&EPA
United States
Environmental Protection
Agency
Office of Transportation EPA420-R-04-013
and Air Quality August 2004
Technical Guidance on the
Use of MOBILE6.2 for
Emission Inventory Preparation
-------
EPA420-R-04-013
August 2004
Technical Guidance on the Use of MOBILE6.2 for
Emission Inventory Preparation
Transportation and Regional Programs 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 an exchange of
technical information and to inform the public of technical developments.
-------
CONTENTS
1.0 Introduction 1
2.0 External conditions 2
2.1 Calendar year of evaluation 3
2.2 Month of Evaluation 3
2.2.1 Guidance 3
2.2.2 Description 4
2.3 Altitude 5
2.4 Temperature 5
2.4.1 Guidance 5
2.4.2 Description 5
2.4.3 Methods 6
2.4.3.1 Methods for assigning values for minimum and maximum
temperature 6
2.4.3.2 Methods for hourly temperature input 7
2.4.4 MOBILE6.2 Commands 7
2.5 Humidity 8
2.5.1 Guidance 8
2.5.2 Description 8
2.5.3 Methods 9
2.5.3.1 Methods 9
2.5.3.1 Methods 10
2.5.4 MOBILE6.2 Command 11
2.6 Other air-conditioning related input options 11
2.6.1 Guidance 11
2.6.2 MOBILE6.2 Commands 11
3.0 Vehicle Fleet Characteristics 12
3.1 Age Distribution of Vehicle Registrations 13
3.1.1 Guidance 13
3.1.2 Description 13
3.1.3 Methods 13
3.1.4 MOBILE6.2 Command 15
3.2 Annual mileage accumulation rates 15
3.2.1 Guidance 15
3.2.2 Description 16
3.2.3 Methods 16
3.2.4 MOBILE6.2 Command 18
3.3 Diesel Fractions 18
3.3.1 Guidance 18
3.3.2 Description 18
3.3.3 Methods 18
3.3.4 MOBILE6.2 Command 19
-------
3.4 Natural Gas Vehicles 19
4.0 Vehicle Activity 20
4.1 Vehicle Miles Traveled Fraction by Vehicle Class 20
4.1.1 Guidance 20
4.1.2 Description 21
4.1.3 Methods 21
4.1.4 Disaggregation of Local Information 22
4.1.5 HPMS or TIUS Data 23
4.1.4 MOBILE6.2 Commands 25
4.2 Vehicle Miles Traveled Fraction by Highway Functional System 28
4.2.1 Guidance 28
4.2.2 Description 28
4.2.3 Method 31
4.2.4 MOBILE6.2 Commands 36
4.3 Vehicle Miles Traveled Fraction by Hour of the Day 36
4.3.1 Guidance 36
4.3.2 Description 36
4.3.3 Method 37
4.3.4 MOBILE6.2 Commands 39
4.4 Vehicle Miles Traveled Fraction by Average Speed 39
4.4.1 Guidance 39
4.4.2 Description 39
4.4.3 Method 40
4.4.4 Idling Emission Rates 44
4.4.5 MOBILE6.2 Commands 45
4.5 Vehicle Engine Starts Per Day 46
4.5.1 Guidance 46
4.5.2 Description 46
4.5.3 Method 47
4.5.4 MOBILE6.2 Commands 47
4.6 Vehicle Engine Starts by Hour of the Day 48
4.6.1 Guidance 48
4.6.2 Description 48
4.6.3 Method 48
4.6.4 MOBILE6.2 Commands 49
4.7 Vehicle Soak Time between Engine Starts 49
4.7.1 Guidance 49
4.7.2 Description 50
4.7.3 Method 50
4.7.4 MOBILE6.2 Commands 50
4.8 Vehicle Hot Soak Time After Engine Shut Down 51
4.8.1 Guidance 51
4.8.2 Description 51
4.8.3 Method 52
4.8.4 MOBILE6.2 Commands 52
-------
4.9 Vehicle Diurnal Soak Time 53
4.9.1 Guidance 53
4.9.2 Description 53
4.9.3 Method 53
4.9.4 MOBILE6.2 Commands 54
4.10 Vehicle Trip Length (Duration) Distributions 55
4.10.1 Guidance 55
4.10.2 Description 55
4.10.3 Method 56
4.10.4 MOBILE6.2 Commands 56
4.11 Weekday and Weekend Day Activity 56
4.11.1 Guidance 56
4.11.2 Description 57
4.11.3 Method 57
4.11.4 MOBILE6.2 Commands 57
5.0 Vehicle Gasoline Specifications 57
5.1 Gasoline Volatility 58
5.1.1 Guidance 58
5.1.2 Description 58
5.1.3 Method 58
5.1.4 MOBILE6.2 Commands 58
5.2 Reformulated Gasoline 59
5.2.1 Guidance 59
5.2.2 Description 59
5.2.3 Method 59
5.2.4 MOBILE6.2 Commands 60
5.3 Gasoline Oxygen Content 60
5.3.1 Guidance 60
5.3.2 Description 60
5.3.3 Method 61
5.3.4 MOBILE6.2 Commands 62
5.4 Gasoline Sulfur Content 62
5.4.1 Guidance 62
5.4.2 Description 63
5.4.3 Method 63
5.4.4 MOBILE6.2 Commands 63
5.5 Diesel Sulfur Content 64
5.5.1 Guidance 64
5.5.2 Description 64
5.5.3 Method 64
5.5.4 MOBILE6.2 Commands 65
6.0 Inspection and Maintenance Programs and Anti-Tampering Programs 65
6.1 Inspection Type, Test Frequency, and Network Type 65
6.2 Model Years 68
-------
6.3 Vehicles 68
6.4 Stringency 69
6.5 Compliance Rate 69
6.6 Waiver Rates 70
6.7 Outpoints 70
6.8 Exemption Age 70
6.9 Grace Period 71
6.10 Technician Training Credits 71
6.11 I/M Effectiveness 71
6.12 Anti-Tampering Programs 71
7.0 Alternative emission regulations and control measures 72
7.1 Disablement of 1990 Clean Air Act Requirements 72
7.2 Heavy-Duty Diesel Vehicle NOx Off-Cycle Emissions Effects
72
7.3 2007 Heavy Duty Standards 73
7.4 Alternative Emission Factors for Passenger Cars and Light-Duty Trucks 73
7.4.1 Guidance for Modeling Alternative NLEV Implementation and Adoption
of California Standards 73
Appendix: References 76
-------
1.0 Introduction
MOBILE6 is a software program designed by the U.S. Environmental Protection Agency to
estimate emission rates for the highway motor vehicle fleet under a range of conditions.
MOBILE6.0 was released in January, 2002. This was the latest in a series of MOBILE models
dating back to 1978 and was the first update in MOBILE since the release on MOBILESb in
1996 and the first major update since the release of MOBILES in 1992.
MOBILE6.2 is the first and latest update to MOBILE6 and adds the capability to estimate
particulate matter (PM) and mobile source air toxics emissions. MOBILE6.2 contains very minor
modifications to MOBILE6 for the parts of the model that estimate emissions of hydrocarbons
(HC), nitrogen oxides (NOx), and carbon monoxide (CO).
The original technical guidance for MOBILE6 was released in 2002. Changes to the model have
necessitated an update to the technical guide. Most of this version of the technical guidance is
unchanged from the 2002 version. The following sections have been changed or added:
Section 2.1 - Calendar Year of Evaluation - Modified to refer to changes in Section 2.2
Section 2.2 - Month of Evaluation - Modified to add method used by EPA in the
National Emission Inventory (NEI) annual inventory calculations
Section 2.4 - Temperature - Modified to be consistent with temperature guidance for
other sources. Users can enter average temperatures for months during the ozone or CO
seasons, rather than temperatures for the 10 highest ozone or CO days.
Section 2.5 - Humidity - Added guidance for new RELATIVE HUMIDITY command in
MOBILE6.2. Modified humidity guidance to be consistent with new temperature
guidance.
Section 4.2 - Vehicle Miles Traveled Fraction by Highway Functional System - In
Table 4.2.1, changed MOBILE6.2 driving cycle for rural local roadways from "local
roadway" to "arterial/collector".
Section 5.5 - Diesel Sulfur Content - Added guidance for new DIESEL SULFUR
command in MOBILE6.2 required for particulate modeling.
The technical guidance in this document can be applied to either MOBILE6 or MOBILE6.2,
with the exception of guidance that applies only to the particulate emissions functions in
MOBILE6.2. This Technical Guidance document does not address use of MOBILE6.2 for air
toxics emissions estimation. Guidance for mobile air toxics is in a separate document,
"Technical Description of the Toxics Module for MOBILE6.2 and Guidance on Its Use for
Inventory Preparation", which can be found at
http://www.epa.gov/otaq/models/mobile6/r02029.pdf.
One of the primary uses of the MOBILE model is to develop emission inventories for State
Implementation Plans (SIPs) and for conformity determinations. These emission inventories are
estimates of total emissions from the highway motor vehicle fleet on a regional level. MOBILE
is also used increasingly for other kinds of analysis ranging from estimating the national impacts
1
-------
of motor vehicle emissions control strategies to estimating human exposure to pollutants at a
specific intersection.
As described in the MOBILE6.2 User's Guide and in MOBILE6.2 technical documents, the
latest versions in the MOBILE series (MOBILE6 and MOBILE6.2) are based on substantial new
test data and accounts for changes in vehicle technology and regulations as well as improved
understanding of in-use emission levels and the factors that influence them. They also
incorporate many new features compared to versions prior to MOBILE6, including many new
input and output options. Most of these features were designed to make MOBILE6 and
MOBILE6.2 more flexible and more capable of being used effectively over the range of uses
mentioned above.
New input options in the MOBILE6 series (MOBILE6 and MOBILE6.2) and changes in the way
the MOBILE6 series handles existing information (for example; changes in the way MOBILE6
handles vehicle classification) may appear to create significant new information burdens for
states preparing submissions for SIP and conformity related purposes. However, many of the
new input options in the MOBILE6 series were designed for very specialized types of analysis
and are not necessary for more general uses in SIP and conformity submissions. The remainder
of this document provides further guidance on the necessity of using particular input options.
This document also provides guidance on how existing local information can be combined with
national default information in cases where local information is not available for all aspects of a
particular command in the MOBILE6 series.
As a general rule, states that have not yet worked with the MOBILE6 series should start with
updated versions of existing local information developed for MOBILES or PARTS. If
MOBILE6 or MOBILE6.2 requires new local information and that new local information is not
readily available, states can use national default information using the methods described in this
document. For some of the input options, states should plan over the long term to develop new
local information for SIP and conformity submissions. EPA will work with the states and other
agencies to develop methods and tools to accomplish this goal.
This document covers most of the input options available in MOBILE6.2. If the guidance here
does not address a particular topic, the most recent previous guidance on the subject will still
apply. MOBILE6.2 users should always check with EPA if there is any question about the
applicability of guidance to any specific situation. The appendix to this document lists previous
sources of guidance on MOBILE modeling for SIP and conformity related purposes.
2.0 External conditions
There are several conditions external to the highway vehicle fleet and the transportation network
which have an impact on emissions. The year and month of evaluation affect the expected fuel
characteristics and composition of the fleet with respect to the phase-in and fleet turnover of new
emission standards. The emissions of vehicles are affected by the local altitude, as well as by
weather conditions such as temperature and humidity. Temperature, humidity, and solar load
-------
conditions such as cloud cover and time of peak sun also affect the amount of air conditioning
usage, which in turn affects emissions. In MOBILE6.2, only the year and month of evaluation
affect direct particulate matter (PM) emissions, through the impact of fuel characteristics and
fleet composition. Altitude, temperature, humidity, and air conditioning usage have no impact
on direct PM emissions in MOBILE6.2.
2.1 Calendar year of evaluation
The value used for calendar year of evaluation in MOBILE6.2 reflects the year for which
emission factors are calculated. MOBILE6.2 can model emission factors for the calendar year
1952 to 2050 inclusive. There is no significant change in this input from MOBILES to
MOBILE6.2 and no change in existing guidance on the use of this input option. Users should
select the calendar year that corresponds to the date needed for the current evaluation. Selection
of the appropriate calendar year may be affected if the user intends to model the months of
October, November and/or December. See section 2.2 of the MOBILE6.2 User's Guide for
more discussion of the interaction between the chosen evaluation month and calendar year. The
calendar year of evaluation is specified in MOBILE6.2 using the CALENDAR YEAR command,
which is required in every MOBILE6.2 input file. See section 2.8.6.1 of the MOBILE6.2 User's
Guide for more information on the use of this command.
2.2 Month of Evaluation
2.2.1 Guidance
MOBILE6.2 allows users to calculate emission factors for January 1 or July 1 of the year of
evaluation. Users should choose the evaluation month most appropriate to the pollutant of
concern. For example, users estimating summer day emissions of VOC and NOx should use
July as the evaluation month. For winter CO emissions, January is more appropriate. Other
months can be estimated by mathematical interpolation between January and July results or
using the method described later in this section, but this type of analysis is not typically needed
for SIP submissions and conformity determinations for ozone or CO.
Users estimating annual particulate matter emissions could develop emissions factors for January
and July and interpolate emission factors for the other ten months of the year, or they could use
the method described later in this section. Emissions for all months should then be summed to
estimate annual emissions. Users should review final regulations and guidance for particulate
matter standards before developing particulate matter inventories.
Users should review the MOBILE6.2 User's Guide for additional information on estimating
emissions in other months.
-------
2.2.2 Description
The month of evaluation in MOBILE6.2 is specified using the EVALUATION MONTH
command, described in Section 2.8.6.2 of the MOBILE6.2 User's Guide. As described in the
MOBILE6.2 User's Guide, this command only affects the composition of the fleet (July 1 has six
additional months of replacement of older vehicles by the current model year) and the effects of
reformulated gasoline (RFG). By default, July 1 uses summer season RFG rules and January 1
uses winter season RFG rules. Other factors which might change between January and July,
such as temperature and fuel volatility, are not affected by this command and must be specified
separately by users.
If interpolation is necessary, use the SEASON command, as described in the MOBILE6.2 User's
Guide to ensure that both points in the interpolation assume the same RFG parameters.
Another method to determine emission rates for months other than January and July has been
used for the National Emission Inventory (NET) annual inventory calculations by EPA. In this
method, a separate MOBILE6.2 run is done representing each of the 12 months of the year.
Under this approach, no interpolation is required. The EVALUATION MONTH and
CALENDAR YEAR used for each month is shown in Table 2.2.2.1.
Table 2.2.2.1
Selecting Evaluation Month and Calendar Year for
Annual Inventory Calculations
Month
January
February
March
April
May
June
July
August
September
October
November
December
1
2
3
4
5
6
7
8
9
10
11
12
EVALUATION MONTH
1
1
1
7
7
7
7
7
7
1
1
1
CALENDAR YEAR
Current Year
Current Year
Current Year
Current Year
Current Year
Current Year
Current Year
Current Year
Current Year
Current Year + 1
Current Year + 1
Current Year + 1
Note that the calendar year used for the months of October, November and December is not the
current calendar year, but the following calendar year. This makes the emission rates calculated
for these months more closely resemble the emission rates for January of the following calendar
year, rather than January or July of the current calendar year. Care must be taken using this
approach to make the other input parameters match as closely as possible the conditions being
modeled. In particular, it may be necessary to use the SEASON, as described in the MOBILE6.2
User's Guide, to ensure that the appropriate RFG parameters (summer or winter) are used.
Using this method will most closely match the results obtained by EPA for the NEI.
-------
Both methods described for modeling months other than January or July are approximations.
Other rational methods to approximate emission rates for months other than January or July may
be used, but must be properly documented and explained.
2.3 Altitude
In MOBILE6.2, users can estimate emissions for either a low-altitude area or a high-altitude area
using the ALTITUDE command, which is described in Section 2.8.6.5 of the MOBILE6.2 User's
Guide. Guidance for the use of this command is unchanged from MOBILES. For the majority
of applications, low-altitude is the appropriate choice. For those areas designated as high-
altitude by EPA for mobile source regulatory purposes only (generally those counties that lie
"substantially" above +4000 ft MSL), high-altitude should be selected. A list of those counties
EPA has designated as high-altitude appears in §86.091-30, paragraphs (a)(5) (ii) and (iv), Code
of Federal Regulations (July 1, 2001).
2.4 Temperature
2.4.1 Guidance
Ambient temperature is key to estimating emission rates for highway vehicles and States must
use local temperatures that are consistent with conditions during an ozone or CO exceedance
episode. The use of minimum and maximum daily temperatures is the recommended approach
for analyses of average summer or winter day conditions that will not be used as input into an air
quality or dispersion model. The use of hourly temperatures is necessary for inventories that will
be used for air quality analysis of specific exceedance episodes. Temperature assumptions used
for transportation conformity analyses should be consistent with those used in the SIP as
required in the transportation conformity rule, §93.122 (a)(6), Code of Federal Regulations (July
1,2001).
In MOBILE6.2, temperature has no effect on direct PM emissions. However, temperature does
affect emissions of some PM precursors. Users should review final regulations and guidance for
PM standards and consult with their EPA Regional Office to determine what PM precursors may
need to be included in local emission inventories.
2.4.2 Description
When a user specifies minimum and maximum daily temperatures, MOBILE6.2 takes those
temperatures and creates a daily temperature profile in which the low temperature is assumed to
occur at 6 a.m. and the high temperature at 3 p.m. The hourly temperature option allows the user
to enter hourly temperatures and thereby specify a different temperature profile than
MOBILE6.2 would create.
The hourly temperature option was designed to give users who want to do detailed air quality
analyses of specific exceedance episodes finer control over the hourly temperatures used by
-------
MOBILE6.2. For SIP and conformity purposes that do not involve air quality modeling, the
hourly temperature option is not required.
2.4.3 Methods
Sources of information on minimum and maximum temperatures and methods for determining
appropriate temperatures for SIP and conformity modeling have not changed substantially from
previous guidance. States that wish to use hourly temperatures must document that the hourly
temperatures used are consistent with existing temperature guidance. Sources of temperature
data and any methods used to adjust them to fit the requirements of MOBILE6.2 must be
documented in any official SIP or conformity-related submission.
2.4.3.1 Methods for assigning values for minimum and maximum temperature
Users who wish to model a specific ozone or CO exceedance episode can use the actual
minimum and maximum temperatures that occurred during that episode. For more generic
modeling of average summer or winter day ozone or CO conditions for SIP or conformity
purposes, average minimum and maximum temperatures during the months when ozone or CO
exceedances typically occur should be used. For ozone season analysis, users can enter the local
average minimum and maximum temperatures for July, or for the three month period that best
represents the area's ozone season (typically June, July and August; or July, August, and
September). For CO season analysis, users can enter the local average minimum and maximum
temperatures for January, or for the three month period that best represents the CO season
(typically December, January, and February, with December being that of the same or previous
year).
For any analysis of PM precursors as part of an annual PM standard, users should model each
month using the methods described in Section 2.2 and using average minimum and maximum
temperatures for each month. Users should review final regulations and guidance for particulate
matter standards before developing particulate matter inventories.
Official minimum and maximum temperature information is contained in the Local
Climatological Data Monthly Summary for the inventory area and is available from:
National Climatic Data Center
Federal Building
Asheville, NC 28801-2696
Telephone: (828)271-4300
It is also available online at the following website:
www.ncdc.noaa.gov
All requests must be prepaid. Checks should be made out to the U.S. Department of Commerce.
Data are available either in digital format or as paper copy. Please note that the NCDC stores
electronic data only for approximately 3-4 years. Older data is either stored in hardcopy or in
-------
microfiche. Data are no longer available in the 1440 format from NCDC. This format is
available from Trinity Consultants, in Dallas, Texas. They may be contacted at: 1-800-229-6655
or at their website: www.trinityconsultants.com
Data are also available from several other data sources. The Federal Depository Library System
has branches in many larger cities. These branches have a document entitled "Climatological
Data" for the area. Daily extremes in temperature on the designated days may be obtained from
that document. There is also a document entitled "Local Climatological Data" with the same
information as well as temperature data collected every three hours at 300 airports around the
country. Additionally, the Regional Depository Library System has branches in many medium
and large cities and can make available the same documents. Hourly data are usually not
available from either the NCDC or the two library systems noted above. Sources of temperature
data and any methods used for calculating average minimum and maximum temperatures must
be documented in any official SIP or conformity related submission.
2.4.3.2 Methods for hourly temperature input
The use of hourly temperature input is only necessary in cases where the user is trying to model
a specific exceedance episode for air quality or dispersion analysis and needs more control over
the daily temperature profile used by MOBILE6.2 by using the actual hourly temperatures
during the episode. Users need not attempt to estimate average hourly temperatures for more
generic modeling of average summer or winter day ozone or CO exceedance conditions.
Average minimum and maximum temperatures are sufficient for that kind of analysis.
Temperatures at three-hour intervals are available from the National Climatic Data Center (see
reference information above). Users who want to model a specific episode using these
temperatures could interpolate intermediate hourly temperatures. However, use of this data
alone might cause the user to miss the daily extreme minimum and maximum temperatures.
Users could adjust for this problem by substituting the maximum and minimum daily recorded
temperatures for the highest and lowest three-hour temperatures. However, an approach that
used actual recorded hourly temperatures would be preferable. Sources of temperature data and
any methods used to adjust them to fit the requirements of MOBILE6.2 must be documented in
any official SIP or conformity related submission.
2.4.4 MOBILE6.2 Commands
The two temperature command options in MOBILE6.2 (MIN/MAX TEMPERATURE and
HOURLY TEMPERATURE) are used to specify daily temperatures used by MOBILE to
estimate exhaust and evaporative emissions. Temperature is a mandatory input for MOBILE6.2
and users must specify either minimum and maximum temperatures or hourly temperatures for
each scenario. Both commands are described in detail in Section 2.8.6.3 and Section 2.8.6.4 of
the MOBILE6.2 User's Guide.
-------
2.5 Humidity
2.5.1 Guidance
MOBILE6 and MOBILE6.2 incorporate important new information on air conditioning effects
that was not available for MOBILES. This information indicates that air conditioning use can
have a significant impact on HC, NOx, and CO emissions. High humidity increases air
conditioning use in hot weather. In addition, humidity has an important impact on NOx
emissions at all temperatures due to the quenching effect of water vapor in the air on combustion
chamber temperatures. Because of these effects, EPA now recommends and encourages states to
use local humidity as an input in MOBILE6.2 for estimates of summer day emissions that will be
used for SIP or conformity purposes, and for estimates of NOx emissions any time of year,
including analysis of secondary nitrate formation for dispersion modeling of particulate matter.
