oversight

Air Pollution: Limitations of EPA's Motor Vehicle Emissions Model and Plans to Address Them

Published by the Government Accountability Office on 1997-09-15.

Below is a raw (and likely hideous) rendition of the original report. (PDF)

                  United States General Accounting Office

GAO               Report to the Chairman, Subcommittee
                  on Oversight and Investigations,
                  Committee on Commerce, House of
                  Representatives

September 1997
                  AIR POLLUTION
                  Limitations of EPA’s
                  Motor Vehicle
                  Emissions Model and
                  Plans to Address Them




GAO/RCED-97-210
                   United States
GAO                General Accounting Office
                   Washington, D.C. 20548

                   Resources, Community, and
                   Economic Development Division

                   B-277440

                   September 15, 1997

                   The Honorable Joe Barton
                   Chairman, Subcommittee on Oversight
                     and Investigations
                   Committee on Commerce
                   House of Representatives

                   Dear Mr. Chairman:

                   To reduce and control air pollution, the Clean Air Act requires that the
                   Environmental Protection Agency (EPA) establish national ambient air
                   quality standards and that the states develop strategies for reaching and
                   maintaining these standards. In order to evaluate the states’ strategies for
                   reducing the atmospheric concentrations of three pollutants emitted by
                   motor vehicles—carbon monoxide, hydrocarbons, and nitrogen
                   oxides1 —EPA has developed a series of complex computer models, known
                   as the MOBILE series, to estimate motor vehicle emissions. The current
                   version is MOBILE5a. Both EPA and the states rely on this model to estimate
                   future emissions after various control strategies are employed.
                   Consequently, to the extent that the model erroneously estimates
                   emissions, EPA could approve control strategies that may not be sufficient
                   to attain the air quality standards, or, conversely, EPA could require that the
                   states implement additional, often expensive control measures that may go
                   beyond what is needed.2 In view of the pivotal role that the MOBILE model
                   plays in the states’ efforts to improve air quality, you asked us to describe
                   the major limitations in EPA’s MOBILE model and EPA’s process for improving
                   the current and future versions of the model.


                   EPA and a group of stakeholders have identified 14 major limitations in the
Results in Brief   current MOBILE model.3 Some vehicle emissions-producing activities are
                   not accounted for in the current model, and other emissions-producing
                   activities may not be adequately represented on the basis of the most

                   1
                    Carbon monoxide (CO) impairs lung and heart functioning, and hydrocarbons (HC) and nitrogen
                   oxides (NOx) combine in sunlight to form ozone, or smog, which has been linked to a variety of health
                   problems, ranging from eye, nose, and throat irritation to bronchitis, emphysema, and other serious
                   lung diseases.
                   2
                    The states must obtain sufficient reductions in emissions or risk Clean Air Act sanctions that could
                   result in losing millions of dollars in federal highway funds and jeopardizing industrial growth. Motor
                   vehicles are one of several categories of sources contributing to air pollution.
                   3
                    EPA and a group of stakeholders working with the agency on modeling issues identified a number of
                   limitations in MOBILE5a and selected 14 of these for potential revision in MOBILE6.



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recent information. According to EPA, much of this information has
become available since MOBILE5a was released. These limitations cause the
model to underestimate vehicle emissions in some cases and overestimate
them in others. For example, the impact of road grade—such as when a
car climbs a hill—is not a part of the current model, although some studies
have indicated that both the increased load on the engine from climbing a
hill and the decreased load that accompanies engine deceleration
significantly increase vehicle emissions. Similarly, other studies indicate
that some activities—such as when someone drives at high speeds,
accelerates quickly, runs the air conditioner, or uses a higher mileage
vehicle4 —are inadequately represented in the model. For example, some
testing indicates that nitrogen oxide emissions from running the air
conditioner may be from 30 to 75 percent greater than estimated by the
MOBILE model. Conversely, another study indicates that carbon monoxide
and hydrocarbon emissions from higher mileage vehicles may be
significantly less than the model’s estimates. EPA plans to address most of
these limitations in its next revision to the MOBILE model; however,
according to agency officials, three of the limitations, including the impact
of road grade, will probably not be addressed until later because of a
combination of factors. According to agency officials, these include the
negligible impact on emissions inventory predictions, a relatively low
priority ascribed by EPA and stakeholders, the cost and length of time
required for these studies relative to the schedule for release of MOBILE6,
and the emergence of new technologies that will make the improvements
more feasible or cost effective in a few years. EPA officials pointed out that
they have updated the estimating capabilities of the MOBILE model 10 times
since it was first introduced in 1978.

Irrespective of these limitations, there are specific actions—most of which
were recommended by the Science Advisory Board in its 1989
resolution5—that, when followed, can enhance a model’s estimating
capabilities. Among other things, these actions involve documenting the
implicit and explicit assumptions that are the basis of the formulas
contained in the model, obtaining external stakeholders’ input during the
model’s development, and having the model peer reviewed before it is
used. EPA officials acknowledged that, primarily because of resource
limitations, until recently such actions have been delayed or forgone.

4
EPA’s current model assumes that the emissions systems of vehicles with 50,000 or more odometer
miles deteriorate much more rapidly than the systems of other vehicles.
5
 EPA’s Science Advisory Board recommended many of these processes in its January 1989 resolution
on models, including one process—peer review—that has been required by EPA policy since
January 1993. The Board is a legislatively established body of independent experts who provide advice
to the EPA Administrator on scientific and engineering issues.



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             However, EPA is developing the next model, MOBILE6, with significantly
             increased openness and input from other stakeholders. EPA also plans to
             carry out the actions recommended by the Science Advisory Board, such
             as peer review, as part of its program for developing MOBILE6, due to be
             issued in late 1998.


             The current MOBILE model, MOBILE5a,6 also known as the EPA mobile source
Background   emissions factor model, is a computer program7 that estimates the
             emissions of carbon monoxide, hydrocarbons, and nitrogen oxides for
             eight different types of gasoline-fueled and diesel highway motor vehicles.8
              The model consists of an integrated collection of mathematical equations
             and assumptions about the emissions from vehicles manufactured from
             1960 to 2020; generally, the cars produced in the 25 most recent model
             years are assumed to be in operation in any given calendar year.9 The first
             MOBILE model was made available for use in 1978; since that time, major
             updates and improvements to the model have been made as more has
             become known about the complexity of the factors affecting vehicle
             emissions, as measurement devices have improved, and as more data have
             been collected. According to agency officials, these improvements have
             resulted in the refinement of emissions estimates for evaporative
             emissions (such as occur when the fuel tank and fuel system heat up on a
             hot summer day); for the uncorrected in-use deterioration (wear and tear)
             that results from poor vehicle maintenance or tampering; and for other
             factors.

             In its simplest form, EPA’s MOBILE model allows the model user to produce
             a number—an estimated quantity of emissions for the three pollutants of
             concern—by multiplying the estimated emissions per mile for an average
             urban trip times the estimated number of trip miles traveled in an area.
             Over the years, however, researchers have learned that vehicle emissions

             6
              MOBILE5a, issued in March 1993, is the current version of the EPA MOBILE model that the states are
             required to use; EPA issued an update in September 1996, known as MOBILE5b, as an option for the
             states to use in calculating selected emissions reduction credits. Unless otherwise indicated, the
             limitations discussed in this report relate to both MOBILE5a and MOBILE5b.
             7
              To facilitate computer programming, EPA created two other models—one for fuels, another for
             inspection and maintenance measures—whose outputs are inputs to MOBILE5a. Unless otherwise
             specified, references to the MOBILE model hereinafter in this report will include these input models.
             8
              The eight classes of vehicles are light duty gasoline-powered vehicles (passenger cars); light duty
             gasoline-powered trucks up to 6,000 pounds gross weight, and from 6,000 to 8,500 pounds gross
             weight; heavy duty gasoline-powered trucks; light duty diesel vehicles; light duty diesel trucks; heavy
             duty diesel trucks; and motorcycles.
             9
              Vehicles more than 25 years old are lumped into one category called “25 years and older.”



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are highly complex. For example, EPA and others have indications today
that as much as half of all hydrocarbon emissions from motor vehicles are
evaporative emissions, under certain conditions. To compensate for the
complexities of these and other emissions-producing activities, EPA has
periodically adjusted its basic formula—through the use of revised
“correction factors”—to approximate vehicle exhaust emissions in a range
of situations. In essence, the correction factor is a multiplier added to the
basic formula (miles traveled times emissions rate per mile) to adjust the
model’s output to more closely reflect actual emissions. Except for
California,10 EPA supplies the baseline emissions rates and correction
factors for other model users—primarily state and local agencies—that
typically supply their own estimates of the number of vehicle miles
traveled, according to agency officials, as well as many other local area
parameters, such as the average ambient temperature, vehicle
classifications, and types of fuels sold.

