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Technical Issues Associated With the MOBILE Mocle' THE FOCUS OF CHAPTER 3 is the technical issues associated with the cur- rent (MOBILES) and upcoming (MOBILE6) versions of the MOBILE mod- el. The introductory portion of this chapter discusses the development of the model and the updates for MOBILE6. The chapter goes on to describe related models for estimating mobile-source emissions (PARTS, NON- ROAD, and others) and previous reviews of the model. A major portion of this chapter then focuses on the technical issues associated with the mod- el, such as how the model handles high emitters, driving cycle, start emis- sions, and many others details. The chapter concludes with a summary and recommendations related to technical aspects of the model. HISTORY AND STATUS OF THE MOBILE MODEL History of the MOBILE Model The MOBILE model for estimating on-road vehicle emissions factors (in grams per mile [g/mi]) was first developed by the U.S. Environmental Pro- tection Agency (EPA) in the late 1970s. Prior to that time, the agency pub- lished simple look-up tables for estimation of mobile-source emissions. The model, originally and still written using the Fortran scientific pro- gramming language, has had significant updates and new releases every few years as new data became available, new regulations were promul- 61

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62 M ODEL/NG M OB![E-SOURCE EMISSIONS gated, emissions standards were established, and sources and processes of vehicle emissions were better understood. Each new version of the model has become more complex in the approach to modeling average in-use ve- hicle emissions, and has provided the user with additional options for tai- Toring emissions-factor estimates to local conditions. The model versions, release dates, and changes in each model update are summarized in Table 3-1 (EPA 1999e). Changes in the databases underlying the models and changes in model- ing methodology in each successive version result in changes to predicted total on-road vehicle emissions. From one model version to the next, these changes can be either increases or decreases in emissions factors, and the changes are not always in the same direction for all three pollutants (NOX, CO, and VOCs). Although these changes created somewhat of a moving target for air-quality planners and the regulated industries, the revised models should provide more accurate analyses of mobile source emissions and of the effects of mobile source control programs. As an example, Figure 3-1 shows emissions for the Baltimore area for calendar years 1988 and 1990 as predicted using three recent official re- lease versions of the model (MOBILE5a, MOBILE4.1, and MOBILES; these comparisons are unaffected by the inclusion of new emissions stan- dards and regulations. Carbon monoxide (CO) emissions and emissions of nitrogen oxides (NOx) increase from one model to the next, albeit in differ- ent proportions. Volatile organic compound (VOC) emissions, though, de- creased from MOBILE4 to MOBILE4.1, and then increased significantly from MOBILE4. 1 to MOBILE5a, while still remaining lower than MOBILE4 levels. MOBILESThe Current Moclel The MOBILES model, released in 1993, provides emission factors for on-road vehicles for the three regulated pollutants: VOCs, CO, and NOX. The model provides emission factors separately for the classes of vehicles listed in Table 3-2, and also for the average on-road fleet using a default national mix of vehicles; the user can optionally input a different fleet mix for the calculation of fleet average emissions. The vehicle classes are fur- ther subdivided into technology classes in MOBILE, to account for emis- sions differences between, for example, vehicles with carburetors and those with fuel injection. To estimate total on-road mobile emissions in a given area, either the vehicle class emissions factor is multiplied by esti- mates of vehicle miles traveled (VMT) by vehicle class for the area and summed, or the fleet average emissions factor is multiplied by total VMT (across vehicles classes) for the area. These VMT estimates are typically provided by local or regional transportation-planning agencies.

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TECHNICAL ISSUES ASSOCIATED WITH THE M OBILE MODEL 63 TABLE 3-1 MOBILE Model Revisions Release Version Date Model Revisions MOBILE 1 1978 Included modeling of exhaust emissions rates as func- tions of vehicle age/mileage (zero-mile levels and deteriora- tion rates). MOBILE2 1981 Updated with substantial data (available for the first time) on emission-controlled vehicles (i.e., catalytic con- verters, model years 1975 and later) at higher ages/mileages. Provided additional user control of input options. MOBILES 1984 Updated with substantial new in-use data. Elimination of California vehicle emissions rates (contin- ued to model low- and high-altitude emissions). Added tampering (rates and associated emissions im- pacts) and anti-tampering program benefits. MOBILE4 1989 In-use emissions-factor estimates for nonexhaust emis- sions adjusted for real-world fuel volatility as measured by Reid vapor pressure (RVP). Updated with new in-use data. Added running losses as distinct emissions source from gasoline-powered vehicles. Modeled fuel volatility (RVP) effects on exhaust emis- sions rates. Continued expansion of user-controlled options for input data. MOBILE4.1 1991 Updated with new in-use data. Added numerous features allowing user control of more parameters affecting in-use emissions levels, including more inspection/maintenance (I/M) program designs. Included effects of various new emissions standards and related regulatory changes (e.g., test procedures). Included impact of oxygenated fuels (e.g., gasohol) on CO . . emlsslons. MOBILES 1993 Updated with new in-use data, including basing new ba- sic emissions-rate equations on much larger database de- rived from state-implemented IM240 test programs. Included effects of new evaporative emissions test proce- dure (impact on in-use nonexhaust emissions levels). Included effects of reformulated gasoline (RF G). Included effects of new NOX standard of 4.0 g/bhp-hr for heavy-duty engines. Included impact of oxygenated fuels on VOC emissions. Included Tier 1 emissions standards under 1990 Clean Air Act Amendments. Added July 1 evaluation option. (Corltirrued)

