United States Air and Radiation EPA420-R-02-012
Environmental Protection March 2002
Agency M6.PM.001
vvEPA MOBILE6.1
Emission Factor Model
Technical Description
DRAFT
yŁu Printed on Recycled
Paper
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EPA420-R-02-012
March 2002
MOBILE6.1 Particulate Emission Factor Model
Technical Description
DRAFT
M6.PM.001
Edward L. Glover
Mitch Cumberworth
Assessment and Standards Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
NOTICE
This technical report does not necessarily represent final EPA decisions or positions.
It is intended to present technical analysis of issues using data that are currently available.
The purpose in the release of such reports is to facilitate the exchange of
technical information and to inform the public of technical developments which
may form the basis for a final EPA decision, position, or regulatory action.
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1. Introduction
Since 1995, EPA has made available the PART5 model, a Fortran
program that estimates particulate air pollution emissions of in-
use gasoline-fueled and diesel-fueled highway motor vehicles. It
calculates particle emission factors in grams per mile (g/mi) for
on-road automobiles, trucks, and motorcycles, for particle sizes of
1-10 microns. The particulate matter (PM) estimates include
emission factors for exhaust particulate, brakewear, and tirewear.
The PART5 model is now outdated.
The MOBILE6.0 model is the most recent EPA emission factor
model. It calculates average in-use fleet emission factors for
three criteria pollutants: hydrocarbons (HC), carbon monoxide (CO) ,
and oxides of nitrogen (NOx). These emission estimates are made
for gas, diesel and natural gas fueled cars, trucks, buses and
motorcycles for calendar years 1952 through 2050. The model
calculates emission factors under a wide variety of conditions
affecting in-use emission levels, e.g., ambient temperatures,
average traffic speeds, etc.
The MOBILE and PARTS models have been used by EPA and other
organizations in a variety of applications. These include
evaluations of highway mobile source control strategies by state,
local and regional planning agencies; emission inventories and
control strategies for State Implementation Plans under the Clean
Air Act; transportation plans and conformity analyzes by
metropolitan planning organizations and state transportation
departments; environmental impact statements by industry
investigators; and academic research efforts.
This document describes the methodology and algorithms used to
combine the PARTS and MOBILE6.0 models to produce an integrated
MOBILE6.2 model. This new model produces the same estimates for
HC, CO and NOx emission as MOBILE6.0, but it also can estimate
particulate emission factors like the PARTS model. The MOBILE6.2
particulate emission estimates differ somewhat from the PARTS
estimates. The principal reasons for these differences are changes
in vehicle registration and technology distributions between PARTS
and MOBILE6 and the fact that some basic particulate emission rates
for future model years have been updated in MOBILE6.2 to reflect
recent rulemakings.
2. Overview of MOBILES.2 Features
The MOBILE6.2 model offers several advantages relative to the
separate MOBILE6.0 and PARTS models. First, the combination
eliminates significant duplication of technical material between
the two models. For instance, both models contain many of the same
data parameters relating to vehicle activity and use. Both models
also have very similar input requirements and produce similar
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output. Second, combining the two models aids users who are now
given a single, consistent interface for both functions, and allows
EPA to support one consistent computer model product rather than
two. Combining PART5 and MOBILE6.0 was a prominent recommendation
of Modeling MOBILE SOURCE Emissions, the National Academy of
Science Research Council's review of MOBILE. This panel concluded
that the process of emission inventory modeling could be improved
by creating a new model or suite of integrated models that could
produce emission factor estimates for a wider range of pollutants
and conditions.
The objective of the MOBILE6.2 project was to produce, in the
relatively short term, a combined model that reflects EPA
particulate emission modeling done for recent vehicle emission
control rulemakings. The project takes into account the fuel
sulfur level reductions that are now mandated, and new vehicle
emission standards.
The project was also originally intended to take into account
particulate test data that have become available since PART5 was
produced, including in-use testing by EPA and other parties. EPA
has concluded, however, that these data are insufficient to update
the basic particulate emission rates at this time.
The foundation of MOBILE6.2 is, therefore, the basic mobile
source particulate emission rates from the PART5 PM model and from
EPA rulemaking modeling sources. These sources are supported by a
large body of engine and vehicle certification test results.
An additional feature of MOBILE6.2 is that it allows the user
to enter alternative basic exhaust particulate rates into the model
as a function of vehicle class, model year, catalyst technology,
and vehicle age. Also, deterioration estimates as a function of
mileage can now be added. This will allow a sophisticated user to
model a specific fleet and perform more complex modeling exercises
if they can supply defensible particulate emission factors.
Section 3 describes the way PART5 and MOBILE6.0 were combined
to produce MOBILE6.2 and the new features added. Here is a brief
summary of these updates:
2.1 Base Emission Rates - The base emission rates for most
vehicle classes and model years are unchanged from PART5.
However, the basic emission rates for heavy-duty diesel
vehicles were updated in MOBILE6.2 to reflect the emission
factors modeled in EPA's 2007 Heavy-Duty Diesel Vehicle
Rulemaking effort. As a result MOBILE6.2 predicts that 2007
and later model year diesel heavy-duty vehicles will meet a
0.01 g/bhp-hr certification standard if low sulfur fuel is
used. The basic PM emission rates for light-duty and heavy-
duty gasoline vehicles were updated to assume compliance with
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EPA's Tier2 vehicle rulemaking requirements in 2004, and with
the 2005 heavy-duty gasoline vehicle rulemaking, if low sulfur
fuel is used.
2.2 Sulfate Particulate and Gaseous S02 Emission Factors - PART5's
calculation of sulphate particulate and gaseous S02 exhaust
emissions were restructured to account for the sulfur levels
of gasoline and diesel fuel, while still using the same basic
algorithms as PART5. This feature of the program now allows
the user to model the effects of different fuels and changes
in EPA fuel regulations.
2.3 Ammonia Emission Factors - MOBILE6.2 adds the ability to
estimate exhaust emissions of ammonia. These estimates are
based on the emission rates and calculation methods described
in EPA Report Number EPA/AA/CTAB/PA/81-20, entitled
"Determination of a Range of Concern for Mobile Source
Emissions of Ammonia". While this report dates from 1981, we
are not aware of a better or significantly different basis for
such calculations.
2.4 ZEVs - MOBILE6.2 allows the user to model the effects of zero
emitting vehicles on particulate emissions whereas PART5 did
not have this capability. In MOBILE6.2 the exhaust
particulate emission factors are assumed to be zero for ZEVs.
However, their tire and brake wear emissions are assumed to be
the same as gasoline-fueled vehicles.
2.5 Natural Gas Vehicles (NGVs)
PART5 did not contain exhaust particulate emission estimates
for NGVs. MOBILE6.2 assumes that the exhaust particulate
emissions of NGVs are the same as gasoline-fueled vehicles
operating on very low sulfur fuel. This assumption is based
on comparisons between NGV and gasoline vehicle hydrocarbon
emission test results. These test results, provided by the
NGV industry (See EPA report EPA420-R-01-033) suggest that
NGVs generally have equivalent or lower emissions than
gasoline vehicles. Based on the similarity between
hydrocarbon and particulate emission formation, the general
assumption of rough equivalence between these vehicle types
was extended to their particulate emission factors. The tire
and brake wear emissions of NGVs are assumed to be the same as
gasoline-fueled vehicles.