The MOBILE model exists because precise information about the emissions
behavior of the approximately 200 million vehicles in use in the United
States is not known, yet the need exists to estimate the impact of motor
vehicles on air quality. For the states, the MOBILE model is a tool for
constructing emissions inventories,11 creating control strategies,
producing state implementation plans (SIP),12 and—subsequently—
demonstrating control strategy effectiveness to EPA and others. For
example, the states are allowed to vary a number of control strategy
features, including the types of fuels used, the type of inspection and
maintenance (I&M) testing network,13 the frequency of I&M testing, the ages
and types of vehicles to be inspected, the stringency of the tailpipe test,
the number and percent of inspected vehicles that may receive a waiver,14
and a host of other factors. The states may choose among a number of



10
  With EPA’s approval, California uses its own emissions factor model to estimate vehicle emissions.
11
 National emissions are estimated annually by EPA on the basis of statistical information about each
of more than 450 source categories, including the emissions factors and control efficiencies for each
source category. These aggregated emissions show emissions trends at national and regional levels for
man-made pollutants.
12
 These plans explain in detail how, and by how much, state and local areas will reduce or control
emissions from both mobile and stationary sources.
13
  Testing networks may be test-only—where inspectors are prohibited from making repairs;
test-and-repair—where inspectors/mechanics are allowed to make repairs; or hybrid—where both
types of facilities are allowed.
14
 The act requires that vehicle owners spend at least $450 toward emissions-related repairs before a
waiver may be granted.



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                     control options as long as the state’s control strategy achieves at least as
                     many reductions as required by the Clean Air Act.15

                     For EPA, the MOBILE model is a tool for evaluating the adequacy of a state’s
                     emissions inventory estimate, motor vehicle control strategies, and
                     implementation plans. In essence, the model’s estimates provide EPA
                     regulators with critical information that is used to evaluate the adequacy
                     of a state’s program and the relative benefits of various policies to control
                     motor vehicle emissions. Additionally, the model’s estimates can affect
                     state policy decisions on issues such as the content and volatility of fuels,
                     and some decisions on highway improvement projects. For example, the
                     1991 Intermodal Surface Transportation Efficiency Act (ISTEA) required,
                     among other things, that state transportation improvement programs in
                     certain nonattainment areas16 conform with the applicable state
                     implementation plan developed under the Clean Air Act. Although the
                     model’s original purpose was to support the development of mobile source
                     emissions inventories, over the years its role and influence have been
                     expanded considerably. Today, its estimates have a substantial influence
                     not only on state and local programs but also on the automobile and oil
                     industries, environmental and trade organizations, the public, and others.
                     According to estimates derived partly from the MOBILE model, motor
                     vehicles produce about 90 percent of the carbon monoxide, 50 percent of
                     the hydrocarbons, and 30 percent of the nitrogen oxides emitted annually
                     in major urban areas.


                     EPA officials are examining 14 areas in the current MOBILE model in which
Limitations in the   major limitations exist. According to agency officials, it is their plan for
Current Model        each new version of the MOBILE model to reflect the most recent testing,
                     data collection, and research that are available. They pointed out that EPA
                     has updated the estimating capabilities of its MOBILE source emissions
                     model 10 times since the model was first introduced in 1978. Table 1
                     briefly summarizes the areas in which major limitations exist, as well as
                     EPA’s plans to address these limitations in its next revision to the model,
                     MOBILE6, due to be issued in late 1998. (Additional information on these 14
                     areas is provided in app. I.)


                     15
                      The states must achieve percentage reductions in HC and, for CO and NOx, must submit plans for
                     attaining the national ambient air quality standards by the dates specified in the act relative to the
                     seriousness of their pollution problems.
                     16
                       Areas that have not yet reached attainment with the national ambient air quality standards are
                     classified by EPA as nonattainment areas, generally ranging in seriousness from marginal to extreme.
                     The more serious the classification, the more stringent the control measures. ISTEA refers specifically
                     to areas that are in nonattainment for ozone or carbon monoxide.



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Table 1: Summary of Major Limitations in the MOBILE5a Model
                                                                                                                          Planned to be
                                                                                                                           addressed in
Area in MOBILE model           Brief description of limitationa                                                               MOBILE6
1. Emission estimates for    Higher speeds produce increasingly larger emissions of CO, HC, and NOx;                                 Yes
higher speeds, especially    technological limits of previous dynamometersc meant most data were collected
                           b
speeds in excess of 65 mph. at speeds below 57 mph; increases in speeds above 65 mph have left data
                             gaps in current model that are not adequately represented by existing
                             correction factors; ongoing work will be used to revise these estimates.
2. Representation of           Rapid acceleration and deceleration can produce large increases in emissions                          Yes
emissions from rapid           of CO and HC; technological limits of previous dynamometers meant most data
acceleration and               were collected at or below acceleration rates of 3.3 mph/second; one EPA
deceleration, including        study indicated that about one-third of trips had acceleration rates of more than
aggressive driving             7 mph/second; concerns exist as to whether these increased emissions are
behaviors.b                    adequately represented by existing correction factors; ongoing work will be
                               used to revise these estimates.
3. Representation of           Until vehicle engines reach normal operating temperatures, emissions typically                        Yes
emissions immediately after    bypass emissions control equipment for 3 to 5 minutes, making cold starts one
engine start-up, known as      of the single largest emissions-producing activities; recent studies indicate that
cold start emissions.b         cold starts occur more frequently than estimated in the Federal Test Procedure,
                               thus increasing emissions; however, newer technology vehicles now reach
                               normal operating temperatures more quickly, thereby decreasing emissions;
                               concerns exist as to whether these activities are adequately represented by the
                               current MOBILE model and whether they should be separately calculated on
                               the basis of activity levels as opposed to representing one portion of EPA’s
                               typical driving cycle; EPA has already announced plans to separate cold starts
                               from other driving cycle activities.
4. Representation of           Air conditioners place an additional load on a vehicle’s engine and cause a                           Yes
emissions from air conditioner vehicle to produce more NOx emissions than when the air conditioner is not
usage.b                        used; although EPA had added 10 percent more load to the dynamometer to
                               simulate the effects of air conditioners on emissions, testing on a few vehicles
                               indicates that the emissions of NOx may be from 30 to 75 percent greater at
                               some speeds than currently represented by the MOBILE modele; the results of
                               recent air conditioner emissions studies are planned to be reflected in the
                               model’s next revisions.
5. Representation of           Increased road grade can result in substantial emissions increases, especially                         No
emissions from road grade,     in CO and NOx; due to the difficulty of testing for this driving activity and the
such as when a car climbs a    difficulty of plotting road grades for millions of miles of highways, this activity
hill.b                         has not been represented in EPA’s model to date; due to the expense and time
                               required for road grade testing and mapping, EPA plans to add road grade
                               emissions estimates in several years when it develops MOBILE7.
6. Representation of high      High emitting vehicles account for a disproportionate amount of vehicle                               Yes
emitting vehiclesd in the      emissions; thus, the estimate of their relative proportion in the overall vehicle
MOBILE model’s supporting      population is critical to accurate inventory emissions estimations; a California
database.                      study found high emitting vehicles underrepresented in its own model by nearly
                               5 times; concerns exist as to whether these high emitting vehicles are
                               appropriately represented in the current EPA model; ongoing I&M lane testing
                               in three states will be used to review these estimates.
                                                                                                                              (continued)




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                                                                                                                              Planned to be
                                                                                                                               addressed in
Area in MOBILE model              Brief description of limitationa                                                                MOBILE6
7. Representation of              Lower volatility fuels produce less emissions, especially HC and CO emissions;                         Yes
emissions from lower              over the years, low volatility fuels have been produced that are lower than the
polluting fuels, especially       lowest level EPA estimated in 1989 when it placed a lower limit restriction in the
fuels with lower volatility.      MOBILE model, below which low volatility fuels received no additional
                                  emissions reductions credits; additionally, one study found that the model
                                  overestimates the CO emissions benefits of oxygenated fuels; EPA plans to
                                  eliminate the lower limit restriction, review correction factors for lower volatility
                                  fuels, and review CO estimates for oxygenated fuels.
8. Representation of              On the basis of a small sample, EPA added new correction factors in 1992 on                            Yes
emissions system                  the assumption that emissions control systems perform much worse after
deterioration for vehicles with   50,000 miles; some testing indicates that this adjustment now causes the
50,000 or more odometer           MOBILE model to overestimate the emissions from fuel-injected vehicles with
miles.                            50,000 or more odometer miles.; EPA plans to lower this adjustment; ongoing
                                  work will determine the extent of change.
9. Emissions estimates and        Vehicle condition is one of the most important factors contributing to excess                          Yes
assumptions for vehicle           emissions; more recent and more complete data are needed about many facets
inspection and maintenance        of I&M programs, including the impact of newer on-board computer technology,
(I&M) programs.                   better emissions control system warranties, the durability of equipment, and
                                  alternative ways to complement inspection programs and improve repair
                                  effectiveness; for MOBILE6, EPA plans to add an I&M flexibility module
                                  covering most of these areas so that individual states’ studies and ongoing EPA
                                  studies of I&M testing in three states can be used to revise these estimates as
                                  study data become available.
10. Estimates and                 Hydrocarbons evaporate from the fuel tank and fuel lines when the vehicle is                           Yes
assumptions for non-tailpipe      parked; improved testing over a longer time period with more realistic
evaporative emissions when        temperature increases is ongoing to better reflect actual emissions and refine
the vehicle is not operating.     existing correction factors in the MOBILE model; however, some vehicles that
                                  leak fuel are now believed by EPA to be so significant that these will become a
                                  separate emissions category in MOBILE6; indications are that vehicles with
                                  such fuel leaks—while a small percentage of the fleet—can exceed the
                                  evaporative emissions of corresponding non-leaking vehicles by one to two
                                  orders of magnitude.
11. Emissions estimates and       EPA’s supporting data on the in-use emissions of this category of vehicles are                          No
assumptions for the               about 20 years old; certification standards are higher for these vehicles than
inspection and maintenance        their light duty counterparts, and they are generally older and driven more miles
(I&M) of heavy duty               annually than their light duty counterparts, although improved emissions
vehicles—those with a gross       technology has lessened the contributions of individual vehicles; some studies
vehicle weight of 8,501           are under way, but agency officials question whether sufficient I&M data will be
pounds or more.                   obtained in time to change emissions estimates for heavy duty vehicles.
12. Data characterizing           Many model assumptions, such as emissions control system deterioration, are                            Yes
vehicle fleet.                    closely tied to accumulated mileage; the MOBILE model’s data on mileage
                                  accumulation has been superseded by more recent data indicating that
                                  passenger cars are driven about 2,000 miles, or 10 percent more, than
                                  currently estimated in the MOBILE model; also, heavy duty vehicles are
                                  believed to constitute a significantly larger percentage of the overall vehicle
                                  fleet than currently estimated in the MOBILE model; EPA plans to revise these
                                  estimates at the national level and notes that this is an area where model users
                                  often supply their own local data.
                                                                                                                                  (continued)