In MOBILE6.2, humidity has no effect on direct PM emissions. Users should review final
regulations and guidance for PM standards and consult with their EPA Regional Office to
determine what PM precursors may need to be estimated included in local emission inventories.
Humidity assumptions used for transportation conformity analyses should be consistent with
those used in the SIP as required in the transportation conformity rule, §93.122 (a)(6), Code of
Federal Regulations (July 1, 2001). If the SIP did not include humidity input, states can use this
input for conformity analysis as determined through the interagency consultation process as
described in §93.105 (c)(l)(i), Code of Federal Regulations (July 1, 2001).
2.5.2 Description
In MOBILE6.2, the humidity input is used to adjust the estimate of NOx production during the
combustion process and determine emissions from additional engine loading with air conditioner
usage. Humidity has its largest effects on NOx emissions. Humidity effects on HC and CO
emissions are smaller.
The humidity input is used directly to correct for humidity's impact on NOx emissions. Water
vapor absorbs some of the heat of combustion in the engine, which in turn results in a reduction
in NOx formation. Humidity effects on HC and CO emissions are indirect. High humidity on
hot days increases air conditioning usage which increases HC and CO emissions. MOBILE6.2
incorporates both of these effects using the humidity input. For air conditioning effects,
humidity and temperature are combined internally in the model to create a heat index which in
turn is used to estimate air conditioning usage.
There are two methods for entering humidity values in MOBILE6.2: RELATIVE HUMIDITY,
which requires the user to enter hourly relative humidity values, or ABSOLUTE HUMIDITY,
which requires just a single absolute humidity value to represent an entire day. Either approach
can be used, but the RELATIVE HUMIDITY command has advantages. Humidity can vary
substantially over the course of a day and use of the RELATIVE HUMIDITY command will
-------
allow a much more accurate estimation of air conditioning usage and the quenching effect on
NOx emissions throughout the day. Use of the ABSOLUTE HUMIDITY command requires less
data but can result in problems if the absolute humidity value used is inconsistent with the
minimum or maximum temperatures used (this is discussed more fully in Sec. 2.5.3).
The RELATIVE HUMIDITY command has been added to MOBILE6.2 to allow the user to
specify hourly relative humidity values and relate these relative humidity values directly to the
hourly temperature values. In the past, users were limited to entering one absolute humidity
value for the 24 hour period. The model then used the temperature range for the day, along with
the barometric pressure to calculate a series of relative humidity values for the day. This
resulted in two problems. First, it did not allow for the fact that both relative humidity and
absolute humidity vary throughout the day. Second, it could result in internal calculations of
relative humidity that exceeded 100%. The addition of the RELATIVE HUMIDITY command
solves both of those problems.
2.5.3 Methods
Methods will vary depending on whether the user will enter humidity as relative humidity or
absolute humidity. Under either approach, users who wish to model a specific ozone, CO, or PM
exceedance episode should use actual humidity values recorded during that episode. For more
generic modeling of average day ozone or CO conditions for SIP or conformity purposes,
average humidity values during the months when ozone, CO, or PM exceedances typically occur
should be used. For ozone season analysis, users can enter the local average minimum and
maximum temperatures for July, or for the three month period that best represents the area's
ozone season (typically June, July and August; or July, August, and September). For CO season
analysis, humidity will typically not be a factor, unless CO exceedances occur in hot months
when air conditioning is used. In that case, users should enter local average humidity values for
a month when exceedances occur.
Users should fully document the relative or absolute humidity values, as well as any temperature
or barometric pressure data used to adjust those values to meet MOBILE6.2 requirements for any
SIP or conformity-related submission.
2.5.3.1 Methods for relative humidity input
The use of the RELATIVE HUMIDITY command requires the user to enter 24 hourly relative
humidity values. It also requires the use of the BAROMETRIC PRESSURE command in order
to internally calculate the NOx quenching effect. The hourly relative humidity values together
with hourly temperature values and barometric pressure are used to calculate hourly absolute
humidity values. Subsequently, hourly humidity effects on NOx emissions can be determined.
Also, the hourly relative humidity values are used directly to determine hourly heat indices
which are used to determine the air conditioning effects on HC, CO, and NOx emissions.
-------
Relative humidity and barometric pressure information at three-hour intervals are available from
the National Climatic Data Center (see reference information in Section 2.4.3). Users who want
to model a specific episode using these humidity values could interpolate intermediate hourly
humidity values. However, use of this data alone might cause the user to miss the daily extreme
minimum and maximum humidity values. Users could adjust for this problem by substituting
the maximum and minimum daily recorded relative humidity for the highest and lowest three-
hour humidity values.
2.5.3.1 Methods for absolute humidity input
The alternate input for humidity in MOBILE6.2 is referred to as absolute humidity, but it might
more accurately be called a humidity ratio. This is defined as the mass of water vapor per unit
mass of dry air. For MOBILE6.2 users who are developing inventories for air quality modeling
purposes, humidity ratios may be available as output from a meteorological model (e.g.,
Fifth-Generation NCAR / Penn State Mesoscale Model (MM5)). For MOBILE6.2, the humidity
ratio input needs to be calculated in terms of grains of water per pound of dry air. This will
likely require unit conversion of the meteorological model output.
For users without access to humidity ratios for the exceedance days being modeled, the humidity
ratio can be calculated from temperature, relative humidity, and barometric pressure. A
spreadsheet and a computer program that perform this calculation are available at the
MOBILE6.2 website. Relative humidity and barometric pressure information at three-hour
intervals are available from the National Climatic Data Center (see reference information above).
Users should take care to ensure that the inputs are converted to the proper units needed to
calculate the humidity ratio in grains of water per pound of dry air.
Whether users are using humidity ratios values produced by a meteorological model or
calculating humidity ratios from relative humidity information, input for the ABSOLUTE
HUMIDITY command in the model should always be the lowest humidity ratio that occurred
during the exceedance day. MOBILE6.2 currently only allows input of a single absolute
humidity value for an entire day, while temperature varies across the range of values entered by
the user. Because both relative and absolute humidity can change significantly during the day,
users need to carefully choose the absolute humidity value used in the model. Absolute humidity
values that are reasonable at one temperature, can produce unreasonable relative humidity values
(i.e., greater than 100%) at a different temperature during the same day. Using the lowest
humidity ratio of the day should prevent this problem. An acceptable alternative approach
would be to use the highest humidity ratio that does not result in a relative humidity value
greater than 100%. Users should fully document the temperatures, barometric pressures, relative
humidity values, and methods used to calculate humidity ratios for any SIP or conformity related
submission.
10
-------
2.5.4 MOBILE6.2 Commands
Relative humidity is specified by using the RELATIVE HUMIDITY command described in
section 1.3.4 of the MOBILE6.2 User's Guide. There is no default value for this command.
Values between 0% and 100% may be entered. If the RELATIVE HUMIDITY command is
used, the BAROMETRIC PRESSURE command must also be used. This command has default
value of 29.92 inches of mercury (Hg).
Absolute humidity is specified using the ABSOLUTE HUMIDITY command which is described
in detail in Section 2.8.6.6 of the MOBILE6.2 User's Guide. The default value used in the
model is 75 grains of water per pound of dry air. MOBILE6.2 allows users to enter values
between 20.0 and 528.0 grains of water per pound of dry air.
2.6 Other air-conditioning related input options
2.6.1 Guidance
MOBILE6.2 incorporates three additional factors that affect air conditioning usage: average
percent cloud cover, period of peak sun, and sunrise/sunset times. These three factors affect the
timing and magnitude of solar loading, which in turn affect the amount of air conditioning use.
MOBILE6.2 allows users to supply local information for all three factors. However, EPA
recommends that users use the MOBILE6.2 default values for most typical SIP or conformity
applications. The default values are consistent with conditions for ozone formation. The impact
of reasonable changes in the default values is likely to be small, and may not justify the burden
of developing and incorporating additional data into the modeling process.
These input options might be of interest to air quality modelers who are analyzing specific ozone
exceedance episodes and wish to have more control over the detailed environmental conditions
assumed in MOBILE6.2. Even in these cases, use of local data for these input options is not
required. States or local agencies who do want to include local inputs for these options in an
official submission must fully document the sources of those inputs including showing that they
are consistent with data used for temperature and humidity input in the model.
2.6.2 MOBILE6.2 Commands
The three commands covered by this section are called CLOUD COVER, PEAK SUN, and
SUNRISE/SUNSET in MOBILE6.2. They are described in detail in Section 2.8.6.7 of the
MOBILE6.2 User's Guide.
3.0 Vehicle Fleet Characteristics
Certain characteristics of the highway vehicle fleet can have a significant impact on overall
emissions from these sources. The age distribution and the average annual mileage
11
-------
accumulation of vehicles in the fleet affect estimates of the deterioration of vehicle emission
control effectiveness. The age distribution of the fleet also determines the fractions of the fleet
that meet different emission standards. The relative fractions of gasoline and diesel-powered
vehicles are important because gasoline and diesel engines have different emissions
characteristics.
The relative activity of different vehicle classes (i.e., fractions of VMT for cars, light duty trucks,
heavy duty trucks, and motorcycles) is also important, but is discussed separately in Section 4.
MOBILE6.2 incorporates a new vehicle classification system that more accurately reflects the
way vehicles are classified for emissions standards than previous versions of MOBILE. The
following table shows how the vehicle classes in MOBILE6.2 correspond to the classes defined
in MOBILES. The table shows that the vehicle classification used in MOBILE6.2 is simply a
finer subdivision of the vehicle classes used in MOBILES. In many cases, methodologies used
to develop fleet characteristic input for MOBILES can still be used in for MOBILE6.2 with
simple modifications.
Table 3.0.1
Comparison of MOBILES and MOBILE6.2 Vehicle Classifications
MOBILES
LDV
LDT1
LDT2
HDV
MC
MOBILE6.2
LDV
LDT1
LDT2
LDT3
LDT4
HDV2B
HDV3
HDV4
HDV5
HDV6
HDV7
HDV8A
HDV8B
HDBS
HDBT
MC
Description
Light-Duty Vehicles (Passenger Cars)
Light-Duty Trucks 1 (0-6,000 Ibs. GVWR, 0-3750 Ibs. LVW)
Light Duty Trucks 2 (0-6,001 Ibs. GVWR, 3751-5750 Ibs. LVW)
Light Duty Trucks 3 (6,001-8500 Ibs. GVWR, 0-5750 Ibs. ALVW)
Light Duty Trucks 4 (6,001-8500 Ibs. GVWR, >5750 Ibs. ALVW)
Class 2b Heavy Duty Vehicles (8501-10,000 Ibs. GVWR)
Class 3 Heavy Duty Vehicles (10,001-14,000 Ibs. GVWR)
Class 4 Heavy Duty Vehicles (14,001-16,000 Ibs. GVWR)
Class 5 Heavy Duty Vehicles (16,001-19,500 Ibs. GVWR)
Class 6 Heavy Duty Vehicles (19,501-26,000 Ibs. GVWR)
Class 7 Heavy Duty Vehicles (26,001-33,000 Ibs. GVWR)
Class 8a Heavy Duty Vehicles (33,001-60,000 Ibs. GVWR)
Class 8b Heavy Duty Vehicles (>60,000 Ibs. GVWR)
School Buses
Transit and Urban Buses
Motorcycles (All)
The gross vehicle weight rating (GVWR) is a value defined by the vehicle manufacturer which
indicates the total weight of the vehicle including it's curb (empty) weight, fluids, driver and the
maximum recommended payload. The adjusted loaded vehicle weight (ALVW) is the numerical
average of the vehicle curb weight and the gross vehicle weight rating (GVWR). The loaded
vehicle weight (LVW) is the curb (empty) weight of the vehicle plus 300 Ibs., representing the
driver and incidental payload weight.
12
-------
3.1 Age Distribution of Vehicle Registrations
3.1.1 Guidance
The age distribution of vehicle fleets can vary significantly from area to area. For SIP and
conformity related purposes, EPA recommends and encourages states to develop local age
distributions. This basic guidance is unchanged from past guidance. However, MOBILE6.2
breaks down the vehicle fleet into many more classes than previous versions of MOBILE. EPA
encourages States to work to develop local age distributions for all of the vehicle classes over the
long-term and EPA will work with the Department of Transportation and with the States to
identify or develop tools, such as vehicle identification number (VIN) decoders to do this. Until
States have this information, for SIP and conformity related submissions there is no alternative
but to assume that the new subclasses of vehicles in MOBILE6.2 will have the same age
distribution as the previous classes they were derived from.
If local registration age distributions are used, they normally should not change across calendar
years. In particular, EPA will not accept SIP or conformity related modeling that includes
assumptions that the vehicle fleet will be newer (have a lower average age) in the future than is
reflected in the latest registration age distribution information currently available. States must
fully document the sources of data and methods used to develop local age distributions used in
modeling for SIP and conformity related purposes.
3.1.2 Description
A typical vehicle fleet includes a mix of vehicles of different ages. MOBILE6.2 covers a 25 year
range of vehicle ages, with vehicles 25 years and older grouped together. MOBILE6.2 allows
the user to specify the fraction of vehicles in each of 25 vehicle ages for each of the 16 vehicle
classes (combined gasoline and diesel) in the model.
Fleets with a higher percentage of older vehicles will have higher emissions for two reasons.
Older vehicles have typically been driven more miles and have experienced more deterioration
in emission control systems. A higher percentage of older vehicles also means that there are
more vehicles in the fleet that do not meet newer, more stringent emissions standards. Surveys
of registration data indicate considerable local variability in vehicle age distributions.
3.1.3 Methods
Local age distributions can be estimated from local registration data or from an analysis of local
inspection and maintenance program data. Methods for estimating registration distributions by
age are described in the reports "Techniques for Estimating MOBILE2 Variables" and
"Additional Techniques for Estimating MOBILE2 Variables" (Energy and Environmental
Analysis, Inc. for EPA Contract Number 68-03-2888). Methods for estimating age distributions
from I/M program data are given in the report "Methodology for Gathering Locality-Specific
Emission Inventory Data" which is incorporated in "Volume IV: Chapter 1, Preferred and
13
-------
Alternative Methods for Gathering and Locating Specific Emission Inventory Data" (June 1996,
Emission Inventory Improvement Program).
In the absence of new information on the age distribution of new vehicle classes in MOBILE6.2,
age distributions for the equivalent MOBILES vehicle classes can be applied. For example,
based on registration data, a state has developed methods for determining age distributions for
vehicles in the LDGT1 category as defined in MOBILES. In the absence of new information,
this age distribution could be applied to all vehicles in the new LDGT1 and LDGT2 categories as
defined in MOBILE6.2. Likewise, age distributions calculated for vehicles in the LDGT2
category in MOBILES could be applied to the new LDGT3 and LDGT4 categories in
MOBILE6.2.
A similar approach can used to estimate age distributions for new heavy duty vehicle classes as
defined in MOBILE6.2. However, an additional adjustment will be necessary here to account
for the fact that MOBILE6.2 requires a single age distribution for both gasoline and diesel heavy
duty trucks while MOBILES allowed for two separate age distributions. Chapter 5 of the
MOBILE6.2 User's Guide includes a method for weighting separate heavy duty gasoline and
diesel age distributions to develop a single composite age distribution. This age distribution
could then be applied to all of the new classes of heavy duty vehicles in MOBILE6.2 except for
the heavy duty transit bus category, which is assumed to be all diesel-fueled.
Having the same age distribution for both gasoline and diesel fueled vehicles of the same class
assumes that the scrappage rate for these two fuel types will be the same within a class. If the
user wishes the age distributions for gasoline and diesel fueled vehicles to be different, each fuel
type will need to be modeled with a separate MOBILE6.2 run using diesel sales fractions that do
not vary by model year or age and with the appropriate age distribution for the fuel type to be
modeled.
The following table summarizes how age distributions developed for MOBILES could be
adapted to MOBILE6.2. However, areas are encouraged to determine separate age distributions
for as many of the new vehicle classes as possible from the available data.
14
-------
Table 3.1.3.1
Matching MOBILE6.2 Vehicle Classes to MOBILES Age Distributions
MOBILE6.2 Vehicle Class
LDV
LDT1
LDT2
LDT3
LDT4
HDV2B
HDV3
HDV4
HDV5
HDV6
HDV7
HDV8A
HDV8B
HDBS
HDBT
MC
MOBILES Age Distribution
LDV
LDGT1
LDGT2
Weighted Combination of HDGV and HDDV
HDDV
ivrr
3.1.4 MOBILE6.2 Command
The age distribution of vehicle registrations is specified in MOBILE6.2 using the REG DIST
command, which is described in detail in Section 2.8.7.1 of the MOBILE6.2 User's Guide.
Users must create an external data file which contains fractions for each of the 25 model years
for each vehicle type specified by the user.
3.2 Annual mileage accumulation rates
3.2.1 Guidance
EPA continues to recommend that States use national default mileage accumulation rates built
into MOBILE6.2. Most local sources of mileage accumulation rate by age are subject to
sampling bias or data entry errors, and the use of such data should be approached with caution.
In the development of highway vehicle emission inventories for SIP or conformity related
purposes, States should obtain prior approval of alternate mileage accumulation rates and their
derivation from EPA before using such rates in their modeling. States that wish to develop local
mileage accumulation rates must fully document data collection and analysis methodologies.
States that have developed approved local mileage accumulation rates based on MOBILES
vehicle classes could apply those rates to the corresponding MOBILE6.2 vehicle classes.
If local mileage accumulation rates are used they normally should not change across calendar
years. In particular, EPA will not accept SIP or conformity related modeling that assumes that
vehicles in the future will accumulate mileage at a different rate than current vehicles.
15
-------
3.2.2 Description
Vehicles accumulate mileage at different rates depending on the type and age of the vehicle.
Trucks tend to be driven more miles per year than cars. Older vehicles tend to be driven fewer
miles per year than newer ones. Annual mileage accumulation affects the rate at which vehicle
emission controls deteriorate and affect the relative emissions contributions of newer and older
vehicles.
MOBILE6.2 assigns separate default mileage accumulation rates and allows user input for all
vehicle classes. For annual mileage accumulation rates, MOBILE6.2 treats gasoline and diesel
vehicles separately for a total of 28 separate vehicle types. MOBILE6.2 allows users to specify
the vehicle classes for which new annual mileage accumulation rates are entered. All other
classes are assumed to have the default rates.
3.2.3 Methods
Methods for estimating annual mileage accumulations from I/M program data are given in the
report "Methodology for Gathering Locality-Specific Emission Inventory Data" which is
incorporated in "Volume IV: Chapter 1, Preferred and Alternative Methods for Gathering and
Locating Specific Emission Inventory Data" (June 1996, Emission Inventory Improvement
Program).
Chapter 5 of the MOBILE6.2 User's Guide includes a description of how to apply local annual
mileage accumulation rates based on MOBILES vehicle classes to the new vehicle classes in
MOBILE6.2. The following table summarizes how annual mileage accumulations developed
using MOBILES vehicle classes could be applied to the MOBILE6.2 vehicle classes. Unlike the
REG DIST command used to enter the registration age distribution, the MILE ACCUM RATE
command used to enter annual mileage accumulation rates separates the fleet into gasoline and
diesel vehicles. This makes the use of MOBILES based rates simpler since it is not necessary to
combine gasoline and diesel numbers. In addition, the MILE ACCUM RATE allows the user to
specify mileage accumulation rates only for the vehicle classes the user wishes to change from
the default. As a result users could easily choose to enter local annual mileage accumulation
rates only for the vehicle classes for which they have local data, and use national defaults for the
rest.
16
-------
Table 3.2.3.1
Matching MOBILE6.2 Vehicle Classes to MOBILES Classes for Annual Mileage
Accumulation
MOBILE6.2
Class
LDGV
LDGT1
LDGT2
LDGT3
LDGT4
HDGV2B
HDGV3
HDGV4
HDGV5
HDGV6
HDGV7
HDGV8A
HDGV8B
LDDV
LDDT12
HDDV2B
HDDV3
HDDV4
HDDV5
HDDV6
HDDV7
HDDV8A
HDDV8B
MC
HDGB
HDDBT
HDDBS
LDDT34
Description
Light-Duty Gasoline Vehicles (Passenger Cars)
Light-Duty Gasoline Trucks 1 (0-6,000 Ibs. GVWR, 0-3750 Ibs. LVW)
Light-Duty Gasoline Trucks 2 (0-6,000 Ibs. GVWR, 3751-5750 Ibs. LVW)
Light-Duty Gasoline Trucks 3 (6,001-8,500 Ibs. GVWR, 0-5750 Ibs. ALVW)
Light-Duty Gasoline Trucks 4 (6,001-8,500 Ibs. GVWR, 5751 Ibs. and greater
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)
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 1 and 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)
MOBILES Class
LDGV
LDGT1
LDGT2
HDGV
LDDV
LDDT
HDDV
MC
HDGV
HDDV
LDDT
The annual mileage accumulation rates are used to determine the number of miles per
day driven by vehicles assumed by MOBILE6.2 by dividing the annual mileage accumulation
rate by 365 days. Changing the annual mileage accumulation rate will change the assumed
number of miles driven each day. However, changing the annual mileage accumulation rate will
affect the "age" of vehicles, since the basic emission rates (and many other estimates) are based
on total mileage accumulation. Users should not attempt to affect the estimate of miles per day
by altering the annual mileage accumulation rates. The MOBILE6.2 database output options
include data fields, such as the assumed miles per day estimates, which can be used in a
spreadsheet or other application to adjust gram per mile estimates to reflect different
assumptions regarding the number of miles driven each day.
17
-------
3.2.4 MOBILE6.2 Command
More details on the use of the command MILE ACCUM RATE can be found in Section 2.8.7.3
of the MOBILE6.2 User's Guide.
3.3 Diesel Fractions
3.3.1 Guidance
Use of local estimates of fractions of diesel vehicles by vehicle class is optional for SIP and
conformity related purposes. Diesel sales fractions may vary from area to area. In addition,
diesel sales fractions do not equate directly with the diesel VMT fraction, since diesel fueled
vehicles tend to have a higher mileage accumulation rate than gasoline fueled vehicles of the
same class. However, local information for all of the MOBILE6.2 vehicle classes is unlikely to
be readily available. Because diesel fractions vary considerable among the heavy duty vehicle
classes in MOBILE6.2, default MOBILE6.2 fractions should be used for any vehicle category
for which there is no direct information. For example, the fraction of diesel heavy duty trucks in
the entire heavy duty fleet cannot be used as a substitute for the fractions of diesel heavy duty
trucks in each of the MOBILE6.2 heavy duty classes (i.e., HDV2b, HDV3, etc.).