Further improvements to the estimation of mobile source
particulate emissions will be made in the course of the longer term
effort to produce a new generation of mobile source air pollution
models (MOVES). The MOVES model is intended to implement the
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recommendations of the National Academy of Science. It will be
based on an extensive database of emission measurements made during
actual operation of in-use vehicles and will provide a framework
for allocating emission estimates to much smaller geographic areas
and time periods.
3 . Technical Description
3.1 Definitions
The MOBILE6.2 model reports separate PM emission factors for
twenty-eight vehicle classes covering model years 1952 through
2050. The PM and PM-related pollutants are:
OCARBON
ECARBON
Sulfate
Lead
The organic carbon portion of diesel exhaust
particulate emissions. It was denoted as SOF in
the PARTS model.
The elemental carbon and residual carbon portion of
diesel vehicle exhaust particulate. It was denoted
as RCP in the PART5 model.
The sulfate particulate emissions. These are based
directly on the sulfur content of the fuel.
The lead particulate emissions. These are based
directly on the quantity of lead in the automotive
fuel. Like PART5, MOBILE6.2 model assumes that
post 1975 model year vehicles and all calendar
years subseqent to 1991 are free from lead PM
emissions.
Total Exhaust Diesel PM
OCARBON + ECARBON + Sulfate + Lead
In MOBILE6.2, Total Exhaust Diesel PM is calculated
by the model and then apportioned to the four
reported constituents: OCARBON, ECARBON, Sulfate,
and Lead.
GASPM
The sum of the organic and elemental carbon portion
and any residual carbon portion of gasoline vehicle
exhaust particulate.
Total Exhaust Gasoline PM = GASPM + Sulfate + Lead
In MOBILE6.2, Total Exhaust Gasoline PM is the sum
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of three constituents GASPM, Sulfate and Lead
emissions.
NH3
Ammonia emission factors. These are new to the
MOBILE6 and PART5 model series. Ammonia is a
gaseous pollutant that is converted in the
atmosphere to an ammonium based particulate
emission. Only the gaseous emissions which are
directly emitted from a vehicle tailpipe are
reported by MOBILE6.2. The model does not contain
any algorithms pertaining to the conversion of
gaseous emissions to particulate emissions. These
reactions and their effects are calculated in other
EPA models.
BRAKE
Particulate emission factors from Brake wear.
TIRE
Particulate emission factors from Tire wear.
S02
Gaseous Sulfur Dioxide Emissions. These are based
directly on the fuel sulfur content.
The emission factors listed above are reported by vehicle
type. The 28 vehicle types are listed and described in Table 3.1.
They are the same classifications used in MOBILE6.0. This is an
expansion from the twelve vehicle classifications that the PART5
model used, but each PART5 vehicle class corresponds directly to
one or to a group of MOBILE6.2 vehicle classes.
Table 3.1
MOBILE6 Vehicle Classifications
Number
1
2
3
4
5
6
7
8
9
MOBILE6
Abbreviation
LDGV
LDGT1
LDGT2
LDGT3
LDGT4
HDGV2b
HDGV3
HDGV4
HDGV5
Description
Light-Duty Gasoline Vehicles (Passenger Cars)
Light-Duty Gasoline Trucks 1 (0-6,000 Ibs. GVWR, 0-3,750 Ibs. LVW)
Light-Duty Gasoline Trucks 2 (0-6,001 Ibs. GVWR, 3,751-5750 Ibs. LVW)
Light-Duty Gasoline Trucks 3 (6,001-8500 Ibs. GVWR, 0-5750 Ibs. ALVW)
Light-Duty Gasoline Trucks 4 (6,001-8500 Ibs. GVWR, 5,751 and greater
Ibs. ALVW)
Class 2b Heavy-Duty Gasoline Vehicles (8501-10,000 Ibs. GVWR)
Class 3 Heavy-Duty Gasoline Vehicles (10,001-14,000 Ibs. GVWR)
Class 4 Heavy-Duty Gasoline Vehicles (14,001-16,000 Ibs. GVWR)
Class 5 Heavy-Duty Gasoline Vehicles (16,001-19,500 Ibs. GVWR)
PARTS
Abbreviation
LDGV
LDGT1
LDGT1
LDGT2
LDGT2
HDGV
HDGV
HDGV
HDGV
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10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
HDGV6
HDGV7
HDGVSa
HDGVSb
LDDV
LDDT12
HDDV2b
HDDV3
HDDV4
HDDV5
HDDV6
HDDV7
HDDVSa
HDDVSb
MC
HDGB
HDDBT
HDDBS
LDDT34
Class 6 Heavy-Duty Gasoline Vehicles (19,501-26,000 Ibs. GVWR)
Class 7 Heavy-Duty Gasoline Vehicles (26,001-33,000 Ibs. GVWR)
Class 8a Heavy-Duty Gasoline Vehicles (33,001-60,000 Ibs. GVWR)
Class 8b Heavy-Duty Gasoline Vehicles (>60,000 Ibs. GVWR)
Light-Duty Diesel Vehicles (Passenger Cars)
Light-Duty Diesel Trucks land 2 (0-6,000 Ibs. GVWR)
Class 2b Heavy-Duty Diesel Vehicles (8501-10,000 Ibs. GVWR)
Class 3 Heavy-Duty Diesel Vehicles (10,001-14,000 Ibs. GVWR)
Class 4 Heavy-Duty Diesel Vehicles (14,001-16,000 Ibs. GVWR)
Class 5 Heavy-Duty Diesel Vehicles (16,001-19,500 Ibs. GVWR)
Class 6 Heavy-Duty Diesel Vehicles (19,501-26,000 Ibs. GVWR)
Class 7 Heavy-Duty Diesel Vehicles (26,001-33,000 Ibs. GVWR)
Class 8a Heavy-Duty Diesel Vehicles (33,001-60,000 Ibs. GVWR)
Class 8b Heavy-Duty Diesel Vehicles (>60,000 Ibs. GVWR)
Motorcycles (Gasoline)
Gasoline Buses (School, Transit and Urban)
Diesel Transit and Urban Buses
Diesel School Buses
Light-Duty Diesel Trucks 3 and 4 (6,001-8,500 Ibs. GVWR)
HDGV
HDGV
HDGV
HDGV
LDDV
LDDT
2BHDDV
LHDDV
LHDDV
MHDDV
MHDDV
MHDDV
HHDDV
HHDDV
MC
BUSES
BUSES
BUSES
LDDT
3.2 Calculation of Particulate Emission Constituents
3.2.1 Calculation of Organic Carbon (OCARBON) Emissions
The pollutant type called OCARBON in MOBILE6.2 was formerly
called Soluble Organic Fraction (SOF) in PART5. This type of
particulate emission is generally a complex mixture of organic
chemical matter that is attached to the 'carbon' core of the
particle. As the former name implies, it is soluble in some
organic solvents. The name was changed to OCARBON in the model
because it was felt that the former name (soluble organic fraction)
was less precise and misleading (i.e., soluble in which solvent?
and the output is in terms of grams per mile not a fraction or
percentage).
Other than the name change, no changes from PART5 were made in
the definition of the pollutant, or in the values of OCARBON-
related parameters in the associated calculation algorithm. The
algorithm and data parameters presented here are used to model all
diesel vehicle classes for all model years. Due to a lack of
consistent and reliable data, gasoline vehicle particulate emission
factors are not broken out into OCARBON and ECARBON, but are
reported as GASPM.
