United States EPA420-B-03-013
Environmental Protection
Agency November2003
Frequently Asked Questions on MOBILE6
Assessment and Standards Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
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Introduction
This document contains EPA's responses to questions that are frequently asked about MOBILE6.
New questions will be added to future versions of this document. Therefore, this is not intended
to be a "final" report; it will be continually evolving.
This version contains responses to questions received through September 5, 2003.
To submit questions for possible inclusion in future versions of this document, send them by
email to: mobiSe(5),epa.gov.
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Table of Contents
(Page down to reach the individual questions)
General Questions / Overview
Program Will Not Run
Files
Files / Names / Path Names
Files / Names / Reading (Opening)
Files / Names / Samples
Files / Location
Files / External
Input
Input File / Structure
Input / Commands
Input / Problems
Input Data
Input / Hourly Temperatures
Input / Fuel Parameters
Input / Time of Day
Input / Air Conditioning
Input / Altitude
Input / I/M Programs
Input / Mileage Accumulation Rates
Input / Natural Gas Vehicle (NGV) Fractions
Input / Starts per Day
Input / Barometric Pressure
Output
Output / Alternate Units
Output / Evaporative Emissions
Output / Vehicle Types
Output / Messages (Error / Warning)
Output / Spreadsheet
Output / Aggregating
MOBILES versus MOBILES
MOBILES v. MOBILES / MOBILES Function Like MOBILES
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Distributions
Distributions / VMT
Distributions / Speed
Distributions / Soak Time
Distributions / Diesel Sales
Distributions / Vehicle Mix
Distributions / Registration Data
Modeling Emissions
Emissions / At Single Average Speed
Emissions / At Idle
Emissions / Start Emissions
Emissions / Refueling Emissions
Emissions / Heavy-Duty / Consent Decrees
Emissions / Carbon Dioxide (C02) Emissions
Emissions / Natural Gas Vehicle (NGV) Emissions
Modeling / For Sensitivity Testing
Miscellaneous
Misc / Programming
Misc / Documentation
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General Questions / Overview
Q. What is MOBILE6?
MOBILE is an EPA model for estimating pollution from highway vehicles. MOBILE
calculates emissions of hydrocarbons (HC), oxides of nitrogen (NOx), and carbon
monoxide (CO) from passenger cars, motorcycles, light- and heavy-duty trucks. The
model accounts for the emission impacts of factors such as changes in vehicle emission
standards, changes in vehicle populations and activity, and variation in local conditions
such as temperature, humidity and fuel quality.
MOBILE is used to calculate current and future emission inventories of these emissions
at the national and local level. These inventories are used to make decisions about air
pollution policy at the local, state and national level. Inventories based on MOBILE are
also used to meet the federal Clean Air Actis State Implementation Plan (SIP) and
transportation conformity requirements, and are sometimes used to meet requirements of
the National Environmental Protection Act (NEPA).
The MOBILE model was first developed in 1978. Since that time, it has been updated
many times to reflect our growing understanding of vehicle emissions, and to cover new
emissions regulations and modeling needs. Although some updates were made in 1996
with the release of MOBILESb, MOBILE6 is the first major revision to MOBILE since
MOBILESa was released in 1993.
Q. How is MOBILE6 different from MOBILES?
MOBILE6 is a major revision of the MOBILE model. The revision is based on much new
data, but also on new understanding of vehicle emission processes. It also includes the
effects of regulations that have been issued since MOBILESb was released, and it
includes new features designed to make the model more useful.
The improvements in the data and calculations have led to improved estimates of
highway vehicle emissions. In some cases, the updated MOBILE6 emissions are
significantly different from the emissions estimated with MOBILES.
The new features of the model make the model easier to use. The modelis input structure
is more straightforward, easing data entry. Users can enter more detailed vehicle activity
information to better tailor their estimates to local area conditions. Also, the modelis new
output structure makes it easier for users to automate the processing of MOBILE6 results
using spreadsheet and database tools.
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Q. What are the major factors driving the change in results between MOBILES and
MOBILE6? (Why are the numbers different?)
There are a number of reasons why emissions estimates are different in MOBILES and
MOBILE6. Some changes drive the estimates down; others drive the estimates up. This
means that the overall comparison can vary greatly depending on the pollutant of interest,
the calendar year being considered, and the local fuel, local driving activity and other
local user inputs.
Here are some of the most important quantitative changes to the MOBILE model and
their general impact on emission results:
Tailpipe emissions from light-duty cars and trucks
Basic Emission Rates
MOBILE6 "basic emission rates" (exhaust emissions expected from driving a "Federal
Test Procedure" (FTP) driving cycle under specific laboratory conditions) for light-duty
cars and trucks are lower than those predicted in MOBILES for late 1980s and early
1990s model year vehicles. This is because new data from in-use vehicles shows that the
pollution control technologies that came into widespread use in the late 1980s are more
durable than had been expected when MOBILES was developed. MOBILE6 also includes
new, lower, basic emission rates for 2001-and-newer vehicles subject to the National Low
Emission Vehicle and Tier 2 emission standards.