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                                                                                                                                   Planned to be
                                                                                                                                    addressed in
Area in MOBILE model          Brief description of limitationa                                                                         MOBILE6
13. Greater distinctions in   Roadway classification (freeway, arterial, feeder/collector street, along with their                                Yes
roadway classifications.      associated level of service) has a substantial impact on emissions; the current
                              model was designed for estimating areawide emissions over an entire trip and
                              has not provided for assessing emissions’ impacts on a smaller scale, yet
                              today’s users need a model that provides different estimates for different
                              roadways; for MOBILE6, EPA plans to provide separate model emissions
                              estimates for at least three roadway classifications.
14. Quantifying the           Traditionally, when estimates are made from limited data, the magnitude of                                          No
uncertainty of the model’s    uncertainty, or range, associated with the estimate is provided; however, this
estimates.                    has not been done for any versions of the MOBILE model. Several studies
                              suggest that the current model’s uncertainty may be large; however, many
                              factors, including cost, will likely prevent EPA from reporting the uncertainty of
                              its MOBILE6 estimates, although the agency plans to move in this direction.

                                             Note: Representatives from EPA and from the Federal Advisory Committee Act (FACA) workgroup
                                             are continuing to work on other less significant limitations in MOBILE5a that are not addressed
                                             above. From an original list of 90 items, an EPA-sponsored FACA subcommittee has identified 47
                                             high-priority items for improvement in the current model; however, many of these items involved
                                             overlapping issues, and in some instances, two or more of these items have been consolidated to
                                             form the 14 categories of limitations discussed above. Additionally, some of these items deal with
                                             suggested procedural changes in the process that EPA should follow in making MOBILE model
                                             revisions. Procedural changes in EPA’s modeling process are discussed in the next section of
                                             this report.
                                             a
                                              In some instances, the suggested emissions impact is based on a few studies of vehicle
                                             activities that EPA is currently trying to corroborate.
                                             b
                                              While many model assumptions are interconnected, these limitations relate primarily to limitations
                                             in the Federal Test Procedure (FTP) for certifying new cars, which was revised in October 1996 to
                                             better estimate emissions from high speeds, aggressive driving, cold engine starts, and air
                                             conditioner use; these changes in the revised FTP have not been reflected in MOBILE5a (issued
                                             in March 1993), nor in MOBILE5b (an optional version that was issued September 1996); the
                                             relationship of FTP assumptions to EPA’s MOBILE model estimates is discussed more fully in
                                             appendix I.
                                             c
                                              A dynamometer is a treadmill-like device that simulates the load placed on a vehicle by
                                             electronically controlling inertial forces while the vehicle’s wheels are driven at various
                                             pre-determined speeds, including acceleration and deceleration.
                                             d
                                              High emitters are generally defined as vehicles with emissions more than twice the applicable
                                             standard for the particular make and model of vehicle, and are often described on the basis of
                                             the amount of excess emissions they produce. For CO and HC, EPA classifies such excess
                                             emitters as high, very high, and super emitters; for NOx, vehicles are classified as either normal
                                             or high.
                                             e
                                              These tests compared actual air conditioner emissions with the FTP’s estimate of such emissions
                                             by adding 10 percent additional load to the dynamometer; MOBILE5a employed essentially the
                                             same correction factor, thereby estimating air conditioner emissions in the same manner and
                                             amount as the FTP.



                                             While acknowledging that some vehicle emissions-producing activities are
                                             not accounted for in the current model and that other emissions-producing



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                    activities are not adequately represented in the current model on the basis
                    of the most recent information, EPA officials said that it is important to
                    note that EPA has conducted and/or partially funded some of the studies
                    that have led to the new data that now question the old estimates and
                    assumptions. Additionally, they said that EPA has work under way to
                    address most of these limitations. For example, since its formation in
                    September 1995, an EPA-sponsored Federal Advisory Committee Act (FACA)
                    subcommittee workgroup has identified 47 high-priority items for
                    improvement in the current model, and EPA and workgroup
                    representatives are examining these limitations. EPA is also in the process
                    of developing new procedures for improving models in general, which are
                    discussed below.


                    Several model experts told us that it is the nature of models such as
EPA’s Process for   MOBILE5a to have limitations and to be in a continuous improvement mode.
Improving the       Agency officials agreed with this assessment, noting that the current
MOBILE Model        model is better than any previous versions and reflects consistent growth
                    in the quality and quantity of information available on very complex
                    issues. Additionally, they pointed out that—through the FACA workgroup
                    process—the revisions to the next MOBILE model, MOBILE6, have been
                    undertaken with significantly increased openness and input from other
                    government agencies, academia, the automobile and oil industries,
                    environmental groups, and others. Our contacts with representatives of
                    these groups confirmed this increased level of external stakeholders’
                    involvement in preparation for MOBILE6. Several commended EPA’s efforts
                    in recent years to reach out to persons outside of the agency, and some
                    noted that the outreach effort had given them a much greater appreciation
                    of the model.

                    While acknowledging that, historically, there have been few firm criteria
                    on the processes that should be followed when creating or revising a
                    model such as the MOBILE model, the executive director of EPA’s Science
                    Advisory Board (SAB) told us that the agency has a project under way to
                    develop agencywide procedures for improving models. According to the
                    project director, the Office of Research and Development (ORD) is planning
                    a workshop in December 1997 to discuss the status of modeling across
                    several media and other modeling issues, including the need for better
                    agencywide modeling procedures. The project director also said that the
                    Science Advisory Board’s January 1989 resolution on models was one of
                    the best documents available on the processes for creating and improving
                    models in general. The SAB executive director and the ORD project director



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    told us that in their opinion, there are specific actions—most of which
    were recommended by the SAB in its 1989 resolution—that, when followed,
    can enhance a model’s predictive capabilities. Among other things, these
    actions include the following:

•   Obtaining external stakeholders’ input to ensure that the model’s
    assumptions and formulas receive critical review by those not involved in
    the model’s development.
•   Documenting the implicit and explicit assumptions so that others can
    evaluate the basis of the formulas embedded in the model.
•   Performing sensitivity analyses over key parameters to identify the most
    sensitive parameters and to establish the areas most in need of further
    research.
•   Verifying the adequacy of the model’s mathematical code.
•   Testing the model’s predictions with laboratory and field data to confirm
    that the model generates results consistent with its underlying theory.
•   Conducting peer review to enhance the quality, credibility, and
    acceptability of the model’s applications.

    The SAB executive director pointed out that because of continuing
    concerns with the quasi-regulatory use of agency models, EPA issued
    agencywide guidance in 1994 specifically calling for the peer review of
    such models. This directive was a follow-on to EPA’s January 1993
    agencywide policy requiring peer review of the scientific and technical
    work products used to support agency decisions. In September 199617 and
    March 1997,18 we reported and testified on the uneven implementation of
    EPA’s peer review policy, including that the MOBILE model had not been
    peer reviewed. EPA agreed with our recommendations for educating staff
    and managers about the merits of and procedures for conducting peer
    review and for ensuring that all relevant products are considered for peer
    review. The agency has set in motion a three-pronged approach to improve
    the implementation of peer review agencywide, including peer review of
    the next MOBILE model. According to the ORD project director, it is too early
    in EPA’s study to predict whether the agency may recommend that EPA
    require its offices to follow the other actions listed above when creating or
    revising models.

    According to Office of Mobile Sources (OMS) officials, they plan to carry
    out all six of the above activities as part of their improvement process for

    17
      Peer Review: EPA’s Implementation Remains Uneven (GAO/RCED-96-236, Sept. 24, 1996).
    18
     Peer Review: EPA’s Implementation Remains Uneven (GAO/T-RCED-97-95, Mar. 11, 1997), testimony
    before the House Subcommittee on Energy and Environment, Committee on Science.