For diesel vehicles, the organic carbon emissions are
calculated by first subtracting the sulfate and lead emission
factors from the total exhaust PM emission factor. The remainder
is then multiplied by the organic carbon fractions (OCFRAC) to
produce the OCARBON emission factor. The values of OCFRAC are the
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same as in the PART5 model.
in Equation Eqn 3.1.
The algorithm is shown mathematically
OCARBON
[Exh PM - Sulfate - Lead] * OCFRAC
Eqn 3.1
The values of OCFRAC are a function of the vehicle class.
following values were taken directly from PART5.
The
Vehicle Class Number
14
15
28
16
17 and 18
19,20,21, 26 and 27
22 and 23
Vehicle Type
LDDV
LDDT1, LDDT2
LDDT3, LDDT4
2b
3 and 4
5 through 7, buses
8a and 8b
OCFRAC
0.18
0.50
0.48
0.51
0.51
0.44
0.24
3.2.2
Calculation of Elemental Carbon (ECARBON) Emissions
The pollutant type called ECARBON or elemental carbon in
MOBILE6.2 was formerly called Remaining Carbon Portion (RCP) in
PARTS. As the former name implies it is the 'elemental carbon'
portion of the particulate after all other constituents have been
removed. Other than the name change no changes were made in the
definition of the pollutant. The algorithm presented here is used
to model all diesel vehicle classes for all model years. Gasoline
vehicle particulate emission factors are not broken out into
OCARBON and ECARBON, but are reported only as the sum GASPM. The
elemental carbon emissions are calculated by subtracting the
sulfate, lead and OCARBON emissions from the total Exhaust
Particulate Emission factor. The algorithm is shown mathematically
in Equation 3.2.
ECARBON = [total diesel exhaust PM - Sulfate -
OCARBON - lead]
Eqn 3.2
3.2.3
Calculation of LEAD Emissions
The lead emission factors are based directly on the quantity
of lead in the automotive fuel. The model assumes that all post-
1975 model year vehicles that were not tampered with and all
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calendar years subsequent to 1991 are free from lead PM emissions.
The algorithm and data coefficients used to calculate LEAD
emissions are the same as those used in the PART5 model. The
frequency of leaded fuel tampering effects (rates of tampering) are
the same as those used in MOBILE6.0. The PART5 documentation
contains a thorough explanation of these calculations. [DRAFT
User's Guide to PART5: A Program for Calculating Particle Emissions
from Motor Vehicles - EPA-AA-AQAB-94-2, pp 48-52.]
3.2.4 Calculation of BRAKE-WEAR Emissions
The PM brake wear emission factor was not updated from PART5.
[See PART5 User Guide page 63.] The brake wear emission factor is
assumed to be the same for all vehicle classes in the model. It is
set equal to:
BRAKE = 0.0128 * PSBRK Eqn 3.3
where
PSBRK = The fraction of particles less than or equal to the
particle size cutoff
3.2.5 Calculation of TIRE-WEAR Emissions
The tire wear emission factor in units of grams per mile was
not updated from PART5. It is given by equation 3.4. This
equation is used for all vehicle classes and model years.
TIRE = 0.002 * PSTIRE * WHEELS Eqn 3.4
where
TIRE is the emission factor in grams per mile
PSTIRE is the fraction of particles less than or equal to the
particle size cutoff.
WHEELS is the number of wheels on a vehicle class.
The value of 0.002 is the emission rate of airborne particles
from tire wear [taken from Compilation of Air Pollutant Emission
Factors, Volume 2, : Stationary Point and Area Sources. EPA (AP-42,
4th Edition) ] .
The tire wear emission factors are the same as those used in
PART5 with one exception. In MOBILE6.2, number of wheels on a
School Bus has been increased to 6 from 4 (the analogous brakewear
number does not change because the number of brake disks or drums
is not increased by the addition of two wheels).
3.2.6 Calculation of Sulfate and Gaseous Sulfur Dioxide
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Emissions
The methodology for calculating sulfate and gaseous sulfur
dioxide emissions (S02) is based on PART5. [See PART5 User Guide -
EPA-AA-AQAB-94-2 pp 50 to 60]. PART5 did not have user inputs for
gasoline or diesel fuel sulfur levels. MOBILE6.2 has user-supplied
fuel sulfur levels and has extended the PART5 algorithm to use
them.
The overall methodology for calculating sulfate particulate
and S02 emissions in MOBILE6.2 is based on the principal of sulfur
conservation and mass balance. This means that the sulfur
contained in the gasoline or diesel fuel must be equal on a mass
basis to the sulfur leaving in the exhaust stream as sulfate and
gaseous S02 emissions. The proportion of the fuel sulfur that is
converted to either sulfate or gaseous S02 emissions is discussed
below.
3.2.6.1 Calculation of Gasoline Vehicle Sulfate Emissions
The gasoline vehicle sulfate emissions are a function of
catalyst availability, catalyst type, air injection availability,
speed and vehicle fuel economy. The calculations require three
parameters: the basic sulfate emission rates (which depend on
speed), the technology weighting factors (air injection type,
catalyst type, etc.), and the fuel economy values. The basic
sulfate emission factors (Table 3.4 Sulfate Emission values) were
taken from the PART5 model, and are not updated for MOBILE6.2. The
vehicle fleet technology weighting factors were taken from
MOBILE6.0 and are slightly different than those used in PART5. The
fuel economy values were also taken from the MOBILE6.0 model, and
are slightly different than those used in PART5.
Basic Sulfate Emission Factors
The basic gasoline vehicle sulfate emission factors for all
model year gasoline vehicles are shown in Tables 3.2 and 3.3. All
emission factors except the Sulfate emission slope (sulfate
emissions versus fuel sulfur level) were taken from PART5 [See
PART5 user guide]. The sulfate emission factors are a function of
catalyst type, air injection type and average speed bin. Two speed
bins are shown in the table: 19.6 MPH and 34.8 MPH. Sulfate
emission levels at intermediate speeds are calculated by linear
interpolation between these two speeds. Speeds below 19.6 MPH are
considered to be 19.6 MPH and speeds above 34.8 are considered to
be 34.8 for this purpose.
The tables contain two columns of emission values. The first
value is the sulfate emission factor in grams per mile at a fuel
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sulfur level of 340 ppm sulfur (0.034 wt%). This value was taken
from PART5 and represents the fuel sulfur level of the underlying
emission tests for all gasoline vehicles. The second 'slope' value
is the sulfate emission rate as a function of the fuel sulfur level
in units of [grams/mile ] per ppm Sulfur. These were calculated
from a linear interpolation of the 340 ppm sulfur point, and the 0
ppm sulfur point. Logically, the 0 ppm gasoline fuel sulfur level
will produce zero sulfate emissions.
As a result of the Tier2 rulemaking for 2004 and later model
years, the 340 ppm fuel sulfur level is no longer representative of
in-use vehicle fuel for these model year vehicles. Thus, the base
sulfate emission factors used in pre-Tier2 vehicles are
unrepresentative as well. Unfortunately, there is also no new test
data at a lower sulfur fuel level such as 30 ppm in which to
develop new sulfate emission factors. To overcome this lack of
representative data, the pre-2004 model year sulfate emission
factors were ratioed down to the 30 ppm sulfur level using the
'Slopes' in Table 3.2 (also shown in Table 3.3). These resulting
sulfate levels based on 30 ppm fuel sulfur and shown in Table 3.3
then become the basis for the 2004 and later model year gasoline
vehicles rather than the sulfate emission factors shown in Table 3-
2 .
