirements for sulfur levels in diesel fuel and more stringent NOX and PM standards for future diesel en- gines. MOBILE and its related models play a major role in assessing these standards. COMMITTEE S CHARGE AND HOW IT ORIGINATED EPA first issued general guidance on model review in 1989 from its Sci- ence Advisory Board. The board recommended that the MOBILE model's predictive capability could be enhanced through: (1) obtaining external stakeholder input; (2) documenting the model's explicit and implicit as- sumptions; (3) performing sensitivity analyses; (4) testing model predic- tions against laboratory and field data; and (~) conducting peer reviews. As noted by the GAO (1996), the EPA's policy on peer review had not been followed during the updating of previous and the current versions of MO- BILE. However, EPA's Office of Transportation and Air Quality (formerly the EPA's Office of Mobile Sources) has made the review process an inte- gral part of the development of MOBILE6. An emissions modeling work group was established as part of the Mobile Source Technical Review Sub- committee to offer technical advice on MOBILE6 development. In addi- tion, the Office of Transportation and Air Quality has, outside the subcom- mittee process, developed a formalized procedure to obtain and respond to extensive stakeholder comments on MOBILE6 development. Congress has taken an interest in the accuracy and reliability of MOBILE because of its implications for the design of vehicle inspection and maintenance (I/M) programs and its role in regulatory decision-mak- ing. These issues were at the forefront during a 1995 hearing of the House Subcommittee on Oversight and Investigation on the effectiveness of vehi- cle I/M programs and how the MOBILE model credits these programs (U.S. Congress 1995~. These concerns prompted Congress to request not only the NRC study reported here, but also a study by GAO that describes the major limitations of the current MOBILE model and EPA's progress towards improving the model. The NRC committee has used the results of the GAO study (1997) as a source of information for reviewing current and future versions of MOBILE. Congress requested the National Research Council to review the MOBILE model in its 1997 appropriation for EPA. The task statement reads as follows: The committee will review the U.S. EPA's mobile source emission factor model (MOBILE). The committee will consider the adequacy of the model's input data, assumptions, structure, and results used to character- ize mobile source emissions. To the extent possible, the committee will

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32 MODEI/NG MOB/1E-SOURCE EM/SS/ONS consider ways to improve the reliability of the MOBILE model as a toot for assisting in the development of emission control strategies to meet air- quality goals. Specifically, the committee will evaluate the current and planned versions of the MOBILE model, as appropriate, with respect to the following aspects: . The types of mobile sources addressed, particularly the amount and quality of data from under-represented and unrepresented categories of sources (e.g., heavy-duty vehicles) and emissions of increasing interest (e.g. particles smaller than 2.5 mm). The strategies and methods for future data gathering in partnerships with other researchers, such as state agencies, industry associations or others with relevant data, in order to increase the amount and range of data in the most cost-effective manner. Alternative data sources and analytical techniques currently used for similar purposes by others (e.g. the California Air Resources Board in their EMFAC and other related mobile source emissions models) or the German government. The latest developments in related areas of modeling and how such advances might be incorporated in a new version of the model. The feasibility and requirements for the incorporation of modal mod- eling in MOBILE7 to reflect the effect on emissions of a variety of driving conditions and vehicle technologies. To the extent practical, the overall accuracy of the current version of the MOBILE model in predicting emissions to the atmosphere. REPORT STRUCTURE This report is the committee's response to its charge. The Executive Summary presents the committee's main recommendations. These recom- mendations encompass improvements to the MOBILE model as well as improvements to the overall process for estimating mobile-source emis- sions. Chapter 2 describes how the MOBILE mode! is used in air-quality planning and regulation. Chapter 3 briefly discusses the history and the technical aspects of the model and describes the major changes to the model for MOBILES. Chapter 4 considers the accuracy and uncertainties of the MOBILE model. Chapter 5 describes alternative modeling ap- proaches, some of which could be used to improve the modeling of mobile- source emissions. Chapter 6 incorporates the materials from the previous five chapters in a proposal for a new approach to modeling mobile-source emissions that relies on a toolkit of models that are better suited to the wide range of applications currently covered by MOBILE.