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64 MODE1/NG MOB/[E-SOURCE EMISSIONS Table 3-1 Model Revisions (Continued) Included impact of low-emission vehicle (LEV) programs patterned after California regulations. Revised speed corrections used to model emissions fac- tors over range of traffic speeds. MOBILE5a 1993 Corrected a number of minor errors in MOBILES. MOBILE5b 1996 Included final on-board vapor-recovery regulations. Included final reformulated gasoline regulations. Added more user options for I/M programs. Source: EPA l999e All of the MOBILES emissions factors are estimated from existing test data, and engineering judgment in the absence of test data. Although there is a detailed User's Guide (EPA 1994) for the model, there is limited documentation from EPA describing the databases and analytical methods used in MOBILES to develop the emissions factors. The user provides inputs (some required and some optionaV to MOBILES that describe typical operating characteristics, fleet character- ization, and mobile-source control programs. These inputs (in addition to vehicle class and VMT mentioned above) are ambient temperature; average vehicle speeds by vehicle class; . fuel characteristics; (include fuel volatility and oxygen content, and if reformulated gasoline is in use); vehicle inspection and maintenance (I/M) program parameters, if such a program is in place; and vehicle age distributions (used to estimate composite emissions across all vehicle model years). Figure 3-2 shows emissions estimates developed using an updated ver- sion of MOBILES and updated travel-activity estimates. It shows the pre- dicted distribution of on-road mobile source emissions of VOCs and NOX in year 2007 by emissions category for New York and Chicago. These inven- tories were generated by EPA using the latest emissions model developed as part of the regulatory impact assessment for the recent Tier 2 vehicle emissions and fuel sulfur standards (EPA 19996~. This model, known as the Tier 2 Model, was developed from MOBILE5b and available elements of the upcoming version, MOBILES (EPA l999c). The Tier 2 model is ac- tually a spreadsheet program derived from MOBILE algorithms, outputs, supplemental test data, and assumptions. The MOBILES elements incor- porated into the Tier 2 Model include updated assessments of in-use vehi- cle deterioration, fuel sulfur impacts, and fleet characteristics. However,

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TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL 65 60 - 50 - - ~o - o ~ 30 o US In - 10 - o - 1E MOBILE5A 20 ~ ~ ll]~: MOBILE4.1 O MOBILE4 HC HC NOX NOX CO/10 CO/10 1988 1990 1988 1990 1988 1990 FIGURE 3-1 Comparison of estimated emissions for Baltimore from three recent versions of the MOBILE model. Note that CO emissions are divided by 10. Also note that this report usually uses the term VOCs as opposed to hydrocarbons (HCs) to refer to the general class of gaseous organic com- pounds the Tier 2 Model does not use the MOBILES methodology and test data for estimating evaporative emissions. MOBILES will likely increase the frac- tion of evaporative to tailpipe emissions of VOCs compared with that ob- tained in MOBILE5b. Both cities depicted in Figure 3-2 show a fairly similar emissions profile, although important differences are clear. Chicago has much greater emis- sions from heavy-duty vehicles. This is especially apparent for NOX emis- sions; Chicago has 37% of emissions from HDDVs whereas New York only has 24%. Chicago also has greater emissions from motorcycles. For exam- ple, Chicago has 7% of their VOC emissions attributed to motorcycles, over twice the percent of emissions from motorcycles in New York. Generally speaking, the Tier 2 Model estimates that about 45% of the total on-road VOC emissions is from light-duty vehicle exhaust, about 30% is from light- duty evaporative emissions, and the remainder is primarily from heavy- duty vehicles. Note that the regulatory impact analysis was performed for four cities Atlanta, Charlotte, Chicago, and New York. However, the emissions profiles for Atlanta and Charlotte were similar to those for New York, and are not shown here.