MOBILE6.2 projects future diesel fractions as constant beginning in 1996. If no local data are
available for years after 1995, States should use the 1996 and later default values in MOBILE6.2
for all years after 1996. If more recent data are available, States must use the estimate of diesel
fractions for the most recent year available as the projected fractions for future years for SIP and
conformity related submissions.
Methods and data sources for diesel fractions used in any SIP or conformity related submission
must be fully documented.
3.3.2 Description
Within any vehicle class, diesel and gasoline vehicles have distinctly different emission rates.
Diesel fractions allow the model to separate gasoline and diesel vehicles within a vehicle class
MOBILE6.2 includes default diesel sales fractions for 14 of the 16 composite vehicle classes -
all except urban/transit busses, which are assumed to be all diesel-fueled, and motorcycles,
which are assumed to be all gasoline-fueled. Users can enter alternative diesel fractions for these
14 vehicle classes for each of 25 vehicle ages.
3.3.3 Methods
Vehicle registration data are the most likely source for diesel fraction information. Diesel sales
fractions developed for MOBILES vehicle class LDV may be applied directly in MOBILE6.2 to
the same class. Diesel sales fractions for MOBILES vehicle class LDT1 can be applied to the
new MOBILE6.2 vehicle classes LDT1 and LDT2. For all other MOBILE6.2 vehicle classes,
18
-------
users need to either develop diesel fractions specific to the class or use the national defaults
estimates. Diesel fractions for all vehicle classes (including those for which national defaults are
assumed) need to be entered if the DIESEL FRACTIONS command is used. Chapter 5 of the
MOBILE6.2 User's Guide includes a description of how to do this. Appendix E of the
MOBILE6.2 User's Guide gives the national default diesel fractions used in the model.
3.3.4 MOBILE6.2 Command
More detail on the use of the DIESEL FRACTIONS command is given in Section 2.8.7.2 of the
MOBILE6.2 User's Guide.
3.4 Natural Gas Vehicles
MOBILE6.2 allows users to specify the fraction of the vehicle fleet by vehicle class (for all
vehicle classes, including urban busses) that consists of vehicles certified to operate on either
compressed or liquefied natural gas and to enter alternative emission rates for those vehicles.
Use of these commands is primarily limited to analyses of the effects of natural gas vehicles on a
small subset of a typical area-wide fleet. For example, they could be used to analyze the
emission effects of changing a small fleet of vehicles to natural gas vehicles (NGVs). Under
normal circumstances for SIP and conformity modeling, in which the fraction of natural gas
vehicles in an area-wide fleet is extremely small, use of these commands may not be necessary.
However areas with a significant population of NGVs as the result of a specific control program
might use this feature to quantify emission reductions from that program.
Two commands affect the emissions associated with NGVs. The NGV FRACTION command is
used to specify the fraction of NGVs in the fleet. When this command alone is used,
MOBILE6.2 relies on a set of default NGV emission rates built into the model. However, these
default emission rates do not include NGV emission rates for vehicles meeting Tier 2 emission
standards. NGVs in model years 2004 and later are assumed to continue to meet ULEV
standards rather than Tier 2 standards. To change this, users must enter the NGV EF command
along with a set of alternate NGV emission factors. EPA does not currently have a set of
approved alternate emission factors for NGVs meeting Tier 2 standards. Further details on the
MOBILE6.2 commands described here are in Section 2.8.7.5 and Section 2.8.7.6 of the
MOBILE6.2 User's Guide.
Use of these commands in any official submission for SIP or conformity related purposes must
be fully documented. Documentation should include Any use of the alternative emission factor
option should be approved by EPA before submission. These commands should only be used for
vehicles originally certified as NGVs and should not be used for retrofits or dual-fueled (gasoline
and natural gas) vehicles.
19
-------
4.0 Vehicle Activity
Travel demand forecasting models (TDFMs) are often the source of information used by MPOs
and State DOTs to estimate VMT. Traffic demand modelers for MPOs and State DOTs
traditionally have had to adjust estimates of vehicle miles of travel generated through the TDFM
process in order to match the Highway Performance Monitoring System (HPMS) estimates of
VMT based on actual vehicle counts. These procedures are required in order to generate
consistent VMT estimates from TDFMs for roadway functional classes within HPMS for use in
regional emissions and air quality analysis. Section 3, "Developing Locality-Specific Inputs
from Travel Demand Models," of the EPA document, "Volume IV: Chapter 2, Use of Locality-
Specific Transportation Data for the Development of Mobile Source Emission Inventories,"
(September 1996), discusses how to reconcile traffic demand model results with HPMS VMT
estimates. However, these methods may not be appropriate for all areas. EPA can be consulted
to determine if other HPMS factoring schemes may be acceptable.
In some cases, MPOs and State DOTs will not have the detailed activity information needed for
emission modeling. In these cases, EPA should be consulted to determine whether other sources
of the data are available or whether national average default values can be used.
In those cases where the default MOBILE6.2 values are not used and are replaced by local
information, the activity estimates used in the inventory modeling should be consistent, where
possible, with assumptions used in other highway vehicle related planning. Some care must be
taken to assure that each local activity estimate is consistent with other local estimates used and,
as much as possible, consistent with MOBILE6.2 defaults that will be used in the inventory
modeling. MOBILE6.2 will not always recognize when conflicting information is provided by
the user.
4.1 Vehicle Miles Traveled Fraction by Vehicle Class
4.1.1 Guidance
The fraction of vehicle miles traveled (VMT) by vehicle class varies from area to area and can
have a significant effect on overall emissions from highway mobile sources. For SIP-related
highway vehicle emission inventory development in moderate and above non-attainment areas,
EPA expects states to develop and use their own specific estimates of VMT by vehicle class.
The VMT fractions by vehicle class used in inventory modeling should be consistent, where
possible, with assumptions used in other highway vehicle related planning. Metropolitan
Planning Organizations (MPOs) and state Departments of Transportation (DOTs) should be
consulted for information to determine the proportion of passenger vehicles and light duty trucks
relative to heavy duty trucks by time of day and facility class.
20
-------
4.1.2 Description
The VMT fractions specify the fractions of total highway VMT that are accumulated by each of
16 combined vehicle classes. The VMT fractions are used in MOBILE6.2 to calculate emission
factor estimates that involve further aggregations of these vehicle classes. These include many
of the emission results found in the default descriptive output. MOBILE6.2 calculates the
default VMT fractions based on national average data characterizing vehicle age distributions
and annual mileage accumulation rates by age for each vehicle class, and the total number of
vehicles of each vehicle class nationally. MOBILE6.2 allows this default distribution to be
replaced by local values.
If the user intends only to use the MOBILE6.2 emission results separately for each of the 28
individual vehicle classes, and will not be using any composite vehicle class results, then the
VMT fractions used by the model will not affect the results and the user is not required to enter
local VMT fractions for the MOBILE6.2 runs. However to appropriately determine inventory
tons from the separate vehicle class emission rates produced by the MOBILE6.2 model, the
estimates of total highway vehicle miles traveled must still be divided by vehicle class using the
methods described in this section.
4.1.3 Methods
MOBILE6.2 requires that local VMT fractions by vehicle class be supplied separately for 16
vehicle class definitions. These vehicle classes are determined by the regulations under which
the vehicles are certified for sale. The sixteen vehicle classes are:
Table 4.1.1
MOBILE6.2 Vehicle Classifications
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Abbreviation
LDV
LDT1
LDT2
LDT3
LDT4
HDV2B
HDV3
HDV4
HDV5
HDV6
HDV7
HDV8A
HDV8B
MDBS
HDBT
MC
Description
Light-Duty Vehicles (Passenger Cars)
Light-Duty Trucks 1 (0-6,000 Ibs. GVWR, 0-3750 Ibs. LVW)
Light Duty Trucks 2 (0-6,001 Ibs. GVWR, 3751-5750 Ibs. LVW)
Light Duty Trucks 3 (6,001-8500 Ibs. GVWR, 0-5750 Ibs. ALVW)
Light Duty Trucks 4 (6,001-8500 Ibs. GVWR, >5750 Ibs. ALVW)
Class 2b Heavy Duty Vehicles (8501-10,000 Ibs. GVWR)
Class 3 Heavy Duty Vehicles (10,001-14,000 Ibs. GVWR)
Class 4 Heavy Duty Vehicles (14,001-16,000 Ibs. GVWR)
Class 5 Heavy Duty Vehicles (16,001-19,500 Ibs. GVWR)
Class 6 Heavy Duty Vehicles (19,501-26,000 Ibs. GVWR)
Class 7 Heavy Duty Vehicles (26,001-33,000 Ibs. GVWR)
Class 8a Heavy Duty Vehicles (33,001-60,000 Ibs. GVWR)
Class 8b Heavy Duty Vehicles (>60,000 Ibs. GVWR)
School Buses
Transit and Urban Buses
Motorcycles (All)
21
-------
The vehicle classes used for entry of the VMT fractions do not differentiate between vehicles by
the type of fuel used (i.e., diesel versus gasoline). The emission certification categories used by
MOBILE6.2 do not easily map into other vehicle classification systems, such as those used by
the US Department of Transportation, which use different criteria (i.e., the number of axles) to
determine vehicle class.
If the available information obtained from Metropolitan Planning Organizations (MPOs) or state
Departments of Transportation (DOTs) has VMT already separated into the sixteen vehicle
classes used in MOBILE6.2, and these values are consistent with assumptions used in other
highway vehicle related planning, then no adjustments need to be made and the values can be
used directly. However, it is likely that the local VMT information obtained will not be detailed
enough to directly determine the VMT of the sixteen vehicle class categories needed.
4.1.4 Disaggregation of Local Information
The default VMT distribution of vehicle classes used in MOBILE6.2 can be used to separate the
available local combined VMT information into the sixteen vehicle classifications needed for use
in MOBILE6.2. This general method is also described in Section 5.3.2 in the MOBILE6.2 User
Guide, which describes how to use the set of 8 vehicle class VMT distribution values used in
MOBILES to estimate the 16 MOBILE6.2 categories. In this method, the MOBILE6.2 national
average distribution values are used to determine the proportions of each subgroup in a more
aggregate classification and these proportions are used to split the VMT for the aggregate class
into the needed subgroup VMT estimates.
For example, an area determines that 20% of the total VMT in calendar year 2000 is from heavy
duty vehicles (HDV) and the remainder (80%) is light duty vehicles (LDV), including
motorcycles. The national average default value is 11.80% for HDV (vehicle classes 6 through
15) and 88.2% for LDV (vehicle classes 1 through 5 and 16). Each default LDV fraction is
multiplied by the ratio of the local total LDV VMT fraction estimate divided by the national total
LDV VMT fraction estimate. Each default HDV fraction is multiplied by the ratio of the local
total HDV VMT fraction estimate divided by the national total HDV VMT fraction estimate.
The calculations needed for this hypothetical example follow:
1. LDV = 0.4858 * (0.80/0.882) = 0.4406
2. LDT1 = 0.0671 * (0.80/0.882) = 0.0608
3. LDT2 = 0.2230 * (0.80/0.882) = 0.2022
4. LDT3 = 0.0690 * (0.80/0.882) = 0.0626
5. LDT4 = 0.0321 * (0.80/0.882) = 0.0291
6. HDV2b = 0.0383 * (0.20/0.118) = 0.0649
7. HDV3 = 0.0038 * (0.20/0.118) = 0.0065
8. HDV4 = 0.0029 * (0.20/0.118) = 0.0049
9. HDV5 = 0.0022* (0.20/0.118) = 0.0037
10. HDV6 = 0.0083 * (0.20/0.118) = 0.0141
11. HDV7 = 0.0099* (0.20/0.118) = 0.0168
22
-------
12. HDV8a = 0.0109* (0.20/0.118) = 0.0185
13. HDVSb = 0.0389 * (0.20/0.118) = 0.0659
14. HDBS = 0.0019 * (0.20/0.118) = 0.0033
15. HDBT = 0.0009 * (0.20/0.118) = 0.0015
16. MC = 0.0051 * (0.80/0.882) = 0.0046
The new set of sixteen values for the VMT fractions must still add to one. If a more detailed
breakout of vehicle class VMT is available, such as a breakout of light-duty truck VMT from
passenger car and motorcycle VMT, then only the portions of the local VMT estimate that need
to be disaggregated will need to be adjusted. In this example, the VMT fractions for each of the
four light-duty truck classes would be estimated by using the default VMT fraction times the
local VMT fraction for all light-duty trucks divided by the sum of the default VMT fractions for
the four light-duty truck classes. The local fractions for passenger cars and motorcycles would
be used directly. The description of this method in Section 5.3.2 in the MOBILE6.2 User Guide
shows an example of how more detailed information (such as values used for MOBILES) can be
used with this method.
Since MOBILE6.2 VMT fractions combine gasoline and diesel vehicle driving, local
information about the fraction of diesel and gasoline VMT must be summed before using the
methodology described in this section. The user cannot enter separate VMT fractions for diesel
and gasoline fueled vehicles as in MOBILES. The combined VMT of gasoline and diesel
vehicles are split internally in the MOBILE6.2 model using the diesel sales fractions, age
distributions and annual mileage accumulation rates. These values may be provided by the user
using the DIESEL FRACTIONS, REG DIST and MILE ACCUM RATE commands in
MOBILE6.2.
The VMT by vehicle class in MOBILE6.2 is a calculated value and varies from calendar year to
calendar year due to uneven growth in the vehicle sales by vehicle class. The default VMT
distribution used in the calculation must be taken from the calendar year that coincides with the
calendar year of the local data used. Table 4.1.2 shows VMT by vehicle class calculated by
MOBILE6.2 for all calendar years using the default national average values. If the user intends
to use non-default values for annual mileage accumulation rates, age distributions or diesel sales
fractions, then MOBILE6.2 can be run with the non-default values to determine the calculated
value for the VMT distribution by vehicle class using the non-default values.
4.1.5 HPMS or TIUS Data
If the local VMT data is obtained using Highway Performance Monitoring System (HPMS) data
or Census Truck Inventory and Use Survey (TIUS) data, then the methods described in the EPA
document, "Volume IV: Chapter 2, Use of Locality-Specific Transportation Data for the
Development of Mobile Source Emission Inventories," (September 1996), must be used to
develop MOBILES-style VMT fractions. However, the HPMS estimates for passenger cars and
light duty trucks are not always a reliable estimate of the split between passenger cars and light
trucks, since many HPMS traffic counting systems cannot properly distinguish between these
23
-------
light duty categories. These two estimates should be combined into a total "light duty" category.
The light duty and heavy duty VMT fractions can then be expanded to the sixteen vehicle classes
needed for MOBILE6.2 using the preferred method described above.
In some cases (such as for heavy duty vehicles) it will be necessary to combine VMT fractions
based on local data before splitting the VMT into the sixteen MOBILE6.2 vehicle classifications.
For example, since MOBILE6.2 VMT fractions combine gasoline and diesel vehicle driving,
local information about the fraction of diesel and gasoline VMT must be summed before using
the methodology described in this section. The combined VMT of gasoline and diesel vehicles
are split internally in the MOBILE6.2 model using the diesel sales fractions, age distributions
and annual mileage accumulation rates. These values may be provided by the user using the
DIESEL FRACTIONS, REG DIST and MILE ACCUM RATE commands in MOBILE6.2.
Table 4.1.2 below shows an example mapping of the HPMS vehicle categories to general vehicle
class categories. These general class categories would then be split into the 16 MOBILE6.2
vehicle class categories using the methodology described above.
Table 4.1.1
MOBILE6.2 Vehicle Classifications
HPMS Category
Motorcycle
Passenger Car
Other 2-Axel, 4-Tire Vehicles
Buses
All Other Trucks:
Single Unit, 2-Axel, 6-Tire
Single Unit, 3 -Axel
Single Unit, 4 or More Axel
Single Trailer, 4 or Fewer Axel
Single Trailer, 5-Axel
Single Trailer, 6 or More Axel
Multi-Trailer, 5 or Fewer Axel
Multi-Trailer, 6 Axel
Multi-Trailer, 7 or More Axel
General Category
Motorcycle (MC)
Passenger car (LDV)
Light Truck (LOT)
Bus (HDB)
Heavy Duty Truck (HDV)
24
-------
For example, using the same default MOBILE6.2 national average VMT distribution from
calendar year 2000 used in the example above, the calculations needed would be:
1. LDV = LDV
2. LDT1 = 0.0671 * (LDT/0.3912)
3. LDT2 = 0.2230 * (LDT/0.3912)
4. LDT3 = 0.0690 * (LDT/0.3912)
5. LDT4 = 0.0321 * (LDT/0.3912)
6. HDV2b = 0.0383 * (HDV/0.1152)
7. HDV3 = 0.0038 * (HDV/0.1152)
8. HDV4 = 0.0029 * (HDV/0.1152)
9. HDV5 = 0.0022 * (HDV/0.1152)
10. HDV6 = 0.0083 * (HDV/0.1152)
11. HDV7 = 0.0099* (HDV/0.1152)
12. HDV8a = 0.0109* (HDV/0.1152)
13. HDV8b = 0.0389* (HDV/0.1152)
14. HDBS = 0.0019 * (HDB/0.0028)
15. HDBT = 0.0009 * (HDB/0.0028)
16. MC = MC
Where the values LDV, LDT, HDV, HDB and MC are the general category VMT fractions from
HPMS. These five values must sum to one. The value 0.3912 is the sum of the LDT1, LDT2,
LDT3 and LDT4 default values. The value 0.1152 is the sum of the HDV2b, HDV3, HDV4,
HDV5, HDV6, HDV7, HDVSa and HDVSb default values. The value 0.0028 is the sum of the
HDBS and HDBT default values. These default values are from calendar year 2000.
The VMT by vehicle class in MOBILE6.2 varies from calendar year to calendar year due to
uneven growth in the vehicle sales by vehicle class. The default VMT distribution used in the
calculation must be taken from the calendar year that coincides with the calendar year of the
local data used. Table 4.1.2 shows the default national average VMT by vehicle class calculated
by MOBILE6.2 for all calendar years.
4.1.4 MOBILE6.2 Commands
The local fraction of vehicle miles traveled by each vehicle class is provided to MOBILE6.2
using the VMT FRACTIONS command. This command requires that the user supply the sixteen
values which represent the fraction of all VMT accumulated by all highway motor vehicles
which is accumulated by each of sixteen vehicle classes. All values must be between zero and
one, and the sixteen values must sum to one. See Section 2.8.8.1 of the MOBILE6.2 User Guide
for more information on how to use this command.