In the MOBILE6.2 model the gasoline sulfur effects in Tables
3-2 and 3-3 are extrapolated linearly to a maximum of 1000 ppm
gasoline fuel sulfur levels (600 ppm maximum sulfur in gasoline
fuel for 2000 and later model years). (The linear sulfur function
was used because no data were available to develop any other
functional response.) This approximation has only a minimal impact
on MOBILE6.2's total exhaust PM emission estimates.
Table 3-2
Gasoline Vehicle Sulfate Emission Factors
thru Model year 2003
Technology
Type
Non Catalyst
Ox Cat/No Air
3W Cat/No Air
Ox Cat / Air
Speed BIN
< 19.6 MPH
< 19.6 MPH
< 19.6 MPH
< 19.6 MPH
Sulfate
Emission (g/mi)
@340 ppm Sulfur
0.002
0.005
0.005
0.016
Sulfate
Emission
(g/mi*ppm S)
SLOPE
5.882e-6
1.471e-5
1.4716-5
4.706e-5
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3W Cat/ Air
Non Catalyst
Ox Cat/No Air
3W Cat/No Air
Ox Cat / Air
3W Cat / Air
< 19.6 MPH
> 34.8 MPH
> 34.8 MPH
> 34.8 MPH
> 34.8 MPH
> 34.8 MPH
0.016
0.001
0.005
0.001
0.020
0.025
4.7066-5
2 .941e-6
1.471e-5
2 .941e-6
5.882e-5
7.353e-5
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Table 3-3
Gasoline Vehicle Sulfate Emission Factors
Model Years 2004 and Later
Technology
Type
Non Catalyst
Ox Cat/No Air
3W Cat/No Air
Ox Cat / Air
3W Cat / Air
Non Catalyst
Ox Cat/No Air
3W Cat/No Air
Ox Cat / Air
3W Cat / Air
Speed BIN
< 19.6 MPH
< 19.6 MPH
< 19.6 MPH
< 19.6 MPH
< 19.6 MPH
> 34.8 MPH
> 34.8 MPH
> 34.8 MPH
> 34.8 MPH
> 34.8 MPH
Sulfate
Emission (g/mi)
@30 ppm Sulfur
0.0002
0.0004
0.0004
0.0014
0.0014
0.0001
0.0004
0.0001
0.0018
0.0022
Sulfate
Emission
(g/mi*ppm S)
SLOPE
5.882e-6
1.471e-5
1.471e-5
4.7066-5
4.7066-5
2 .941e-6
1.4716-5
2 .941e-6
5.882e-5
7.3536-5
Gasoline Sulfate Emission Technology Weighting Factors
The gasoline sulfate emission factors shown in Tables 3.2 and
3.3 by technology type are combined into a composite all technology
factor based on the technology weighting factors already present in
the MOBILE6.0 model. Equation 3.5 is the general equation used to
calculate these.
Sulfate
SUM[EF(i) * Frac(i)]
Eqn 3.5
Where EF(i) are the sulfate emission factors in Table 3-2 dn 3-3,
Frac(i) are the technology fractions, and indexing by 'i'
represents summation over the technology categories and MOBILE6
vehicle speed bins. The technology fractions are functions of
vehicle type and model year that are calculated in MOBILE6.2 based
on vehicle technology distributions already present in MOBILE6.0.
The technology fraction topic is discussed in detail in EPA report
M6.FLT.008A.
3.2.6.2 Calculation of Gasoline Vehicle S02 Emissions
The model assumes that all of the sulfur in the fuel is
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exhausted either as sulfate emissions or gaseous sulfur dioxide
emissions (S02). Thus, once the sulfate emissions are calculated,
the remaining sulfur in the fuel is considered to be exhaust S02.
The first step in this calculation is to determine the
fraction of the gasoline fuel sulfur that is converted to sulfate
emissions (DCNVRT) . This is done by using the gasoline fuel sulfur
relationship from PART5 shown in Equation 3.6. A value of DCNVRT
is calculated for each of the technology and speed groups.
DCNVRT = Sulfate * FE / [UNITS * (l.+WATER) * FDNSTY *SWGHT ] Eqn 3.6
Where:
DCNVRT - percent of sulfur in the fuel that is directly converted
to sulfate.
Sulfate - is the direct sulfate emission factor of a vehicle in
g/mi calculated from Table 3.4a or Table 3.4b.
WATER - is the constant 1.2857 (see PART5 User Guide).
FDNSTY - is the fuel density. It is a constant value of 6.09
Ib/gal.
FE - is the fuel economy in miles per gallon. (These values come
from MOBILE6) . They are a function of model year and vehicle
class.
SWGHT - is the weight percent of sulfur in the fuel. (i.e., 0.034
= 340 ppm gasoline fuel sulfur).
UNITS - is the constant 13.6078. This is the units conversion
factor. Calculated by (453.592 * 3)/100. Where 453.592 is the
number of grams in a pound, 3 is the weight ratio of S04 to sulfur,
and the 100 is to correct for the weight percent of sulfur.
The gaseous S02 emissions are calculated as in PARTS by
plugging the values of DCNVRT into the S02 emission equation (Eqn
3.7), and solving for S02 for each technology and speed group.
S02 = UNITS#2 * FDNSTY * SWGHT * (I. - DCNVRT) / FE Eqn 3.7
Where:
UNITS#2 = 9.072. This is the units #2 conversion factor.
Calculated by (453.592 * 2)/100. Where 453.592 is the number of
grams in a pound, 2 is the weight ratio of S02 to sulfur, and the
100 is to correct for the weight percent of sulfur.
The final composite S02 emission factor is calculated by
weighing together the individual technology and speed S02 emission
factors. The same weighting factors are used for both Sulfate and
S02 emissions.
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Mathematically, it is shown in Equation 3.8.
Composite S02 = SUM[S02(i) * Frac(i)] Eqn 3.8
Where S02(i) are the emission factors calculated in the gaseous S02
Equation X, Frac(i) are the technology fractions, and indexing by
i represents that the summation is over the technology types and
MOBILE6 speed bins.
3.2.6.3 Gasoline Vehicle Sulfate and S02 Emission Sample
Calculation
This section provides a sample calculation for the gasoline
fueled vehicle sulfate and S02 emission factors for two technology
and speed groups (no weighting factors will be used) . It is
provided to give the reader a feel for the relative size of the
Sulfate and gaseous S02 emission factors.
Sulfate emission conversion:
Speed Bin > 34.8 MPH
DCNVRT = Sulfate * FE / [UNITS * (l.+WATER) * FDNSTY *SWGHT ]
DCNVRT = (0.001 g/mi * 25 mile/gal) /
[ 13.6078 * (1+1.2857) * 6.09 Ib/gal * 0.034%
DCNVRT = 0.00384 or 0.39% for the 3-way catalyst no air pump
group.
DCNVRT = 0.0970 or 9.70% for the 3-way catalyst with air
pump group.