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66 MODELING MOB/LE-SOURCE EMISSIONS TABLE MOBILE b Vehicle Classes Vehicle Class Light-duty gasoline vehicles (passenger cars) Light-duty gasoline trucksa (pick-ups, minivans, passenger vans, and sport-utility vehicles) MOBILE Code LDGV Weight Description Up to 6000 lb gross vehicle weight (GV\iV) Up to 6000 lb GVVV LDGT1 LDGT2 Heavy-duty gasoline vehicles HDGV Light-duty diesel vehicles (passenger cars) Light-duty diesel trucks Heavy-duty diesel vehicles Motorcyclesb Emissions for light-duty trucks are modeled separately for two weight classes with different emissions standards in the Clean Air Act bHighway-certif fed motorcycles only are included in the model. OiT-road motor- cycles, such as dirt bikes, are modeled as a non-road mobile source in EPA's NON- ROAD model. LDDN7 LDDT HDDV MC 6001-8500 lb GVVV 8501 lb and higher GVW equipped with heavy-duty . gave .lne engines Up to 6000 lb GVVV Up to 8500 lb GVW 8501 lb and higher GVW MOBILE6- The Next Generation Model EPA's Office of Transportation and Air Quality (OTAQ) has for the last several years been working on the next generation of the MOBILE model, referred to as MOBILE6. This model will be significantly different from MOBILES in almost all model components, and will be based on an enor- mous amount of recent vehicle-emissions testing data from EPA, the Cali- fornia Air Resources Board (CARB), automobile manufacturers, and petro- leum refiners. At this point, MOBILE6 is expected to be released in the year 2000. However, EPA has already released substantial documentation and held workshops describing the model revisions, allowing the agency to gather feedback on its proposed modifications. This documentation and public outreach process will be discussed in the following section. The sig- nificant changes being incorporated into MOP;TT,F:Hi into t.h~ fallowing dramatically lower basic emissions rates, based on analyses of the Dayton, Ohio I/M program data; Reparation of start and running-exhaust emissions; addition of so-called off-cycle emissions (aggressive driving and air- conditioning operation, which are not included in the Federal Test Proce- dure [FTP]~;

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TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL 67 2007 VOC New York 6% 3% 1%~ - ~...3 ~ l ~~ ~ 9% ~ l ~ ,.~ ~ Ft., ~ ~ ~ .. -.- :::- :.-. ~ ~ ~ ~ ~ 48% .:.:...: .-.:.:.:.:.:: :.: ::.:: :.::t ~ ~ ~ ~ (~ ! i ~ 33%~. 11 ~ i New York 24% ~ ~ I...*....*. I,-.--..--.. '* *-* ****,.*.* i.S**..SSSSSS... Fat. -*an. .~*~**~. ****~-*-~*-**. ~0.2% 2007 NOx 4% Chicago 10% _ ` - ~ ~_ , _ I,,..,,_ ~SSSSSSS. 29% '>-an--. ~~..~., _,-----... ~SSS.SSS:SS ~ err ~- ..- ~- - ~ `5 / /0 ~ a***-----*-* ~~~~ 1*~.are- - - h. SSSS--SSS. S SSSSSSSSSSSSSS +*~- - - - - - - .~-~. I---------- ~- ~.~ 22222222222-.* I-.-.-.-. . '. ''I. . ~ Chicago Legend LDV Exhaust ~ LDV Evap. ~HDGV Exhaust ~ ~ HDGV Evap. ~ HDDV Exhaust ~ MC Exhaust & I Evap. 58% FIGURE 3-2 VOC and NOx emissions inventories for New York and Chicago. Source: EPA 19994. control of off-cycle emissions with the Supplemental FTP (SFTP) in future years; . emissions factor estimates for different roadway types (e.g., highways arterials, locals); evaporative diurnal emissions factors estimated *om real-time diur- nal test data previously unavailable,

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cards. 68 MODELING MOBILE-SOURCE EMISSIONS revised (lower) estimates of the effects of oxygenated fuels on CO . . emlsslons; revised (lower) effects of I/M programs on vehicle emissions; addition of off-cycle NOx emissions for heavy-duty diesel vehicles; effects of in-use fuel sulfur content on all emissions; and effects of national low-emissions vehicle (NLEV) and Tier 2 stan- Although the MOBILE6 documentation provides numerical results for changes in specific model components, overall changes to average in-use fleet emissions factors will not be known until the full model is released. Thus, it is not yet known whether regional emissions estimates from MOBILE6 will increase or decrease relative to MOBILES, though it is like- ly that they will increase at least for VOCs in order to be in better agree- ment with the findings of evaluation studies that are discussed in Chapter 4. FEDERAL ADVISORY COMMITTEE ACT PROCESS AND PUBLIC OUTREACH An important part of the developmental process for MOBILE6 has been public outreach. This includes input from EPA advisory committees, com- ments *om stakeholders and the interested public, and the release of tech- nical documentation describing model modifications. The Clean Air Act Amendments of 1990 (CAAA90) established the Clean Air Act Advisory Committee to advise EPA on issues of implementa- tion this law. One of the many subcommittees of the Clean Air Act Advi- sory Committee is the Mobile Sources Technical Review Subcommittee (MSTRS), often referred to as a FACA subcommittee because it is char- tered under the Federal Advisory Committee Act (FA CA). The MSTRS advises EPA's OTAQ on technical issues specific to the control of emissions from mobile-sources. It is composed of experts on mobile-source emissions from industry, academia, state agencies, and nongovernmental organiza- tions. Meetings are held quarterly and are open to the public. One of the MSTRS working groups is the Modeling Working Group, which provides on-going advice on the development and improvement of MOBILE and other emissions models. The specific charge for this work group includes helping to set priorities for developments to MOBILE6 and developing procedures for EPA to use when obtaining outside review for products used to support MOBILE6. This group also is producing a com- prehensive report on the MOBILE modeling process, problems, and oppor-