Real World Driving
In addition to the basic emissions, MOBILE calculates corrections to the FTP driving
cycle estimates to account for various factors that impact emissions in the real world.
Many of these factors have changed in MOBILE6.
Air Conditioning and Acceleration: One major improvement is the inclusion in
MOBILE6 of high emissions due the effects of air conditioning and high acceleration
driving. MOBILE6 adds an "off-cycle" correction to account for these emissions. Studies
show these off-cycle emissions are high for vehicles built before 2001, but they are
expected to decline as the "Supplemental Federal Test Procedure" regulations to reduce
these emissions are phased in. This tends to make emission estimates for older vehicles
higher in MOBILE6 than in MOBILES.
Fuel Content: The emission corrections to account for fuel content are also improved.
MOBILE6 explicitly accounts for the damage inflicted by high levels of sulfur in gasoline
in vehicles with advanced catalysts. This leads to increased emission estimates in the late
1990s and early 2000s. This effect declines as the Tier 2 regulations phase in lower sulfur
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fuel. Meanwhile, evidence shows that oxygenated fuels do not improve CO emissions
from advanced catalyst vehicles; so, in MOBILE6 the CO benefits of oxygenated fuel
programs will phase out as these more technologically advanced vehicles become more
prominent in the fleet.
Speed and driving habits: Other changes also affect the light-duty results in MOBILE6.
New data on speed effects shows that vehicle emissions are generally less sensitive to
speed changes than previously thought. New data on driving activity shows that todayis
vehicles are driven more miles-per-year than in the past. Todayis vehicles are used to
make more trips, increasing the number of vehicle starts per day and the associated start
emissions. EPA also changed the default car/truck ratio in MOBILE6 to account for the
growing number of light-duty trucks.
Heavy-Duty Exhaust
In MOBILE6, basic emissions for 1987-and-later heavy-duty trucks and buses were
derived from new certification data that shows emissions are much lower than predicted
by MOBILES. Also, since MOBILES was developed, EPA learned that in certain driving
conditions, many 1988-2000 model year heavy-duty trucks emit much more NOx than
they do in certification testing. MOBILE6 takes account of the lower base-rate emissions,
the excess NOx emissions under steady state driving conditions, and the reduction in
NOx emissions expected in future years as a result of a consent decree with engine
manufacturers. Thus, MOBILE6 heavy-duty truck emissions are significantly higher than
MOBILES for some model years and pollutants, and significantly lower for others.
Evaporative Emissions
Evaporative emissions are hydrocarbon emissions from evaporated fuel. These emissions
are due to changes in ambient and engine temperatures and to small leaks and seepages in
the fuel system (in contrast to "tailpipe" emissions of exhaust from combustion.) Much
new data has been collected on evaporative emissions. MOBILE6 takes account of new
data that show that a small fraction of vehicles with leaks in their fuel systems contribute
a large quantity of evaporative emissions. At the same time, MOBILE6 also takes
advantage of data from new tests that better mimic real-world evaporative conditions and
accounts for new regulations that require lower emissions and more durable fuel systems-
-effects that reduce evaporative emissions.
Q. How do we know MOBILE6 estimates are better than MOBILES?
Every MOBILE model is based on the best information available at the time the model is
developed. Since vehicle technology changes over time, and our understanding of
emissions improves, it is not surprising that there were areas in MOBILES that needed
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improvement. In time, MOBILE6 will also become outdated. That said, MOBILE6 is the
most thoroughly documented and tested model in the MOBILE series, and it provides a
valuable tool for national and local-area mobile source emission inventories.
EPA has gone to great lengths to assure that MOBILE6 is based on the best data and
calculation methodologies available. EPA staff produced nearly 50 technical reports
explaining the data analysis behind the MOBILE6 estimates and the methods to be used
in the model. These are posted on the EPAis MOBILE6 Web site (
www.epa.gov/otaq/m6.htm) for comment and review. All papers were extensively
reviewed within EPA, and many were sent to external experts for an independent peer
review. All comments were considered, and MOBILE6 methodologies were revised in
response to comment as necessary.
After MOBILE6 was coded, the model was subject to a battery of tests to make sure the
results were consistent with the underlying data. The model was also released for limited
pilot testing to make sure the model could be used in real-life conditions by real-life
modelers.
Even after the release of MOBILE6, EPA will continue to validate MOBILE estimates by
comparing them with real-world emission measurements.