Gaseous S02 Emissions:
S02 = UNITS#2 * FDNSTY * SWGHT * (1. - DCNVRT) / FE
S02 = 9.072 * 6.09 Ib/gal * 0.034% * (1-0.00384) / 25
S02 = 0.0748 g/mi for the 3-way catalyst no air pump group at
340 ppm gasoline fuel sulfur.
3.2.6.4 Calculation of Diesel Vehicle Sulfate Emissions
The diesel vehicle sulfate emissions are a function of the
basic user supplied diesel fuel sulfur level (a required input for
PM emission calculation in MOBILE6.2), and the diesel vehicle fuel
economy values. The fuel economy values currently in use for the
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for diesel vehicles were taken from the MOBILE6 emission model.
Future versions of the MOBILE6 model (Version MOBILE6.3) may
contain updated fuel economy estimates and allow user input of
alternative values.
Sulfate emissions are calculated for diesel fueled vehicles in
MOBILE6.2 by using Equation 3.9.
Sulfate = UNITS * (1.+WATER)*DFDNSTY*DWGHT*DCNVRT / FE Eqn 3.9
Where:
Sulfate is the direct sulfate emission factor of a vehicle in g/mi.
WATER is the constant 1.2857.
DFDNSTY is the constant 7.11 Ib/gal.
FE is the fuel economy in miles per gallon. (These values are to
come from MOBILE6). They are a function of model year and vehicle
class.
DWGHT weight percent of sulfur in the fuel. (i.e., 0.050 = 500 ppm
diesel fuel sulfur).
DCNVRT percent of sulfur in the fuel that is directly converted to
sulfate. MOBILE6.2 retrains the 2% value of this parameter from
PARTS.
UNITS is the constant 13.6078.
Sulfate emissions for diesel vehicles are calculated using the
assumption from PARTS that 2 percent of the sulfur in the diesel is
converted into sulfate compounds, and the remaining sulfur is
converted to S02 compounds.
3.2.6.5 Calculation of Diesel Vehicle Gaseous S02 Emissions
The diesel vehicle gaseous S02 emissions are calculated using
equation 3.10. The methodology assumes that the 98 percent of the
fuel sulfur is converted to gaseous S02 emissions. Like the
calculation for the diesel vehicle sulfate emissions, the gaseous
S02 emissions are a function of user input fuel sulfur level and
the vehicle fuel economies.
S02 = UNITS#2 * FDNSTY * SWGHT * (1. - DCNVRT) / FE Eqn 3.10
3.2.6.6 Diesel Sulfate Emissions on Vehicles with Particulate
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Trap Technology
It is anticipated that future technology needed to meet strict
particulate matter standards for diesel vehicles in model years
2007 and later will include particulate traps. Such traps may take
a variety of designs; however, the basic principle is for the trap
to collect virtually all of the particulate matter present in the
exhaust stream, and to either burn it off at high temperature or to
otherwise remove it from the exhaust stream. Currently, no data
exist as to efficiency of this process nor do any sulfate emission
factor data exist to suggest the magnitude of such emissions.
Thus, the model will set in calendar years 2007 and later, a very
low base diesel fuel sulfur level of 10 ppm as required in the
Heavy-Duty 2007 Rule, and continue to predict that, fleet-wide, 2
percent of this fuel stream is emitted as sulfate emissions.
3.2.7 Calculation of Total Exhaust PM Emissions
3.2.7.1 Diesel Vehicles
Total Exh PM Calculation
The general equation for total exhaust particulate emissions
is shown in Eqn 3.10b. It includes OCARBON, ECARBON and Sulfate
emissions. Calculation details on these sub-components have been
previously discussed in Sections 3.2.6.1 through 3.2.6.6.
Total Exh PM = OCARBON + ECARBON + Sulfate Eqn 3.1Ob
The default total exhaust particulate emission rates are
represented as a linear function with respect to mileage. For the
light-duty diesel vehicles the rates were taken from the PART5
model (See EPA report EPA-AA-AQAB-94-2). The default total exhaust
particulate parameters for heavy-duty diesel vehicles are also a
linear function, and are shown in the MOBILE6.0 technical support
materials - see EPA reports M6.HDE.001, M6.HDE.002, and M6.HDE.004.
They can also be found in the support materials section of the EPA
2007 heavy-duty rule making docket.
The total exhaust PM emission rates in MOBILE6.2 are a
function of vehicle class (all diesel classes can have a separate
emission factor), model year (1950 - 2020+), and mileage. The
mileage relationship is linear with a zero mile emission level, two
possible slopes and a user supplied inflection point between the
two slopes (Equation 3.11).
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Exh PM = ZML + DETl*mileagel + DET2(mileage2-mileagel) Eqn 3.11a
The default values of these parameters are provided in the
Excel Spreadsheets PMDIES_ZML.xls, PMDIES_DETl.xls and
PMDIES_DET2.xls. Examination of the heavy-duty emission rates in
these spreadsheets shows that in virtually all cases the zero mile
emission level is assumed to be the certification standard, and the
deterioration rates with respect to mileage are zero.
Total Exh PM Size Correction Factors
The total exhaust PM emission factors are computed on the
basis of the entire amount of PM material that is collected on an
EPA test filter during the emission tests. This is referred to as
PM30. Exh PM calculated in Eqn 3.11a is in terms of PM30.
For use in the MOBILE6.2 model, the particulate emissions must
be converted from PM30 terms into particulate size terms that can
range from PM1 to PM10. The general equation for any size in the
range of 1 micron to 10 microns (x) for this transformation is
given in Eqn 3.lib.
Exh PM(x) = Exh PM(30) * SIZE CF Eqn 3.lib
This value is not allowed to exceed the certification standard
applicable to future years if future rulemakings are being modeled.
The values for the SIZE CF used in Eqn 3. lib are shown in
Table 3.4. Correction factors are provided for the range of
pollutant type, vehicle/fuel classes and particle sizes. Linear
interpolation should be used to calculate correction factors for
particle sizes between those listed in the Table.
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Table 3.4
Fraction of Particles Less than or Equal to the Particle Size Cutoff
Vehicle Type/ Particle Size Fraction of Particles less than or
Particulate Component Cutoff (PSO Equal to the Particle Size Cutoff
Gasoline vehicles using 10.0 0.64
leaded fuel/ Lead, Carbon 2.0 0.43
0.2 0.23
Gasoline vehicles with catalyst, 10.0 0.97
using unleaded fuel/Lead, Carbon 2.0 0.89
0.2 0.87
Gasoline vehicles without a 10.0 0.90
catalyst, using unleaded fuel/ 2.0 0.66
Lead, Carbon 0.2 0.42
Diesel vehicles/Exhaust PM 10.0 1.00
2.5 0.92
2.0 0.90
1.0 0.86
All vehicles/Brake-wear 10.0 0.98
7.0 0.90
4.7 0.82
1.1 0.16
0.43 0.09
All vehicles/Tire-wear 10.0 1.00
0.10 0.01
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Calculation of OCARBON and ECARBON
The total exhaust particulate emission factor corrected for
particulate size (Exh PM) calculated in Equation 3.lib is
substituted into Equations 3.12 and 3.13 (rewrites of Equation 3.1
and 3.2 where lead is zero for diesel vehicles) to calculate
OCARBON and ECARBON emission factors. The appropriate sulfate
emission factors corrected for particulate size are also
substituted into the two equations to account for these
constituents. The sulfate emission factor is the "base" diesel
fuel sulfur level 'Sulfate [b] ' . For pre-2007 model years this base
level is 500 ppm sulfur. For 2007+ it is 8 ppm sulfur. It is
subtracted from the OCARBON and ECARBON emission factors
CJDCARBON = [C_EXH_PM - Sulfate[b]] * OCFRAC Eqn 3.12
C_ECARBON = [C_EXH_PM - Sulfate[b] - OCARBON] Eqn 3.13
3.2.7.2 Gasoline Vehicles
The GASPM emission factors are supplied as a function of
vehicle class, catalyst technology, model year (1950 - 2020+), and
mileage. The mileage relationship is linear with a zero mile
emission level, two possible slopes and a user supplied inflection
point between the two slopes (Equation 3.14). The default values
of these parameters are provided in Excel Spreadsheets
PMGAS_ZML.xls, PMGAS_DET1.xls and PMGAS_DET2.xls.