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TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL 69 "unities for improvement on which this committee was briefed (this paper, "Big Picture Modeling Issues," is currently in draft form). Additionally, EPA has increased public feedback and public involvement in the development of MOBILE6 in several other ways. These steps were taken to make the model better understood by the user community and to counter criticism that the model was inadequately documented and peer reviewed. Although the draft MOBILE6 model is not expected to be avail- able until later in 2000, EPA has released detailed technical documenta- tion for most of the proposed changes in the Stakeholder Review Docu- ments on MOBILE6 web page (http://www.epa.gov/OMSWWW/m6.htm). EPA is to be commended for documenting the databases used and the de- velopment of revised emissions factors in the Stakeholder Review Docu- ments. This documentation, although not always complete in describing the full details of the analyses, is a major improvement from all previous versions of MOBILE. EPA provides a 60-day review period for each docu- ment as it is posted, and has stated its intention to provide all comments and responses to comments with each document in the final version. EPA has also held three workshops discussing the new version of the model and created an e-mail list server to update interested parties on new model developments. The workshops were open to the public. They were intended to update interested parties on EPA's plans for the model as well as solicit input and reaction to those plans. The workshops included both technical presentations describing changes to the model methodology and presentations oriented to model users describing changes to data in- put and output. The e-mail list server is used to announce the workshop agendas, the release of new documentation concerning the MOBILE6 mod- el, updates to the current version of MOBILE5b, and other information. RELATED MODELS There are several emissions models and databases related to EPA's MO- BILE model, which are used to estimate mobile-source emissions invento- ries and provide inputs for air-quality models. These are PARTS estimates particulate matter (PM) emissions factors for on- road vehicles; Complex Model estimates emissions impacts of reformulated fuel compositional changes on 1990 light-duty gasoline vehicles; MOBTOX estimates on-road mobile-source toxic emissions factors; and SPECIATE, and related databases and models provide VOC speciation profiles for complex photochemical grid modeling.

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70 MODE[/NG MOBI[E-SOURCE EMISSIONS These models are each described briefly below. References are provided for readers who desire more detailed information on any of these models. PARTS EPA's PARTS mode! estimates PM emissions factors in g/mi for 12 vehi- cle classes. Emissions factor estimates are provided for particle diameter sizes from less than or equal to 1.0 to 10.0 ,um (micrometers). The model is referred to as PARTS to indicate consistency with MOBILES in fleet char- acterization data and in the general methods used to estimate basic emis- sions rates. PARTS estimates aD PM emissions associated with on-road travel: exhaust emissions, brake-wear emissions, tire-wear emissions, and fleet-average paved and unpaved road-dust emissions. For HDDVs, PARTS also provides estimates of idle emissions. The PARTS model has been updated in only very minor ways since the original model develop- ment in the mid-1980s. The emissions factors in the model are based on either engine certification data or on ratios from VOC emissions. Al- though there is a User's Guide for the model, there is no documentation that explains the derivation of the emissions factors in the model. The emissions-factor estimates in the PARTS model are seriously out of date. New PM test data have recently become available for both light- duty vehicles (LDVs) and heavy-duty vehicles (HDVs). Although on-road mobile-source emissions-factor estimates will be needed for state and local air-quality planning agencies to develop PM emissions inventories and air- quality management plans for the new PM standards, EPA's OTAQ has not focused as much effort on updates to the PARTS model as on MOBILE. However, many of the revisions developed for MOBILES can be easily in- corporated into PARTS, and OTAQ has done so for estimating PM emis- sions as part of the Tier 2 rulemaking (EPA 1998d). But the major im- provement required for the model is the inclusion of recent testing data for the revision of emissions factors. Because most of the methods for estimating PM emissions in PARTS are similar to the methods used in MOBILES, and because PARTS needs major revision, an updated version of MOBILES should incorporate re- vised PM emissions-factor estimates. For most users, it would be much more desirable to have one integrated model that provides emissions-fac- tor estimates for PM as well as VOC, NOX, and CO. Complex Moclel The Complex Model is used by petroleum refiners and other interested parties to estimate how gasoline composition affects vehicle emissions.