25
-------
Table 4.1.2
National Average Vehicle Miles Traveled Fractions By Vehicle Class
Using MOBILE6.2
(Part 1)
Calendar
Year
1952-
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
LDV
1
0.6833
0.6833
0.6833
0.6833
0.6834
0.6839
0.6843
0.6844
0.6843
0.6841
0.6837
0.6813
0.6764
0.6695
0.6621
0.6548
0.6460
0.6357
0.6284
0.6212
0.6109
0.6009
0.5910
0.5815
0.5721
LDT1
2
0.0374
0.0374
0.0374
0.0374
0.0373
0.0372
0.0371
0.0370
0.0370
0.0370
0.0370
0.0367
0.0365
0.0369
0.0377
0.0387
0.0396
0.0408
0.0420
0.0435
0.0456
0.0477
0.0497
0.0517
0.0534
LDT2
3
0.1245
0.1245
0.1245
0.1245
0.1243
0.1238
0.1234
0.1232
0.1231
0.1230
0.1231
0.1222
0.1216
0.1228
0.1255
0.1287
0.1317
0.1358
0.1397
0.1448
0.1518
0.1587
0.1655
0.1721
0.1776
LDT3
4
0.0517
0.0517
0.0517
0.0517
0.0517
0.0517
0.0517
0.0517
0.0517
0.0517
0.0517
0.0517
0.0520
0.0520
0.0524
0.0533
0.0548
0.0565
0.0566
0.0560
0.0555
0.0551
0.0546
0.0542
0.0547
LDT4
5
0.0237
0.0237
0.0237
0.0237
0.0238
0.0238
0.0238
0.0238
0.0238
0.0238
0.0238
0.0238
0.0239
0.0239
0.0241
0.0245
0.0252
0.0260
0.0260
0.0257
0.0255
0.0253
0.0251
0.0249
0.0252
HDV2B
6
0.0238
0.0238
0.0238
0.0238
0.0238
0.0238
0.0238
0.0238
0.0238
0.0238
0.0238
0.0251
0.0270
0.0290
0.0302
0.0309
0.0317
0.0325
0.0332
0.0336
0.0342
0.0348
0.0354
0.0358
0.0362
HDV3
7
0.0021
0.0021
0.0021
0.0021
0.0021
0.0021
0.0021
0.0021
0.0021
0.0021
0.0021
0.0023
0.0025
0.0027
0.0029
0.0031
0.0032
0.0033
0.0034
0.0035
0.0036
0.0036
0.0037
0.0037
0.0037
HDV4
8
0.0013
0.0013
0.0013
0.0013
0.0013
0.0013
0.0013
0.0013
0.0013
0.0013
0.0013
0.0014
0.0015
0.0016
0.0017
0.0017
0.0018
0.0019
0.0020
0.0021
0.0022
0.0023
0.0024
0.0025
0.0025
HDV5
9
0.0012
0.0012
0.0012
0.0012
0.0012
0.0012
0.0012
0.0012
0.0012
0.0012
0.0012
0.0012
0.0013
0.0014
0.0015
0.0015
0.0015
0.0016
0.0016
0.0017
0.0017
0.0018
0.0018
0.0019
0.0019
HDV6
10
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0039
0.0039
0.0040
0.0042
0.0045
0.0049
0.0053
0.0056
0.0058
0.0060
0.0062
0.0064
0.0066
0.0068
0.0070
0.0072
0.0073
0.0075
HDV7
11
0.0042
0.0042
0.0042
0.0042
0.0042
0.0042
0.0043
0.0044
0.0046
0.0048
0.0050
0.0054
0.0059
0.0064
0.0068
0.0070
0.0074
0.0076
0.0079
0.0081
0.0083
0.0085
0.0087
0.0089
0.0090
HDV8A
12
0.0069
0.0069
0.0069
0.0069
0.0070
0.0070
0.0070
0.0070
0.0070
0.0070
0.0070
0.0073
0.0078
0.0083
0.0086
0.0088
0.0090
0.0092
0.0094
0.0095
0.0097
0.0098
0.0100
0.0101
0.0102
HDV8B
13
0.0252
0.0252
0.0252
0.0252
0.0252
0.0252
0.0252
0.0252
0.0252
0.0252
0.0252
0.0262
0.0279
0.0296
0.0307
0.0313
0.0321
0.0329
0.0337
0.0341
0.0346
0.0350
0.0355
0.0360
0.0364
MDBS
14
0.0012
0.0012
0.0012
0.0012
0.0012
0.0012
0.0012
0.0013
0.0012
0.0013
0.0012
0.0013
0.0014
0.0015
0.0015
0.0015
0.0016
0.0016
0.0017
0.0017
0.0017
0.0017
0.0018
0.0018
0.0018
HDBT
15
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0007
0.0007
0.0007
0.0007
0.0008
0.0008
0.0008
0.0008
0.0008
0.0008
0.0008
0.0009
0.0009
MC
16
0.0092
0.0092
0.0092
0.0092
0.0092
0.0092
0.0092
0.0092
0.0092
0.0092
0.0092
0.0091
0.0088
0.0085
0.0082
0.0079
0.0077
0.0075
0.0073
0.0072
0.0071
0.0070
0.0070
0.0069
0.0068
Table 4.1.2
26
-------
National Average Vehicle Miles Traveled Fractions By Vehicle Class
Using MOBILE6.2
(Part 2)
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020-
2050
0.5569
0.5360
0.5153
0.4953
0.4785
0.4646
0.4507
0.4365
0.4231
0.4096
0.3952
0.3807
0.3669
0.3544
0.3428
0.3325
0.3231
0.3145
0.3071
0.3004
0.2944
0.2892
0.2846
0.2793
0.0557
0.0590
0.0622
0.0655
0.0683
0.0706
0.0729
0.0752
0.0774
0.0797
0.0822
0.0846
0.0869
0.0891
0.0911
0.0928
0.0944
0.0959
0.0971
0.0982
0.0992
0.1001
0.1008
0.1017
0.1853
0.1963
0.2071
0.2179
0.2273
0.2349
0.2425
0.2503
0.2577
0.2654
0.2735
0.2817
0.2894
0.2965
0.3031
0.3090
0.3143
0.3191
0.3233
0.3270
0.3304
0.3332
0.3357
0.3384
0.0571
0.0605
0.0638
0.0672
0.0700
0.0724
0.0748
0.0771
0.0794
0.0818
0.0843
0.0868
0.0892
0.0914
0.0934
0.0952
0.0969
0.0983
0.0996
0.1008
0.1018
0.1027
0.1035
0.1043
0.0263
0.0278
0.0294
0.0309
0.0322
0.0333
0.0344
0.0355
0.0365
0.0376
0.0388
0.0399
0.0410
0.0420
0.0430
0.0438
0.0445
0.0452
0.0458
0.0463
0.0468
0.0472
0.0476
0.0480
0.0367
0.0372
0.0377
0.0380
0.0381
0.0382
0.0384
0.0386
0.0387
0.0387
0.0387
0.0388
0.0389
0.0390
0.0390
0.0390
0.0390
0.0391
0.0391
0.0392
0.0392
0.0393
0.0394
0.0396
0.0037
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0038
0.0039
0.0039
0.0039
0.0039
0.0039
0.0039
0.0026
0.0027
0.0028
0.0029
0.0029
0.0030
0.0030
0.0030
0.0031
0.0031
0.0031
0.0031
0.0032
0.0032
0.0032
0.0032
0.0032
0.0032
0.0032
0.0033
0.0033
0.0033
0.0033
0.0033
0.0020
0.0021
0.0021
0.0022
0.0022
0.0022
0.0023
0.0023
0.0023
0.0023
0.0023
0.0024
0.0024
0.0024
0.0024
0.0024
0.0024
0.0024
0.0024
0.0024
0.0024
0.0024
0.0025
0.0025
0.0077
0.0079
0.0081
0.0082
0.0083
0.0084
0.0085
0.0085
0.0086
0.0086
0.0086
0.0087
0.0087
0.0087
0.0087
0.0087
0.0087
0.0088
0.0088
0.0088
0.0088
0.0088
0.0088
0.0089
0.0092
0.0095
0.0097
0.0098
0.0099
0.0100
0.0100
0.0101
0.0102
0.0102
0.0102
0.0102
0.0103
0.0103
0.0103
0.0103
0.0103
0.0103
0.0104
0.0104
0.0104
0.0104
0.0104
0.0105
0.0104
0.0106
0.0107
0.0108
0.0109
0.0109
0.0110
0.0111
0.0111
0.0111
0.0111
0.0111
0.0112
0.0112
0.0112
0.0112
0.0112
0.0112
0.0112
0.0112
0.0113
0.0113
0.0113
0.0114
0.0370
0.0376
0.0382
0.0386
0.0388
0.0390
0.0392
0.0394
0.0395
0.0396
0.0396
0.0397
0.0398
0.0399
0.0398
0.0399
0.0399
0.0400
0.0400
0.0400
0.0401
0.0402
0.0403
0.0405
0.0018
0.0019
0.0019
0.0019
0.0019
0.0019
0.0019
0.0019
0.0020
0.0020
0.0020
0.0020
0.0020
0.0020
0.0020
0.0020
0.0020
0.0020
0.0020
0.0020
0.0020
0.0020
0.0020
0.0020
0.0009
0.0009
0.0009
0.0009
0.0009
0.0009
0.0009
0.0009
0.0009
0.0009
0.0009
0.0009
0.0010
0.0010
0.0010
0.0010
0.0010
0.0010
0.0010
0.0010
0.0010
0.0010
0.0010
0.0010
0.0067
0.0065
0.0064
0.0062
0.0061
0.0060
0.0059
0.0058
0.0057
0.0056
0.0056
0.0055
0.0054
0.0054
0.0053
0.0053
0.0053
0.0052
0.0052
0.0052
0.0051
0.0051
0.0051
0.0051
27
-------
4.2 Vehicle Miles Traveled Fraction by Highway Functional System
4.2.1 Guidance
The fraction of vehicle miles traveled (VMT) by highway functional system (also called
"roadway type" or "facility type") varies from area to area and can have a significant effect on
overall emissions from highway mobile sources. For SIP-related highway vehicle emission
inventory development in moderate and above non-attainment areas, EPA expects states to
develop and use their own specific estimates of VMT by highway functional system.
The VMT fractions by highway functional system used in inventory modeling should be
consistent, where possible, with assumptions used in other highway vehicle related planning.
Metropolitan Planning Organizations (MPOs) and state Departments of Transportation (DOTs)
should be consulted for information to determine the proportion of vehicle VMT by time of day
and facility class.
4.2.2 Description
Often different groups will use different terms to describe the same highway functional system
(or roadway type or facility). As a result, states should take care in determining the proper
allocation of VMT among the driving cycles available in MOBILE6.2. The release of
MOBILE6.2 is not intended to redefine roadway types or to change any reporting requirements
of EPA or other organizations. MOBILE6.2, however, does estimate different emission levels
for a limited range of driving cycles. These differences must be accounted for in SIP-related
highway vehicle emission inventory development.
There are four sets of driving cycles that are modeled separately in MOBILE6.2. States may
choose to run each driving cycle set separately and combine the results outside the MOBILE6.2
model, or they may use the ability of MOBILE6.2 to combine the results into a single emission
rate. In either case, states will need to determine which of the four driving cycles available in
MOBILE6.2 will be used to model each portion of the total VMT. The four driving cycle sets
available in MOBILE6.2 are (using the MOBILE6.2 terminology):
Freeway Driving Cycles
Arterial/collector Driving Cycles
Local roadway Driving Cycle
Freeway ramp Driving Cycle
These driving cycles are intended to include all VMT by all highway motor vehicles. Each
driving cycle set implies different assumptions about vehicle activity and different emission
estimates in MOBILE6.2.
28
-------
Freeway Driving Cycles
In MOBILE6.2, "freeway" VMT refers to driving that occurs on roadways that do not have
traffic signals, that usually have limited access (via converging ramps) and have free flow speeds
greater than 50 miles per hour. These roadways are usually divided and have more than one lane
in each direction. This definition does not include short sections (less than two miles) of a
roadway between signals, but could include longer roadway segments that effectively act as
freeways. Driving on roadways of this sort normally include no vehicle stops except in the most
extreme congested conditions. VMT associated with freeway-type roadways are best modeled in
MOBILE6.2 using the freeway set of driving cycles.
The freeway driving cycles in MOBILE6.2 do not include the special VMT that occurs on
freeway ramps. Driving activity that occurs on freeway ramps is modeled separately as another
separate driving cycle. Determining what portion of overall freeway VMT is considered freeway
ramp VMT for MOBILE6.2 is discussed later in this section.
Arterial/Collector Driving Cycles
In MOBILE6.2, "arterial/collector" VMT refers to driving that occurs on roadways that have
signalized traffic control. These roadways are not freeways, because they have traffic signals,
but they may be divided, multiple lanes, one-way and have high free flow speeds. However,
traffic will be stopped periodically by traffic signals and will be further affected by access to the
roadway by driveways and un-signalized intersections. Even in free flow, the driving on
arterial/collector roadways will be characterized by cruising periods interrupted by traffic
signals. VMT associated with arterial/collector-type roadways are best modeled in MOBILE6.2
using the arterial/collector set of driving cycles.
Standard USDOT definitions of arterial and collector roadways may include some roadways
which are more characteristic of freeways for purposes of MOBILE6.2 emission modeling.
Areas should consider if some VMT assigned to arterial and collector roadways would be better
modeled as freeways in MOBILE6.2.
Local Roadway Driving Cycle
In MOBILE6.2, "local roadway" VMT refers to driving on roadways which are not normally
considered as part of the traffic network. These roadways do not have traffic lights and rarely
have more than one lane in each direction. They usually allow vehicle parking on the roadway
surface and traffic control is handled via stop/yield signs. Speed limits are normally 30 miles per
hour or less. The driving cycle used in MOBILE6.2 to model local roadways is fixed at an
average speed of 12.9 miles per hour. Driving on local roadways is characterized by extremely
low speeds and frequent stops at intersections. Most local roadways will likely be in residential
areas. VMT associated with local-type roadways are best modeled in MOBILE6.2 using the
local roadway driving cycle.
29
-------
Note that the MOBILE6.2 driving cycle used for local roadways may differ from the range of
activity on the roadways defined as local by USDOT Publication No. FHWA-ED-90-006 revised
March 1989 (Highway Functional Classification) for both rural and urban local VMT. The local
roadway driving cycle used in MOBILE6.2 likely constitutes only a small subset of urban local
VMT as defined by FHWA. When in doubt, EPA recommends that any roadway VMT that does
not clearly match the MOBILE6.2 driving cycle for local roadways be included with
MOBILE6.2 arterial/collector VMT rather than included with MOBILE6.2 local roadway VMT.
Freeway Ramp
In MOBILE6.2, "freeway ramp" VMT refer to the activity on access roadways for freeways. It
includes both traffic entering the freeway and exiting. Driving on freeway ramps is
characterized by rapid acceleration from stop or low speeds to freeway speeds and decelerations
from freeway speeds to low speeds or stop.
Freeway ramp activity is not included in the MOBILE6.2 the freeway driving cycle set.
Therefore, all "freeway" VMT in MOBILE6.2 must include a corresponding amount of freeway
ramp VMT in MOBILE6.2 to account for acceleration and deceleration to and from freeway
speeds. VMT associated with freeway ramp-type activity are best modeled in MOBILE6.2 using
the freeway ramp driving cycle.
Most areas do not explicitly account for freeway ramp VMT separately. If this is the case, then
the default fraction used in MOBILE6.2 should be used. MOBILE6.2 models freeway ramp
VMT based on the assumption that freeway ramps are 8% of all VMT assigned to both freeways
and freeway ramps. MOBILE6.2 models all freeway ramps at a fixed average speed of 34.6
miles per hour. If the freeway ramp VMT is accounted for in other driving cycle sets (i.e.,
collector roadways), then the VMT in those roadways must be reduced by the amount of VMT
assigned to the freeway and freeway ramp combination. In the following parts of this section, it
is assumed that all freeway ramp VMT is originally assigned only to freeway groupings.
The emission estimates based on the freeway ramp driving cycle do not include the effects of
roadway grade on emissions. This means that any driving activity that involves rapid
acceleration from stop or low speeds to freeway speeds and decelerations from freeway speeds to
low speeds or stop can be considered as ramp activity for purposes of emission modeling, since it
does not matter whether such accelerations and decelerations occur on graded ramps or on flat
roadways themselves. Any roadways with freeway-like speeds (greater than 50 miles per hour),
even if they do not have limited access, will have accelerations and decelerations that could be
considered as ramp activity.
For purposes of emission modeling, VMT assigned to classes of roadways used by states,
regardless of the terminology used, should be carefully matched to the driving cycle sets
available in MOBILE6.2 which best matches the characteristics of the roadway links in that
class.
30
-------
4.2.3 Method
It was not necessary for previous versions of the MOBILE model to differentiate between
roadway classes. By design, these earlier models were not intended to evaluate the differences
between the vehicle activity on different roadway types. MOBILE6.2 was designed to account
for differences in vehicle activity on different roadway types and allow for more precise
modeling of emissions for individual roadway links.
At a minimum, the Highway Performance Monitoring System (HPMS) provides vehicle count
data by roadway type representative of urban areas in each state ("Highway Performance
Monitoring System Field Manual," December 2000, OMB No. 21250028, Office of Highway
Policy Information, Federal Highway Administration). However, Metropolitan Planning
Organizations (MPOs) and state Departments of Transportation (DOTs) should be consulted for
information to determine the proportion of vehicle VMT by time of day and facility class. The
VMT fractions by highway functional system used in inventory modeling should be consistent,
where possible, with assumptions used in other highway vehicle related planning. The report,
"Guidance for the Development of Facility Type VMT and Speed Distributions," Systems
Applications International, Inc., September 1998 (EPA 420-P-99-004, M6.SPD.004), provides
some discussion of obtaining and using data for this purpose.
MOBILE6.2 does not require the creation of new roadway classifications or change the
definition of existing roadway classifications. States will be expected to carefully match the
roadway type classifications that they use with the driving cycle sets available in MOBILE6.2.
The descriptions of the MOBILE6.2 driving cycle sets are discussed in Section 4.2.2, above.
The U.S. Department of Transportation Federal Highway Administration (FHWA) has a set of
standard roadway (highway functional system) groupings. These 12 categories are used for
many of FHWA reporting requirements and state DOT's usually report to US DOT in these
categories. However, state air agencies, and DOT's that have to do air inventories, may develop
other categorizations that serve their needs or may have different terminology for these same
classifications.
If we consider MOBILE6.2 driving cycles used for freeways and freeway ramps as a single
driving cycle set, the resulting three classifications can be matched in general to each of the 12
types used by FHWA, described in the following table:
-------
Table 4.2.1
Matching FHWA Roadways to MOBILE6.2
FHWA Highway Functional System
rural interstate
rural other principle arterial
rural minor arterial
rural major collector
rural minor collector
rural local
urban interstate
urban other freeways
urban other principle arterial
urban minor arterial
urban collector
urban local
MOBILE6.2 Driving Cycle
Set
freeway and freeway ramp
freeway and freeway ramp
arterial/collector
arterial/collector
arterial/collector
arterial/collector
freeway and freeway ramp
freeway and freeway ramp
arterial/collector
arterial/collector
arterial/collector
local roadway
By default in MOBILE6.2, 8% of VMT in any freeway and freeway ramp category will be the
freeway ramp VMT. For example, if the urban interstate category has a VMT fraction of 10%, a
MOBILE6.2 VMT for this roadway grouping would be 0.8% (8% of the 10%) freeway ramp and
9.2% (10%-0.8%) freeway without ramps. If the VMT for freeway ramps is accounted for
explicitly in additional highway functional systems provided by the state, then this
approximation method is not needed.
This mapping of FHWA standard roadway groupings to MOBILE6.2 driving cycle sets is
presented as an example. Users should critically consider the actual roadway segments that are
classified into the FHWA system and compare their characteristics with the descriptions of the
MOBILE6.2 driving cycle sets in Section 4.2.2 above before making a final determination of the
VMT distribution by driving cycle set in MOBILE6.2. The criteria used for mapping of any
roadway system to the MOBILE6.2 driving cycle sets must be clearly documented.
If results for each of the driving cycle sets are needed separately, then the database output option
of MOBILE6.2 can be used to determine the results for each of the four driving cycle sets
available in MOBILE6.2 from a single MOBILE6.2 run. A separate MOBILE6.2 run for each
driving cycle set can also be used. This approach may be necessary if a more detailed breakout
of roadways (i.e., the FHWA classifications) is needed. This can be done by assigning all VMT
(100%) to the driving cycle set in MOBILE6.2 corresponding to the roadway grouping. For
freeways, which include freeway ramps, a fraction of freeway VMT without ramps and the
freeway ramp fraction must both be entered. However, the sum of both fractions (freeway and
freeway ramp) must equal 100%.
If only an overall result is needed, then the VMT assigned to each MOBILE6.2 driving cycle set
can be divided by the total VMT to give a fraction for each of the four MOBILE6.2 driving cycle
32
-------
sets. These four values must equal 100%. Then a single MOBILE6.2 run, including a VMT
distribution for all driving cycle sets, can be done to give the overall result.
Hourly VMT by Highway Functional System
The data obtained from Metropolitan Planning Organizations (MPOs) and state Departments of
Transportation (DOTs) may be available on an hourly, or at least peak/off peak basis.
MOBILE6.2 requires an estimate for each hour of the day. If hourly results are not available,
states must distribute the known daily or peak/off peak results to the individual hours of the day.
If no information is available regarding the temporal distribution of vehicle travel, EPA should
be consulted before using national average values.
The default national average VMT distribution by MOBILE6.2 driving cycle set (roadway) is
different for each hour of the day. The derivation of these default values is described in the
report, "Development of Methodology for Estimating VMT Weighting by Facility Type,"
Systems Applications International, Inc., September 1998 (EPA 420-P-99-006, M6.SPD.003).
The results are shown in the table below:
Table 4.2.2
MOBILE6.2 Default Distribution of VMT
By Driving Cycle Set in Each Hour
Hour
1
2
o
J
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Description
6 a.m. through 6:59 a.m.
7 a.m. through 7:59 a.m.
8 a.m. through 8:59 a.m.
9 a.m. through 9:59 a.m.
10 a.m. through 10:59 a.m.
11 a.m. through 11:59 a.m.
12 Noon through 12:59 p.m.
1 p.m. through 1:59 p.m.
2 p.m. through 2:59 p.m.
3 p.m. through 3:59 p.m.
4 p.m. through 4:59 p.m.
5 p.m. through 5:59 p.m.
6 p.m. through 6:59 p.m.
7 p.m. through 7:59 p.m.
8 p.m. through 8:59 p.m.
9 p.m. through 9:59 p.m.
10 p.m. through 10:59 p.m.
11 p.m. through 11:59 p.m.
12 Midnight through 12:59 a.m.
1 a.m. through 1:59 a.m.
2 a.m. through 2:59 a.m.
3 a.m. through 3:59 a.m.
4 a.m. through 4:59 a.m.
5 a.m. through 5:59 a.m.
Distribution by Driving Cycle Set in Each Hour
Non-Ramp
Freeway
0.427
0.443
0.457
0.461
0.453
0.418
0.392
0.344
0.338
0.349
0.346
0.333
0.324
0.334
0.334
0.320
0.330
0.312
0.295
0.310
0.329
0.343
0.381
0.405
Arterial/
Collector
0.418
0.403
0.394
0.392
0.400
0.434
0.457
0.497
0.497
0.492
0.497
0.509
0.516
0.506
0.506
0.518
0.506
0.521
0.538
0.527
0.510
0.497
0.460
0.437
Local
Roadway
0.118
0.115
0.109
0.107
0.108
0.112
0.117
0.129
0.135
0.129
0.127
0.129
0.132
0.131
0.131
0.134
0.135
0.140
0.141
0.136
0.133
0.130
0.126
0.123
Freeway
Ramp
0.037
0.039
0.040
0.040
0.039
0.036
0.034
0.030
0.030
0.030
0.030
0.029
0.028
0.029
0.029
0.028
0.029
0.027
0.026
0.027
0.028
0.030
0.033
0.035
All
Roadways
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
33
-------
If hourly values are not available, but the full daily VMT distribution by the MOBILE6.2 driving
cycle set is known, the known distribution should be used for every hour of the day. For
example, we might assume that it is known that on a daily basis that:
40% of VMT occurs on non-ramp freeways
40% of VMT occurs on arterial/collector roadways
15% of VMT occurs on local roadways
5% of VMT occurs on freeway ramps
In this case, every hour of the day would use the same distribution of VMT by MOBILE6.2
driving cycle set. Similarly, if the full VMT distribution by the MOBILE6.2 driving cycle set is
known for certain periods of the day, such as peak and off-peak, then those same distributions
should be used for each hour of the day within the time periods.
In the case where the proportions of VMT assigned to arterial/collector and local roadway
driving cycle sets is known, however, the proportion of total freeway VMT which is freeway
ramps is not known, the MOBILE6.2 default value of 8% may be used to determine the freeway
ramp VMT proportion. For example, if the freeway category (including freeway ramps) has a
VMT fraction of 40%, a MOBILE6.2 VMT for this roadway grouping would be 3.2% (8% of the
40%) freeway ramp and 36.8% (40%-3.2%) freeway without ramps. Once the full distribution is
known, the same distribution would be used for all hours of the day (for daily values) or within
the time period (for peak/off-peak values).
If hourly values are not available and the proportions of VMT assigned to arterial/collector and
local roadway driving cycle sets is not known, the daily VMT values for each MOBILE6.2
driving cycle set can be distributed in each hour using the proportions in Table 4.2.2 above.
Similarly, peak and off peak averages can be redistributed to the corresponding individual hours
using the proportions in those hours shown in the table. For example, it is known that 40% of
the VMT occurs on freeways (including ramps) and the remaining 60% of the VMT occurs on
non-freeways on a daily basis. Between 6:00 and 6:59 a.m., the Table 4.2.2 shows that 41.8% of
total VMT occurs on arterial/collector roadways and 11.8% of total VMT occurs on local
roadways. These values can be used to distribute the known proportion of VMT which does not
occur on freeways (60%):
Arterial/Collectors : 60% * (0.418 / (0.418 + 0.118)) = 46.8%
Local Roadways : 60% * (0.118 / (0.418 + 0.118)) = 13.2%
The sum of the two fractions must still equal 60%. This calculation is repeated for each hour of
the day. The known proportion of total freeway VMT is divided into freeway ramp and non-
ramp freeway values using the MOBILE6.2 default value of 8% as described above. Similarly,
cases where there are peak/off-peak estimates, the proportions can be determined within each
time period.
