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OCR for page 61
Technical Issues Associated
With the MOBILE Mocle'
THE FOCUS OF CHAPTER 3 is the technical issues associated with the cur-
rent (MOBILES) and upcoming (MOBILE6) versions of the MOBILE mod-
el. The introductory portion of this chapter discusses the development of
the model and the updates for MOBILE6. The chapter goes on to describe
related models for estimating mobile-source emissions (PARTS, NON-
ROAD, and others) and previous reviews of the model. A major portion of
this chapter then focuses on the technical issues associated with the mod-
el, such as how the model handles high emitters, driving cycle, start emis-
sions, and many others details. The chapter concludes with a summary
and recommendations related to technical aspects of the model.
HISTORY AND STATUS OF THE MOBILE MODEL
History of the MOBILE Model
The MOBILE model for estimating on-road vehicle emissions factors (in
grams per mile [g/mi]) was first developed by the U.S. Environmental Pro-
tection Agency (EPA) in the late 1970s. Prior to that time, the agency pub-
lished simple look-up tables for estimation of mobile-source emissions.
The model, originally and still written using the Fortran scientific pro-
gramming language, has had significant updates and new releases every
few years as new data became available, new regulations were promul-
61
OCR for page 62
62 M ODEL/NG M OB![E-SOURCE EMISSIONS
gated, emissions standards were established, and sources and processes of
vehicle emissions were better understood. Each new version of the model
has become more complex in the approach to modeling average in-use ve-
hicle emissions, and has provided the user with additional options for tai-
Toring emissions-factor estimates to local conditions. The model versions,
release dates, and changes in each model update are summarized in Table
3-1 (EPA 1999e).
Changes in the databases underlying the models and changes in model-
ing methodology in each successive version result in changes to predicted
total on-road vehicle emissions. From one model version to the next, these
changes can be either increases or decreases in emissions factors, and the
changes are not always in the same direction for all three pollutants (NOX,
CO, and VOCs). Although these changes created somewhat of a moving
target for air-quality planners and the regulated industries, the revised
models should provide more accurate analyses of mobile source emissions
and of the effects of mobile source control programs.
As an example, Figure 3-1 shows emissions for the Baltimore area for
calendar years 1988 and 1990 as predicted using three recent official re-
lease versions of the model (MOBILE5a, MOBILE4.1, and MOBILES;
these comparisons are unaffected by the inclusion of new emissions stan-
dards and regulations. Carbon monoxide (CO) emissions and emissions of
nitrogen oxides (NOx) increase from one model to the next, albeit in differ-
ent proportions. Volatile organic compound (VOC) emissions, though, de-
creased from MOBILE4 to MOBILE4.1, and then increased significantly
from MOBILE4. 1 to MOBILE5a, while still remaining lower than
MOBILE4 levels.
MOBILES—The Current Moclel
The MOBILES model, released in 1993, provides emission factors for
on-road vehicles for the three regulated pollutants: VOCs, CO, and NOX.
The model provides emission factors separately for the classes of vehicles
listed in Table 3-2, and also for the average on-road fleet using a default
national mix of vehicles; the user can optionally input a different fleet mix
for the calculation of fleet average emissions. The vehicle classes are fur-
ther subdivided into technology classes in MOBILE, to account for emis-
sions differences between, for example, vehicles with carburetors and
those with fuel injection. To estimate total on-road mobile emissions in a
given area, either the vehicle class emissions factor is multiplied by esti-
mates of vehicle miles traveled (VMT) by vehicle class for the area and
summed, or the fleet average emissions factor is multiplied by total VMT
(across vehicles classes) for the area. These VMT estimates are typically
provided by local or regional transportation-planning agencies.
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TECHNICAL ISSUES ASSOCIATED WITH THE M OBILE MODEL 63
TABLE 3-1 MOBILE Model Revisions
Release
Version Date
Model Revisions
MOBILE 1 1978 Included modeling of exhaust emissions rates as func-
tions of vehicle age/mileage (zero-mile levels and deteriora-
tion rates).
MOBILE2 1981 Updated with substantial data (available for the first
time) on emission-controlled vehicles (i.e., catalytic con-
verters, model years 1975 and later) at higher
ages/mileages.
Provided additional user control of input options.
MOBILES 1984 Updated with substantial new in-use data.
Elimination of California vehicle emissions rates (contin-
ued to model low- and high-altitude emissions).
Added tampering (rates and associated emissions im-
pacts) and anti-tampering program benefits.
MOBILE4 1989
In-use emissions-factor estimates for nonexhaust emis-
sions adjusted for real-world fuel volatility as measured by
Reid vapor pressure (RVP).
Updated with new in-use data.
Added running losses as distinct emissions source from
gasoline-powered vehicles.
Modeled fuel volatility (RVP) effects on exhaust emis-
sions rates.
Continued expansion of user-controlled options for input
data.
MOBILE4.1 1991 Updated with new in-use data.
Added numerous features allowing user control of more
parameters affecting in-use emissions levels, including
more inspection/maintenance (I/M) program designs.
Included effects of various new emissions standards and
related regulatory changes (e.g., test procedures).
Included impact of oxygenated fuels (e.g., gasohol) on CO
. .
emlsslons.