GASPM = ZML + DETl*mileagel + DET2(mileage2-mileagel) Eqn 3.14
The default particulate parameters (zero mile and
deterioration rates) are taken from PARTS values for gasoline
vehicles (See EPA report EPA-AA-AQAB-94-2) . As a result, in all
cases the deterioration rates DET1 and DET2 are assumed to be zero.
The sulfate emission factors for gasoline vehicles are
calculated according to the equations discussed in Section 3.2.6.1.
The calculated sulfate emission factor is based on the user
specified fuel sulfur level [i] rather than on the "base" level at
which original emission factor testing was done. The lead emission
factors are calculated according to the algorithm referenced in
Section 3.2.3. They are a function of technology, model year,
existence of tampering and calendar year. They cannot be changed
by the user.
The Exhaust PM emission factor for gasoline vehicles is the
sum of the GASPM, sulfate and lead emission factors, and is shown
mathematically in Equation 3.15.
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Exhaust PM (gas vehicles) = GASPM + sulfate[i] + lead Eqn 3.15
Like the diesel vehicles, the Exhaust PM emission factor for
gasoline vehicles is compared against the certification standard
level and capped at this level if it exceeds it. This will
typically not happen except in the case of the 2004+ Tier2 emission
vehicles which have stringent PM standards.
The Total Exhaust PM emissions for gasoline vehicles are also
adjusted for particle size using a particle size distribution
function. These particle size correction factors are taken
directly from PART5 and are tabulated in the PART5 User Guide.
Mathematically, the calculation is shown in Equation 3.16.
Total Exhaust PM = Exhaust PM * Particle Size Corr Eqn 3.16
3.3 Ammonia Emission Calculations
3.3.1 Ammonia Emission Factors
The MOBILE6.2 model calculates a composite, FTP test based
(composite running and start emissions) gaseous ammonia emission
factor for all vehicle types and model years. The base ammonia
emission factors built into the MOBILE6.2 model were taken from the
EPA report EPA/AA/CTAB/PA/81-20 "Determination of a Range of
Concern for Mobile Source Emissions of Ammonia" by Robert Garbe,
August, 1981. They can also be found in SAE paper 830987. They
were selected for use in MOBILE6.2 because of their established use
in EPA's National Trends modeling for many years, and a lack of new
ammonia emission test results. Because the emission factors are
about 20 years old, a literature search was conducted to verify
that they are still representative of current vehicles. A
description of this literature search is contained in Appendix A.
The ammonia emission factor values used in the MOBILE6.2 model
are shown in Table 3.5. All units are milligrams per mile.
Table 3.5
Ammonia Emission Factors by Vehicle Class and Catalyst Type
Intercept Values in Regression
(all UNITS are Milligrams per Mile)
MOBILE 6
Vehicle Types
All
Non
Catalyst
Ox
Catalyst
3 -Way
Catalyst
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1-5 (LDG)
24 (MC)
6 - 13, 25
(HDG)
14, 15, 28
(LDD)
16 - 23, 26, 27
(HDD)
11.265
mg/mi
45.062
mg/mi
6.759
mg/mi
27.037
mg/mi
11.265
mg/mi
15.128
mg/mi
101.711
mg/mi
LDG are the light-duty gasoline vehicles
MC is the motorcycle class
HDG are the heavy-duty gas vehicles
LDD are the light-duty diesel vehicles
HDD are the heavy-duty diesel vehicles.
Based on the literature search, EPA concluded that these
numbers are in the same general range as the limited FTP test
results, and thus are appropriate for use in MOBILE6.2. However,
there is substantial variation in ammonia measurements and ammonia
is likely a function of sulfur level, test cycle (FTP versus US06),
advancing catalyst technology, and other factors. Additional
research is recommended on this topic.
The gaseous ammonia emission factors are reported by the
MOBILE6.2 model in the particulate section because gaseous ammonia
reacts with sulfates and/or nitrates to form ammonium sulfate and
ammonium nitrate in the atmosphere. These ammonium compounds are
classified as particulate emissions. The MOBILE6.2 model
calculates and reports only the gaseous emissions emitted directly
from a vehicle tailpipe. It makes no attempt to model the
atmospheric chemistry of ammonia conversion to other ammonium based
compounds or estimate the direct emissions from ammonium compounds.
These types of calculation are left to atmospheric chemistry
models.
3.4 Indirect Sulfate Emission Calculations
In addition to the direct sulfate emission factors discussed
above, the previous model (PARTS) estimated an indirect sulfate
emission factor by assuming that a fraction of the gaseous sulfur
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dioxide emissions are later converted in the atmosphere to sulfate
material. Based on ambient sulfur and sulfate measurements in 11
cities, EPA estimated that 12 percent of all gaseous sulfur is
converted to sulfate.
During the update process for MOBILE6.2 it was decided to drop
this calculation from the model and not report an estimate for
indirect sulfate emission production. The reasoning for this
decision is that the MOBILE6.X series of models are vehicle
emission models not atmospheric models. They are best used for
estimating emission factors for pollutants directly emitted from
vehicles through pathways such as exhaust, evaporation, brake and
tire, and engine draft (PCV), rather than atmospheric chemical
reactions.
3.5 Fugitive Dust Emission Calculations
MOBILE6.2 does not include estimates of fugitive road dust
emissions. These will be covered by a simple calculation tool
being developed separately by EPA's Office of Air Quality Planning
and Standards (OAQPS) . They were removed from the MOBILE6.2
because the new tool is available and because MOBILE6 cannot
properly account for the facility / roadway type - unpaved roads.
Since dust emissions on an unpaved road are usually considerably
higher than on a paved road, the issue of paved versus unpaved
roads is critical in any modeling or discussion of fugitive dust
emissions.
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4. Results from the MOBILES.2 Model
Some limited and preliminary results from the MOBILE6.2 model
are shown at the end of this document, and are discussed in this
section.
4.1 Emissions Versus Calendar Year
The results are shown in a series of Figures (Figures 1
through 15) . They are shown in terms of total particulate
emissions (TOTEX), total carbon emissions from gasoline vehicles
(GASPM), sulfate emissions and lead emissions. All of the results
in these figures are shown as a function of calendar year. The
results are also shown for individual vehicles types: light-duty
gasoline vehicles, light-duty truck class 4 vehicles, heavy-duty
gasoline vehicles, light-heavy, medium-heavy, and heavy-heavy duty
diesel vehicles and transit diesel buses. The emission results in
all of the figures are the average emission levels for each
calendar year from 1970 through 2020. A calendar year includes the
weighted average emission result of the previous 25 model years.