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MOBILE6 and noted that some of these activities are already well under way,
such as involving external stakeholders. For example, OMS held its first
stakeholder meeting in June 1994, established a FACA mobile modeling
workgroup in July 1995, and has held five meetings since that time to
obtain external views by those not involved in the model’s development,
according to agency officials. OMS officials acknowledged that some of the
key formulas in the current model have not been properly documented,19
that full-scale sensitivity analyses have not been performed since May 1990
(when they were performed for MOBILE4.1),20 that fewer resources have
resulted in fewer confirming data, and that the MOBILE model has not been
peer reviewed. However, they said they have efforts under way or planned
to address these and other modeling needs. For example, one of the
recommendations of the FACA mobile modeling workgroup—made up of
representatives from EPA, state and local agencies, industry, environmental
groups, and academia—is that EPA more fully document the model’s
assumptions. Additionally, OMS plans to perform sensitivity analyses for
the next version of the model and to have the studies supporting key
changes for MOBILE6 peer reviewed. Also, OMS officials explained that as
changes are proposed for each area of major limitations in the model, they
plan to have the entire area peer reviewed.21

Agency officials explained that declining modeling resources have affected
the pace of model improvements over the years, particularly their ability to
confirm the model’s estimates with large numbers of vehicle tests. For
example, a study of the emissions characteristics of 100 passenger cars for
both exhaust and evaporative emissions could cost from $1.4 to
$1.6 million, according to the agency’s current estimates, and still not
address the emissions impacts of road grade, air conditioning, or most fuel
studies. Studies of heavy duty trucks and other larger vehicles would cost
considerably more. As an illustration of the magnitude of the task
compared with available resources, we obtained EPA’s estimate of mobile
modeling needs in response to the mobile source requirements envisioned
for the 1990 Clean Air Act amendments. This June 1990 analysis indicated
that the Office of Mobile Sources would need about $60 million for the
modeling improvements known at that time. Since then, because of higher

19
  Reports by Sierra Research, Systems Applications International, and the Department of
Transportation’s Federal Highway Administration, among others, found that some of the key formulas
in the MOBILE model are not properly documented, making it difficult for researchers to know what
data or assumptions were used in developing the formulas.
20
 This was the last sensitivity analysis EPA published; agency officials indicated that limited sensitivity
analyses were performed over selected parameters—such as key speeds, temperatures, and fuels—in
MOBILE5 (Dec. 1992) and MOBILE5a (Mar. 1993) but that these analyses were limited and not
published.
21
 OMS officials explained that they are still early in the peer review process and have not yet
developed a peer review plan for the MOBILE model.
Page 11                                                              GAO/RCED-97-210 Air Pollution
                  B-277440




                  priority needs, OMS has been allocated only $21.8 million, cumulatively, for
                  modeling improvements, although the research needs have increased.
                  However, many more groups have become involved in non-EPA-funded
                  vehicle emissions studies than in the past, allowing EPA to benefit from
                  their studies and observations. Additionally, in some instances,
                  researchers have sought EPA’s input on study protocols beforehand and
                  have shared the data collected with EPA afterwards. While concerned
                  about resources, OMS officials explained that making model improvements
                  is an ongoing, continuous process—and one that will continue after
                  MOBILE6 is issued in 1998. They pointed out that their goal is for each new
                  version of the MOBILE model to reflect the latest testing, data collection,
                  and research. While still not able to quantify the improvements, they said
                  that in their opinion, each new version of the MOBILE model is better than
                  its predecessor.


                  We provided copies of a draft of this report to the Environmental
Agency Comments   Protection Agency for its review and comment. We obtained comments
                  from EPA officials, including the Director of the EPA Office of Mobile
                  Sources. EPA agreed with the overall message of the report but expressed
                  concerns with imprecise language and suggested several changes to clarify
                  information in the report. For example, EPA suggested that in lieu of
                  describing some limitations associated with its use of the Federal Test
                  Procedure as a basis for estimating emissions as “FTP assumptions,” since
                  the FTP is a specific, codified test cycle, these would be better described as
                  “FTP parameters.” EPA also suggested that we provide specific citations for
                  four studies referred to in appendix I. We made the language changes
                  suggested by EPA, including adding the citations.

                  EPA was also concerned that the report did not clearly distinguish between
                  limitations that may result in only trivial emissions impacts and those that
                  could be significant. Where it was possible to do so, we believe the
                  estimated emissions impact had already been quantified or qualitatively
                  described in the report. Additionally, as noted in the section on the
                  uncertainty limitation, one of the 14 limitations of the MOBILE model is that
                  it does not currently have information about, nor estimates of, the
                  uncertainty associated with its emissions estimates. However, we agree
                  that researchers viewed some limitations as having a more significant
                  impact on emissions than others, and we have provided this view in the
                  report. Appendix II contains the agency’s overall written comments.




                  Page 12                                            GAO/RCED-97-210 Air Pollution
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We conducted our review from October 1996 through August 1997 in
accordance with generally accepted government auditing standards. A
detailed discussion of our scope and methodology is provided in appendix
III.

As arranged with your office, unless you publicly announce its contents
earlier, we plan no further distribution of this report until 15 days after the
date of this letter. At that time, we will send copies to the Administrator of
EPA and other interested parties. We will also make copies available to
others upon request.

Please call me at (202) 512-9692 if you or your staff have any questions.
Major contributors to this report are listed in appendix IV.

Sincerely yours,




Lawrence J. Dyckman
Associate Director, Environmental
  Protection Issues




Page 13                                             GAO/RCED-97-210 Air Pollution
Contents



Letter                                                                                          1


Appendix I                                                                                     16

Major Limitations in
MOBILE5a
Appendix II                                                                                    30

Comments From the
Environmental
Protection Agency
Appendix III                                                                                   32

Objectives, Scope,
and Methodology
Appendix IV                                                                                    34

Major Contributors to
This Report
Table                   Table 1: Summary of Major Limitations in the MOBILE5a Model             6




                        Page 14                                      GAO/RCED-97-210 Air Pollution
Contents




Abbreviations

API        American Petroleum Institute
AQIRP      Air Quality Improvement Research Program
CAA        Clean Air Act
CARB       California Air Resources Board
CO         carbon monoxide
CRC        Coordinating Research Council
EPA        Environmental Protection Agency
FACA       Federal Advisory Committee Act
FTP        Federal Test Procedure
GAO        General Accounting Office
HC         hydrocarbons
ISTEA      Intermodal Surface Transportation Efficiency Act
I&M        inspection and maintenance
OMS        Office of Mobile Sources
ORD        Office of Research and Development
NOx        nitrogen oxides
OBD        on-board diagnostics
RVP        reid vapor pressure
SAB        Science Advisory Board
SIP        state implementation plan


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Major Limitations in MOBILE5a


                              According to agency officials and model experts we contacted, it is the
                              nature of models to have limitations and to be in a continuous
                              improvement mode. As a result, the Environmental Protection Agency
                              (EPA) has periodically updated the estimating capabilities of its mobile
                              source emissions model to reflect new information as data have become
                              available; MOBILE5a reflects the 10th major revision since the model was
                              first introduced in 1978. The following sections provide additional
                              information on 14 areas in which major limitations exist in EPA’s current
                              MOBILE model, MOBILE5a.



Five Limitations Related to   The underlying basis for EPA’s original model, and all subsequent versions,
Use of the Federal Test       has been the Federal Test Procedure (FTP),1 a laboratory dynamometer2
Procedure as a Basis for      test used to certify new cars against new-car emissions standards. The FTP
                              is roughly based on a typical urban area trip, complete with starts and
Inventory Modeling            stops, covering 7.5 miles in the Los Angeles urban area in the late 1960s.
                              Such a trip is known as a driving cycle, which can be approximated on a
                              dynamometer.3 Primarily because of the limitations in past dynamometers,4
                               the FTP driving cycle parameters stipulate, among other things, that
                              vehicles average 19.6 miles per hour (mph) over the 7.5 mile trip, do not
                              exceed 57 mph, accelerate gradually (not to exceed 3.3 mph/second), and
                              travel on a flat surface. Additionally, EPA added 10 percent to the FTP
                              dynamometer load in an attempt to simulate the effects of air conditioner
                              usage. However, five of the major limitations in the current MOBILE model
                              relate to FTP parameters. These five are (1) emissions from road grade,
                              (2) emissions from air conditioner usage, (3) emissions at higher speeds,


                              1
                               Since 1991, EPA has used dynamometer test data from thousands of vehicles tested in IM240
                              inspection lanes in Hammond, Indiana, to improve the basic emissions rate estimates in the MOBILE
                              model. The IM240 test is a 240-second test designed to replicate certain segments of the longer FTP
                              test and, as such, has the same basic benefits and limitations as the FTP test.
                              2
                               A dynamometer is a treadmill-like device that simulates the load placed on a vehicle by electronically
                              controlling inertial forces while the vehicle’s wheels are driven at various pre-determined speeds,
                              including acceleration and deceleration. The new FTP requires improved single-roll dynamometers, 48
                              inches in diameter, with higher power absorption capacity to provide better replication of actual
                              emissions (or a system that provides equivalent or superior results), replacing the smaller (about 9
                              inch) double-roll types.
                              3
                               EPA’s driving cycle, known as the EPA Urban Dynamometer Driving Schedule, tests a vehicle over
                              several driving scenarios during a 1,877-second, or about a 31-minute dynamometer test. “Cold-start”
                              emissions are represented by the first 505 seconds of emissions, after which so-called “hot stabilized”
                              emissions are collected to represent the period when the vehicle is fully warmed up and both the
                              engine and catalytic converter have reached typical operating temperatures (the 1,877-second driving
                              cycle also includes a “hot start” component).
                              4
                               According to EPA, FTP driving cycle parameters, such as top speed and acceleration rate, until
                              recently were substantially constrained from approximating actual driving conditions by the physical
                              limitations in dynamometer technology.