MOBILES 1993 Updated with new in-use data, including basing new ba-
sic emissions-rate equations on much larger database de-
rived from state-implemented IM240 test programs.
Included effects of new evaporative emissions test proce-
dure (impact on in-use nonexhaust emissions levels).
Included effects of reformulated gasoline (RF G).
Included effects of new NOX standard of 4.0 g/bhp-hr for
heavy-duty engines.
Included impact of oxygenated fuels on VOC emissions.
Included Tier 1 emissions standards under 1990 Clean
Air Act Amendments.
Added July 1 evaluation option.
(Corltirrued)
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64 MODE1/NG MOB/[E-SOURCE EMISSIONS
Table 3-1 Model Revisions
(Continued) Included impact of low-emission vehicle (LEV) programs
patterned after California regulations.
Revised speed corrections used to model emissions fac-
tors over range of traffic speeds.
MOBILE5a 1993 Corrected a number of minor errors in MOBILES.
MOBILE5b 1996 Included final on-board vapor-recovery regulations.
Included final reformulated gasoline regulations.
Added more user options for I/M programs.
Source: EPA l999e
All of the MOBILES emissions factors are estimated from existing test
data, and engineering judgment in the absence of test data. Although
there is a detailed User's Guide (EPA 1994) for the model, there is limited
documentation from EPA describing the databases and analytical methods
used in MOBILES to develop the emissions factors.
The user provides inputs (some required and some optionaV to
MOBILES that describe typical operating characteristics, fleet character-
ization, and mobile-source control programs. These inputs (in addition to
vehicle class and VMT mentioned above) are
ambient temperature;
average vehicle speeds by vehicle class;
.
fuel characteristics; (include fuel volatility and oxygen content, and if
reformulated gasoline is in use);
vehicle inspection and maintenance (I/M) program parameters, if
such a program is in place; and
vehicle age distributions (used to estimate composite emissions
across all vehicle model years).
Figure 3-2 shows emissions estimates developed using an updated ver-
sion of MOBILES and updated travel-activity estimates. It shows the pre-
dicted distribution of on-road mobile source emissions of VOCs and NOX in
year 2007 by emissions category for New York and Chicago. These inven-
tories were generated by EPA using the latest emissions model developed
as part of the regulatory impact assessment for the recent Tier 2 vehicle
emissions and fuel sulfur standards (EPA 19996~. This model, known as
the Tier 2 Model, was developed from MOBILE5b and available elements
of the upcoming version, MOBILES (EPA l999c). The Tier 2 model is ac-
tually a spreadsheet program derived from MOBILE algorithms, outputs,
supplemental test data, and assumptions. The MOBILES elements incor-
porated into the Tier 2 Model include updated assessments of in-use vehi-
cle deterioration, fuel sulfur impacts, and fleet characteristics. However,
OCR for page 65
TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL 65
60 -
50 -
—- ~o
-
o
~ 30
o
US
In
-
10 -
o -
1E MOBILE5A
20 ~ ~ ll]~:
· MOBILE4.1
O MOBILE4
HC HC NOX NOX CO/10 CO/10
1988 1990 1988 1990 1988 1990
FIGURE 3-1 Comparison of estimated emissions for Baltimore from three
recent versions of the MOBILE model. Note that CO emissions are divided
by 10. Also note that this report usually uses the term VOCs as opposed to
hydrocarbons (HCs) to refer to the general class of gaseous organic com-
pounds
the Tier 2 Model does not use the MOBILES methodology and test data for
estimating evaporative emissions. MOBILES will likely increase the frac-
tion of evaporative to tailpipe emissions of VOCs compared with that ob-
tained in MOBILE5b.
Both cities depicted in Figure 3-2 show a fairly similar emissions profile,
although important differences are clear. Chicago has much greater emis-
sions from heavy-duty vehicles. This is especially apparent for NOX emis-
sions; Chicago has 37% of emissions from HDDVs whereas New York only
has 24%. Chicago also has greater emissions from motorcycles. For exam-
ple, Chicago has 7% of their VOC emissions attributed to motorcycles, over
twice the percent of emissions from motorcycles in New York. Generally
speaking, the Tier 2 Model estimates that about 45% of the total on-road
VOC emissions is from light-duty vehicle exhaust, about 30% is from light-
duty evaporative emissions, and the remainder is primarily from heavy-
duty vehicles. Note that the regulatory impact analysis was performed for
four cities Atlanta, Charlotte, Chicago, and New York. However, the
emissions profiles for Atlanta and Charlotte were similar to those for New
York, and are not shown here.
OCR for page 66
66 MODELING MOB/LE-SOURCE EMISSIONS
TABLE
MOBILE b Vehicle Classes
Vehicle Class
Light-duty gasoline vehicles
(passenger cars)
Light-duty gasoline trucksa
(pick-ups, minivans, passenger
vans, and sport-utility vehicles)
MOBILE
Code
LDGV
Weight Description
Up to 6000 lb gross vehicle
weight (GV\iV)
Up to 6000 lb GVVV
LDGT1
LDGT2
Heavy-duty gasoline vehicles HDGV
Light-duty diesel vehicles
(passenger cars)
Light-duty diesel trucks
Heavy-duty diesel vehicles
Motorcyclesb
Emissions for light-duty trucks are modeled separately for two weight classes
with different emissions standards in the Clean Air Act
bHighway-certif fed motorcycles only are included in the model. OiT-road motor-
cycles, such as dirt bikes, are modeled as a non-road mobile source in EPA's NON-
ROAD model.