34
-------
If the distribution of VMT by vehicle class in each hour of the day is known, but the proportion
of freeway ramp VMT is not known and/or the proportions of arterial/collector and local
roadways is not known, then, the appropriate distribution of total VMT in each of the four
driving cycle sets needed for input to MOBILE6.2 can be determined using the methods
described above for each individual hour.
After the VMT for assigned to each MOBILE6.2 driving cycle set has been determined, the four
values must sum to one in each hour of the day.
VMT by Vehicle Class and Highway Functional System
MOBILE6.2 does not have different values for the VMT by driving cycle set (roadway) for each
vehicle class. The same VMT by driving cycle set may be used for every vehicle class.
However, MOBILE6.2 will allow the use of different estimates for the VMT by driving cycle set
for each vehicle class if the data is available. The methods described above can be used for each
vehicle class (or for groupings of vehicle classes) independently. Separate VMT by driving
cycle set for each vehicle class is preferred. This will only be possible if the information
regarding VMT on each roadway class is split by vehicle class as well.
4.2.4 MOBILE6.2 Commands
The VMT by driving cycle set is entered into MOBILE6.2 using the VMT BY FACILITY
command. This command requires that the user supply the 2,688 values which represent the
fraction of all VMT accumulated by all highway motor vehicles in each hour of the day (24
hours) which is accumulated on each of the four driving cycle sets by each of the 28 vehicle
classes. MOBILE6.2 does not require that values for all vehicle classes be entered, so that only
the vehicle classes that need to be changed must be entered. All values must be between zero
and one, and the four values in each hour for each vehicle class must sum to one. See Section
2.8.8.2a of the MOBILE6.2 User Guide for more information on how to use this command.
This command is closely related to the VMT BY HOUR and SPEED VMT commands, which
are used to provide the temporal distribution of VMT and the average speed distributions for
each driving cycle set. Determination of VMT by hour and average speeds are discussed in
Sections 4.3 and 4.4 below.
4.3 Vehicle Miles Traveled Fraction by Hour of the Day
4.3.1 Guidance
The fraction of vehicle miles traveled (VMT) by hour of the day varies from area to area, but for
most purposes, this variation has a negligible effect on overall daily emissions from highway
mobile sources. States may choose to use the default values for VMT by hour of the day instead
of developing local values. However, results from photochemical models are sensitive to
differences in the estimated inventory by time of day. For SIP-related highway vehicle emission
35
-------
inventory development in moderate and above non-attainment areas, where photochemical
modeling is needed, EPA encourages states to develop and use their own specific estimates of
VMT by hour of the day.
The VMT fractions by hour used in inventory modeling should be consistent, where possible,
with assumptions used in other highway vehicle related planning. Metropolitan Planning
Organizations (MPOs) and state Departments of Transportation (DOTs) should be consulted for
information to determine the proportion of vehicle VMT by time of day.
4.3.2 Description
The amount of vehicle travel varies significantly during the day. MOBILE6.2 has a national
average default distribution for the daily VMT. States may choose to use the default values for
VMT by hour of the day instead of developing local values. The distribution is simply the
fraction of all daily VMT by all vehicle classes which occurs in each hour of the day.
4.3.3 Method
Development of an estimate for the VMT by hour of day is optional. However, results from
photochemical models are sensitive to differences in the estimated inventory by time of day.
The report, "Guidance for the Development of Facility Type VMT and Speed Distributions."
Systems Applications International. Inc.. September 1998 (EPA 420-P-99-004. M6.SPD.0041
provides some discussion of obtaining and using data for this purpose. The VMT fractions by
hour of the day used in inventory modeling should be consistent, where possible, with
assumptions used in other highway vehicle related planning.
If available data on the temporal distribution of VMT is not hourly specific (i.e., peak and off
peak), the data may be disaggregated into hourly estimates using the MOBILE6.2 default values.
These values are shown in Table 4.3.1 below.
36
-------
Table 4.3.1
MOBILE6.2 Default Distribution of VMT
By Hour of Day
Hour
1
2
O
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Description
6 a.m. through 6:59 a.m.
7 a.m. through 7:59 a.m.
8 a.m. through 8:59 a.m.
9 a.m. through 9:59 a.m.
10 a.m. through 10:59 a.m.
11 a.m. through 11:59 a.m.
12 Noon through 12:59 p.m.
1 p.m. through 1:59 p.m.
2 p.m. through 2:59 p.m.
3 p.m. through 3:59 p.m.
4 p.m. through 4:59 p.m.
5 p.m. through 5:59 p.m.
6 p.m. through 6:59 p.m.
7 p.m. through 7:59 p.m.
8 p.m. through 8:59 p.m.
9 p.m. through 9:59 p.m.
10 p.m. through 10:59 p.m.
11 p.m. through 11:59 p.m.
12 Midnight through 12:59 a.m.
1 a.m. through 1:59 a.m.
2 a.m. through 2:59 a.m.
3 a.m. through 3:59 a.m.
4 a.m. through 4:59 a.m.
5 a.m. through 5:59 a.m.
Total
Fraction of Total VMT
0.0569
0.0740
0.0656
0.0555
0.0540
0.0582
0.0608
0.0571
0.0598
0.0636
0.0777
0.0730
0.0501
0.0389
0.0308
0.0264
0.0194
0.0144
0.0108
0.0086
0.0081
0.0080
0.0098
0.0186
1.0000
Peak and off peak averages can be redistributed to individual hours using the proportions shown
in the table. For example, if 30% of the VMT occurs between 7 a.m and 9:59 a.m., then using
the default fractions the distribution of the 30% would be:
7 a.m. - 7:59 a.m. : 30% * (0.0740 / (0.0740 + 0.0656 + 0.0555)) = 11.4%
8 a.m. - 8:59 a.m. : 30% * (0.0656 / (0.0740 + 0.0656 + 0.0555)) = 10.1%
9 a.m. - 9:59 a.m. : 30% * (0.0555 / (0.0740 + 0.0656 + 0.0555)) = 8.5%
Similarly, the off peak values can be distributed. The sum of all 24 values must still add to one
(100%).
37
-------
4.3.4 MOBILE6.2 Commands
The VMT by hour is entered into MOBILE6.2 using the VMT BY HOUR command. This
command requires that the user supply 24 values which represent the fraction of all VMT
accumulated by all highway motor vehicles in each hour of the day (24 hours). All values must
be between zero and one, and the 24 values must sum to one. See Section 2.8.8.2b of the
MOBILE6.2 User Guide for more information on how to use this command.
This command is closely related to the VMT BY FACILITY and SPEED VMT commands,
which is used to provide the driving cycle set distribution of VMT and the average speed
distributions for each driving cycle set. Determination of VMT by driving cycle set and average
speeds is discussed in Sections 4.2 and 4.4, respectively.
4.4 Vehicle Miles Traveled Fraction by Average Speed
4.4.1 Guidance
The transient behavior of vehicles as they travel has a significant effect on vehicle emissions.
This effect is captured in the MOBILE6.2 model using average speed (for particular roadway
types) as a surrogate for the transient driving behavior. For this reason, the distribution of
vehicle miles traveled (VMT) by average speed has a significant effect on overall daily
emissions from highway mobile sources. For SIP and conformity related highway vehicle
emission inventory development in moderate and above non-attainment areas, EPA expects
states to develop and use their own specific estimates of VMT by average speed.
Results from photochemical models are sensitive to differences in the estimated inventory by
time of day. For SIP related highway vehicle emission inventory development in moderate and
above non-attainment areas, where photochemical modeling is needed, EPA encourages states to
develop and use their own specific estimates of VMT by average speed by hour of the day.
However, hourly estimates are not required.
The VMT fractions by average speed used in inventory modeling should be consistent, where
possible, with assumptions used in other highway vehicle related planning. Metropolitan
Planning Organizations (MPOs) and state Departments of Transportation (DOTs) should be
consulted for information used to determine the proportion of vehicle VMT by average speed.
4.4.2 Description
Average speed, as defined for use in MOBILE6.2, is the distance traveled (in miles) divided by
the time (in hours). This is not the same as the instantaneous velocity of vehicles or the nominal
speed limit on the roadway link. The MOBILE6.2 definition of speed includes all operation of
vehicles including intersections and other obstacles to travel which may result in stopping and
idling. As a result, average speeds, as used in MOBILE6.2, will tend to be less than nominal
speed limits for individual roadway links.
38
-------
Selection of vehicle speeds is a difficult and complex process. Although it is appropriate for
some purposes to use an average speed for all vehicle trips and vehicle types within urban areas
as a whole, such an approach is not suitable for SIP inventory preparation. Instead, VMT
fractions should be estimated for similar roadway segments that have roughly equal average
speeds. At a minimum, speeds should be estimated separately by roadway functional class.
MOBILE6.2 has been designed to allow the entry of VMT associated with roadways with
roughly equal average speeds separated by roadway functional class and, by default, will
aggregate the emission results into an area wide emission estimate. However, MOBILE6.2 can
also be used to generate emission estimate separately by average speed and roadway type (i.e.,
emissions by roadway link), if post-processing of the emission estimates outside of MOBILE6.2
is preferred.
4.4.3 Method
The recommended approach to estimating average speeds is to post-process the output from a
local travel demand network model. Travel demand models that do not meet the performance
and validation requirements for use in forecasting VMT growth may nevertheless be suitable for
deriving speed estimates. However, reasonable efforts and success in validating the model are
still required. In most transportation models, speed is estimated primarily to allocate travel
across the roadway network. Speed is used as a measure of impedance to travel rather than as a
prediction of accurate travel times. For this reason, speed results from most travel demand
models must be adjusted to properly estimate actual average speeds.
Average speed estimates for calendar years other than the calendar year on which the average
speed estimates are based must be logically related to the current year methodology and
estimates, with no arbitrary or unsupported assumptions of changes in average speeds. Future
average speed estimates should account for the effect of growth in overall fleet VMT on roadway
congestion and average speeds.
Section 3.3, "Speed Estimation Methods," of the EPA document, "Volume IV: Chapter 2, Use of
Locality-Specific Transportation Data for the Development of Mobile Source Emission
Inventories," (September 1996), discusses the methods used to determine average speeds for
roadways. Additionally, the report, "Guidance for the Development of Facility Type VMT and
Speed Distributions," Systems Applications International, Inc., September 1998 (EPA 420-P-99-
004, M6.SPD.004), provides some discussion of obtaining and using data for this purpose. The
average speeds and VMT estimates used in inventory modeling should be consistent, where
possible, with assumptions used in other highway vehicle related planning.
MOBILE6.2 uses average speed distributions to represent the driving on all of the roadway links
within the area to be modeled for each hour of the day. The average speed distributions have
fourteen values which represent the fraction of all VMT which occurs at 2.5 miles per hour
(MPH), and 5 to 65 mph in 5 mph increments. Each of the fourteen values must be within the
range of zero through one and the sum of the fourteen values must equal one. There is a separate
39
-------
speed distribution for roadways classified as freeways and for roadways classified as
arterial/collectors. The definition of roadway classifications is discussed in Section 4.2 above.
Table 4.4.1
Definition of MOBILE6.2 Speed Bins
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Abbreviation
2.5 mph
5 mph
10 mph
15 mph
20 mph
25 mph
30 mph
35 mph
40 mph
45 mph
50 mph
55 mph
60 mph
65 mph
Description
Miles with average speed 0-2.5 mph
Miles with average speed 2.5-7.5 mph
Miles with average speed 7.5-12.5 mph
Miles with average speed 12.5-17.5 mph
Miles with average speed 17.5-22.5 mph
Miles with average speed 22.5-27.5 mph
Miles with average speed 27.5-32.5 mph
Miles with average speed 32.5-37.5 mph
Miles with average speed 37.5-42.5 mph
Miles with average speed 42.5-47.5 mph
Miles with average speed 47.5-52.5 mph
Miles with average speed 52.5-57.5 mph
Miles with average speed 57.5-62.5 mph
Miles with average speed >62.5 mph
Once the average speed is determined for each roadway segment in the area to be modeled, the
VMT for each segment can be assigned to one of the average speed bins described in the Table
4.4.1 above.
The average speed VMT distributions are constructed from the average speed estimates made for
each roadway segment. Knowing the traffic volume and segment length, the VMT (miles
traveled by vehicles on the link) can be calculated.
VMT = Roadway segment length (miles) * Roadway segment volume (number of vehicles)
The VMT from each roadway segment is then added to the appropriate MOBILE6.2 average
speed "bin" for the appropriate driving cycle set. Once the VMT for all of the roadway network
segments had been placed in bins, the VMT in each bin would be divided by the total VMT for
that driving cycle set (the sum of all of the bins) to give the fraction of VMT in that bin. The
sum of the fractions must be one.
MOBILE6.2 input will require a speed distribution for each hour of the day. If available average
speed data is not hourly specific (i.e., peak and off peak or daily average), the same values may
be used for multiple hours in order to make a complete set of hourly values. However, in
MOBILE6.2. average speed by hour accounts for the effects of congestion on emissions and
results from photochemical models are sensitive to differences in the estimated inventory by time
40
-------
of day. EPA encourages states to develop and use their own specific estimates of VMT by
average speed by hour of the day.
Freeway Average Speed
MOBILE6.2 requires a freeway average speed distribution which does not include VMT which
occurs on freeway ramps. If the calculation of average speed for freeways does not include the
effects of activity on freeway ramps, then no further adjustment is necessary. EPA recommends
that states separate the activity on freeway ramps from the remaining activity on freeways for
this reason. Users may, however, use the EPA default VMT fractions for freeway ramp activity
instead.
If the average speed distribution to be used for freeways includes freeway ramp activity, the
VMT associated with freeway ramps must be removed from the freeway average speed
distribution before it can be used in MOBILE6.2.
First, the fraction of all freeway VMT which occurs on ramps must be determined. If freeway
ramp VMT has been included in total freeway VMT, this may indicate that a separate estimate of
freeway ramp VMT is not available. Areas which cannot separate freeway ramp VMT from
non-ramp freeway travel may assign freeway ramp VMT using the default national average of
eight percent (8%) of total freeway VMT.
Using the default fraction of freeway ramp VMT and the default national average freeway ramp
speed of 34.6 mph, the average speed of non-ramp freeway travel can be calculated from the
total freeway average speed value for each freeway segment.
Since the average speed on freeway ramps is not a user input, the appropriate average speed for
non-ramp segments can be determined once the ramp VMT fractions (and thus, non-ramp VMT)
are known.
Non-Ramp Speed = (1-Ramp VMT fraction)/(l/Freeway Speed - Ramp VMT
fraction/Average Ramp Speed)
Because ramp speed is a constant (34.6 miles per hour), if the default ramp VMT fraction is used
(8 percent), this calculation simplifies to:
Non-Ramp Speed = 0.927(1 /Freeway Speed - 0.00231)
For example, if my average speed on a freeway segment (including ramps) is 40.0 mph, and I am
using the default ramp VMT fraction of 8 percent, the non-ramp freeway speed is:
Non-Ramp Speed = 0.927(1/40 - 0.00231) = 40.6 mph
41
-------
Freeway average speeds which are greater than 34.6 mph will tend to be increased and freeway
average speeds less than 34.6 mph will tend to be decreased using this method to remove the
influence of freeway ramps on average speed. Since the average speed for freeway ramps is
fixed at 34.6 mph and since the maximum allowed value for freeway average speeds is 65 mph,
the theoretical maximum combined non-ramp and ramp average speed in MOBILE6.2 is 60.73
mph, using the default ramp VMT fraction of 8 percent. All combined freeway speeds above the
theoretical level should have the same emission results as for 60.73 mph. For this reason,
average speeds above 60.73 mph should not be entered using the AVERAGE SPEED command
with the FREEWAY option when using MOBILE6.2.
After the average speed for each freeway segment has been adjusted to account for freeway
ramps, the VMT associated with each segment must be reduced by the ramp VMT fraction as
well. This VMT is then assigned to the freeway ramp category. See Section 4.2 above. Using
this method, the overall fraction of freeway ramp VMT (as a fraction of the combined freeway
and freeway ramp VMT) will be the same as the value (i.e., default 8% or the user supplied
value) used in this method.
Arterial/Collector Roadways
MOBILE6.2 has a separate roadway classification for the arterial/collector roadway functional
class with their own distribution of average speeds. No adjustment is needed to the average
speeds estimated for arterial/collector roadway links before they are used in MOBILE6.2.
Individual Roadway Estimates
MOBILE6.2 is designed to estimate emissions for regions, including a variety of roadway links.
If the user wishes to estimate emissions of an individual roadway link (or obtain separate results
for a set of individual roadway links for use in a table), a single average speed estimate for each
particular link or roadway segment must be used for each model run, instead of an average speed
distribution. Modeling of individual roadway segments in MOBILE6.2 should not use more than
one average speed bin in the speed distribution input, other than to interpolate between average
speed bins used in the model. MOBILE6.2 assumes a default speed distribution for each average
speed input and cannot be used to model the observed speed distribution of individual roadway
links. For modeling purposes, each roadway link has only a single average speed in any hour of
the day.
MOBILE6.2 has an "AVERAGE SPEED" command which is intended specifically to assist
users in modeling individual roadway links. This command can automatically make the
necessary interpolations to get specific average speed values and will automatically restrict
driving cycle sets to those intended by the user. EPA recommends that users consider using the
AVERAGE SPEED command when modeling individual roadway links for specific time periods
less than a day. Since daily emission estimates usually require that the average speed vary, the
AVERAGE SPEED command may not be appropriate for estimating daily emission rates for
individual roadway links.
42
-------
The results of individual roadway segments may be later combined outside the MOBILE6.2
model to generate area wide emission estimates, if desired. The average speeds used for
individual roadways must be consistent with values used to generate average speed distributions
used for related area wide estimates. If the average speeds and VMT estimates are consistent,
summing the emission results from individual roadway segments or running a single
MOBILE6.2 run using VMT distributions by average speed will result in the same overall
emission estimate.
Freeway Ramps and Local Roadways
Freeway ramps and local roadways do not have user inputs for average speeds. The default
national average speeds for freeway ramps and local roadways is 34.6 mph and 12.9 mph,
respectively. The local VMT weighted average speed for freeway ramps and local roadways
should resemble the national averages used in MOBILE6.2.
If the VMT for local roadways is a significant fraction of total VMT and the average speed on
those roadways does not resemble the default national average of 12.9 mph, it may be necessary
to model local roadways as arterial/collector roadways. However, users should first check to
assure that the local roadway links have been properly matched to MOBILE6.2 driving cycle
sets.
See Section 4.2 for a description of local roadways as they are used in MOBILE6.2. States
should consult with EPA before defining all local roadways as arterial/collector roadways for
modeling.
MOBILE6.2 cannot model freeway ramp conditions with average speeds significantly different
from the national average default speed of 34.6 mph. Users should use the MOBILE6.2 freeway
ramp emission estimates for any necessary freeway ramp modeling.
4.4.4 Idling Emission Rates
Idle emission rates are a special case for vehicle emission modeling. Idle emission rates are not
normally needed to model area wide emission inventories, since the MOBILE6.2 emission rates
already include vehicle idling in proportion to normal driving. However, in some special cases,
such as intersection modeling, it is necessary to estimate the emissions from idling explicitly.
MOBILE6.2 does not directly model idle emission rates except for PM idle emissions for heavy
duty diesel trucks as described later in this section. However, as with MOBILES, idling
emission rates (in grams per hour) are assumed to be the same as for driving at 2.5 mph.
MOBILE6.2 is run using an average speed distribution which assigns all VMT to the 2.5 mph
average speed bin and the arterial/collector driving cycle set. The resulting emission rate (in
grams per mile) is multiplied by the average speed (2.5 miles per hour) to give the idling
emission rate (in grams per hour).
Idling emission rate = Emissions at 2.5 mph * Average speed (2.5 mph)
43
-------
All other inputs should be selected to best model the idling situation to be modeled. These
conditions must be consistent with the values used for inventory modeling generally and for the
time of day specifically.
Idle emission rates, in some cases, may include effects from an engine start. However,
accounting for the effects of engine starts on exhaust emissions during vehicle operation (such as
at intersections) is very difficult. Since the effects of engine starts diminish rapidly with the time
since the engine start, all of the effects of an engine start on exhaust emissions will disappear
within about two minutes from the engine start time on most vehicles. In addition, most of the
effects of an engine start on exhaust emissions will occur in the first minute. For this reason,
EPA recommends that the calculation of idling emissions at intersections not include any effects
from engine starts.
Special modeling situations (such as parking lots) may require the modeling of the effects of
engine starts. In these cases, the engine start emissions must be modeled separately from idling
emission rates and can be converted from units of grams per hour to grams per engine start using
the database output option available in MOBILE6.2. In some cases, it may be necessary to
combine the contribution of engine start emissions from parking with vehicle operation
emissions in the evaluation of intersections near parking to properly estimate ambient emission
levels.
Special handling of idle emissions may also be required if temperatures go below 20 degrees
Fahrenheit. If significant idle emissions occur below 20 degrees Fahrenheit, States should
consult with their EPA regional offices.
MOBILE6.2 provides an option for producing PM idle emissions for HD trucks in its output.
Instructions for this can be found in section 2.8.4.1.fin the MOBILE6.2.2 User's Guide. This
option is available for PM only, and should be used if separate PM idle emission factors are
needed for inventory analysis. It is important to note that the idle emission factors derived using
the methods described here are only applicable to short-duration idling. If guidance is needed to
estimate emission reductions associated with reductions in long-duration idling the user should
contact EPA.
4.4.5 MOBILE6.2 Commands
The distribution of VMT by average speed is entered into MOBILE6.2 using the SPEED VMT
command. This command requires that the user supply 14 values which represent the fraction of
all VMT accumulated by all highway motor vehicles in each average speed bin for each hour of
the day (24 hours) for freeways and arterial/collector roadways, a total of 672 values. All values
must be between zero and one, and the 14 values for each hour and driving cycle set must sum to
one. See Section 2.8.8.2c of the MOBILE6.2 User Guide for more information on how to use
this command. This command is closely related to the VMT BY FACILITY command, which is
44
-------
used to provide the distribution of VMT by driving cycle set. Determination of VMT by driving
cycle set is discussed in Sections 4.2 above.
The AVERAGE SPEED command is used to enter a single average speed, instead of an average
speed distribution. This command is discussed in Section 2.8.8.2d of the MOBILE6.2 User
Guide. Average speed for a single roadway segment can also be entered using the SPEED VMT
command by determining the weighting necessary for two adjacent average speed bins available
in the MOBILE6.2 average speed bin distribution and restricting the results to a single driving
cycle set. This method allows a different average speed to be used for each hour of the day. The
method for determining the weighting factors and restricting the driving cycle set is described in
Chapter 5 of the MOBILE6.2 User Guide.