With the exception of the Ammonia results, all the figures
were constructed as comparisons of the MOBILE6.2 and PARTS emission
results.
Figures 1 through 4 show the results from the light-duty
gasoline vehicles. Figure 1 shows the TOTEX (total exhaust
particulate emission) results from both MOBILE6.2 and PART5. As
can be seen from the figure, only relatively small differences are
observed. The differences occur mostly in the pre-1980 years and in
the post 1996 calendar years. In the early years they are caused
by differences in underlying methodology of modeling misfueling
and tampering effects on lead particulate emission in between
MOBILE6 and PART5. They are not due to changes in the basic lead
emission factors. The differences in the later years are due to
different fuel sulfur levels that create differences in sulfate
emission factors. The PART5 model does not allow alternate
gasoline fuel sulfur levels to be modeled and fixes this fuel
parameter at 343 ppm. However, the MOBILE6.2 model allows
alternative sulfur levels to be modeled. The fuel sulfur level was
set at 30 ppm for all 2000 and later calendar years. Figure 3
illustrates the impact of different fuel sulfur levels on the
sulfate emission factors. Once the different lead and sulfur
influences are removed, the carbon particulate emission (GASPM) is
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shown to be very similar between MOBILE6.2 and PART5 (see Figure
2) .
Figures 5 and 6 show the average TOTEX results for light-duty
gas truck class 4 and heavy-duty gas trucks for the MOBILE6.2 and
PART5 models. As for the light-duty vehicles (i.e. cars), the
results for MOBILE6.2 and PART5 for the trucks are relatively close
for most calendar years. The differences for the light-duty gas
trucks can explained in terms of truck size and different fleet and
technology distributions between the two models. For instance, the
figure shows the results for a light-duty gas truck class 4 in
MOBILE6.2, but an average result for light-duty gas truck class 3
and 4 for PART5 (PART5 did not separate class 3 and 4 trucks).
The heavy-duty gas truck result comparison shows differences
that are mostly technology related (different fleet phase-ins for
fuel injected, air injection and catalyst technology) for emission
in the 1990 through 2005 calendar years, and EPA Tier2 standards
related effects for the 2008 and later calendar years.
Figures 7 through 10 show the TOTEX comparisons for the diesel
vehicles. For heavy-heavy duty vehicles there are only slight
differences, but for most vehicle classes there are some
significant differences. The differences arise because the
MOBILE6.2 model follows the analysis done to support the EPA heavy-
duty diesel 2007 rule. In general, the MOBILE6.2 results are
higher than those predicted by the PART5 model (this is
particularly true for the transit buses - see Figure 10) . An
exception is the 2007 and later model years which reflect the
implementation of the new stringent 2007 diesel rule. The effects
of this rule were not accounted for in the PART5 model.
Figure 11 compares the MOBILE6.2 and PART5 model results for
sulfate emissions on heavy-heavy duty diesel trucks. The PART5
curves are the default emission results that cannot be modified by
the user. They typically are based on very high diesel fuel sulfur
levels of 2500 ppm, and then a lower level of 500 ppm sulfur for
all 1993 and later model years. The MOBILE6.2 results are based on
500 ppm sulfur for pre-2007 calendar years and 8 ppm diesel fuel
sulfur for 2007 and later calendar years. Note that the MOBILE6.2
and PARTS sulfate curves agree when the fuel sulfur levels are the
same at 500 ppm. If all the calendar-year MOBILE6.2 runs had been
done at the same fuel sulfur levels as the PARTS runs, the curves
would agree for all calendar years. Instead, alternate fuel sulfur
levels were modeled, and shown in Figure 11, to compare the effects
of fuel sulfur between the two models, and to show the effect of
different diesel fuel sulfur level on sulfate emissions.
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Figure 12 shows the Ammonia emission factors as a function of
calendar year and vehicle class. As can be observed, the diesel
emission factors are not a function of calendar year, but the
gasoline vehicle factors are. The gasoline vehicles show a rising
and then a flattening curve of ammonia as calendar year progresses.
This rising curve is due to the fact that modern fuel injection and
3-way catalyst technology has a greater tendency to produce ammonia
than the older non catalyst or oxygenated only catalyst equipped
vehicles. The flattening aspect of the curve reflects the almost
complete penetration of fuel injected and 3-way catalyst vehicles
into the fleet.
Figure 13 compares the MOBILE6.2 and PART5 exhaust carbon
particulate emissions for motorcycles. The figure shows close
agreement between MOBILE6.2 and PART5. In both figures the
emissions start out at fairly high levels in the 1970s and drop to
considerably lower levels in the 1990 and beyond due to technology
improvements.
Figure 14 compares the MOBILE6.2 and PART5 exhaust carbon
emissions for light-duty diesel vehicles. The figure shows fairly
good agreement between the models with similar overall trends. The
models diverge after 2007 because of the incorporation of the
effects of 2007 diesel rule on the MOBILE6.2 emission factors and
the lack of such effect in PART5. The PART5 graph shows an unusual
'dip and increase' in emission factors in the 1980 to 1989 calendar
years. This effect is not due to rising general emission factors in
the model, but changing registration distributions between
individual model years. For example, in the calendar years where
the emission rate is increasing the overall LDDV fleet is getting
older (new model years are replacing older vehicles at a slower
rate).
Figure 15 compares the MOBILE6.2 and PART5 exhaust carbon
particulate emissions for light-duty diesel trucks. This figure is
analogous to Figure 14 for the LDDVs. It also shows fairly good
agreement between the models with similar overall trends. The
models diverge after 2007 because of the effects of 2007 diesel
rule on the MOBILE6.2 emission factors and the lack of such effect
in PART5.
4.2 Emissions Versus Model Year
Figure 16 (the only figure based on model year instead of
calendar year) presents Total Exhaust particulate emissions versus
model year for the 8B heavy-duty diesel vehicles in calendar year
2010. These results show the basic emission factor for 8B diesel
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vehicles for each individual model year prior to the application of
weighting factors and correction factors. In comparison, the
results shown in Section 4.1 are by calendar year where each
calendar year is a weighted average of the emission factors from
the previous 25 model years.
The results in Figure 16 show that the 8B and other heavy-duty
diesel vehicle basic emission factors are NOT precisely the same as
those from PART5. The differences in Figure 16 occur because
different emission factors were used to model heavy-duty diesel
vehicles in the EPA 2007 Heavy-Duty Rulemaking effort than in
PART5. The differences are most notable in model years 1984
through 1989 where the new MOBILE6.2 emission factors now include
the effects of deterioration of particulate emissions versus
vehicle odometer. Also, the MOBILE6.2 particulate emission factors
for the 2007 and later model years are lower than the corresponding
PART5 emission factors due to the effects of the 2007 rulemaking.
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Appendix A
Literature Search on Vehicle Ammonia Emissions
The ammonia emission factors used in the MOBILE6.2 model are
based on a 1981 EPA study which tested only limited numbers of 3-
way catalyst vehicles. Thus, as part of the MOBILE6.2 update, EPA
did a literature search to determine if other ammonia emission
estimates were available, and to determine if the MOBILE6.2
estimates based on this study were appropriate.