LDDN7
LDDT
HDDV
MC
6001-8500 lb GVVV
8501 lb and higher GVW
equipped with heavy-duty
.
gave .lne engines
Up to 6000 lb GVVV
Up to 8500 lb GVW
8501 lb and higher GVW
MOBILE6- The Next Generation Model
EPA's Office of Transportation and Air Quality (OTAQ) has for the last
several years been working on the next generation of the MOBILE model,
referred to as MOBILE6. This model will be significantly different from
MOBILES in almost all model components, and will be based on an enor-
mous amount of recent vehicle-emissions testing data from EPA, the Cali-
fornia Air Resources Board (CARB), automobile manufacturers, and petro-
leum refiners. At this point, MOBILE6 is expected to be released in the
year 2000. However, EPA has already released substantial documentation
and held workshops describing the model revisions, allowing the agency to
gather feedback on its proposed modifications. This documentation and
public outreach process will be discussed in the following section. The sig-
nificant changes being incorporated into MOP;TT,F:Hi into t.h~ fallowing
dramatically lower basic emissions rates, based on analyses of the
Dayton, Ohio I/M program data;
Reparation of start and running-exhaust emissions;
addition of so-called off-cycle emissions (aggressive driving and air-
conditioning operation, which are not included in the Federal Test Proce-
dure [FTP]~;
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TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL 67
2007 VOC
New York
6% 3%
1%~
- ~...3
~ l ~~ ~
9% ~ l ~ ,.~ ~
Ft., ~ ~ ~
.. -.- :::- :.-. ~ ~ ~ ~ ~ 48%
.:.:...: .-.:.:.:.:.:: :.: ::.:: :.::t ~ ~ ~ ~
(~ ! i ~
33%~. 11
~ i
New York
24% ~ ~
I...*....*.
I,-.--..--..
'* *-* ****,.*.*
i.S**..SSSSSS...
Fat——. -—*an.
.~*~**~.
·****~-*-~*-**.
~0.2%
2007 NOx
4%
Chicago
10% _ ` -
~ ~_
, _
I,,..,,_
~SSSSSSS.
29%
'>-an--.
~~..~.,
_,-----...
~SSS.SSS:SS
~ err ~—- ..—- ~—- - ~—
`5 / /0 ~ a***-----*-*
——~——~—~—~
1*~.—are- - -
h. SSSS--SSS. S
SSSSSSSSSSSSSS
·—+*~—- - - - - - -—
.~-~.
I----------
~—- ~.—~—
22222222222-.*
I-.-.-.-. . '. ''I. . ~
Chicago
Legend
LDV Exhaust ~ LDV Evap. ~HDGV Exhaust
~ ~ HDGV Evap. ~ HDDV Exhaust ~ MC Exhaust &
I Evap.
58%
FIGURE 3-2 VOC and NOx emissions inventories for New York and
Chicago. Source: EPA 19994.
control of off-cycle emissions with the Supplemental FTP (SFTP) in
future years;
.
emissions factor estimates for different roadway types (e.g., highways
arterials, locals);
evaporative diurnal emissions factors estimated *om real-time diur-
nal test data previously unavailable,
OCR for page 68
cards.
68 MODELING MOBILE-SOURCE EMISSIONS
revised (lower) estimates of the effects of oxygenated fuels on CO
. .
emlsslons;
revised (lower) effects of I/M programs on vehicle emissions;
addition of off-cycle NOx emissions for heavy-duty diesel vehicles;
effects of in-use fuel sulfur content on all emissions; and
effects of national low-emissions vehicle (NLEV) and Tier 2 stan-
Although the MOBILE6 documentation provides numerical results for
changes in specific model components, overall changes to average in-use
fleet emissions factors will not be known until the full model is released.
Thus, it is not yet known whether regional emissions estimates from
MOBILE6 will increase or decrease relative to MOBILES, though it is like-
ly that they will increase at least for VOCs in order to be in better agree-
ment with the findings of evaluation studies that are discussed in Chapter
4.
FEDERAL ADVISORY COMMITTEE ACT
PROCESS AND PUBLIC OUTREACH
An important part of the developmental process for MOBILE6 has been
public outreach. This includes input from EPA advisory committees, com-
ments *om stakeholders and the interested public, and the release of tech-
nical documentation describing model modifications.
The Clean Air Act Amendments of 1990 (CAAA90) established the
Clean Air Act Advisory Committee to advise EPA on issues of implementa-
tion this law. One of the many subcommittees of the Clean Air Act Advi-
sory Committee is the Mobile Sources Technical Review Subcommittee
(MSTRS), often referred to as a FACA subcommittee because it is char-
tered under the Federal Advisory Committee Act (FA CA). The MSTRS
advises EPA's OTAQ on technical issues specific to the control of emissions
from mobile-sources. It is composed of experts on mobile-source emissions
from industry, academia, state agencies, and nongovernmental organiza-
tions. Meetings are held quarterly and are open to the public.