Idling emission rates are best calculated using the AVERAGE SPEED command for the
arterial/collector driving cycle set with the average speed set to 2.5 miles per hour. Only the
"running" exhaust emission results (not including engine start emissions) are used to calculate
the idle emission rates. These are obtained using the EXPAND EXHAUST command or the
database output option.
4.5 Vehicle Engine Starts Per Day
4.5.1 Guidance
The number of vehicle engine starts per day will vary from area to area, but for most purposes,
EPA will not expect States to develop local estimates. States may choose to use the default
values for engine starts per day instead of developing local values.
Small scale and "hot spot" analysis of specific locations may require specific local estimates be
used. EPA should be consulted when planning small scale analysis to determine if the use of
national average default values is still appropriate for the emission modeling.
If the default values are not used, the number of vehicle engine starts per day used by states in
inventory modeling should be consistent, where possible, with assumptions used in other
highway vehicle related planning. Metropolitan Planning Organizations (MPOs) and state
Departments of Transportation (DOTs) should be consulted for information to determine the
number of vehicle engine starts per day.
4.5.2 Description
The nearly uncontrolled exhaust emissions that occur immediately after a cold engine start can
account for a significant fraction of the emissions that occur during a vehicle trip. MOBILE6.2
contains default values for the number of engine starts that occur each day by vehicle class (but
not by vehicle age). There will be at least one engine start for each vehicle trip. Any
intermediate destination, which results in an engine shut down, will add another engine start to a
45
-------
total trip. For this reason, caution must be used in interpreting traffic demand model counts of
trips when determining the number of engine starts.
Only light duty vehicles (passenger cars), light duty trucks and motorcycles account for engine
starts separately. The emission rates for heavy duty vehicles and busses include engine starts
and the number of engine starts and the soak time distribution cannot be adjusted by the user.
Accounting for engine starts separately from vehicle miles traveled in MOBILE6.2 allows
changes in both parameters independently. The effect of engine soak time between trips on
engine start emissions is also accounted for separately from the raw count of engine starts.
4.5.3 Method
Development of an estimate for the number of engine starts per day is optional. If the default
MOBILE6.2 values are not used, the number of engine starts per day used in inventory modeling
should be consistent, where possible, with assumptions used in other highway vehicle related
planning.
EPA recommends that states base their estimate of the average number of engine starts per day
by vehicle class and vehicle age on instrumented vehicle data. Although travel demand
forecasting models and trip diaries can account for the number of trips, instrumented vehicles
best account for the effect of intermediate destinations which add engine starts to each
source/destination trip. Care should be taken in the design and implementation of an
instrumented vehicle study to assure that a representative sampling of vehicles are recruited for
instrumentation. States should consult with EPA before attempting an instrumented vehicle
study.
States should consult with EPA before using any data (traffic demand modeling results, trip
diaries or instrumented vehicle data) to change the default values of the average number of
engine starts per day by vehicle class and vehicle age in MOBILE6.2 for use in SIP-related
highway vehicle emission inventory development.
4.5.4 MOBILE6.2 Commands
The number of engine starts per day is entered into MOBILE6.2 using the STARTS PER DAY
command. This command requires that the user supply 25 values which represent the average
number of engine starts that occur each day for each vehicle class. Only vehicle classes for
which new values are provided need to be included. The default values for all 28 vehicles
classes (a total of 700 values) may be entered. All values must be greater than zero and may be
fractional values. See Section 2.8.8.3 of the MOBILE6.2 User Guide for more information on
how to use this command.
This command is closely related to the START DIST and SOAK DISTRIBUTION commands,
which are used to provide the distribution of engine starts by hour of the day and the distribution
46
-------
of engine starts by engine soak time between engine starts. The engine start temporal
distribution and soak distribution are discussed in Sections 4.6 and 4.7 below.
4.6 Vehicle Engine Starts by Hour of the Day
4.6.1 Guidance
The distribution of the number of vehicle engine starts by hour of the day will vary from area to
area, but for most purposes, it is expected that this variation has a negligible effect on overall
daily emissions from highway mobile sources. States may choose to use the default values for
the distribution of the engine starts by hour of the day instead of developing local values.
However, results from photochemical models are sensitive to differences in the estimated
inventory by time of day. For SIP-related highway vehicle emission inventory development in
moderate and above non-attainment areas, where photochemical modeling is needed, EPA
encourages states to develop and use their own specific estimates for the distribution of the
engine starts by hour of the day.
Small scale and "hot spot" analysis of specific locations may require specific local estimates be
used. EPA should be consulted when planning small scale analysis to determine if the use of
national average default values is still appropriate for the emission modeling.
If the default values are not used, the distribution of the engine starts by hour of the day used by
states in inventory modeling should be consistent, where possible, with assumptions used in
other highway vehicle related planning. Metropolitan Planning Organizations (MPOs) and state
Departments of Transportation (DOTs) should be consulted for information to determine the
distribution of the engine starts by hour of the day.
4.6.2 Description
The nearly uncontrolled exhaust emissions that occur immediately after a cold engine start can
account for a significant fraction of the emissions that occur during a vehicle trip. MOBILE6.2
contains default values for the distribution of the engine starts by hour of the day for a weekday
and a weekend day.
Only light duty vehicles (passenger cars), light duty trucks and motorcycles account for engine
starts separately. The emission rates for heavy duty vehicles and busses include engine starts
and the number of engine starts and the soak time distribution cannot be adjusted by the user.
4.6.3 Method
Development of an estimate for the distribution of the engine starts by hour of the day is
optional. If the default MOBILE6.2 values are not used, the distribution of the engine starts by
47
-------
hour of the day used in inventory modeling should be consistent, where possible, with
assumptions used in other highway vehicle related planning.
Metropolitan Planning Organizations (MPOs) and state Departments of Transportation (DOTs)
should be consulted for information to determine the distribution of the engine starts by hour of
the day. A distribution of trip starts by hour of the day may be part of the results from traffic
demand modeling. The distribution of trip starts by hour of the day used for highway vehicle
related planning may be used to distribute the engine starts per day in MOBILE6.2.
4.6.4 MOBILE6.2 Commands
The distribution of the engine starts by hour of the day is entered into MOBILE6.2 using the
STARTS DIST command. This command requires that the user supply 48 values which
represent the fraction of engine starts that occur in each hour of the day (24 values) for average
week days and weekend days. The same distribution is used for all light duty vehicle classes and
motorcycles. All values must be greater than zero and less than one. The sum of the first 24
values (week days) must equal one and the sum of the last 24 values (weekend days) must equal
one. See Section 2.8.8.4 of the MOBILE6.2 User Guide for more information on how to use this
command.
This command is closely related to the STARTS PER DAY and SOAK DISTRIBUTION
commands, which is used to provide the number of engine starts and the distribution of engine
starts by engine soak time between engine starts. The number of engine starts and the engine
start soak distribution are discussed in Section 4.5 above and Section 4.7 below.
4.7 Vehicle Soak Time between Engine Starts
4.7.1 Guidance
The distribution of the soak times before an engine start will vary from area to area, but for most
purposes, it is expected that this variation has a negligible effect on overall daily emissions from
highway mobile sources. States may choose to use the default values for the distribution of the
soak times before an engine start instead of developing local values.
Small scale and "hot spot" analysis of specific locations may require specific local estimates be
used. EPA should be consulted when planning small scale analysis to determine if the use of
national average default values is still appropriate for the emission modeling.
If the default values are not used, the distribution of the soak times before an engine start used by
states in inventory modeling should be consistent, where possible, with assumptions used in
other highway vehicle related planning. Metropolitan Planning Organizations (MPOs) and state
Departments of Transportation (DOTs) should be consulted for information to determine the
distribution of the soak times before an engine start.
48
-------
4.7.2 Description
The nearly uncontrolled exhaust emissions that occur immediately after a cold engine start can
account for a significant fraction of the emissions that occur during a vehicle trip. However, not
all engine starts occur on cold engines. Soak time is defined as the time between when the
engine is turned off to the next time it is restarted. The soak time can have a significant effect on
the emissions associated with an engine start. MOBILE6.2 contains default values for the
distribution of the soak times before an engine start by hour of the day.
Only light duty vehicles (passenger cars), light duty trucks and motorcycles account for engine
starts separately. The emission rates for heavy duty vehicles and busses include engine starts
and the number of engine starts and the soak time distribution cannot be adjusted by the user.
4.7.3 Method
Development of an estimate for the distribution of the soak times before an engine start is
optional. If the default MOBILE6.2 values are not used, the distribution of the soak times before
an engine start used in inventory modeling should be consistent, where possible, with
assumptions used in other highway vehicle related planning.
EPA recommends that states base their estimate of the distribution of the soak times before an
engine start by hour of the day on instrumented vehicle data. Although travel demand
forecasting models can account for the number of trips, in many cases it is not possible to
account for the soak time of individual vehicles. Trip diaries and surveys could be used, but
these rely on vehicle owners correctly reporting values and may exclude starts that occur at
intermediate destinations during trips. Section 3.5, "Cold Start/Hot Start/Hot Stabilized
Weighting Factors," of the EPA document, "Volume IV: Chapter 2, Use of Locality-Specific
Transportation Data for the Development of Mobile Source Emission Inventories," (September
1996), discusses methods for using travel demand forecasting models and surveys to obtain
information about soak distributions.
Care should be taken in the design and implementation of an instrumented vehicle study to
assure that a representative sampling of vehicles are recruited for instrumentation. States should
consult with EPA before attempting an instrumented vehicle study.
States should consult with EPA before using any data (traffic demand modeling results, trip
diaries or instrumented vehicle data) before changing the default values in MOBILE6.2 for the
distribution of the soak times for use in SIP-related highway vehicle emission inventory
development.
4.7.4 MOBILE6.2 Commands
The distribution of the soak times before an engine start is entered into MOBILE6.2 using the
SOAK DISTRIBUTION command. This command requires that the user supply 3,360 values
49
-------
which represent the 70 soak time periods for each hour of the day (24 values) for average week
days and weekend days. The same distributions are used for all vehicle classes. All values must
be greater than zero and less than one. The sum of each set of 70 values must equal one. See
Section 2.8.8.5 of the MOBILE6.2 User Guide for more information on how to use this
command.
This command is closely related to the STARTS PER DAY and START DIST commands, which
is used to provide the number of engine starts and the distribution of engine starts by hour of the
day. The number of engine starts and the engine starts by hour of the day is discussed in Section
4.5 and Section 4.6 above.
4.8 Vehicle Hot Soak Time After Engine Shut Down
4.8.1 Guidance
The distribution of the hot soak time after an engine shut down will vary from area to area, but
for most purposes, it is expected that this variation has a negligible effect on overall daily
emissions from highway mobile sources. States may choose to use the default values for the
distribution of the hot soak time after an engine shut down instead of developing local values.
Small scale and "hot spot" analysis of specific locations may require specific local estimates be
used. EPA should be consulted when planning small scale analysis to determine if the use of
national average default values is still appropriate for the emission modeling.
If the default values are not used, the distribution of the hot soak time after an engine shut down
used by states in inventory modeling should be consistent, where possible, with assumptions
used in other highway vehicle related planning. Metropolitan Planning Organizations (MPOs)
and state Departments of Transportation (DOTs) should be consulted for information to
determine the distribution of the hot soak time after an engine shut down.
4.8.2 Description
Immediately after an engine is shut down (key off), while the engine is still hot, fuel vapors in
the intake manifold and other locations in the fuel system are driven off the vehicle by the heat
of the engine. These hydrocarbon losses are referred to as hot soak emissions. If the vehicle is
restarted, the active emission control systems begin again and the full hot soak effect is
interrupted, resulting in fewer hot soak emissions. MOBILE6.2 contains default values for the
distribution of the hot soak time after an engine shut down by hour of the day.
The actual number of hot soaks that occur is a function of the number of engine starts per day
that occur. Changing the number of engine starts per day will automatically change the number
of hot soaks in a day. The number of engine starts per day is discussed in Section 4.5 above.
50
-------
Only gasoline fueled vehicles are affected by the hot soak activity. Diesel vehicles and natural
gasoline vehicles are assumed to have negligible hot soak evaporative emissions. The same
distribution of hot soak times is used for all vehicle classes.
4.8.3 Method
Development of an estimate for the distribution of the hot soak time after an engine shut down is
optional. If the default MOBILE6.2 values are not used, the distribution of the hot soak time
after an engine shut down used in inventory modeling should be consistent, where possible, with
assumptions used in other highway vehicle related planning.
EPA recommends that states base their estimate of the distribution of the hot soak time after an
engine shut down by hour of the day on instrumented vehicle data. Since hot soak distributions
are a measure of time between engine shut down and the next engine start, this information is the
same as information gathered for engine start soak times and diurnal activity. Although travel
demand forecasting models can account for the number of trips, in many cases it is not possible
to account for the soak time of individual vehicles. Trip diaries and surveys could be used, but
these rely on vehicle owners correctly reporting values and may exclude starts and hot soaks that
occur at intermediate destinations during trips. Section 3.5, "Cold Start/Hot Start/Hot Stabilized
Weighting Factors," of the EPA document, "Volume IV: Chapter 2, Use of Locality-Specific
Transportation Data for the Development of Mobile Source Emission Inventories," (September
1996), discusses methods for using travel demand forecasting models and surveys to obtain
information about soak distributions.
Care should be taken in the design and implementation of an instrumented vehicle study to
assure that a representative sampling of vehicles are recruited for instrumentation. States should
consult with EPA before attempting an instrumented vehicle study.
States should consult with EPA before using any data (traffic demand modeling results, trip
diaries or instrumented vehicle data) before changing the default values in MOBILE6.2 for the
distribution of the soak times for use in SIP-related highway vehicle emission inventory
development.
4.8.4 MOBILE6.2 Commands
The distribution of the hot soak time after an engine shut down is entered into MOBILE6.2 using
the HOT SOAK ACTIVITY command. This command requires that the user supply 840 values
which represent the 60 possible one minute soak time periods for each of 14 hours of the day for
average week days and weekend days. The hours between 7 p.m. and 5 a.m. the next day are
assumed to have the same hot soak time distribution. The same distributions are used for all
vehicle classes. All values must be greater than zero and less than one. The sum of each set of
60 values must equal one. See Section 2.8.8.6 of the MOBILE6.2 User Guide for more
information on how to use this command.
51
-------
This command is closely related to the STARTS PER DAY, SOAK DISTRIBUTION and
DIURN SOAK ACTIVITY commands, which is used to provide the number of engine starts, the
distribution of soak times before engine starts and the distribution of soak time between engine
starts. The number of engine starts, soak times before engine starts and the diurnal soak
distribution are discussed in Section 4.5 and Section 4.7 above and Section 4.9 below.
4.9 Vehicle Diurnal Soak Time
4.9.1 Guidance
The distribution of the diurnal soak time will vary from area to area, but for most purposes, it is
expected that this variation has a negligible effect on overall daily emissions from highway
mobile sources. States may choose to use the default values for the distribution of the diurnal
soak time instead of developing local values.
Small scale and "hot spot" analysis of specific locations may require specific local estimates be
used. EPA should be consulted when planning small scale analysis to determine if the use of
national average default values is still appropriate for the emission modeling.
If the default values are not used, the distribution of the diurnal soak time used by states in
inventory modeling should be consistent, where possible, with assumptions used in other
highway vehicle related planning. Metropolitan Planning Organizations (MPOs) and state
Departments of Transportation (DOTs) should be consulted for information to determine the
distribution of the diurnal soak time.
4.9.2 Description
While the engine is shut down (key off) and during times of day when the ambient temperature is
rising, fuel vapors will be driven off the vehicle from the increasing temperature of the fuel in
the tank and other locations on the vehicle. The ability of the vehicle emission control
components to adsorb these vapors depends on how long the vehicle has been subjected to
diurnal emission generation. The resulting hydrocarbon losses are referred to as diurnal
emissions. If the vehicle is restarted, the active emission control systems begin again and the full
diurnal effect is interrupted, resulting in fewer diurnal emissions. MOBILE6.2 contains default
values for the distribution of the diurnal soak time by hour of the day.
Only gasoline fueled vehicles are affected by the diurnal soak activity. Diesel vehicles and
natural gas vehicles are assumed to have negligible diurnal evaporative emissions. The same
distribution of diurnal soak times is used for all vehicle classes.
4.9.3 Method
Development of an estimate for the distribution of the diurnal soak time is optional. If the
default MOBILE6.2 values are not used, the distribution of the diurnal soak time used in
52
-------
inventory modeling should be consistent, where possible, with assumptions used in other
highway vehicle related planning.
EPA recommends strongly that states base their estimate of the distribution of the diurnal soak
time by hour of the day on instrumented vehicle data. Since diurnal soak distributions are
partially a measure of time between engine shut down and the next engine start, this information
is the same as information gathered for engine start soak times and hot soak activity. Although
travel demand forecasting models can account for the number of trips, in many cases it is not
possible to account for the soak time of individual vehicles. Trip diaries and surveys could be
used, but these rely on vehicle owners correctly reporting values and may exclude starts and
soaks that occur at intermediate destinations during trips. Section 3.5, "Cold Start/Hot Start/Hot
Stabilized Weighting Factors," of the EPA document, "Volume IV: Chapter 2, Use of Locality-
Specific Transportation Data for the Development of Mobile Source Emission Inventories,"
(September 1996), discusses methods for using travel demand forecasting models and surveys to
obtain information about soak distributions.
Diurnal soak distributions do not need to add to one, since the distribution is of all vehicles
within an hour and not all vehicles are experiencing a diurnal soak. Some vehicles will be
driving and some vehicles will be experiencing a hot soak. However, a consistent distribution of
diurnal soak activity will have the total fraction of vehicles soaking in a given hour that is less
than one (indicating vehicles that are driving or in hot soak mode). The diurnal activity will also
have an equal or decreasing fraction of vehicles soaking in the next hour at the next soak time
increment. This shows the reduction in soaking vehicles as they are started and begin to operate
in subsequent hours of the day. The EPA report, "Soak Length Activity Factors for Diurnal
Emissions," March 1999 (EPA 420-P-98-019, M6.FLT.006) discusses how EPA used
instrumented vehicle data to derive the national average default values for MOBILE6.2.
Care should be taken in the design and implementation of an instrumented vehicle study to
assure that a representative sampling of vehicles are recruited for instrumentation. States should
consult with EPA before attempting an instrumented vehicle study.
States should consult with EPA before using any data (traffic demand modeling results, trip
diaries or instrumented vehicle data) before changing the default values in MOBILE6.2 for the
distribution of the soak times for use in SIP-related highway vehicle emission inventory
development.
4.9.4 MOBILE6.2 Commands
The distribution of the diurnal soak time is entered into MOBILE6.2 using the DIURN SOAK
ACTIVITY command. This command requires that the user supply 1,296 values which
represent the 72 possible one hour soak time periods for each of 18 hours of the day. The hours
between 11 p.m. and 6 a.m. the next day are assumed to have the same diurnal soak time
distribution. The same distributions are used for all vehicle classes. All values must be greater
than zero and less than one. The sum of each set of 72 values, representing the fraction of
53
-------
vehicles within an hour which are experiencing a diurnal soak, do not need to add to one, but
must be less than one. See Section 2.8.8.7 of the MOBILE6.2 User Guide for more information
on how to use this command.
This command is closely related to the SOAK DISTRIBUTION and HOT SOAK ACTIVITY
commands, which is used to provide distributions of the soak times before engine starts and the
distribution of soak time after engine starts. The number of engine starts and the diurnal soak
distribution are discussed in Section 4.7 and Section 4.8 above.
4.10 Vehicle Trip Length (Duration) Distributions
4.10.1 Guidance
The distribution of the vehicle trip duration (lengths) will vary from area to area, but for most
purposes, it is expected that this variation has a negligible effect on overall daily emissions from
highway mobile sources. States may choose to use the default values for the distribution of the
vehicle trip lengths instead of developing local values.
Small scale and "hot spot" analysis of specific locations may require specific local estimates be
used. EPA should be consulted when planning small scale analysis to determine if the use of
national average default values is still appropriate for the emission modeling.
If the default values are not used, the distribution of the vehicle trip lengths used by states in
inventory modeling should be consistent, where possible, with assumptions used in other
highway vehicle related planning. Metropolitan Planning Organizations (MPOs) and state
Departments of Transportation (DOTs) should be consulted for information to determine the
distribution of the vehicle trip lengths.
4.10.2 Description
Fuel evaporation occurs during trips due to the heating of the fuel system, especially including
the fuel tank. These emissions are affected by the length of time the vehicle has been in
operation. Longer trips mean more evaporative running loss emissions due to increased fuel
system temperatures. The EPA report, "Trip Length Activity Factors for Running Loss and
Exhaust Running Emissions," February 1998 (M6.FLT.005) defines trip lengths in more detail.
MOBILE6.2 contains default values for the distribution of the vehicle trip lengths by hour of the
day.
Only gasoline fueled vehicles (not including motorcycles) are affected by the distribution of trip
lengths in MOBILE6.2. Diesel vehicles and natural gas vehicles are assumed to have negligible
evaporative running loss emissions. The same distribution of vehicle trip lengths is used for all
vehicle classes.
54
-------
4.10.3 Method
Development of an estimate for the distribution of the vehicle trip lengths is optional. If the
default MOBILE6.2 values are not used, the distribution of the vehicle trip lengths used in
inventory modeling should be consistent, where possible, with assumptions used in other
highway vehicle related planning. MOBILE6.2 default values for the distribution of trip lengths
assume the same distribution for each hour of the day.
Section 3.6, "Trip Duration," of the EPA document, "Volume IV: Chapter 2, Use of Locality-
Specific Transportation Data for the Development of Mobile Source Emission Inventories,"
(September 1996), discusses methods for using travel demand forecasting models and surveys to
obtain information about trip length distributions. MOBILE6.2 default values for the
distribution of trip lengths assume the same distribution for each hour of the day.
States should consult with EPA before using any data (traffic demand modeling results, trip
diaries or instrumented vehicle data) before changing the default values in MOBILE6.2 for the
distribution of the trip lengths for use in SIP-related highway vehicle emission inventory
development.
4.10.4 MOBILE6.2 Commands
The distribution of the vehicle trip lengths is entered into MOBILE6.2 using the WE DA TRI
LEN DI and WE EN TRI LEN DI commands for week days and weekend days, respectively.
Each command requires that the user supply 84 values which represent the six possible trip
lengths for each of 14 hours of the day. The hours between 7 p.m. and 6 a.m. the next day are
assumed to have the same vehicle trip length distribution. The same distributions are used for all
vehicle classes. All values must be greater than zero and less than one. The sum of each set of
six values, representing the fraction of vehicles within an hour within each trip length bin, must
equal one. See Section 2.8.8.10 and Section 2.8.8.9 of the MOBILE6.2 User Guide for more
information on how to use these commands.