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Major Limitations in MOBILE5a




(4) emissions from aggressive driving, and (5) emissions immediately after
engine start-up (cold- start). Agency officials have long recognized that
some of these original FTP parameters were not representative of actual
driving conditions and, to compensate for these limitations, have added
correction factors to the MOBILE model to estimate what emissions would
be for speeds in excess of 57 mph, for rapid acceleration beyond 3.3
mph/second, and for other scenarios, such as different temperatures or
different fuels. For example, the impact of temperature on emissions can
be substantial. Consequently, while FTP testing has been performed
between 68 and 86 degrees Fahrenheit, EPA’s MOBILE model used a
correction factor to estimate that 1995 exhaust emissions of hydrocarbons
would be 3 times greater at 25 degrees than at the FTP temperatures.

However, MOBILE5a does not account for the impact of road grade—such as
when a car climbs a hill—although some studies have indicated that both
the increased load on the engine from climbing a hill and the decreased
load that accompanies engine deceleration significantly increase vehicle
emissions. According to agency officials, it is not expected that MOBILE6
will have adjustments for road grade, although such adjustments are being
planned for MOBILE7. In addition to being uncertain about the amount of
emissions related to road grade, EPA officials explained that obtaining the
basic data from instrumented cars and chase cars to make such
adjustments would be expensive at this time and that because of the cost
and length of time required for these studies, the impact of road grade will
probably not be addressed until MOBILE7. Additionally, an equally important
consideration is that once these basic data on the effects of road grade on
emissions are obtained, state and local agencies would have to plot road
grades for millions of miles of roadways in their jurisdictions, also a costly
and time-consuming activity. However, as global positioning technology5
for vehicles becomes less costly and more widely available and used, it is
envisioned that the impacts of road grade emissions will be modeled in the
future. According to EPA officials, the amount of data needed to estimate
the impact of road grade is still years away. They also noted that time
would be needed to develop consistent guidance on how state and local
agencies should go about collecting these data.

With respect to the representation of other emissions-producing activities
not represented by the FTP, EPA officials have made periodic adjustments
but recognize that the current model’s correction factor adjustments may
not reflect the most up-to-date information. For example, as noted in the

5
 Global positioning system and on-board vehicle computer technology allow a driver to electronically
determine his location on earth, among other things. Appropriately instrumented vehicles would use
these technologies to plot and record both grade and location.



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Major Limitations in MOBILE5a




EPA-FACA materials disseminated in March 1997, the 10-percent additional
load intended to simulate the effect of air conditioner usage “is obsolete.”
More recent information indicates that nitrogen oxide emissions may be
from 30 to 75 percent greater at some speeds than the current model’s
estimates when the air conditioner is used.

Additionally, increases in speeds above 65 mph have left data gaps in the
current model that are not adequately represented by existing correction
factors. Others have recognized that the MOBILE model’s estimates are
inextricably tied to the FTP’s parameters, and some studies have
questioned the representativeness of key assumptions as they relate to the
FTP. For example, the California Air Resources Board (CARB)
commissioned Sierra Research in 1993 to develop an improved driving
cycle—known as the Unified Driving Cycle—by using an instrumented
“chase car” to better characterize typical urban driving patterns. Among
other things, this driving cycle allows cars to travel up to 67.2 mph (versus
57 mph for the FTP), allows for acceleration at a rate of up to 6.9
mph/second (versus a maximum of 3.3 mph/second for the FTP), and uses
an average speed of 24.6 mph (versus 19.2 for the FTP). Several model
experts believe these parameters more closely approximate actual driving
conditions today. According to a 1993 CARB study, the FTP may
underestimate hydrocarbon, carbon monoxide, and nitrogen oxide
emissions by 27, 68, and 17 percent, respectively.

A 1993 study sponsored by EPA’s FTP improvement project found that more
than one-third of the trips studied had acceleration rates of more than 7
mph/second—more than double the FTP’s maximum of 3.3 mph/second
rate. Similarly, another 1993 EPA-sponsored study of instrumented vehicles
in the Baltimore area found that 18 percent of total driving time in the area
was composed of higher speeds and sharper accelerations than those
represented on the FTP test. Also, a 1995 National Research Council report
noted that aggressive driving with many accelerations resulted in
hydrocarbon and carbon monoxide emissions being 14 and 15 times
higher, respectively, than the emissions from average driving over the
same 7-mile trip.6 According to the 1995 National Research Council report,
“Virtually all motor vehicle testing has been based on a limited set of
driving test cycles that inadequately represent current urban driving
conditions.” However, one model expert told us that it took more than 1
year to evaluate one component of the model and that collecting vehicle
emissions data on large data sets is very costly. EPA officials pointed out

6
 Expanding Metropolitan Highways: Implications for Air Quality and Energy Use, Special Report 245,
Transportation Research Board, National Research Council, National Academy of Sciences, 1995.



Page 18                                                          GAO/RCED-97-210 Air Pollution
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Major Limitations in MOBILE5a




that there is not a consistent definition of what constitutes aggressive
driving, that aggressive driving happens only over a portion of the trip and
is highly variable among drivers, and that the above observations are not
representative of average driving patterns.

The Congress has also recognized that the FTP may not reflect actual
driving conditions. Concerned about the gap between emissions as
measured by the FTP and actual, real-world emissions, in 1990 the
Congress added Section 206(h) to the Clean Air Act, which required EPA to
review and revise the FTP within 18 months “to insure that vehicles are
tested under circumstances which reflect the actual current driving
conditions under which motor vehicles are used.” EPA’s October 1996 final
rule on FTP revisions7 addressed four emissions-producing activities that,
according to the rule’s preamble, are not adequately represented in the
current FTP. These emissions-producing activities include (1) aggressive
driving behavior (such as high acceleration rates and high speeds),
(2) rapid speed fluctuations (such as quick deceleration8), (3) emissions
immediately after engine start-up, a period when—because of the fact that
engines are designed to operate at higher temperatures—emissions
typically bypass emissions controls for an estimated 3 to 5 minutes until
the engine reaches normal operating temperatures, and (4) actual air
conditioner usage.

EPA has not yet revised the MOBILE model to reflect the results of recent
studies that have led to these FTP rule revisions but has work ongoing in all
four areas. According to agency officials, although adjustments had been
made to the MOBILE model for most of these activities prior to issuing the
revised FTP rule, given the state of knowledge today, it appears that these
activities may not be adequately represented in MOBILE5a. EPA officials told
us that incorporating new estimates for these emissions-producing
activities would be a high priority for MOBILE6, due to be issued in late 1998.
For example, in March 1997 agency officials announced their plans to
substantially revise the cold-start segment of the next MOBILE model,
moving—for the first time ever—from an areawide, trip-based model to a
roadway-specific model that also separately accounts for start-up
emissions. Under this revised model, the magnitude of start-up emissions
will not depend on vehicle speed or the driving cycle. Instead, EPA is
proposing to allow model users to model the emissions impacts of cold

7
 Final Regulations for Revisions to the Federal Test Procedure for Emissions From Motor Vehicles, 61
Fed. Reg. 54852 (Oct. 22, 1996).
8
 Rapid deceleration causes excess emissions because more fuel is fed into the engine than the engine
now demands, resulting in improperly combusted fuel.



Page 19                                                           GAO/RCED-97-210 Air Pollution
                            Appendix I
                            Major Limitations in MOBILE5a




                            starts on the basis of local areas’ estimates of the number of such starts.
                            Additionally, model users will be able to estimate emissions for three
                            different types of roadways—freeways, arterials, and local roadways.


Eight Limitations Related   In addition to concerns about the representativeness of the FTP
to Other Areas of the       parameters, the following issues were also identified by model experts,
MOBILE Model                workgroup participants, and/or stakeholders we contacted. In each
                            instance, EPA officials agreed that the limitation is an area of concern and
                            in most cases noted that the agency has ongoing work to address the issue,
                            which is discussed below.

                            One concern is the representation of high emitters9 in EPA’s MOBILE model
                            database, since the data now indicate that this group of vehicles accounts
                            for a disproportionate amount of an area’s overall emissions and that if
                            this subset of the overall vehicle population is underrepresented, the
                            impact on the emissions estimates can be substantial. For example, Sierra
                            Research testified in 1995 that the worst polluting 22 percent of the
                            vehicles produce about 50 percent of the emissions, and EPA estimates
                            that, overall, from 10 to 30 percent of the vehicles cause the bulk of the
                            pollution problems.