One of the MSTRS working groups is the Modeling Working Group,
which provides on-going advice on the development and improvement of
MOBILE and other emissions models. The specific charge for this work
group includes helping to set priorities for developments to MOBILE6 and
developing procedures for EPA to use when obtaining outside review for
products used to support MOBILE6. This group also is producing a com-
prehensive report on the MOBILE modeling process, problems, and oppor-
OCR for page 69
TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL 69
"unities for improvement on which this committee was briefed (this paper,
"Big Picture Modeling Issues," is currently in draft form).
Additionally, EPA has increased public feedback and public involvement
in the development of MOBILE6 in several other ways. These steps were
taken to make the model better understood by the user community and to
counter criticism that the model was inadequately documented and peer
reviewed. Although the draft MOBILE6 model is not expected to be avail-
able until later in 2000, EPA has released detailed technical documenta-
tion for most of the proposed changes in the Stakeholder Review Docu-
ments on MOBILE6 web page (http://www.epa.gov/OMSWWW/m6.htm).
EPA is to be commended for documenting the databases used and the de-
velopment of revised emissions factors in the Stakeholder Review Docu-
ments. This documentation, although not always complete in describing
the full details of the analyses, is a major improvement from all previous
versions of MOBILE. EPA provides a 60-day review period for each docu-
ment as it is posted, and has stated its intention to provide all comments
and responses to comments with each document in the final version.
EPA has also held three workshops discussing the new version of the
model and created an e-mail list server to update interested parties on
new model developments. The workshops were open to the public. They
were intended to update interested parties on EPA's plans for the model as
well as solicit input and reaction to those plans. The workshops included
both technical presentations describing changes to the model methodology
and presentations oriented to model users describing changes to data in-
put and output. The e-mail list server is used to announce the workshop
agendas, the release of new documentation concerning the MOBILE6 mod-
el, updates to the current version of MOBILE5b, and other information.
RELATED MODELS
There are several emissions models and databases related to EPA's MO-
BILE model, which are used to estimate mobile-source emissions invento-
ries and provide inputs for air-quality models. These are
PARTS estimates particulate matter (PM) emissions factors for on-
road vehicles;
Complex Model estimates emissions impacts of reformulated fuel
compositional changes on 1990 light-duty gasoline vehicles;
MOBTOX estimates on-road mobile-source toxic emissions factors;
and
SPECIATE, and related databases and models provide VOC
speciation profiles for complex photochemical grid modeling.
OCR for page 70
70 MODE[/NG MOBI[E-SOURCE EMISSIONS
These models are each described briefly below. References are provided
for readers who desire more detailed information on any of these models.
PARTS
EPA's PARTS mode! estimates PM emissions factors in g/mi for 12 vehi-
cle classes. Emissions factor estimates are provided for particle diameter
sizes from less than or equal to 1.0 to 10.0 ,um (micrometers). The model is
referred to as PARTS to indicate consistency with MOBILES in fleet char-
acterization data and in the general methods used to estimate basic emis-
sions rates. PARTS estimates aD PM emissions associated with on-road
travel: exhaust emissions, brake-wear emissions, tire-wear emissions, and
fleet-average paved and unpaved road-dust emissions. For HDDVs,
PARTS also provides estimates of idle emissions. The PARTS model has
been updated in only very minor ways since the original model develop-
ment in the mid-1980s. The emissions factors in the model are based on
either engine certification data or on ratios from VOC emissions. Al-
though there is a User's Guide for the model, there is no documentation
that explains the derivation of the emissions factors in the model.
The emissions-factor estimates in the PARTS model are seriously out of
date. New PM test data have recently become available for both light-
duty vehicles (LDVs) and heavy-duty vehicles (HDVs). Although on-road
mobile-source emissions-factor estimates will be needed for state and local
air-quality planning agencies to develop PM emissions inventories and air-
quality management plans for the new PM standards, EPA's OTAQ has
not focused as much effort on updates to the PARTS model as on MOBILE.
However, many of the revisions developed for MOBILES can be easily in-
corporated into PARTS, and OTAQ has done so for estimating PM emis-
sions as part of the Tier 2 rulemaking (EPA 1998d). But the major im-
provement required for the model is the inclusion of recent testing data for
the revision of emissions factors.
Because most of the methods for estimating PM emissions in PARTS
are similar to the methods used in MOBILES, and because PARTS needs
major revision, an updated version of MOBILES should incorporate re-
vised PM emissions-factor estimates. For most users, it would be much
more desirable to have one integrated model that provides emissions-fac-
tor estimates for PM as well as VOC, NOX, and CO.
Complex Moclel
The Complex Model is used by petroleum refiners and other interested
parties to estimate how gasoline composition affects vehicle emissions.