The command, WE VEH US, is used to determine whether the week day or weekend day values
are to be used. Use of weekend values is discussed in Section 4.11 below.
4.11 Weekday and Weekend Day Activity
4.11.1 Guidance
The distribution of the vehicle activity will vary from day to day during the week, but for most
purposes, EPA will not expect States do develop local estimates that vary by day of the week.
States may choose to use a single set of vehicle activity values instead of developing separate
local values by day of the week.
55
-------
Small scale and "hot spot" analysis of specific locations may require specific local estimates be
used. EPA should be consulted when planning small scale analysis to determine if the use of
national average default values is still appropriate for the emission modeling.
The vehicle activity values used by states in inventory modeling should be consistent, where
possible, with assumptions used in other highway vehicle related planning. Metropolitan
Planning Organizations (MPOs) and state Departments of Transportation (DOTs) should be
consulted for information to determine whether local vehicle activity information is based on an
average week day or weekend day values. If the local data is not week day or weekend day
specific, then default MOBILE6.2 week day values should be used.
4.11.2 Description
The number of engine starts per day, the distribution of engine starts by hour of the day, the
distribution of hot soak times and the distribution of the vehicle trip duration (lengths) have
MOBILE6.2 separate default values for both week days and weekend days. This means that
choosing to model a week day will give different daily values than choosing to model a weekend
day. Users are allowed to replace the MOBILE6.2 default values with local data.
The default MOBILE6.2 condition is a week day case. If users are modeling weekend days, then
these runs should explicitly choose the weekend day values for modeling.
4.11.3 Method
Development of separate local values for vehicle activity by day of the week is optional. In any
case (default or local), the vehicle activity data used in inventory modeling should be consistent,
where possible, with assumptions used in other highway vehicle related planning regarding the
use of week day or weekend day values. In some cases, MOBILE6.2 has separate default values
depending on the day of the week. The user should select week day or weekend day consistent
with the other local data supplied for MOBILE6.2 modeling. If the local data is not week day or
weekend day specific, then default MOBILE6.2 week day values should be used.
4.11.4 MOBILE6.2 Commands
The selection of week day or weekend day values for use in MOBILE6.2 is indicated using the
WE VEH US command. The default condition in MOBILE6.2 is week day. Using this
command, default (or user supplied) weekend day values will be used. See Section 2.8.8.10 of
the MOBILE6.2 User Guide for more information on how to use this command.
5.0 Vehicle Gasoline Specifications
The MOBILE6.2 model does not currently include commands that would allow States to vary
the fuel properties of diesel fuel for purposes of modeling emissions from highway mobile
56
-------
sources. However, states must specify the gasoline fuel properties for areas being modeled for
SIP-related highway vehicle emission inventory development.
5.1 Gasoline Volatility
5.1.1 Guidance
The volatility of gasoline has a significant effect on overall daily emissions from highway
mobile sources. States must determine the appropriate average gasoline volatility for gasoline
used by the highway mobile sources for the areas to be modeled.
5.1.2 Description
The volatility of gasoline in MOBILE6.2 is indicated by the Reid Vapor Pressure (RVP) of the
dispensed fuel. The RVP of the fuel is a commonly reported statistic for gasoline. One average
RVP value is used for all gasoline fueled vehicles for all hours of the day for each MOBILE6.2
run. There is no default value for gasoline RVP in MOBILE6.2. MOBILE6.2 cannot be run
without providing an estimate of average gasoline RVP.
5.1.3 Method
The EPA report, "Procedures for Emission Inventory Preparation, Volume IV: Mobile Sources,"
1992 (EPA 450-4-8 l-026d, Revised) describes how States choose appropriate values for RVP
for use in SIP-related highway vehicle emission inventory development. See 40CFR80.27(a)
and 40CFR80.27(b) for RVP values.
The nominal value for gasoline RVP provided to MOBILE6.2 should not include the effects of
oxygenates added, since these effects are determined within MOBILE6.2. See Section 5.3 below
for the discussion of added oxygenates to gasoline.
User supplied values for RVP in evaluations done for the month of July (or when the season is
summer) for areas which have opted into the federal Reformulated Gasoline (RFG) program
(beginning in the 1995 calendar year) will be overridden by default RFG values for RVP.
However, an RVP value must be supplied to MOBILE6.2, even if it is not used in this case.
5.1.4 MOBILE6.2 Commands
The average RVP value for use in MOBILE6.2 is indicated using the FUEL RVP command. A
MOBILE6.2 input file will not work if it does not contain this command. See Section 2.8.10.4
of the MOBILE6.2 User Guide for more information on how to use this command.
The gasoline RVP used by the model may be affected when the user specifies Reformulated
Gasoline. Reformulated Gasoline is discussed in Section 5.2 below.
57
-------
5.2 Reformulated Gasoline
5.2.1 Guidance
Areas which have opted into or are part of the federal Reformulated Gasoline program must
include this fact in MOBILE6.2 runs for use in SIP-related highway vehicle emission inventory
development.
5.2.2 Description
Reformulated Gasoline (RFG) is a federal fuel emission performance specification program.
RFG programs may include a wide variety of fuel specifications within the RFG area. However,
every one of these fuels must have specific fuel properties and must meet the same emission
performance specifications. MOBILE6.2 models a single representative set of fuel properties
that meets these specifications to represent all formulations for RFG. The user only needs to
specify the season of the year (winter or summer) and the RFG region (North or South) and
MOBILE6.2 will select the appropriate fuel formulation.
Winter time RFG emission results will be affected by the user supplied values for the fuel sulfur
content, oxygen content and Reid Vapor Pressure (RVP) of gasoline.
5.2.3 Method
Reformulated Gasoline only affects emissions in calendar years 1995 and later. The Clean Air
Act defines the RFG "Summer" as May 1 through September 15. "Winter" is the rest of the year.
In the summer season, user supplied values for gasoline sulfur content, gasoline RVP and
gasoline oxygen content are overridden by MOBILE6.2 default RFG values. In the winter
season, users must supply appropriate values for gasoline sulfur content (See Section 5.5),
gasoline RVP (See Section 5.1) and gasoline oxygen content (See Section 5.3). Since season is
an important parameter for RFG, although MOBILE6.2 will choose the appropriate season based
on the user supplied value for month, we recommend users specify season as well as month
when modeling RFG.
It is not appropriate to use the RFG command option in MOBILE6.2 for calendar year 2000 or
later for RFG programs in western states (i.e., Alaska, Colorado, Idaho, Montana, New Mexico,
North Dakota, Utah, Wyoming and bordering counties in other states) in the Tier 2 geographic
phase-in area. See 66 FR 19306, April 13, 2001, for a full list of the relevant counties and
Federal Indian reservations. Users wishing to model RFG in these western states will need to
specify all of the appropriate gasoline parameters (sulfur content, oxygen content and RVP)
separately for each calendar year they wish to evaluate. The MOBILE6.2 User Guide and EPA
should be consulted for further guidance on how to model RFG in these western states.
58
-------
5.2.4 MOBILE6.2 Commands
A Reformulated Gasoline program in MOBILE6.2 is indicated using the FUEL PROGRAM
command. See Section 2.8.10.1 of the MOBILE6.2 User Guide for more information on how to
use this command. The SEASON command is used to precisely determine the appropriate
effects of RFG. See Section 2.8.10.5 of the MOBILE6.2 User Guide for more information on
how to use this command.
The effects of Reformulated Gasoline may be affected by user specified values for gasoline
sulfur content, gasoline Reid Vapor Pressure and gasoline oxygen content using the SULFUR
CONTENT, FUEL RVP and OXYGENATED FUELS commands. See Sections 5.1, 5.3 and 5.4
for more information on these parameters.
5.3 Gasoline Oxygen Content
5.3.1 Guidance
The addition of oxygenates to gasoline has a significant effect on overall daily HC and CO
emissions from highway mobile sources.
Areas that are known to have significant market penetration of ether blends and/or alcohol
blends should characterize the relative market shares and oxygen content of these fuel blends and
account for them in their mobile source emission inventory. If, together, ethanol blends account
for less than 2% of total gasoline sales within an inventory area, and if there is no mandatory or
locally endorsed voluntary program for ether blends, oxygenated fuels need not be explicitly
modeled. There must also have been no significant sales of other oxygenated blend types (e.g.,
methanol).
Areas which have opted into or are part of the federal Reformulated Gasoline (RFG) program do
not need to explicitly account for oxygenated fuels in their mobile source inventory estimates.
When RFG is indicated, MOBILE6.2 will automatically account for oxygenated gasoline as part
of the RFG gasoline formulation. However, if areas have additional gasoline oxygen content
requirements that supercede RFG regulations, these requirements must be accounted for
explicitly in the mobile source emission inventory modeling.
5.3.2 Description
There are a variety of compounds that are added to gasoline in order to add oxygen to engine
during combustion. These compounds fall into two general categories, ethers and alcohols. The
added oxygen in the gasoline will moderate rich fuel/air operation modes and can reduce
emissions. However, adding alcohol to gasoline also affects the volatility of the gasoline. This
will affect emissions, both from engine combustion and from vehicle evaporative hydrocarbon
losses. To estimate emissions correctly, it is important to know the proper amount of oxygen
59
-------
added to gasoline and the market share of both ether and alcohol blends in areas to be modeled.
The default value for gasoline oxygen content in MOBILE6.2 is zero.
Since adding alcohol to gasoline increases the fuel volatility, unless fuel volatility is explicitly
limited (as it is in most nonattainment areas in the summer), it must be assumed that the added
alcohol will increase the nominal Reid Vapor Pressure (RVP) of the dispensed gasoline. In
addition, EPA allows waivers for gasoline volatility limits in some areas, which allow the
addition of alcohol to gasoline to increase RVP. It is important to know whether volatility limits
are in place and whether waivers have been allowed in order to properly model oxygenated
gasoline. Gasoline/ether blends are assumed to have the same nominal RVP value as non-
oxygenated gasoline.
5.3.3 Method
MOBILE6.2 models the effects of two types of oxygenated fuels, gasoline/alcohol blends and
gasoline/ether blends. States must provide the following information:
• Ether blend market share (as a fraction of all gasoline sold);
• Average oxygen content of ether blend fuels (percent weight of oxygen,
expressed as a fraction);
• Alcohol blend market share (as a fraction of all gasoline sold);
• Average oxygen content of alcohol blend fuels (percent weight of oxygen,
expressed as a fraction);
• Presence of any volatility waivers.
MOBILE6.2 assumes a 1 psi increase in RVP from alcohol blends if the presence of a volatility
waiver is indicated in the input file. Scenarios where volatility waivers are allowed must
indicate the presence of volatility waivers. Because areas where volatility limits are not in place
(i.e., some winter scenarios) would also expect a 1 psi increase in RVP from alcohol blends,
users modeling such areas should model as if a volatility waiver were in place. This will affect
the nominal RVP levels supplied by the user. Market shares for ethanol blends are readily
available by state.
EPA should be contacted for assistance in modeling the effects of oxygenated fuels if any of the
following situations apply:
• the fuels available in an area include blends containing both ether(s) and
alcohol(s) in the same fuel;
• an RVP waiver greater than 1.0 psi is applicable to oxygenated fuels in an area;
• no RVP waiver is in effect, but the volatility of base gasoline is currently below
the regulated limit (in this situation, the practical effect may be same as if a
waiver were in effect);
60
-------
• if two or more types of alcohol blends are marketed under different RVP waiver
treatment (for example, gasoline/methanol blends might not be given the same
waiver as gasoline/ethanol blends).
These requirements and procedures are essentially identical to those in the EPA report,
"Procedures for Emission Inventory Preparation, Volume IV: Mobile Sources," 1992 (EPA 450-
4-81-026d, Revised) used for earlier models.
5.3.4 MOBILE6.2 Commands
The oxygen content of gasoline for use in MOBILE6.2 is indicated using the OXYGENATED
FUELS command. See Section 2.8.1034 of the MOBILE6.2 User Guide for more information
on how to use this command.
Oxygenated fuels are required as part of Reformulated Gasoline (RFG). User supplied values of
gasoline oxygen content in RFG areas may be affected by RFG rules. See Section 2.8.10.1 in the
MOBILE6.2 User Guide for more information on the interactions between gasoline oxygen
content and RFG in MOBILE6.2. Also, Reformulated Gasoline is discussed in Section 5.2
above.
5.4 Gasoline Sulfur Content
5.4.1 Guidance
The sulfur content of gasoline has a significant effect on overall daily emissions from modern
highway mobile sources. States must determine the appropriate average gasoline sulfur content
of gasoline used by the highway mobile sources for the areas to be modeled.
Areas which have opted to participate in, or are part of, the federal Reformulated Gasoline
(RFG) program do not need to explicitly account for gasoline sulfur content in their mobile
source inventory estimates after the 1995 calendar year in summertime (July) scenarios. When
RFG is indicated and the season is summer, MOBILE6.2 will automatically account for gasoline
sulfur content as part of the RFG gasoline formulation. However, RFG does not control sulfur
levels in years 1999 and earlier when the season is winter (January). In the winter season states
must determine the appropriate average gasoline sulfur content for gasoline and account for this
gasoline sulfur content in the mobile source emission inventory modeling.
Beginning with the 2000 calendar year, local gasoline sulfur content will be affected by federal
controls on gasoline sulfur content. If there are no local fuel control programs which supercede
the federal gasoline regulations for the gasoline sulfur content of fuel, states may opt to use the
appropriate default gasoline sulfur content values for their area instead of developing specific
local gasoline sulfur content levels.
61
-------
5.4.2 Description
Sulfur is a naturally occurring contaminant in petroleum used to refine gasoline. Sulfur has been
shown to reduce the ability of the catalytic converters used on all modern highway mobile
sources to reduce tailpipe emissions of controlled pollutants. This effect has become increasingly
important as vehicles depend more on catalytic convenors to obtain the low exhaust emission
levels demanded by the latest emission certification standards for highway mobile sources.
MOBILE6.2 explicitly accounts for the effects of the sulfur content of gasoline on the emission
estimates for gasoline fueled highway mobile sources. The default value for gasoline sulfur
content in MOBILE6.2 before calendar year 2000 is 300 parts per million (ppm).
Since calendar year 2000, federal regulations have controlled the amount of sulfur that can occur
in gasoline sold in the United States. These regulations are intended to reduce the sulfur content
of gasoline to levels appropriate for the introduction of Tier 2 exhaust emission standards in the
2004 model year. There are two sulfur control regions, eastern and western, defined by Tier 2
regulations. The western Tier 2 sulfur content phase in states (i.e., Alaska, Colorado, Idaho,
Montana, New Mexico, North Dakota, Utah, Wyoming) and bordering counties in other states,
have a longer phase in than the eastern areas. See 66 FR 19306, April 13, 2001, for a full list of
the relevant counties and Federal Indian reservations. MOBILE6.2 has default values for eastern
and western Tier 2 sulfur content phase in areas for each calendar year, beginning with 2000.
5.4.3 Method
Methods used for obtaining information about the specific local formulation of gasolines are
described in the EPA report, "Procedures for Emission Inventory Preparation, Volume IV:
Mobile Sources," 1992 (EPA 450-4-8 l-026d, Revised) used for earlier models. MOBILE6.2
does contain default values for gasoline sulfur content. If local gasoline sulfur content is not
known, EPA should be consulted for guidance on whether default sulfur values may be used for
calendar years before 2000.
Beginning with the 2000 calendar year, states may choose to use the default values for gasoline
sulfur content. However, states must determine the appropriate Tier 2 sulfur content phase in
area applies to their scenario. It is possible, however, for states to override the default values
and enter local gasoline sulfur content levels. For calendar year 2000 or later scenarios, EPA
should be consulted before non-default values for gasoline sulfur content are used.
5.4.4 MOBILE6.2 Commands
The sulfur content of gasoline for use in MOBILE6.2 is indicated using the SULFUR
CONTENT and FUEL PROGRAM commands. See Section 2.8.10.2 and Section 2.8.10.1 of the
MOBILE6.2 User Guide for more information on how to use these commands.
The sulfur content of gasoline is affected by the use of Reformulated Gasoline (RFG). User
supplied values of gasoline sulfur content in RFG areas may be affected by RFG rules. See
62
-------
Section 2.8.10.1 in the MOBILE6.2 User Guide for more information on the interactions
between gasoline sulfur content and RFG in MOBILE6.2. Also, Reformulated Gasoline is
discussed in Section 5.2 above.
5.5 Diesel Sulfur Content
5.5.1 Guidance
The sulfur content of diesel fuel has a significant effect on emissions of particulates from diesel
vehicles. Input of proper diesel fuel sulfur levels is essential to accurate PM calculations. Diesel
sulfur content information will be required for past year modeling as well as for present and
future years. Diesel sulfur content for past and present years may be determined from local fuel
survey data or using information in an EPA spreadsheet described in Section 5.5.3.1 below.
Diesel sulfur content for future years should be drawn from the table in Section 5.5.3.2 below.
5.5.2 Description
Sulfur is a naturally occurring contaminant in the petroleum from which diesel fuel is refined.
Most sulfur in fuels is emitted as sulfur dioxide and eventually converted to sulfates in the
atmosphere. A small amount of fuel sulfur is emitted directly as sulfates (generally hydrated
sulfuric acid). Sulfates are a significant precursor for the formation of secondary particulates,
particularly PM2 5.
Sulfur in fuels has also been shown to reduce the ability of the catalytic converters to reduce
tailpipe emissions of controlled pollutants. This effect has become increasingly important as
diesel vehicles depend more on catalytic converters to obtain the low exhaust emission levels
demanded by the latest emission certification standards for highway mobile sources.
5.5.3 Method
The most accurate method for determining diesel sulfur levels for past and present values is the
use of survey data. Fuel sulfur levels for past years are available from various surveys (e.g.,
Alliance of Automobile Manufactures, Northrup Grumman Mission Systems ). This data
generally covers several metropolitan areas and goes back several years. Areas modeling future
diesel sulfur levels should assume that the most recent fule survey values apply through May
2006.
If areas are implementing local fuel programs it is expected that local fuel survey data will be
required as a part of this program. In order to implement a local program and take credit for its
benefits, it will be necessary to have baseline data as well as ongoing data regarding average
diesel sulfur levels in the program area. Where applicable, this data should be used as inputs
when modeling particulates with MOBILE6.2.
In the absence of survey data, EPA recommends that past data be taken from an EPA
spreadsheet called "Diesel Sulfur Levels by County" located at
63
-------
http://www.epa.gov/otaq/tn6.httn. This data is arranged by county and contains diesel sulfur
levels for summer and winter, going back to 1999 and forward through May 2006.
Beginning during the 2006 calendar year, federal controls on highway diesel sulfur content will
become more stringent. Sulfur content will change from a current average of 500 ppm to 15
ppm. This rule will be phased in between June, 2006 through 2010. Modelers should use the
49-state average sulfur values listed below as future year values. For years that are no longer in
the future, past year guidance from above should be followed.
Period Highway Diesel Sulfur Level (ppm)
June, 2006-May, 2010 43
June, 2010-2015 11
Note that areas modeling calendar years 2006 and 2010 will need to use different values for
sulfur depending on the month of evaluation.
5.5.4 MOBILE6.2 Commands
The sulfur content of diesel for use in MOBILE6.2 is indicated using the DIESEL SULFUR
command. See Section 2.8.10.3 of the MOBILE6.2 User Guide for more information on how to
use these commands. The value is specified in units of parts per million. This value can range
from 0.01 ppm to 5000 ppm.
6.0 Inspection and Maintenance Programs and Anti-Tampering Programs
During the 1990's, the number and variety of inspection and maintenance programs increased
significantly. MOBILE6.2 includes a number of new features that will increase the flexibility
and ease of use compared to MOBILES when modeling the range of I/M program options
currently being implemented. MOBILE6.2 has the ability to model up to seven separate I/M
programs simultaneously, which simplifies the modeling of programs in which different model
years are subject to different requirements. It includes new program options including OBD
exhaust and evaporative I/M checks. It also adds the ability to specify grace periods and
exemption ages.
MOBILE6.2 also allows for the modeling of anti-tampering programs. However, there is little
change in this input option compared to MOBILES. Users should note that anti-tampering
programs are treated separately from I/M programs in MOBILE6.2 and do not count against the
seven separate I/M programs that can be modeled simultaneously.
6.1 Inspection Type, Test Frequency, and Network Type
The command I/M PROGRAM is used to define the start and end years, the inspection type, test
frequency, and network type of the local I/M program. Details on the use of this command are
given in Section 2.8.9.4.a of the MOBILE6.2 User's Guide.
64
-------
MOBILE6.2 users can choose from eleven different types of exhaust I/M inspections and four
different types of evaporative I/M inspections. The following table summarizes the exhaust and
evaporative program options:
65
-------
Table 6.1.1
I/M Inspection Test Types in MOBILE6.2
Exhaust I/M Inspections
IDLE
2500/IDLE
LOADED IDLE
IM240
ASM 2525 PHASE-IN
ASM 2525 FINAL
ASM 50 15 PHASE-IN
ASM 50 15 FINAL
ASM 2525/5015 PHASE-IN
ASM 2525/50 15 FINAL
OBDI/M
Evap I/M Inspections
EVAP OBD
EVAP OBD & GC
FP&GC
GC
Description
Test performed while vehicle idles in park or neutral.
Test performed while vehicles idles and at 2500 rpm.
Test performed while vehicle operates on a chassis dynamometer at
constant load.
Test performed on a dynamometer, under load, through a pre-defined
"transient" driving cycle of up to 240 seconds.
Test performed on a dynamometer, under load, through a defined
"steady state" driving cycle at 25 mph and 25% load, at phase-in
cutpoints.
Test performed on a dynamometer, under load, through a defined
"steady state" driving cycle at 25 mph and 25% load, at final cutpoints.
Test performed on a dynamometer, under load, through a defined
"steady state" driving cycle at 15 mph and 50% load, at phase-in
cutpoints.
Test performed on a dynamometer, under load, through a defined
"steady state" driving cycle at 15 mph and 50% load, at final cutpoints.
Test performed on a dynamometer, under load, through two defined
"steady state" driving cycles at 25 mph and 25% load, and 15 mph and
50% load, at phase-in cutpoints.
Test performed on an inertia-weighted dynamometer through two
defined "steady state" driving cycles at 25 mph and 25% load, and 15
mph and 50% load, at final cutpoints.