                            As noted by the nonfederal co-chair of the FACA modeling workgroup in a
                            1993 study, “The general problem of failing to control for significant
                            factors is compounded by the likelihood that a small fraction of the
                            vehicle fleet are currently responsible for a large percentage of vehicle
                            emissions.”10 He told us that he still believes this to be one of the most
                            significant issues facing EPA today, primarily because a very small number
                            of vehicles can potentially be responsible for unusually high levels of
                            pollution. This was also a significant issue for one of the Session Chairs
                            for the Coordinating Research Council’s (CRC) April 1997 Workshop,11 who
                            noted that, in his opinion, this is the single greatest issue that EPA
                            faces—how to identify and repair high emitters and properly represent
                            them in the modeling database. The nonfederal co-chair also noted that if
                            the occurrence of such vehicles is not properly represented in the model,

                            9
                             High emitters are generally defined as cars with emissions more than twice the applicable standard
                            and are often described on the basis of the amount of excess emissions they produce. For CO and HC,
                            EPA classifies such excess emitters as high, very high, and super emitters; for NOx, vehicles are either
                            normal or high.
                            10
                             R. Guensler, “Data Needs For Evolving Motor Vehicle Emission Modeling Approaches” (Institute of
                            Transportation Studies, Univ. of California - Davis, Aug. 1993).
                            11
                             Proceedings of the Seventh Coordinating Research Council’s On-Road Vehicle Emissions Workshop,
                            April 9-11, 1997; published June 30, 1997.



                            Page 20                                                             GAO/RCED-97-210 Air Pollution
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Major Limitations in MOBILE5a




the model’s emissions estimates can be seriously flawed. He and others
have concerns that the existing database may underrepresent high
emitters because, among other reasons, the owners of such vehicles may
avoid surrendering such vehicles for inspection and maintenance (I&M)
and other testing at a higher rate than the normal population. As noted in a
February 1996 study, “individuals with intentionally tampered or poorly
maintained vehicles may be less likely to offer their vehicles for testing.”12
Additionally, a 1993 CARB study of 186 vehicles indicated that high emitters
could represent 16.8 percent of the California fleet, or nearly 5 times the
assumption in the California model.13 EPA officials have some concerns
with the study, its reliance on remote sensing devices, and its applicability
to other states. Additionally, EPA officials believe that the larger data sets
provided by their ongoing I&M lane testing in three other states properly
identify most high emitters. However, they agreed that appropriate
representation of high emitters is important to the model’s emissions
estimates and noted that this is also a high priority issue currently being
addressed by EPA and one of the subgroups of the mobile modeling
workgroup.

A second concern is the current correction factors for lower volatility
fuels and for oxygenated fuels. For example, a February 1997 report by
Sierra Research14 found, among other things, that MOBILE5a likely
underestimates the impact of low reid vapor pressure15 (RVP) fuels on
hydrocarbon and carbon monoxide emissions at temperatures above 75
degrees. The report notes that the correction factor for low RVP fuels has
not changed since February 1989, when limited data on fuels with RVPs
lower than 9.0 pounds per square inch (psi) caused EPA to place a
constraint code in the model precluding users from being able to calculate
reductions below this level. The limited data collected since that time
indicate that reducing fuel RVP from 9.0 psi to 7.0 psi may reduce
hydrocarbon and carbon monoxide exhaust emissions from 18 to
27 percent more than the model estimates, respectively. The January 1997
Auto/Oil Air Quality Improvement Research Program (AQIRP) Final Report16
 suggested that reducing fuel volatility by 1 psi, from 9.0 to 8.0 psi, would

12
 “Analysis of Causes of Failure in High Emitting Cars,” Publication Number 4637, Health and
Environmental Sciences Department, American Petroleum Institute, February 1996.
13
 M. Carlock, “An Analysis of High Emitting Vehicles in the On-Road Vehicle Fleet,” California Air
Resources Board, 1993.
14
  This study was sponsored by the American Petroleum Institute (API).
15
  RVP is a measure of fuel volatility, expressed as pounds per square inch, with higher pressures
resulting in higher volatility and, therefore, more hydrocarbon emissions.
16
  Auto/Oil Air Quality Improvement Research Program Final Report, January 1997.


Page 21                                                             GAO/RCED-97-210 Air Pollution
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Major Limitations in MOBILE5a




reduce exhaust CO by 9 percent, exhaust HC by 4 percent, and total
evaporative HC by 34 percent (NOx remained unchanged). The RVPs for most
fuels used to be higher than 9.0 psi,17 but today they can go lower than 7.0
psi. Similarly, the model currently has no emissions reduction credits for
low sulfur fuels (except, according to agency officials, for the lower sulfur
effect in reformulated gasoline), although recent studies suggest that
lowering the concentration of sulfur in fuel reduces the emissions of
hydrocarbons and nitrogen oxides.

EPA officials said that they plan to eliminate the constraint code in the next
model, MOBILE6, which will allow users to receive credit for correction
factors for fuels lower than 9.0 psi; however, they noted that work in this
area is still ongoing and that the data on the emissions benefits of lower
RVP fuels, as well as low sulfur fuels, are limited. In addition, a 1996
National Research Council study suggested that the model may
overestimate the benefits of oxygenated fuels. For example, the study
noted that EPA’s MOBILE model “apparently overpredicts the oxygenated
fuel effect by at least a factor of two” when the model’s estimate of carbon
monoxide reductions is compared with observed data. Similarly, a 1997
study18 of wintertime oxygenated fuels suggested that the observed
oxygenated fuel benefits were much lower than the 20 to 30 percent
estimated by EPA’s model. EPA officials agreed that this is also an area that
needs more study, but one which they plan to address in MOBILE6.

A third concern is MOBILE5a’s estimates of emissions system deterioration
for vehicles with more than 50,000 odometer miles. This concern stems
from studies that have questioned the rate and quantity of the data
supporting EPA’s significantly higher rate of emissions system deterioration
once vehicles reach 50,000 odometer miles.19 For example, prior to
MOBILE5, the model assumed that a vehicle with 100,000 miles emitted
about 1.0 grams of hydrocarbons for each mile driven, or about 4 times the
amount a new car would emit. However, EPA adjusted the deterioration
rates for vehicles with more than 50,000 miles beginning with MOBILE5
(Dec. 1992) so that the MOBILE model’s deterioration formula now
calculates that the same car was emitting about 2.0 grams of hydrocarbons
for each mile driven, or about 8 times the amount a new car would emit.

17
 According to a Senior Environmental Scientist, Health and Environmental Sciences Department,
American Petroleum Institute, from 1990 to 1992, average gasoline RVP was about 10 psi.
18
 L. Anderson, R. Jones, P. Wolfe, “Assessing the Effectiveness of Oxygenated Fuels Use for the
Reduction of Ambient Carbon Monoxide” (Univ. of Colorado at Denver), Seventh CRC On-Road
Vehicle Emissions Workshop, April 9-11, 1997.
19
  According to EPA, 50,000 miles was the regulatory “useful-life” standard for light-duty vehicles at
the time MOBILE5a was developed.



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Major Limitations in MOBILE5a




EPA acknowledges that these adjustments were made on the basis of
limited data and that only recently have 1990-technology vehicles become
old enough to accurately assess their emissions deterioration.

An October 1996 Sierra Research study20 of 75 vehicles with over 100,000
odometer miles questioned whether EPA had perhaps adjusted the formula
too much, resulting in a model that currently overestimates the emissions
from vehicles with 50,000 or more odometer miles. Among other things,
the study found that EPA’s current model estimated that 80 percent or
more21 of these higher mileage vehicles (which, on average, had
accumulated 123,900 odometer miles) would be high emitters, whereas the
study found that only 32 percent of the vehicles fell into this category.
Similarly, an April 1997 study22 of 227 vehicles (model years 1991 to
1993) with more than 50,000 odometer miles found no significant changes
in emissions or deterioration as indicated by the current model. OMS
officials said they used all the data that were available to them at the time
(1991-1992) to estimate the deterioration rate of such vehicles and that
researchers since then had had more time and more vehicles to test than
were available to EPA. They also said the model’s correction factor for
vehicles with 50,000 or more odometer miles would likely be lowered in
MOBILE6, but they were uncertain at the time of our audit how much this
emissions estimate would be reduced. A special subgroup of the FACA
mobile model workgroup has been established to address this issue, and
their work is still ongoing.

A fourth concern is MOBILE5a’s emissions credits and assumptions about
inspection and maintenance programs. According to a 1995 National
Research Council report, vehicle condition—whether the vehicle is well
maintained, or has been tampered with or is malfunctioning—is more
important than vehicle age in determining emissions. Among other issues,
there is a need to update the basic data supporting I&M emissions
reduction credits to reflect a growing population of vehicles in which rates
of tampering may be diminishing since tampering with newer vehicles


20
   This study was prepared by Sierra Research for API using data collected under another API-sponsored
study, the High Mileage Vehicle project.
21
 This range results from differences in the model’s assumptions about emissions because of the type
of fuel-delivery systems used. The model estimates that 80 percent of port fuel-injected vehicles would
be high emitters, and that 97 percent of throttle-body fuel-injected vehicles would be high emitters.
Carbureted vehicles were not a part of this study because this fuel delivery technology is no longer
used on light duty vehicles, according to the study.
22
 D. Berens (Ford Motor Company), H. Haskew (GM Powertrain Group), R. Ortega (Chrysler
Corporation), “FTP Emissions From 1991-1993MY In-Use Vehicles,” Seventh CRC On-Road Vehicle
Emissions Workshop, April 9-11, 1997.