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TECHN/CA~ ISSUES ASSOCIATED WITH THE M OBILE MODEL 7 7 The model was developed in a regulatory negotiation process between EPA and the affected industry. The model is fully described in EPA's reformu- lated gasoline Regulatory Impact Analysis dated December 13, 1993 (EPA 1993b). The Complex Model, which is a spreadsheet model downloadable from EPA's reformulated gasoline web page (http://www.epa.gov/ OMSWWW/rfg.htm), predicts percent change in 1990 technology vehicle emissions for a target reformulated gasoline (RFG) relative to U.S. 1990 baseline gasoline. Emissions are a function of the following input parame- ters: MTBE (methyl tertiary-butyl ether, weight percent oxygen twt%~), ETBE (ethyl tertiary-butyl ether, wt%), ethanol (wt%), TAME (tertiary-amyl methyl ether, wt%), sulfur (parts per million [ppm]), RVP (Reid vapor pressure, pounds per square inch [psi]), E200 (percent of fuel that evaporates at 200 F), E300 (percent of fuel that evaporates at 300 F), aromatics (percent by volume), olefins (percent by volume), and benzene (percent by volume). U.S. baseline emissions are calculated from MOBILES runs with U.S. industry-average gasoline. The model calculates changes from baseline emissions to emissions for the target fuel for exhaust and evaporative VOCs, air taxies (benzene, formaldehyde, acetaldehyde, and 1,3-buta- diene), and exhaust NOX. (The model does not estimate the effects of fuel reformulation on exhaust CO emissions.) The model is a statistical model based on testing data from several major programs measuring the emis- sions effects of the various fuels tested. The present and planned versions of MOBILE allow for the specification of limited fuel properties (e.g., whether or not RFG is used) but do not al- low for the specification of the detailed fuel properties that are available in the Complex Model. This means that if states or nonattainment areas choose to require a fuel with greater emissions reductions than required of federal RFG, the Complex Model must be run first to generate scaling fac- tors to apply to MOBILE output. Even then, there are questions as to how the results would be used given that COMPLEX was developed for 1990 vehicle technologies only. The fuel effects now calculated in the Complex Model should be updated so that they can appy to all model years and technology groups, not just 1990 technology vehicles, and added to future versions of MOBILE. CARB's Predictive Model estimates fuel effects for all on-road light-duty vehicles based on a broader database than that used in the development of the Complex Model; EPA should review and consider

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Pre-1981 .315 .193 .368 .193 1981- 1985 .486 .025 .493 .025 1986- 1990 .172 .006 .122 .006 1991- 1997 .018 .004 .035 .004 724 MODELING MOB/LE-SOURCE EMISSIONS TABLE 3-10 PM Emissions Rates in g/mi from PARTS and from the CRC Project E-24-2 Passenger Cars Light-Duty Trucks Model Year CRC PARTS CRC PARTS _ . Source: Cadle et al. 1998. report prepared for EPA (Weaver et al. 1998) shows significant deteriora- tion for 1994 and later trucks and transit buses. In addition, work by Graboski et al. (1998) concluded that PARTS was "significantly underesti- mating" PM emissions from heavy-duty vehicles. It is important that EPA revise PARTS to reflect in-use PM emissions. This will likely require extensive field measurements. Data are also need- ed to assess differences between HDDV PM measurements obtained in the laboratory compared to in-use emissions; EPA should review work cur- rently being conducted by the CRC and the National Cooperative Highway Research Program in this area. In addition, data are needed relative to systems of the engine and the vehicle that reflect maintenance problems that affect emissions. Studies are also needed on the effectiveness of die- sel I/M programs and whether smoke I/M programs tend to increase NOX TABLE 3-11 Passenger Car PM Emissions Rates in g/mi from PARTS and from the CRC Projects E-24-1 and CRC E-46 Model Year CRC E-24-1 PARTS Pre-1981 .955 .193 1981- 1985 .474 .025 1986-1990 .444 .006 1991-1997 .028 .004 CRC E-46 PARTS Tier 0 .083 .025 Tier 1 .038 .006 l Source: Norbeck et al. 1998; Cadle et al. 1999.

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TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL ~ 25 emissions and reduce PM emissions. Ways of screening and characterizing the in-use vehicle population for high-emitting vehicles are needed. EPA needs to determine the effectiveness of the catalytic oxidation devices used since 1994 to reduce PM emissions by reducing the SOF. There is no infor- mation on the long-term effectiveness, maintenance practices, or tamper- ing for these devices. Particulate Emissions from Tire and Brake-Wear PARTS estimates emissions from tire wear based on the assumption that the emissions rate of airborne particulate is 0.002 grams per mile per wheel (EPA 1998j). This reference, known as AP-42, lists two studies as the basis of the estimate for tire-wear emissions Williams and Cadle 1978; Brachaczek and Pierson 1974~. The single emissions rate is based on tests of LDVs and no estimate for the airborne particle size distribution for tire-wear is offered. Tire-wear emissions less than 10 ,um are based on interpolation. The dated references for the PARTS emissions factors sug- gests that these factors are based on tests of older biased-ply tires rather than longer-wearing tire technologies currently in use. PARTS reports brake wear as a separate emissions factor of 0.0128 grams per mile, based on a paper by Cha et al. (1983~. Brake-wear partic- ulate emissions are higher than for tire wear because a larger fraction is assumed to be less than 10 ,um in diameter. Brake-wear emissions factors in PARTS are assumed to be the same for all vehicle classes, although it could be assumed, as with tire wear, that the number of wheels, the weight of the vehicle, and the driving cycle would be significant contribut- ing factors related to the per mile emissions rate. As with tire wear, the dated reference suggests that the emissions factor is based on older mate- rials and needs to be updated. Issues for Model Revision EPA is planning to update PARTS after it completes the updates for MOBILES. As recommended above, such a revision should become part of MOBILE rather than being issued as a separate model. Issues that need to be addressed while updating PARTS are listed below: Data on particulate emissions from HDVs need to be updated to in- clude the effects of deterioration in emissions, adjustment for the benefits of I/M, and variations due to actual driving conditions.