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TECHN/CA~ ISSUES ASSOCIATED WITH THE M OBILE MODEL 7 7
The model was developed in a regulatory negotiation process between EPA
and the affected industry. The model is fully described in EPA's reformu-
lated gasoline Regulatory Impact Analysis dated December 13, 1993 (EPA
1993b). The Complex Model, which is a spreadsheet model downloadable
from EPA's reformulated gasoline web page (http://www.epa.gov/
OMSWWW/rfg.htm), predicts percent change in 1990 technology vehicle
emissions for a target reformulated gasoline (RFG) relative to U.S. 1990
baseline gasoline. Emissions are a function of the following input parame-
ters:
MTBE (methyl tertiary-butyl ether, weight percent oxygen twt%~),
ETBE (ethyl tertiary-butyl ether, wt%),
ethanol (wt%),
TAME (tertiary-amyl methyl ether, wt%),
sulfur (parts per million [ppm]),
RVP (Reid vapor pressure, pounds per square inch [psi]),
E200 (percent of fuel that evaporates at 200° F),
E300 (percent of fuel that evaporates at 300° F),
aromatics (percent by volume),
olefins (percent by volume), and
benzene (percent by volume).
U.S. baseline emissions are calculated from MOBILES runs with U.S.
industry-average gasoline. The model calculates changes from baseline
emissions to emissions for the target fuel for exhaust and evaporative
VOCs, air taxies (benzene, formaldehyde, acetaldehyde, and 1,3-buta-
diene), and exhaust NOX. (The model does not estimate the effects of fuel
reformulation on exhaust CO emissions.) The model is a statistical model
based on testing data from several major programs measuring the emis-
sions effects of the various fuels tested.
The present and planned versions of MOBILE allow for the specification
of limited fuel properties (e.g., whether or not RFG is used) but do not al-
low for the specification of the detailed fuel properties that are available in
the Complex Model. This means that if states or nonattainment areas
choose to require a fuel with greater emissions reductions than required of
federal RFG, the Complex Model must be run first to generate scaling fac-
tors to apply to MOBILE output. Even then, there are questions as to how
the results would be used given that COMPLEX was developed for 1990
vehicle technologies only. The fuel effects now calculated in the Complex
Model should be updated so that they can appy to all model years and
technology groups, not just 1990 technology vehicles, and added to future
versions of MOBILE. CARB's Predictive Model estimates fuel effects for
all on-road light-duty vehicles based on a broader database than that used
in the development of the Complex Model; EPA should review and consider
OCR for page 124
Pre-1981 .315 .193 .368 .193
1981- 1985 .486 .025 .493 .025
1986- 1990 .172 .006 .122 .006
1991- 1997 .018 .004 .035 .004
724 MODELING MOB/LE-SOURCE EMISSIONS
TABLE 3-10 PM Emissions Rates in g/mi from PARTS and from the CRC
Project E-24-2
Passenger Cars Light-Duty Trucks
Model Year CRC PARTS CRC PARTS
_ .
Source: Cadle et al. 1998.
report prepared for EPA (Weaver et al. 1998) shows significant deteriora-
tion for 1994 and later trucks and transit buses. In addition, work by
Graboski et al. (1998) concluded that PARTS was "significantly underesti-
mating" PM emissions from heavy-duty vehicles.
It is important that EPA revise PARTS to reflect in-use PM emissions.
This will likely require extensive field measurements. Data are also need-
ed to assess differences between HDDV PM measurements obtained in the
laboratory compared to in-use emissions; EPA should review work cur-
rently being conducted by the CRC and the National Cooperative Highway
Research Program in this area. In addition, data are needed relative to
systems of the engine and the vehicle that reflect maintenance problems
that affect emissions. Studies are also needed on the effectiveness of die-
sel I/M programs and whether smoke I/M programs tend to increase NOX
TABLE 3-11 Passenger Car PM Emissions Rates in g/mi
from PARTS and from the CRC Projects E-24-1 and CRC E-46
Model Year CRC E-24-1 PARTS
Pre-1981 .955 .193
1981- 1985 .474 .025
1986-1990 .444 .006
1991-1997 .028 .004
CRC E-46 PARTS
Tier 0 .083 .025
Tier 1 .038 .006
l
Source: Norbeck et al. 1998; Cadle et al. 1999.
OCR for page 125
TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL ~ 25
emissions and reduce PM emissions. Ways of screening and characterizing
the in-use vehicle population for high-emitting vehicles are needed. EPA
needs to determine the effectiveness of the catalytic oxidation devices used
since 1994 to reduce PM emissions by reducing the SOF. There is no infor-
mation on the long-term effectiveness, maintenance practices, or tamper-
ing for these devices.
Particulate Emissions from Tire and Brake-Wear
PARTS estimates emissions from tire wear based on the assumption
that the emissions rate of airborne particulate is 0.002 grams per mile per
wheel (EPA 1998j). This reference, known as AP-42, lists two studies as
the basis of the estimate for tire-wear emissions Williams and Cadle
1978; Brachaczek and Pierson 1974~. The single emissions rate is based
on tests of LDVs and no estimate for the airborne particle size distribution
for tire-wear is offered. Tire-wear emissions less than 10 ,um are based on
interpolation. The dated references for the PARTS emissions factors sug-
gests that these factors are based on tests of older biased-ply tires rather
than longer-wearing tire technologies currently in use.
PARTS reports brake wear as a separate emissions factor of 0.0128
grams per mile, based on a paper by Cha et al. (1983~. Brake-wear partic-
ulate emissions are higher than for tire wear because a larger fraction is
assumed to be less than 10 ,um in diameter. Brake-wear emissions factors
in PARTS are assumed to be the same for all vehicle classes, although it
could be assumed, as with tire wear, that the number of wheels, the
weight of the vehicle, and the driving cycle would be significant contribut-
ing factors related to the per mile emissions rate. As with tire wear, the
dated reference suggests that the emissions factor is based on older mate-
rials and needs to be updated.