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Major Limitations in MOBILE5a




adversely affects gas mileage and vehicle performance. Also, vehicle
owners often replace older, carbureted vehicles with newer fuel-injected
vehicles.

Additionally, according to agency officials, the current model provides no
additional I&M credits for vehicles equipped with on-board diagnostics23
(OBD), a requirement for all 1994 and later light duty vehicles and trucks.
This vehicle computer technology alerts a car owner when an emissions
system malfunctions, permitting quicker repairs than when such
malfunctions are identified through an I&M testing program, and diagnostic
trouble codes assist mechanics in making better repairs. Additionally,
newer vehicles have up to 8 years or 80,000-mile emissions control system
warranties for two components (the on-board computer and catalytic
converter), which should equate to less-polluting vehicles as a result of
more durable emissions control systems and the requirement that
manufacturers cover the costs of certain repairs. The current model does
not provide specific credits for this growing population of OBD-equipped
vehicles designed and believed to have less in-use deterioration than their
predecessors.

Also, more recent and more complete data are needed on the effectiveness
of repairs in an I&M program, including the adequacy and durability of
these repairs, actual participation rates, and impact of remote sensing
efforts. Except for remote sensing, the current model’s estimates for these
parameters is based on aging and limited data.24 For example, EPA has not
performed any tampering surveys since 1992, and agency officials said that
as a result of this lack of data, the tampering assumptions for MOBILE6 will
remain unchanged from MOBILE5a. However, EPA’s goal for MOBILE6 is to
provide users with greater flexibility in designing I&M programs, as long as
the state or local programs’ estimated I&M credits can be substantiated
with state or local data. With respect to newer vehicles equipped with
on-board diagnostics, because of the limited data on the longer-term
emissions impact of this technology, the agency has provided credit for
these OBD-equipped vehicles equal to that provided for operating an
enhanced I&M program. EPA officials said that, in addition to the states’ own
studies, the agency currently has I&M effectiveness studies being carried
out in three states, but they were unsure whether sufficient data would be
available in time to further revise the I&M assumptions in MOBILE6.

23
 All 1994 and later light duty vehicles and light duty trucks must be equipped with an emissions
control diagnostic system capable of identifying deterioration and/or malfunction of the emissions
control system, referred to in EPA’s regulations as on-board diagnostics .
24
  EPA’s remote sensing data are limited, but recent, according to agency officials.



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Appendix I
Major Limitations in MOBILE5a




A fifth concern is the proper representation of diurnal emissions. Diurnal
emissions refer only to hydrocarbons and are a form of evaporative
emissions that occur when a vehicle is parked and the ambient
temperature is fluctuating. For all previous versions of the MOBILE model,
the data supporting these 8- to 24-hour emissions estimates were collected
during a 1-hour period during which temperatures were forcibly increased
over a range of temperatures. More recent testing over 24-hour and longer
periods without constraining temperature increases to a one-hour period
indicates some differences from the MOBILE5a estimates for such
evaporative emissions. Also, evaporative emissions from vehicles with fuel
leaks are now believed to be so significant that, for MOBILE6, EPA plans to
model these emissions separately from other evaporative emissions.
According to EPA’s most recent data, indications are that some vehicles
with fuel leaks—similar to super emitters of exhaust/tailpipe
emissions—can exceed the evaporative emissions of corresponding
vehicles by one to two orders of magnitude. A 1996 automotive industry
study25 of 150 vehicles found that 24-hour diurnal emissions ranged from
0.6 grams of HC to 777.2 grams of HC, with vehicles with liquid fuel leaks
providing the vast majority of the emissions. EPA officials explained that
while they may develop a separate category for some vehicles with
significant fuel leaks, this does not necessarily mean there will be a
significant overall increase in evaporative emissions estimates because,
until more data are collected, there is no clear indication that these
emissions were significantly underestimated in prior diurnal estimates. EPA
has testing under way to determine how to better define this category of
vehicles with significant fuel leaks and also plans tests to estimate their
distribution within the current fleet, rate of occurrence as a function of
accumulated mileage, vehicle age, and/or vehicle technology, such as fuel
tank design. Agency officials are uncertain at this time whether the
correction factors for other evaporative emissions estimates will be
revised for MOBILE6.

A sixth concern is the adequacy of the data supporting MOBILE5a’s
assumptions about emissions for in-use26 I&M heavy duty27 vehicles.
According to EPA’s June 1994 workshop on state needs, the in-use credits
for heavy duty gasoline-powered vehicles are based on data approximately

25
 H. Haskew (GM Powertrain Group), “Diurnal Emissions From In-Use Vehicles,” Seventh CRC
On-Road Vehicle Emissions Workshop, April 9-11, 1997.
26
  In-use emissions estimates attempt to assess actual vehicle emissions over time, mileage, vehicle
type, and other factors, such as whether the vehicle is subjected to an I&M program, according to EPA
officials.
27
  Generally defined as vehicles 8,501 pounds or more in gross vehicle weight.



Page 25                                                            GAO/RCED-97-210 Air Pollution
Appendix I
Major Limitations in MOBILE5a




20 years old, and there has been much change in the technology and
emissions rates of these vehicles since that time. Still, the certification
standards are higher for heavy duty vehicles than their light duty
counterparts, and they are generally older and driven more miles annually
than their light duty counterparts. EPA officials said that testing heavy duty
vehicles is difficult and quite expensive and agreed that there is a lack of
recent data on the in-use emissions from this category of vehicles once
they have been put in service. While some studies are under way, EPA does
not envision at this time that significant changes in the in-use emissions
rates for heavy duty vehicles will be included in MOBILE6.

A seventh concern is the fleet characterization data in EPA’s database,
stemming from a concern that much of the data used for MOBILE5a are quite
old. For example, MOBILE5a’s estimates are based on the assumption that,
on average, light duty vehicles are driven about 14,000 miles annually
when new, decreasing to less than 10,000 miles annually after 10 years.
More recent data from the U.S. Department of Transportation indicates
that passenger cars are driven about 2,000 miles more annually than
currently estimated by EPA’s MOBILE model, or nearly a 10-percent increase
over MOBILE5a. According to a 1996 report,28 because of the linkage
between odometer mileage and I&M program assumptions, a small change
in mileage accumulation rates can result in a large impact on emissions
estimations. EPA officials pointed out that the agency’s guidance
encourages model users to provide their own accumulated mileage
estimates; thus, they said the default values for accumulated mileage in
MOBILE5a would be a problem only in those cases in which model users fail
to provide their own accumulated mileage estimates. Additionally, heavy
duty gasoline-powered vehicles, which have higher certification standards
than their light duty counterparts, are believed to comprise a significantly
larger percentage of the overall vehicle fleet than currently estimated by
the MOBILE model. According to OMS, the agency plans to update the fleet
characterization data for MOBILE6, including reflecting the increases in the
heavy-duty vehicle population.

Similarly, another fleet characterization issue involves urban buses. For
example, the current model does not have a separate classification for
urban buses, although this is a growing vehicle category in many urban
areas with unique operating characteristics, such as very frequent starts
and stops. EPA officials explained that while buses have not been a
separate category in MOBILE5a, EPA plans to expand the current list of

28
  J. Heiken, B. Austin, and A. Pollack (Environ International Corporation) and D. Coe, L. Chinkin, and
D. Eisinger (Sonoma Technology, Inc.), “Estimation of Local Fleet Characteristics and Activity Data
for Improved Emission Inventory Development,” March 18, 1996.



Page 26                                                             GAO/RCED-97-210 Air Pollution
                             Appendix I
                             Major Limitations in MOBILE5a




                             vehicle categories from 8 to 20, one of which will be a separate category
                             for buses.

                             An eighth concern is the level of distinctions in roadway classifications.
                             The MOBILE model was originally designed only for estimating areawide
                             emissions on the basis of assumptions associated with an entire trip. It
                             was not designed for making decisions for various roadway classifications,
                             such as transportation improvement projects for urban interstate, rural
                             arterial, or urban feeder/collector streets. Several model experts have
                             pointed out that the same average travel speed—35 mph, for
                             instance—would indicate smooth traffic flow on a local street but severe
                             congestion on a freeway. EPA officials pointed out that MOBILE6 will allow
                             users to separate start emissions from any linkage to the FTP driving cycle
                             assumptions and will also provide different correction factors for speed
                             and driving cycle for three different types of roadways—freeways,
                             arterials, and local roadways. Additionally, while not planned for MOBILE6,
                             the agency plans to partially fund ongoing research with the Department
                             of Transportation to develop a modal emissions model that may one day
                             allow users to model additional parameters, such as the relative emissions
                             impact of sequencing traffic signals to enhance traffic flow.

                             As noted above, an EPA-sponsored FACA mobile model workgroup made up
                             of representatives from other federal, state, and local government
                             agencies, academia, the automobile and oil industries, environmental
                             groups, and others has been assisting EPA in improving the current model,
                             and much of the research to fill data gaps and update aging databases was
                             still ongoing at the time of our audit. Agency officials said that it is their
                             plan for each new version of the MOBILE model to reflect the most recent
                             testing, data collection, and research that are available. Except for the
                             impact of road grade on emissions and revising the in-use credits for heavy
                             duty vehicles, EPA officials said they plan to address each of the above
                             limitations in the next revision, MOBILE6. However, as discussed below, the
                             agency will not be able to quantify the uncertainty associated with its
                             MOBILE model estimates, primarily because of the complexity and timing of
                             factors affecting vehicle emissions and the high cost of vehicle studies.