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726 MODELING MOBILE-SOURCE EMISSIONS Data from new studies on emissions from light-duty gasoline-powered and diesel-powered vehicles should be included in the updated particulate emissions model. Data collection efforts should be expanded to ensure that the effects of deterioration, I/M, and off-cycle driving conditions are included in the model. Data on modern tires and brake materials must be obtained for inclu- sion in future particulate emissions inventories. FLEET CHARACTERIZATION In MOBILE, the fleet is characterized by three parameters: age or regis- tration distribution, mileage-accumulation rate, and fleet or VMT mix. The age distribution gives the fraction of all vehicles in a particular class that are of a certain age. Because MOBILES accounts for 25 different ages (except for motorcycles), 25 fractions are required for each vehicle class, with the fractions summing to unity within each class. The mileage accu- mulation rate is the annual number of miles a vehicle is expected to be driven. It varies by vehicle age and class. The fleet mix gives the fraction of the fleet total VMT traveled by each of the eight vehicle classes. Again these fractions must sum to one. MOBILE calculates a vehicle class's emissions factor by computing the emissions factors for each of the model years, weights these by each model year's contribution to the vehicle class's total annual VMT, and then sums the weighted emissions factors. The weighting factors are termed travel fractions. The travel fraction, TFm, represents the fraction of the total VMT that is accounted for by a vehicle of age m years. It is calculated *om the fraction of vehicles registered that are m years old, REGm, and the annual mileage accumulation for these vehicles, MILESm, TF = REGm * MILES m MaxYears ~ (REGk M}~ESk k = ~ (3-7) where the summation is over all model years k. Once the vehicle-ciass- specific emissions factors are computed, the model then weights each of them by the corresponding fleet mix fraction and sums the results to pro- duce the fleet emissions factor. Although the user is allowed to enter custom registration distributions, mileage- accumulation rates, and fleet mixes, the model contains default (or for fleet mix, internally calculates) values for these parameters. The

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TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL 727 default registration distributions are obtained from sales fractions through the 1980s. Mileage accumulation rates in MOBILES are based on 1990 National Purchase Diary (NPD) data. Fleet mix is internally calculated using registration distributions, mileage accumulation rates, diesel sales fractions, and total vehicle counts by class. MOBILE6 will see changes in all of the above parameters. These changes are documented in an EPA report (1999x). They are brought about by the availability of new data as well as shifts in methodology in some cases. The new model expands the previous 8 vehicle classes to 28, thus requiring much more detailed fleet characterization data. New mileage-accumulation rates for light-duty vehicles and light- and heavy- duty trucks are derived from the 1995 National Personal Travel Survey (NPTS) and the 1992 Truck Inventory and Use Survey (TIUS), respec- tively. Revised registration distributions are obtained from 1996 data compiled by the R.L. Polk company. Vehicle counts are based on data from various sources including the 1996 Polk data, 1998 Certification and Fuel Economy Information System (CFEIS) database, the Annual Energy Out- look (Energy Information Administration 1998), and a report by Ward's Communications (Pemberton 1996) which gives scrappage rates. Figures 3-16, 3-17, and 3-18 give sample comparisons of the MOBILES and MOBILE6 mileage-accumulation rates, registration distributions, and vehicle counts. Note that the new registration distribution is smoothed. This is a significant change from the former approach, which reflected ac- tual historical trends in sales and perpetuated them in all future-year cal- culations. SUMMARY AND RECOMMENDATIONS EPA is currently developing MOBILE6, the newest version of the MO- BILE model. This version is scheduled for release in the year 2000. In developing MOBILE6, EPA has used a more-open process, involving many stakeholders, and has been published much documentation for general review. In the development of MOBILE6, EPA has addressed many shortcom- ings of MOBILES, particularly those identified in the GAO (1997) report. The extent to which MOBILE6 has addressed those concerns is summa- rized in Table 3-12. The table also notes some improvements that can be made in future versions of MOBILE. In addition to the recommendations in Table 3-12, the committee offers the following recommendations for the improvement of MOBILE. These recommendations begin with changes to components in the existing