Test of exhaust-related systems and components performed by visual
check of Malfunction Indicator Light (MIL) and scan of on-board
(OBD) computer for system readiness, MIL status and stored trouble
codes, on 1996 and newer OBD-equipped vehicles only.
Description
Test of the evaporative emission related systems and components
performed by visual check of the MIL and scan of the OBD computer
system for readiness, MIL status, and stored trouble codes, on 1996
and newer, OBD-equipped vehicles.
The evap OBD test performed in conjunction with a separate check of
the gas cap (GC) for the purpose of identifying leaks in the gas cap not
otherwise identified by the evap OBD check. This combination of
tests can only be conducted on 1996 and newer, OBD-equipped
vehicles. The gas cap pressure test is the only test that can be
conducted in addition to the OBD-I/M test for additional credit under
MOBILE6.2.
A pair of tests to identify leaks in the gas cap (GC) and the rest of the
vehicle's evaporative system. The latter test is conducted by
pressuring the evaporative system by way of the fuel tank's fillneck
and is referred to as the fillneck pressure (FP) test.
A test conducted by pressurizing the gas cap for the purpose of
identifvins leaks in the sas cap.
MOBILE6.2 allows users to enter either annual or biennial inspection frequencies. Users must
also define the network type for the I/M program. Options for network type are:
TRC Computerized Test and Repair program
66
-------
TRM Manual Test and Repair program
T/O Test only Program
Users can modify the effectiveness assumed in MOBILE6.2 for the TRC and TRM options using
the I/M EFFECTIVENESS command discussed below.
States must model the start and end years, inspection type, test frequency, and network type that
matches the local I/M program. In cases where there is any question of which MOBILE6.2
inspection type matches the local program, states should consult with EPA for further guidance.
MOBILE6.2 assumes that an I/M program starts on January 1 of the start year entered by the
user. Users modeling programs that start on other dates should consult with EPA for further
guidance.
In MOBILE6.2, I/M programs that have both exhaust and evaporative inspection components,
including OBD programs, are modeled as two separate, simultaneous programs identified using
different I/M program numbers in separate I/M PROGRAM command lines. Users should be
careful to include both the exhaust and evaporative components to ensure proper credit for the
program. Likewise, an I/M program that applies different tests to different vehicles (e.g., an idle
program that applies to early model years together with an EVI240 program that applies to later
model years) is also modeled as two separate, simultaneous programs identified using different
I/M program numbers in separate I/M PROGRAM command lines and using the I/M MODEL
YEARS command to differentiate which program covers what model years.
6.2 Model Years
Whenever an I/M program is modeled in MOBILE6.2, users must enter the vehicle model years
covered by that program. Users cannot model exhaust or evaporative OBD programs on vehicles
prior to the 1996 model year. Users modeling programs that exempt vehicles older than a certain
calendar age or provide a grace period for vehicles less than a certain calendar age should use the
I/M EXEMPTION AGE and I/M GRACE PERIOD commands, described in sections 6.8 and 6.9
respectively.
Vehicle model year coverage for I/M programs is entered using the command I/M MODEL
YEARS which is described in Section 2.8.9.4.b of the MOBILE6.2 User's Guide.
6.3 Vehicles
MOBILE6.2 requires that users specify the vehicle classes covered by the I/M program. Only
I/M programs that affect gasoline vehicles are modeled in MOBILE6.2. A total of 14 gasoline
vehicle types are covered. Vehicle classes covered by the I/M program are specified using the
I/M VEHICLES command, which is described in Section 2.8.9.4.b of the MOBILE6.2 User's
Guide. The vehicle classes are described in the following table:
67
-------
Table 6.3.1
MOBILE6.2 Vehicle Classes Covered by I/M
MOBILE6.2
Class
LDGV
LDGT1
LDGT2
LDGT3
LDGT4
HDGV2B
HDGV3
HDGV4
HDGV5
HDGV6
HDGV7
HDGV8A
HDGV8B
HDGB
Description
Light-Duty Gasoline Vehicles (Passenger Cars)
Light-Duty Gasoline Trucks 1 (0-6,000 Ibs. GVWR, 0-3750 Ibs. LVW)
Light-Duty Gasoline Trucks 2 (0-6,000 Ibs. GVWR, 3751-5750 Ibs. LVW)
Light-Duty Gasoline Trucks 3 (6,001-8,500 Ibs. GVWR, 0-5750 Ibs. ALVW)
Light-Duty Gasoline Trucks 4 (6,001-8,500 Ibs. GVWR, 5751 Ibs. and greater
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)
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)
Gasoline Buses (School, Transit and Urban)
6.4 Stringency
For exhaust I/M programs, MOBILE6.2 users must specify a stringency rate which is defined as
the expected exhaust inspection failure rate among pre-1981 model year vehicles. This is
required by MOBILE6.2 for exhaust I/M programs regardless of model year coverage. Failure
rates for these vehicles should be based on actual failure rates reported by a program. For I/M
programs that do not affect pre-1981 vehicles, users can enter any value for stringency that is
within the acceptable range allowed by MOBILE6.2. For I/M programs that do not affect pre-
1981 vehicles, the stringency value entered will have no effect on model results. The command
I/M STRINGENCY is described in Section 2.8.9.4.d of the MOBILE6.2 User's Guide.
6.5 Compliance Rate
The compliance rate is the percentage of vehicles in the fleet that complete the I/M program and
receive either a certificate of compliance or a waiver. Historical compliance should be
determined by sticker surveys, license plate surveys, or a comparison of the number of final tests
to the number of vehicles subject to the I/M requirement. Planners should not assume a
compliance rate of 100%. An area planning to implement an I/M program using a registration
denial system that automatically generates compliance documents that uniquely identify the
complying vehicle and that are serially numbered and accounted for, that relies on centralized
processing by government clerks with management oversight, may assume a 96% rate for
modeling purposes prior to program implementation. Once the program begins implementation,
however, the compliance rate should be based on operating program data.
68
-------
The I/M COMPLIANCE command is described in Section 2.8.9.4.e of the MOBILE6.2 User's
Guide. MOBILE6.2 requires that a compliance rate be entered for exhaust I/M programs, but
compliance rates are optional for evaporative I/M programs. If an exhaust compliance rate is
entered, MOBILE6.2 will assume that same rate for an evaporative program that covers the same
vehicle classes and model years. We strongly recommend entering a separate compliance rate for
any evaporative I/M program, even if the compliance rate is identical to the exhaust program, to
avoid confusion about what was assumed in MOBILE6.2 based on the input file.
6.6 Waiver Rates
The waiver rate is the percentage of vehicles that fail an initial I/M test and do not pass a retest,
but do receive a certificate of compliance. Actual historical waiver rates should be used as the
basis for estimating future waiver rates.
The I/M WAIVER RATES command is described in Section 2.8.9.4.f of the MOBILE6.2 User's
Guide. MOBILE6.2 requires that a wavier rate be entered for exhaust I/M programs, but waiver
rates are optional for evaporative I/M programs. If an exhaust waiver rate is entered,
MOBILE6.2 will assume that same rate for an evaporative program that covers the same vehicle
classes and model years. We strongly recommend entering a separate waiver rate for any
evaporative I/M program, even if the waiver rate is identical to the exhaust program, to avoid
confusion about what was assumed in MOBILE6.2 based on the input file.
6.7 Cutpoints
If the I/M program inspection type is IM240, MOBILE6.2 users must enter the I/M cutpoints
used for the EVI240 program. Cutpoints are specified with the I/M CUTPOINTS command and
entered using an external file as described in Section 2.8.9.4.g of the MOBILE6.2 User's Guide.
States should consult with EPA if there is any question about appropriate cutpoints to use in
modeling an IM240 program. Users should specify the cutpoints that will apply to Tier 0
vehicles (i.e., model years prior to 1996) to all ages of the vehicle fleet. When the Tier 0
cuptoints are entered, MOBILE6.2 will apply the proper credit for equivalently stringent
cutpoints on Tier 1 and later vehicles. The I/M CUTPOINTS command only affects IM240
programs. Cutpoints for ASM programs are determined by the particular ASM test selected with
the I/M PROGRAM command. Users cannot enter separate ASM cutpoints.
6.8 Exemption Age
Some I/M programs provide exemptions for all vehicles over a specified age. The command I/M
EXEMPTION AGE, described in Section 2.8.9.4.h of the MOBILE6.2 User's Guide, provides a
simple mechanism for specifying at what age vehicles are exempt from an I/M program. Users
enter the age at which vehicles are no longer subject to mandatory I/M requirements. The
exemption age entered for an exhaust I/M program will also apply to an evaporative I/M
program that covers the same vehicle classes and model years. However, we strongly
recommend entering a separate exemption age for any evaporative I/M program, even if the
69
-------
exemption age is identical to the exhaust program, to avoid confusion about what was assumed
in MOBILE6.2 based on the input file.
6.9 Grace Period
Some I/M programs provide a grace period for newer vehicles during which they are exempt
from the program. The command I/M GRACE PERIOD, described in Section 2.8.9.4.1 of the
MOBILE6.2 User's Guide provides a simple mechanism for specifying how long the grace
period applies to newer vehicles. Users enter the age at which vehicles first become subject to
mandatory I/M requirements. For example, if a user enters "2", MOBILE6.2 assumes that
vehicles first become subject to I/M two years after purchase.
The grace period entered for an exhaust I/M program will also apply to an evaporative I/M
program that covers the same vehicle classes and model years. However, we strongly
recommend entering a separate grace period for any evaporative I/M program, even if the grace
period is identical to the exhaust program, to avoid confusion about what was assumed in
MOBILE6.2 based on the input file.
6.10 Technician Training Credits
As a default assumption, MOBILE6.2 assigns full I/M credit for technician training. States that
do not conduct, and have no plans to conduct, a technician training program should use the
command NO I/M TTC CREDITS (Section 2.8.9.4.J of the MOBILE6.2 User's Guide) to
eliminate credits for this program. This command has no effect on the benefits of an evaporative
I/M program.
6.11 I/M Effectiveness
For I/M programs with a test and repair network type, states must specify effectiveness rates for
the program. Separate effectiveness rates are entered for HC, CO, and NOx. Because the 1995
National Highway Systems Designation Act allows for states to make a demonstration of
network effectiveness, states should consult with EPA on the appropriate effectiveness level
given the specifics of the local I/M program. Program effectiveness is specified using the I/M
EFFECTIVENESS command described in Section 2.8.9.4.k of the MOBILE6.2 User's Guide.
6.12 Anti-Tampering Programs
Anti-tampering programs are treated separately from I/M programs and are not counted against
the seven I/M program limit in MOBILE6.2. Basic guidance on appropriate inputs for anti-
tampering programs is unchanged from previous MOBILES guidance with the exception of the
wider range of vehicle weight classes covered in MOBILE6.2. Users can refer to the MOBILES
User's Guide and to Procedures for Emission Inventory Preparation, Volume IV: Mobile Sources
(EPA, 1992, reference number EPA-450/4-81-026d) for further information. The command
ANTI-TAMP PROG (Section 2.8.9.3 of the MOBILE6.2 User's Guide) is used specify the
70
-------
details of the anti-tampering program. Users should note that there is not tampering effect or
tampering reduction benefit on 1996 or newer vehicles in MOBILE6.2.
7.0 Alternative emission regulations and control measures
MOBILE6.2 contains a number of commands that allow the user to turn off or change the effects
of national motor vehicle standards and regulations. These commands have very specific
applications such as estimating baseline emissions for rate of progress calculations, estimating
incremental benefits of federal motor vehicle standards, and estimating the impacts of adopting
California vehicle standards in other states. These commands should not be used outside of
these specific applications.
7.1 Disablement of 1990 Clean Air Act Requirements
The Clean Air Act Amendments of 1990 include provisions known as rate of progress
requirements that call for regular incremental reductions in VOC emissions over the time period
prior to the attainment deadline for an ozone nonattainment area. Proper implementation of
these provisions requires the estimation of an adjusted base year emission inventory, which is
essentially an estimation of what motor vehicle emissions would have been in the absence of any
motor vehicle standards, fuel standards, or state or local motor vehicle programs required by the
Clean Air Act Amendments of 1990. Details of the methodology for calculating adjusted base
year emissions and the incremental reductions needed to meet the rate of progress requirements
are given in "Guidance on the Adjusted Base Year Emission Inventory and the 1996 Target for
the 15 Percent Rate of Progress Plans" (October 1992, US EPA, Office of Air Quality Planning
and Standards, reference number EPA-452/R-92-005).
The command NO CLEAN AIR ACT in MOBILE6.2 is used to disable all motor vehicle
requirements of the 1990 Clean Air Act Amendments. Details of the motor vehicle requirements
affected by this command are given in Sect. 2.8.11.1 of the MOBILE6.2 User's Guide. States
using MOBILE6.2 to generate rate of progress plans will need to use this command to estimate
adjusted base year emissions. More generally, comparing MOBILE6.2 runs with and without
this command can also be used for estimating the total benefits of all control programs required
by the Clean Air Act Amendments of 1990.
7.2 Heavy-Duty Diesel Vehicle NOx Off-Cycle Emissions Effects
MOBILE6.2 includes estimates of excess emissions of heavy-duty diesel engines produced in the
late 1980's through most of the 1990's that implement engine control strategies that caused
engine emissions to be higher in-use than over the Federal Test Procedure. MOBILE6.2 also
includes offsets of those emissions achieved through early pull-ahead of 2004 heavy duty diesel
emission standards and through low emission rebuilds of existing engines.
MOBILE6.2 includes several commands that allow users to change the effects of these excess
emissions and the programs put in place to offset them. These commands are called NO
71
-------
DEFEAT DEVICE, NO NOX PULL AHEAD, NO REBUILD, and REBUILD EFFECTS.
These commands are described in detail in Section 2.8.11.2 of the MOBILE6.2 User's Guide.
EPA is currently developing separate guidance on the use of these commands.
7.3 2007 Heavy Duty Standards
MOBILE6.2 assumes the implementation of new standards for heavy duty vehicles beginning in
2007. The command NO 2007 HDDV RULE (Section 2.8.11.5 of the MOBILE6.2 User's
Guide) can be used to override effects of these standards in order to estimate their incremental
benefits. This command should not be used in inventories estimate for SIP or conformity related
submissions.
7.4 Alternative Emission Factors for Passenger Cars and Light-Duty Trucks
MOBILE6.2 assumes that emissions standards for passenger cars and light-duty trucks phase in
according to schedules specified in the Tier 1, NLEV, and Tier 2 regulations. MOBILE6.2 also
provides five commands (NO TIER2, T2 EXH PHASE-IN, T2 EVAP PHASE-IN, T2 CERT,
and 94+ LDG IMPLEMENTATION) that allow users to analyze the impacts of alternate
standards and implementation schedules or to analyze the incremental benefits of federal
standards. For most states, no adjustments to these standards or schedules are necessary and
these commands should not be used for typical SIP and conformity related submissions. States
that have adopted or plan to adopt California standards and states in the Northeast that had an
earlier phase-in of NLEV standards will need to use these commands following guidance in the
next section of this document. Details on the commands discussed in this section are given in
Section 2.8.11.3 and Section 2.8.11.4 of the MOBILE6.2 User's Guide.
7.4.1 Guidance for Modeling Alternative NLEV Implementation and Adoption of
California Standards
The MOBILE6.2 default case for post-Tier 1 emission control programs assumes that the
National LEV program applicable to the non-Northeastern states is implemented in model year
2001, and that the Federal Tier 2 program begins implementation in model year 2004. Users
who wish to use MOBILE6.2 with alternate control program scenarios will need to invoke
additional input features as described in this guidance.
The earlier phase-in NLEV would be modeled using an alternate input file in conjunction with
the 94+ LDG IMP command, detailed in the MOBILE6.2 User's Guide. An alternate data file
(NLEVNE.D), developed by EPA and provided with the final model release, reflects the
appropriate phase-in of NLEV standards in the 1999 and 2000 models years. Northeastern
states subject to the earlier phase-in provisions on NLEV should use the NLEVNE.D file instead
of the MOBILE6.2 default for accurate representation of the NLEV program in their area.
Under Section 177 of the Clean Air Act, states have the option to adopt California emission
control programs instead of the federal program if the California programs will help achieve the
72
-------
air quality goals of that state. The focus of this option has been the evaluation of programs
which are alternatives to the Federal Tier 1 and Tier 2 programs. California's emission control
program for light-duty vehicles consists of the LEV I and LEV II programs, which are
considered separate programs for states outside of California, and in MOBILE6.2's modeling
approach.
California's LEV I program affects light-duty vehicles from model year 1994 through the
implementation of the LEV II program starting in 2004. Some northeastern states adopted
California's LEV I program as an alternative to the Federal Tier 1 and NLEV programs.
Because the specific implementation schedules of the LEV I program vary from state to state,
users wishing to model the LEV I program in a specific non-California state will need to develop
a custom input file to be used in conjunction with the 94+ LDG IMP command. This custom file
should be based on the default file P94IMP.D provided with the final model release, modified as
appropriate to reflect the appropriate phase-in schedule of vehicles under the LEV I program (i.e.
TLEV, LEV, ULEV and ZEV) for that state. The modified phase-in schedule should only affect
model years 1994 through 2003; beginning in model year 2004, the model assumes
implementation of the Tier 2 program.
Beginning in 2004, the MOBILE6.2 default case is the federal Tier 2 program under the phase-in
presented in the MOBILE6.2 report M6.EXH.004, "Accounting for the Tier 2 and
Heavy-Duty 2005/2007 requirements in MOBILE6". States have the option of adopting
California's LEV II program, which also begins implementation in 2004. Because the LEV II
program has separate provisions for the required number of vehicles complying with zero-
exhaust emissions and zero evaporative emission standards, exhaust VOC emissions and
evaporative VOC must be calculated using separate MOBILE6.2 runs. The "EXPAND EVAP"
command must be used in order to separate out the exhaust and evaporative VOC emission
components. For exhaust VOC and all other pollutants, analysis of the LEV II option in
MOBILE6.2 should be performed using alternative input files in conjunction with three
commands:
T2 EXH PHASE-IN, which provides phase-in percentages by exhaust certification bin, vehicle
class and pollutant.
T2 CERT, which defines the 50,000 mile standard levels by exhaust certification bin, vehicle
class and pollutant.
94+ LDG IMP, which is used only to establish the fraction of zero-emitting exhaust vehicles
(ZEVs). The remainder of vehicles are categorized as "Tier 2" and allocated according to the bin
phase-in fractions provided with the T2 EXH PHASE-IN command. Users who would like to
model the LEV I or northeastern NLEV and LEV II programs in conjunction can create a single
input file for this command which reflects the phase-in for the appropriate programs from 1994
onward. Users must contact EPA for alternative input files and guidance on how to modify them
to reflect local conditions.
For evaporative (non-exhaust) VOC emissions, analysis of the LEV II option in MOBILE6.2
must be performed with a different input file used in conjunction with 94+ LDG IMP command.
73
-------
A single input file cannot accurately reflect the impact of zero-emitting vehicles on both exhaust
and evaporative emissions. Analysis of the evaporative VOC emissions under the LEV II option
in MOBILE6.2 should therefore be performed using alternative input files in conjunction with
two commands. The command T2 EVAP PHASE-IN provides phase-in percentages for
evaporative standards by vehicle class. The command 94+ LDG IMP is used only to establish
the fraction of zero-emitting evaporative vehicles (ZEVs and Partial ZEVs). The remainder of
vehicles are categorized as "Tier 2" and allocated according to the phase-in fractions provided
with the T2 EVAP PHASE-IN commands. Users must contact EPA for alternative input files
and guidance on how to modify them to reflect local conditions.
74
-------
Appendix: References
This guidance is not intended to explain fully how to use MOBILE6.2. While reading this
document, MOBILE6.2 users should refer to the User's Guide to MOBILE6.2 (US EPA Office
of Air an Radiation, 2001, EPA420-D-01-003) for details on each of the commands discussed
here. This document is available at:
http ://www. epa.gov/otaq/m6. htm
The technical reports created to document MOBILE6.2 will be helpful in understanding how
EPA developed national default estimates for many of the factors that states may want or need to
modify. The following technical reports may be of most interest:
Guidance for the Development of Facility Type VMT and Speed Distributions, Systems
Applications International, Inc., September 1998 (EPA 420-P-99-004, M6.SPD.004).
Trip Length Activity Factors for Running Loss and Exhaust Running Emissions, April
2001 (EPA420-R-01-013, M6.FLT.005).
Fleet Characterization Data for MOBILE6.2: Development and Use of Age Distributions,
Average Mileage Accumulation Rates and Projected Vehicle Counts for Use in
MOBILE6.2, September 2001 (EPA420-R-01-047, M6.FLT.007).
All of these reports are available on the EPA web site at:
http://www.epa.gov/otaq/models/MOBILE6/m6tech.htm
The primary sources of past guidance on the use of MOBILE for estimating highway vehicle
inventories for SIP and conformity related purposes are the following documents:
Procedures for Emission Inventory Preparation, Volume IV: Mobile Sources, US EPA
Office of Air and Radiation, 1992, EPA-450/4-81-026d (Revised).
(http://www.epa.gov/otaq/models.htm)
User's Guide to MOBILES, US EPA Office of Air and Radiation, 1994, EPA-AA-
AQAB-94-01.
(http://www.epa.gov/otaq/m5.htm)
Preferred and Alternative Methods for Gathering and Locating Specific Emission
Inventory Data: Methodology for Gathering Locality-Specific Emission Inventory Data,
Report prepared for US EPA, June 19, 1996.
(http://www.epa.gov/ttnchiel/eiip/techreport/volume04/)
75
-------
Use of Locality-Specific Transportation Data for the Development of Mobile Source
Emission Inventories, Report prepared for US EPA, September, 1996.
(http://www.epa.gov/ttn/chief/eiip/techreport/volume04/index.html)
Techniques for Estimating MOBILE2 Variables, Energy and Environmental Analysis,
Inc. for EPA Contract Number 68-03-2888.
(ORA0022.PDF at http://www.epa.gov/OMS/models/mob-arch/)
Additional Techniques for Estimating MOBILE2 Variables, Energy and Environmental
Analysis, Inc. for EPA Contract Number 68-03-2888.
(ORA0023.PDF at http://www.epa.gov/OMS/models/mob-arch/)
An additional source of information about roadways is:
The Highway Performance Monitoring System Field Manual, December 2000, OMB No.
21250028, Office of Highway Policy Information, Federal Highway Administration.
(http://www.fhwa.dot.gov/ohim/hpmsmanl/hpms.htm)
Additional information about the Highway Performance Monitoring System can be found at:
http://www.fhwa.dot.gov/ohim/hpmspage.htm
76
------- |