Recent studies on vehicle ammonia emissions by various
researchers have suggested that gaseous exhaust ammonia emissions
may be dependent on catalyst type, vehicle operation and fuel
sulfur levels. The 1981 study does take different catalyst types
into account (although, the 1981 3-way catalyst may not reflect
modern technology). However, it did not address ammonia emissions
as a function of vehicle operation or fuel sulfur levels. As a
result, the ammonia emission factors in the MOBILE6.2 model may be
only partially representative of modern vehicles.
Various Studies
1989 Volkswagon Study - Several gasoline and diesel vehicles were
studied using the FTP test. The gasoline sulfur level was 330 ppm.
Non-catalyst gasoline vehicles reported results of 3.52 mg/mi,
diesel vehicles 1.88 mg/mi and 3-way catalyst vehicles 137.4 mg/mi.
Preliminary CE-CERT Work in Calendar Year 2000 - Seven vehicles
tested so far over three different fuel sulfur levels (324 ppm and
30 ppm and California reformulated fuel) . The vehicles were a 1991
Dodge, a 1997 Ford, a 2001 Buick, a 1999 Ford Tierl, a 2001 Suzuki
NLEV, a 1999 GM Sonoma TLEV, a 2000 Ford Winstar ULEV. All were 3-
way catalyst technology.
Table A-l
CE-CERT Vehicle Test Results of Ammonia Emissions
Vehicle
1991 Dodge
1997 Ford
2001 Buick*
1999 Ford*
FTP
30 ppm
Sulfur
118 mg/mi
38 mg/mi
70 mg/mi
324 ppm
Sulfur
86 mg/mi
5 mg/mi
US06
30 ppm
Sulfur
210 mg/mi
237 mg/mi
160 mg/mi
242 mg/mi
324 ppm
Sulfur
161 mg/mi
146 mg/mi
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2001 Suzuki*
1999 GM*
2000 Ford*
12 mg/mi
73 mg/mi
415 mg/mi
82 mg/mi
307 mg/mi
* Tested on California Reformulated Fuel rather than the fuels
with the specified sulfur levels of 30 and 324 ppm.
ORD National Risk Management Research Laboratory Work - One 1993
Chevrolet Lumina (3-way catalyst) was tested over various driving
conditions (FTP, steady state, hard acceleration, partial and major
enrichment, and some on road data) . The FTP ammonia emissions were
about 30 mg/mi. This is lower than other studies. However, the
hard acceleration results were 282 mg/mi, and the major enrichment
results were 2,450 mg/mi.
G. Cass Work - California Institute of Technology - These were
roadway tunnel studies in Los Angeles in 1998. The results were 98
mg/mi ammonia for the fleet as a whole. 116 mg/mi for LDGV.
A. Kean Work - Lawrence Berkeley Labs - A San Francisco Bay area
tunnel study in 1999. Results 79 mg/mi overall fleet result.
M. Baum Work - Oak Crest Institute of Science, CA. - They used
remote sensing measurements. Emissions were measured from vehicles
during acceleration in parking lots and freeway ramps. Results
showed very high emissions. The results are available only in ppm
(78.6 ppm average). 66% of the ammonia emissions are emitted by 10%
of the fleet. M-85 fueled vehicles had slightly higher ammonia
emissions.
Future Work - EPA Office of Research and Development studies, CE-
CERT under EPA cooperative agreement, and CRC testing project.
Future focus will be on determining fuel effects, and how ammonia
emissions change as NOx emissions are controlled.
Appendix B
References
EPA-AA-AQAB-94-2 "Draft User's Guide to PARTS: A Program for
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Calculating Particle Emissions from Motor Vehicles", February,
1995.
Garbe, Robert, EPA/AA/CTAB/PA/81-20 "Determination of a Range of
Concern for Mobile Source Emissions of Ammonia", August, 1981.
EPA 2007 Heavy-Duty Diesel Rulemaking Docket.
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0.30
Figure 1
Total Exhaust PM10 Emissions from MOBILE6.1 and
PARTS forLDGVs
•MOBILE6.2
-PARTS
0.00 -l-r
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
Calendar Year
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0.14
0.12
0.10
I 0.08
W 0.06
<
O
0.04
0.02
0.00
Figure 2
MOBILE6.1 GASPM Emissions Versus
PARTS Carbon Emissions for LDGVs
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
Calendar Year
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0.012
Figure 3
Comparison of MOBILE6.1 and PARTS SULFATE
Emissions for LDGVs
0.000
-0-MOBILE6.2
-m- PARTS
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
CALENDAR YEAR
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Figure 4
Comparison of MOBILE6.1 and PARTS LEAD Emissions
for LDGVs
0.16
0.00
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
CALENDAR YEAR
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0.25
0.00
Figure 5
GASPM Emissions from LDGT4 in MOBILE6.1 and
LDGT2 in PARTS
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
CALENDAR YEAR
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Figure 6
MOBILE6.1 and PARTS GASPM from Heavy-Duty
Gasoline Vehicles
0.25
•MOBILE6.2
PARTS
0.00
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
Calendar Year
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Figure 7
MOBILE6.1 and PARTS Total Exhaust PM10 Emissions
from 2B Heavy-Duty Diesel Vehicles
0.70
-0-MOBILE6.2
-m- PARTS
0.00
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
CALENDAR YEAR
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Figure 8
MOBILE6.1 and PARTS Total Exhaust PM10 Emissions
from Medium Heavy-Duty Diesel Vehicles
-0-MOBILE6.2
-•- PARTS
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
Calendar Year
-38-
-------
Figure 9
Comparison of MOBILE6.1 and PARTS TOTAL
EXHAUST PM10 for Heavy-Heavy Duty Diesels
-0-MOBILE6.2
-m- PARTS
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
CALENDAR YEAR
-39-
-------
Figure 10
MOBILE6.1 and PARTS Total Exhaust PM10 Emissions from
Urban Diesel Buses
3.50
3.00
0.00
•MOBILE6.2
- PARTS
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
Calendar Year
-40-
-------
0.30
0.25
0.20
"3)
0.15
w
0.10
0.05
0.00
Figure 11
MOBILE6.1 and PARTS SULFATE Emissions for
HHDDVs
000000°0oooooooooooo
•MOBILE6.2
PARTS
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
CALENDAR YEAR
-41-
-------
Figure 12
Ammonia Emissions from MOBILE6.1
0.12
ooooooooooooooooooooooooooooooo
0.00
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
Calendar Year
-42-
-------
Figure 13
MOBILE6.1 and PARTS GASPM from Motorcycles
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
Calendar Year
-43-
-------
Figure 14
MOBILE6.1 and PARTS Carbon Emissions from LDDV
0.80
•MOBILE6.2
• PARTS
0.00
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
Calendar Year
-44-
-------
Figure 15
MOBILES.1 and PARTS Carbon Emissions for LDDT3,4
and LDDT (Respectively)
0.80
•MOBILE6.2
•PARTS
0.00
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
Calendar Year
-45-
-------
2.25
0.00
Figure 16
PM10 Emission Factors for 8B Diesel Vehicles
In Calendar Year 2010
1980 1985 1990 1995 2000 2005 2010 2015
Model Year
-46-
------- |