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~ 28 M ODE1`NG M OB!LE-SOURCE EMISSIONS 20,000 1 8,000 1 6,O00 1 4,000 1 2,000 1 0,000 8,000 6,000 4,000 2,O00 + 1995 NPTS MOBILE5a (LDGV only) MOBI' E5a (LDDV only) Expon. (1995 NPTS) y = 1 s6g4e-o.oso6x O. ~ , 1 1 1 0 5 10 15 20 25 30 35 Age FIGURE 3-16 Light-duty vehicle annual mileage-accumulation rates. MOBILES uses the curve developed from the 1995 NPTS data. Source: EPA 1999x. MOBILE model. The next set of recommendations pertains to models that are closely related to MOBILE, such as those that estimate PM and air toxic emissions. The final recommendation deals with the need for long- range planning to guide the future development of the model. Obtain Better Data on High-Emitting Vehicles Establish a long-term testing program to characterize in-use deteriora- tion of representatively aged new-vehicle technology using a driving cycle more representative of actual driving conditions. This should focus on de- termining the nature of both exhaust and evaporative high emitters. Im- proved data on both the emissions rate and the *action of the vehicle pop- ulation that are high emitters are required. Inclusion of Road-Grade Effects in MOBILE The emissions increase from road grade is similar to that from accelera- tion and should be included in the model. This will be particularly impor-

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TECHN/CAI ISSUES ASSOCIATED WITH THE M OBILE M ODEL ~ 2 9 0.1 - JO 0.08 a_ ~ 0~06 lo 0.04 - 0.02 Lo _~ , , , 'it O - 0 5 10 15 20 25 Age MOBILES ~ MOBILE61 FIGURE 3-17 Comparison of MOBILE 5 and MOBILES light-duty vehicle registration distribution. Source: EPA 1999x. tent for areas where there is a significant amount of grade such as Denver and Spokane. Planning for this feature should include input from local regions that use MOBILE to ensure that grade information is available to potential users and that the model revisions are consistent with the avail- able formats of the grade data. Improve the Start-Emissions Database Routine tests of start emissions should be made as part of ongoing mea- surement programs unless there is confirmation that regression tech- niques, similar to those used for MOBILES, provide an effective estimation of start emissions. Additional measurements of the effects of ambient tem- perature, wind speed, and soak time on start emissions should be made to get a better representation of these important factors. Another factor that should be considered when estimating start emissions is the operating mode of a vehicle during the first minutes of operation. Modeling of Inspection and Maintenance Programs Benefits In particular, the treatment of vehicles that failed emission tests but never appeared for a retest, owners who never have their vehicles in-

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30 MODEL/NO MOB!LE-SOURCE EMISSIONS 350,000,000- 300,000,000 250,000,000 200,000,000 150,000,000 100,000,000 50,000,000 7 - ,.~ O- . 1982 2002 2022 2042 Calendar Year MOBILE 5 Linear Interpolation MOBILES FIGURE 3-18 Comparison of light-duty vehicle counts, 1982-2050. Source: EPA 1999x. spected, and the deterioration of vehicles after repair needs to be im- proved. The modeling of repaired vehicles' deterioration should be based on data from actual repaired vehicles. Improve the Emissions Factors for Heavy-Duty Vehicles Emissions factors for HDVs are woefully outdated and there are ques- tions about the conversion of engine dynamometer data into on-road gram- per-mile emissions. Appropriate chassis dynamometer cycles need to be developed for HDVs and data must be obtained on such cycles. Appropri- ate corrections for the effects of humidity and temperature, currently un- der development, should be incorporated into MOBILE. Data should to be generated for in-use conditions that might have significantly different emissions from those predicted based on engine certification tests. Updating of Fleet Characterization In recent years, there has been a significant increase in the use of light- duty trucks (especially sport-utility vehicles) instead of and in addition to passenger cars. EPA has updated fleet characterization data for MOBILES to reflect these current trends. EPA should at regular intervals (every 2 years or so) review the fleet characterization data, both current and pro-

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TECHNICAL ISSUES ASSOCIATED WITH THE M OBILE MODEL 7 3 7 TABLE 3-12 Summary of Expected Changes to MOBILE6 That Respond to Problems Identified in GAO Report (1997) Area of Concern Regarding MOBILE Model Cited in GAO Report MOBILE6 Treatment of Issue 1. Emissions estimates for higher speeds, especially speeds in excess of 65 mph. Representation of emissions from rapid acceleration and deceleration, including aggressive clrlvlng behaviors. Representation of emissions immediately after engine start-up, known as cold-start emissions. 4. Representation of emissions *om air conditioner use. Representation of emissions from road grade, such as when a car climbs a hill. 6. Representation of high- emitting vehicles in the MOBILE's supporting database. 7. Representation of exhaust emissions from lower- polluting fuels, especially fuels with lower volatility (low RVP); representation of emissions from oxygenated fuels. MOBILE6 uses data obtained from recent studies on real-world driving conditions to develop facility-specific speed-correction cycles, which include higher speeds and aggressive driving behavior. The facility-specific speed-correction factors also provide greater distinction in roadway classifications. Start emissions have an improved treatment in MOBILE6; more study should be done to provide additional data for the approach proposed. MOBILE6 has an improved model of air-conditioner use. Additional data and model modifications could improve the estimates of this effect. Not addressed in MOBILE6. EPA used data from IM240 lanes to correct FTP data for recruitment bias in exhaust-emissions data. Special studies should be done to determine effect of high emitters. MOBILE6 exhaust emissions effects of low RVP fuels has not changed from MOBILES. MOBILE6 shows reduced benefits from oxygenated fuels, based on EPA analysis of more recent test data.