Issues for Model Revision
EPA is planning to update PARTS after it completes the updates for
MOBILES. As recommended above, such a revision should become part of
MOBILE rather than being issued as a separate model. Issues that need
to be addressed while updating PARTS are listed below:
Data on particulate emissions from HDVs need to be updated to in-
clude the effects of deterioration in emissions, adjustment for the benefits
of I/M, and variations due to actual driving conditions.
OCR for page 126
726 MODELING MOBILE-SOURCE EMISSIONS
Data from new studies on emissions from light-duty gasoline-powered
and diesel-powered vehicles should be included in the updated particulate
emissions model. Data collection efforts should be expanded to ensure
that the effects of deterioration, I/M, and off-cycle driving conditions are
included in the model.
Data on modern tires and brake materials must be obtained for inclu-
sion in future particulate emissions inventories.
FLEET CHARACTERIZATION
In MOBILE, the fleet is characterized by three parameters: age or regis-
tration distribution, mileage-accumulation rate, and fleet or VMT mix.
The age distribution gives the fraction of all vehicles in a particular class
that are of a certain age. Because MOBILES accounts for 25 different ages
(except for motorcycles), 25 fractions are required for each vehicle class,
with the fractions summing to unity within each class. The mileage accu-
mulation rate is the annual number of miles a vehicle is expected to be
driven. It varies by vehicle age and class. The fleet mix gives the fraction
of the fleet total VMT traveled by each of the eight vehicle classes. Again
these fractions must sum to one.
MOBILE calculates a vehicle class's emissions factor by computing the
emissions factors for each of the model years, weights these by each model
year's contribution to the vehicle class's total annual VMT, and then sums
the weighted emissions factors. The weighting factors are termed travel
fractions. The travel fraction, TFm, represents the fraction of the total
VMT that is accounted for by a vehicle of age m years. It is calculated
*om the fraction of vehicles registered that are m years old, REGm, and
the annual mileage accumulation for these vehicles, MILESm,
TF = REGm * MILES
m MaxYears
~ (REGk M}~ESk
k = ~
(3-7)
where the summation is over all model years k. Once the vehicle-ciass-
specific emissions factors are computed, the model then weights each of
them by the corresponding fleet mix fraction and sums the results to pro-
duce the fleet emissions factor.
Although the user is allowed to enter custom registration distributions,
mileage- accumulation rates, and fleet mixes, the model contains default
(or for fleet mix, internally calculates) values for these parameters. The
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TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL 727
default registration distributions are obtained from sales fractions through
the 1980s. Mileage accumulation rates in MOBILES are based on 1990
National Purchase Diary (NPD) data. Fleet mix is internally calculated
using registration distributions, mileage accumulation rates, diesel sales
fractions, and total vehicle counts by class.
MOBILE6 will see changes in all of the above parameters. These
changes are documented in an EPA report (1999x). They are brought
about by the availability of new data as well as shifts in methodology in
some cases. The new model expands the previous 8 vehicle classes to 28,
thus requiring much more detailed fleet characterization data. New
mileage-accumulation rates for light-duty vehicles and light- and heavy-
duty trucks are derived from the 1995 National Personal Travel Survey
(NPTS) and the 1992 Truck Inventory and Use Survey (TIUS), respec-
tively. Revised registration distributions are obtained from 1996 data
compiled by the R.L. Polk company. Vehicle counts are based on data from
various sources including the 1996 Polk data, 1998 Certification and Fuel
Economy Information System (CFEIS) database, the Annual Energy Out-
look (Energy Information Administration 1998), and a report by Ward's
Communications (Pemberton 1996) which gives scrappage rates.
Figures 3-16, 3-17, and 3-18 give sample comparisons of the MOBILES
and MOBILE6 mileage-accumulation rates, registration distributions, and
vehicle counts. Note that the new registration distribution is smoothed.
This is a significant change from the former approach, which reflected ac-
tual historical trends in sales and perpetuated them in all future-year cal-
culations.
SUMMARY AND RECOMMENDATIONS
EPA is currently developing MOBILE6, the newest version of the MO-
BILE model. This version is scheduled for release in the year 2000. In
developing MOBILE6, EPA has used a more-open process, involving many
stakeholders, and has been published much documentation for general
review.
In the development of MOBILE6, EPA has addressed many shortcom-
ings of MOBILES, particularly those identified in the GAO (1997) report.
The extent to which MOBILE6 has addressed those concerns is summa-
rized in Table 3-12. The table also notes some improvements that can be
made in future versions of MOBILE.
In addition to the recommendations in Table 3-12, the committee offers
the following recommendations for the improvement of MOBILE. These
recommendations begin with changes to components in the existing
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~ 28 M ODE1`NG M OB!LE-SOURCE EMISSIONS
20,000
1 8,000
1 6,O00
1 4,000
1 2,000
1 0,000
8,000
6,000
4,000
2,O00
+ 1995 NPTS
MOBILE5a (LDGV only)
MOBI' E5a (LDDV only)
Expon. (1995 NPTS)
y = 1 s6g4e-o.oso6x
O. ~ ,
1 1 1
0 5 10 15 20 25 30 35
Age
FIGURE 3-16 Light-duty vehicle annual mileage-accumulation rates.