The Uncertainty Limitation   Another significant limitation involves the lack of information about the
                             range, or magnitude, of the uncertainty29 associated with the model’s
                             estimates. Uncertainty occurs for several reasons but often is the result of

                             29
                              Traditionally, this has been referred to as the 95-percent confidence interval, indicating with
                             95-percent certainty that the true number is within a specified range.



                             Page 27                                                              GAO/RCED-97-210 Air Pollution
Appendix I
Major Limitations in MOBILE5a




omissions, or gaps, in the understanding of emissions-producing activities
and of errors in assumptions that are the result of data gaps or data
limitations, as well as mathematical and statistical variabilities. However,
according to the nonfederal co-chair of the FACA workgroup, information
about the uncertainty of the model’s estimates is critical to the ability of
policymakers to make good decisions today and for researchers to address
the most significant areas in route to a better future model. The co-chair
and other model experts told us that the current MOBILE model has no
information about, nor estimates of, the uncertainty associated with its
predictions. According to one researcher, the model “just produces a
number for the user, with no documentation of the reliability of the
output.” Similarly, according to a 1994 National Research Council special
report30 addressing the MOBILE model,

“Uncertainty is pervasive in all three emission modeling components: vehicle activity,
activity-specific emission rates, and emission rate correction factors. Uncertainty is
compounded in the methodologies used to develop the emission inventory. That is, vehicle
activity uncertainty is combined with emission rate uncertainty that has already been
combined with correction factor uncertainty.”31


Additionally, the limited work in this area indicates there are significant
uncertainties associated with the current MOBILE model’s estimates. For
example, one study32 found that “the range of uncertainty is huge” for a
change in one variable—average vehicle speed—of the many variables
contained in the MOBILE model. According to the study, most model users
generally believe that increasing average vehicle speed from 30 mph to 50
mph will reduce vehicle emissions (because of less congested driving, with
more driving at cruising speeds). The study noted that EPA’s MOBILE model
estimates a 24-percent reduction in carbon monoxide emissions by
increasing average vehicle speed from 30 mph to 50 mph. However, when
a 95-percent confidence interval is applied, the change in emissions can
range from a 72-percent decrease in carbon monoxide emissions to a




30
  R. Guensler and D. Sperling, Congestion Pricing and Motor Vehicle Emissions: An Initial Review,
Institute of Transportation Studies, University of California at Davis, commissioned paper for the
Transportation Research Board, National Research Council, National Academy of Sciences.
31
  With respect to MOBILE, according to EPA, only two of these modeling components—emissions
rates and correction factors—are subject to concerns about uncertainty; model users supply their own
vehicle activity data from other sources.
32
  R. Guensler, Vehicle Emission Rates and Average Vehicle Operating Speeds, Ph. D. dissertation,
Institute of Transportation Studies, University of California at Davis, 1993, excerpted for the 1994
commissioned paper for the Transportation Research Board, National Research Council, National
Academy of Sciences.



Page 28                                                              GAO/RCED-97-210 Air Pollution
Appendix I
Major Limitations in MOBILE5a




75-percent increase in such emissions. Similarly, a 1996 study33 of EPA’s
speed correction factors for vehicle exhaust emissions found substantial
uncertainty in EPA’s current MOBILE model. Among other things, the study
concluded that the MOBILE model may significantly underestimate carbon
monoxide and hydrocarbon emissions—“by up to 3 orders of
magnitude”—as the model relates to changes in vehicle speed.

According to Office of Mobile Sources officials, EPA has been unable to
quantify the model’s uncertainty primarily because of the cost and time
associated with such quantification, the fact that the on-road vehicle
population is a constantly changing universe of differing emissions control
devices and levels, technological limitations in measurement devices, and
because there is substantial naturally occurring variability in vehicle
emissions (leading to further data gaps/limitations). Agency officials
pointed out that there is substantial variability across (1) vehicle types
(such as model year, emissions control system, engine type), (2) vehicle
operating conditions (cold start, load, speed), (3) the external
environment (road grade, temperature, humidity, altitude), (4) vehicle
fuels (reformulated, oxygenated, reid vapor pressure), and (5) driver
behavior (quick starts and stops, timing and frequency of trips). For these
reasons, OMS officials told us they do not plan to develop uncertainty
ranges with the next revision to the MOBILE model. Similar to the global
positioning issue for road grade, they said significant technological
advancement may be needed before it becomes cost-effective to address
this issue. For example, future vehicles may have on-board computers
with the ability to instantaneously record and later report emissions under
different operating scenarios. EPA officials pointed out that it is their plan,
at some point in the future, to report uncertainty ranges for some model
estimates. They said they are currently saving both qualitative and
quantitative descriptors for the data being collected by and for the agency
in order to perform these calculations in the future.




33
 M. Kini, Probabilistic Modeling of Exhaust Emissions From Light Duty Gasoline Vehicles, Graduate
Thesis, Department of Civil Engineering, North Carolina State University, Oct. 1996.



Page 29                                                          GAO/RCED-97-210 Air Pollution
Appendix II

Comments From the Environmental
Protection Agency




              Page 30         GAO/RCED-97-210 Air Pollution
Appendix II
Comments From the Environmental
Protection Agency




Page 31                           GAO/RCED-97-210 Air Pollution
Appendix III

Objectives, Scope, and Methodology


               The Chairman, Subcommittee on Oversight and Investigations, House
               Committee on Commerce, asked us to (1) describe the major limitations in
               the current version of EPA’s MOBILE model and (2) describe EPA’s process
               for improving both current and future versions of the MOBILE model.

               To describe the major limitations in the current model, we obtained and
               reviewed the MOBILE5a User’s Guide; EPA/OMS model documentation; the
               most recent published sensitivity analyses (MOBILE4.1, May 1990); relevant
               EPA guidance and memorandums on the MOBILE model; selected vehicle
               studies; the results of stakeholders meetings about the model; and the
               charter, objectives, minutes, and proceedings of the EPA-Federal Advisory
               Committee Act mobile modeling workgroup. We also reviewed five
               electronic databases1 for studies pertaining to the EPA MOBILE model;
               attended one mobile sources symposium where modeling issues were
               discussed; and attended the March 1997 FACA public workshop. We also
               obtained and discussed studies relating to potential MOBILE model
               limitations with selected representatives of state and local agencies,
               academia, industry, environmental groups, consulting firms, and other
               government agencies. Additionally, we interviewed officials and obtained
               documents from EPA’s Office of Mobile Sources in Ann Arbor, Michigan;
               Office of Research and Development in Research Triangle Park, North
               Carolina, and Athens, Georgia; and the EPA Science Advisory Board in
               Washington, D.C. We also discussed model limitations with individuals
               identified to us by EPA or other representatives, as well as through our own
               efforts as noted above.

               To describe EPA’s process for improving both current and future versions
               of the MOBILE model, we obtained and discussed information from
               knowledgeable EPA/OMS air quality officials relative to ongoing activities
               and documented plans for making model revisions. We also discussed
               EPA’s past, ongoing, and planned actions with representatives of academia,
               industry, environmental groups, consulting firms, and other government
               agencies and observed one process—EPA’s open solicitation of input by


               1
                The electronic databases reviewed included “Enviroline,” which covers more than 5,000 international
               primary and secondary environmental publications on all aspects of the environment; “EiCompendex
               Plus,” the electronic version of The Engineering Index, which provides worldwide coverage of
               approximately 2,600 journals and selected government reports and books on the environment and
               other issues; “Pollution Abstracts,” a leading resource for references to environment-related literature
               on pollution, its sources, and its control; “Energy Science and Technology,” one of the world’s largest
               sources of literature references on energy and related topics, including the environment, with coverage
               of journal articles, report literature, conference papers, books, patents, dissertations, and translations;
               and EPA’s Technology Transfer Network, a worldwide network of electronic bulletin boards providing
               information and technology exchange in areas pertaining to air pollution control, with emphasis on
               EPA’s Mobile Sources Information bulletin board, which covers information pertaining to mobile
               source emissions, regulations, test results, models, and guidance.



               Page 32                                                               GAO/RCED-97-210 Air Pollution
Appendix III
Objectives, Scope, and Methodology




external stakeholders not involved in the model’s development—at work.
We also obtained documents and discussed EPA’s process for improving
models in general with the Science Advisory Board and EPA’s Office of
Research and Development.

We conducted our review from October 1996 though August 1997 in
accordance with generally accepted government auditing standards.




Page 33                                       GAO/RCED-97-210 Air Pollution
Appendix IV

Major Contributors to This Report


                        Lawrence J. Dyckman, Associate Director
Resources,              William F. McGee, Assistant Director
Community, and          Judy K. Pagano, Senior Operations Research Analyst
Economic                James R. Beusse, Evaluator-in-Charge
                        Hamilton C. Greene, Jr., Staff Evaluator
Development             DeAndrea M. Leach, Staff Evaluator
Division, Washington,
D.C.




(160369)                Page 34                                       GAO/RCED-97-210 Air Pollution
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