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7 32 MODE1JNG M OBILE-SOURCE EMISSIONS TABLE 3-12 (Continued) Area of Concern Regarding MOBILE Model Cited in GAO Report MOBILE6 Treatment of Issue 8. Representation of emissions system deterioration for ve- hicles with 50,000 or more miles. 9. Emissions estimates and assumptions for vehicle I/M programs. 10. Estimates and assumptions for nontailpipe evaporative emissions when the vehicle is not operating. 11. Emissions estimates and assumptions for the inspec- tion and maintenance of HDVsthose with a gross vehicle weight of 8,501 pounds or more. Data characterizing vehicle fleet. Greater distinctions in road- way classifications. Quantifying the uncertainty of the model's estimates. New data have shown much lower emissions rates for such vehicles. These data have been used in MOBILE6. MOBILE6 shows reduced benefits from I/M programs. Questions re- main about the assumed benefits for OBD and the assumed deterioration of repaired vehicles. MOBILE6 includes updates to rest- ing loss emissions based on real- time (24-hr) test data. Not included in MOBILE6. EPA has updated fleet characteris- tics (fleet mix and age and mileage- accumulation distributions by vehi- cle classy for MOBILE6. See response to items one and two. Not included in MOBILE6. jected, to ensure that changes in the vehicle fleet are properly recognized in the emissions model. Complete Documentation of all Databases and Analyses EPA has done an excellent job of improving their documentation of the basic steps in the MOBILE model. Additional documentation should be provided to explain all the details of the analyses so that interested parties

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TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL 7 33 can readily check the analyses. Placing these detailed analyses and all the databases used on the internet would facilitate external review of the da- tabases and methods used in MOBILE. Integration of the PARTS and MOBILE Models PARTS and MOBILE do basically the same thing: compute actual emis- sions from on-road mobile sources. The separation of gaseous emissions in MOBILE and particulate emissions in PARTS is not necessary. It requires users to run two models instead of one and leads to the possibility that the on-road motor vehicle fleet and other important factors will not be treated consistently between the two models. When incorporating the PARTS model into MOBILE, several problems with PARTS need to be addressed. Updated emissions factors should be developed incorporating data on the effects of high-emitting vehicles (smoking vehicles), in-use deterioration, I/M programs, speed variations, and off-cycle emissions. Incorporation of the COMPLEX Model into MOBILE The effects of reformulated gasolines are currently estimated in a sepa- rate model called the COMPLEX model. Incorporating a COMPLEX-like model into MOBILE would allow states and regions to directly model gaso- line formulations with more stringent requirements than federal reformu- lated gasoline requirements. It would require the impacts of reformulated gasolines to be extended to include impacts on emissions from all vehicles and technology groups as well as the impacts on CO emissions. Incorporation of Toxic-Emissions Factors into MOBILE These emission factors are currently in a separate model, MOBTOX. The rationale for this recommendation is the same as that for the two pre- vious recommendations: convenience and consistency. More Emphasis on the NONROAD Model Although the EPA model, NONROAD, for off-road emissions sources is not part of MOBILE, the committee notes that this critical emissions model is lacking in data on emissions factors and activity levels. As more

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~ 34 M ODE[/NG MOBILE-SOURCE EMISSIONS controls are placed on on-road sources, the off-road sources will become more important in the future, and the NONROAD model will play a larger role in estimating regional emissions inventories. Develop Long-Range Plans for the Evolution of the MOBILE Model The future implementation of new emissions and fuel standards, grow- ing concerns about PM and air tonics emissions, and the rising cost of con- trol strategies will increase the focus on MOBILE. Users will need im- proved accuracy and reliability from MOBILE, even as the regulatory set- ting, vehicle technologies, and fleet characteristics are changing. This poses a daunting challenge for EPA's Office of Transportation and Air Quality (OTAQ) to develop an accurate model that reflects uncertain regu- lations, unproven technologies, and shifting preferences among consumers. To address these demands, EPA must develop a long-range plan for ad- dressing critical modeling issues. EPA first should determine the most appropriate uses for the model and develop improvements to support these specific uses. This plan should then set priorities for model improvements that have the largest impact on emissions and develop a plan for collecting the necessary data to support these improvements. Most importantly, EPA must also address how close the modeling of mobile-source emissions should be to the development of regulations. The model must be seen as an accurate reflection of mobile-source emissions, not as a tool that is used to support proposed regulations.