MOBILES uses the curve developed from the 1995 NPTS data. Source:
EPA 1999x.
MOBILE model. The next set of recommendations pertains to models that
are closely related to MOBILE, such as those that estimate PM and air
toxic emissions. The final recommendation deals with the need for long-
range planning to guide the future development of the model.
Obtain Better Data on High-Emitting Vehicles
Establish a long-term testing program to characterize in-use deteriora-
tion of representatively aged new-vehicle technology using a driving cycle
more representative of actual driving conditions. This should focus on de-
termining the nature of both exhaust and evaporative high emitters. Im-
proved data on both the emissions rate and the *action of the vehicle pop-
ulation that are high emitters are required.
Inclusion of Road-Grade Effects in MOBILE
The emissions increase from road grade is similar to that from accelera-
tion and should be included in the model. This will be particularly impor-
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TECHN/CAI ISSUES ASSOCIATED WITH THE M OBILE M ODEL ~ 2 9
0.1 -
JO 0.08
a_
~ 0~06
lo
0.04 -
0.02
Lo
_~
, , , 'it
O -
0 5 10 15 20 25
Age
· MOBILES ~ MOBILE61
FIGURE 3-17 Comparison of MOBILE 5 and MOBILES light-duty vehicle
registration distribution. Source: EPA 1999x.
tent for areas where there is a significant amount of grade such as Denver
and Spokane. Planning for this feature should include input from local
regions that use MOBILE to ensure that grade information is available to
potential users and that the model revisions are consistent with the avail-
able formats of the grade data.
Improve the Start-Emissions Database
Routine tests of start emissions should be made as part of ongoing mea-
surement programs unless there is confirmation that regression tech-
niques, similar to those used for MOBILES, provide an effective estimation
of start emissions. Additional measurements of the effects of ambient tem-
perature, wind speed, and soak time on start emissions should be made to
get a better representation of these important factors. Another factor that
should be considered when estimating start emissions is the operating
mode of a vehicle during the first minutes of operation.
Modeling of Inspection and Maintenance Programs Benefits
In particular, the treatment of vehicles that failed emission tests but
never appeared for a retest, owners who never have their vehicles in-
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30 MODEL/NO MOB!LE-SOURCE EMISSIONS
350,000,000-
300,000,000
250,000,000
200,000,000
150,000,000
100,000,000
50,000,000
7
-
,.~
O- .
1982 2002 2022 2042
Calendar Year
—MOBILE 5
Linear Interpolation
MOBILES
FIGURE 3-18 Comparison of light-duty vehicle counts, 1982-2050.
Source: EPA 1999x.
spected, and the deterioration of vehicles after repair needs to be im-
proved. The modeling of repaired vehicles' deterioration should be based
on data from actual repaired vehicles.
Improve the Emissions Factors for Heavy-Duty Vehicles
Emissions factors for HDVs are woefully outdated and there are ques-
tions about the conversion of engine dynamometer data into on-road gram-
per-mile emissions. Appropriate chassis dynamometer cycles need to be
developed for HDVs and data must be obtained on such cycles. Appropri-
ate corrections for the effects of humidity and temperature, currently un-
der development, should be incorporated into MOBILE. Data should to be
generated for in-use conditions that might have significantly different
emissions from those predicted based on engine certification tests.
Updating of Fleet Characterization
In recent years, there has been a significant increase in the use of light-
duty trucks (especially sport-utility vehicles) instead of and in addition to
passenger cars. EPA has updated fleet characterization data for MOBILES
to reflect these current trends. EPA should at regular intervals (every 2
years or so) review the fleet characterization data, both current and pro-
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TECHNICAL ISSUES ASSOCIATED WITH THE M OBILE MODEL 7 3 7
TABLE 3-12 Summary of Expected Changes to MOBILE6 That
Respond to Problems Identified in GAO Report (1997)
Area of Concern Regarding
MOBILE Model Cited in GAO
Report
MOBILE6 Treatment of Issue
1. Emissions estimates for
higher speeds, especially
speeds in excess of 65 mph.
Representation of emissions
from rapid acceleration and
deceleration, including
aggressive clrlvlng
behaviors.
Representation of emissions
immediately after engine
start-up, known as cold-start
emissions.
4. Representation of emissions
*om air conditioner use.
Representation of emissions
from road grade, such as
when a car climbs a hill.
6. Representation of high-
emitting vehicles in the
MOBILE's supporting
database.
7.
Representation of exhaust
emissions from lower-
polluting fuels, especially
fuels with lower volatility
(low RVP); representation of
emissions from oxygenated
fuels.
MOBILE6 uses data obtained from
recent studies on real-world driving
conditions to develop facility-specific
speed-correction cycles, which
include higher speeds and
aggressive driving behavior. The
facility-specific speed-correction
factors also provide greater
distinction in roadway
classifications.
Start emissions have an improved
treatment in MOBILE6; more study
should be done to provide additional
data for the approach proposed.
MOBILE6 has an improved model of
air-conditioner use. Additional data
and model modifications could
improve the estimates of this effect.
Not addressed in MOBILE6.
EPA used data from IM240 lanes to
correct FTP data for recruitment
bias in exhaust-emissions data.
Special studies should be done to
determine effect of high emitters.
MOBILE6 exhaust emissions effects
of low RVP fuels has not changed
from MOBILES. MOBILE6 shows
reduced benefits from oxygenated
fuels, based on EPA analysis of
more recent test data.
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7 32 MODE1JNG M OBILE-SOURCE EMISSIONS
TABLE 3-12 (Continued)
Area of Concern Regarding
MOBILE Model Cited in GAO
Report
MOBILE6 Treatment of Issue
8. Representation of emissions
system deterioration for ve-
hicles with 50,000 or more
miles.
9. Emissions estimates and
assumptions for vehicle I/M
programs.
10. Estimates and assumptions
for nontailpipe evaporative
emissions when the vehicle
is not operating.
11. Emissions estimates and
assumptions for the inspec-
tion and maintenance of
HDVs—those with a gross
vehicle weight of 8,501
pounds or more.
Data characterizing vehicle
fleet.
Greater distinctions in road-
way classifications.
Quantifying the uncertainty
of the model's estimates.
New data have shown much lower
emissions rates for such vehicles.
These data have been used in
MOBILE6.
MOBILE6 shows reduced benefits
from I/M programs. Questions re-
main about the assumed benefits for
OBD and the assumed deterioration
of repaired vehicles.
MOBILE6 includes updates to rest-
ing loss emissions based on real-
time (24-hr) test data.
Not included in MOBILE6.
EPA has updated fleet characteris-
tics (fleet mix and age and mileage-
accumulation distributions by vehi-
cle classy for MOBILE6.
See response to items one and two.
Not included in MOBILE6.
jected, to ensure that changes in the vehicle fleet are properly recognized
in the emissions model.
Complete Documentation of all Databases and Analyses
EPA has done an excellent job of improving their documentation of the
basic steps in the MOBILE model. Additional documentation should be
provided to explain all the details of the analyses so that interested parties
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TECHNICAL ISSUES ASSOCIATED WITH THE MOBILE MODEL 7 33
can readily check the analyses. Placing these detailed analyses and all the
databases used on the internet would facilitate external review of the da-
tabases and methods used in MOBILE.
Integration of the PARTS and MOBILE Models
PARTS and MOBILE do basically the same thing: compute actual emis-
sions from on-road mobile sources. The separation of gaseous emissions in
MOBILE and particulate emissions in PARTS is not necessary. It requires
users to run two models instead of one and leads to the possibility that the
on-road motor vehicle fleet and other important factors will not be treated
consistently between the two models.
When incorporating the PARTS model into MOBILE, several problems
with PARTS need to be addressed. Updated emissions factors should be
developed incorporating data on the effects of high-emitting vehicles
(smoking vehicles), in-use deterioration, I/M programs, speed variations,
and off-cycle emissions.
Incorporation of the COMPLEX Model into MOBILE
The effects of reformulated gasolines are currently estimated in a sepa-
rate model called the COMPLEX model. Incorporating a COMPLEX-like
model into MOBILE would allow states and regions to directly model gaso-
line formulations with more stringent requirements than federal reformu-
lated gasoline requirements. It would require the impacts of reformulated
gasolines to be extended to include impacts on emissions from all vehicles
and technology groups as well as the impacts on CO emissions.
Incorporation of Toxic-Emissions Factors into MOBILE
These emission factors are currently in a separate model, MOBTOX.
The rationale for this recommendation is the same as that for the two pre-
vious recommendations: convenience and consistency.
More Emphasis on the NONROAD Model
Although the EPA model, NONROAD, for off-road emissions sources is
not part of MOBILE, the committee notes that this critical emissions
model is lacking in data on emissions factors and activity levels. As more
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~ 34 M ODE[/NG MOBILE-SOURCE EMISSIONS
controls are placed on on-road sources, the off-road sources will become
more important in the future, and the NONROAD model will play a larger
role in estimating regional emissions inventories.
Develop Long-Range Plans for the Evolution
of the MOBILE Model
The future implementation of new emissions and fuel standards, grow-
ing concerns about PM and air tonics emissions, and the rising cost of con-
trol strategies will increase the focus on MOBILE. Users will need im-
proved accuracy and reliability from MOBILE, even as the regulatory set-
ting, vehicle technologies, and fleet characteristics are changing. This
poses a daunting challenge for EPA's Office of Transportation and Air
Quality (OTAQ) to develop an accurate model that reflects uncertain regu-
lations, unproven technologies, and shifting preferences among consumers.
To address these demands, EPA must develop a long-range plan for ad-
dressing critical modeling issues. EPA first should determine the most
appropriate uses for the model and develop improvements to support these
specific uses. This plan should then set priorities for model improvements
that have the largest impact on emissions and develop a plan for collecting
the necessary data to support these improvements. Most importantly,
EPA must also address how close the modeling of mobile-source emissions
should be to the development of regulations. The model must be seen as
an accurate reflection of mobile-source emissions, not as a tool that is used
to support proposed regulations.
Representative terms from entire chapter:
emissions factors