Q. What will be different for MOBILE6 users?
MOBILE6 has a different input structure than MOBILES. Where MOBILES and previous
versions of MOBILE used fixed formats, numerical flags and obscure acronyms,
MOBILE6 uses English language cues, external data files, and flexible formatting.
Transforming old input files into the new structure will require effort, but the new
structure is much more "friendly" and should be much easier for new users to learn.
More importantly, MOBILE6 allows users to provide much more detailed local inputs
than were allowed in MOBILES. This permits users to adapt model to local conditions
and to model special situations that are not reflected in the modelis defaults.
MOBILE6 also has an updated structure that allows users to output emissions by hour of
the day and to segregate start and running emissions. The new output uses standard
database formats to allows users to easily post-process their results. These features will be
useful for entering the emissions data into air quality models and other tools that make
use of mobile source emission inventories.
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Q. Where can I learn more about MOBILE6?
(1) Check our Web site: www.epa.gov/otaq/m6.htm
(2) Read the User Guide for MOBILE6 (available on that MOBILE6 Web site).
(3) Try the computer based training (CBT) (also available on that MOBILE6 Web
site).
(4) See the MOBILE6 Technical Guidance document (available on the MOBILE6
Web site) on the use of MOBILE6 for emission inventory preparation.
(5) If these resources do not address your questions or concerns, send questions or
comments to: mobiSe(5),epa.gov
Program Will Not Run
Q. We recently converted to Windows XP. When we attempt to run MOBILE6 in
Windows XP, we get an error message about "Memory Protection Faults."
We have tried to address this problem by recompiling the source code. We found that we
eliminated the problem by recompiling *without* using the "trace" option. The version of
MOBILE6 currently on our Web site should run without having this problem. Download
the current version of MOBILE6 and try it.
Files
Files / Names / Path Names
Q. Input file names that are read by a batch file cannot have more than 8 characters
before the decimal. Is this also true for all other external files?
Yes this is true for all MOBILE6 file names. MOBILE6 is really a "DOS" program and so
really only understands the DOS convention of file naming.
Q. With Windows95 and later Windows operating systems, directories/folders can have
spaces in their names. Does this create a problem for MOBILE6 when reading the
path for a file? Are there any restrictions on path names other than the 80-character
limit?
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Spaces in pathnames seem likely to cause problems. Again, MOBILE6 is really a DOS
program and so really only understands the DOS convention of file naming.
Files / Names / Reading (Opening)
Q. MS Windows applications will not automatically open the MOBILE6 database
output file, since it uses a non-standard "TB1" extension. Why doesnit MOBILE6
use a standard extension?
There really is no "standard" extension that will work with all, or even most, applications.
If the user prefers a different extension, the user may alter the output file name extension
after the MOBILE6 model has run. (Please see the following question.)
Q. How can we open the database reports? (Should we rename the file with a new
extension rather than use the "TB1" extension?)
The files with the "TB1" extension are simply ASCII text files containing a table with
fields separated by tab characters. The extension TB1 is *not* a standard software
extension, so most software (with default configuration) will not know how to open them.
A solution for some applications is to rename the file with an extension of "TXT"
indicating that the file is a text file. However, since this new name would duplicate the
file name being used for the MOBILE6 descriptive output, the descriptive output file
should first be moved (or copied) into another directory.
Another solution for some applications is to associate the "tbl" extension with a specific
application.
Files / Names / Samples
Q. What does the file extension "DEF" mean?
Files ending with the DEF extension are example MOBILE6 external data files. These
files have been created to replicate the same default (DEF) values that are contained in
the MOBILE6 code. Use of these inputs should produce results identical to runs without
the input. These files are included to provide an example of what an external file of that
type might look like. They can be used as a template for user supplied values. External
files may have *any* file extension the user wishes. However, all external data files we
provide end with either a DEF (external data, default values) extension or D (external
data) extension.
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Q. Where can I find brief overview and explanation of the example files? Could one
example be a detailed template of what an input file contains, with suitable
explanation for each entry?
The README.TXT file distributed with MOBILE6 contains a listing and brief
explanation of each of the example files. The MOBILE6.IN file is intended to be a kind
of template showing a nearly full range of what an input file can do. It is not a "typical"
input file, however, and contains non-default data. (We have received different comments
on what people would like this file to be).
Files / Location
Q. How do I get the model to put my input and output files in different directories?
Can I use a "relative path" with two dots?
No. MOBILE6 can process the input file name assuming a default *.IN extension. In
order to do this, it must note the location of the decimal ("dot") character in the input files
with non-default extensions. Thus, DOS navigation commands that include decimals may
not operate correctly.
Q. The User Guide suggests that "all example external data files be placed in the
directory containing the MOBILE6 executable application file." Elsewhere, it is
suggested that the user put example input files in the EXAMPLES directory for use
as references. Which statement is correct? Is the second statement actually directing
users to keep copies of input files in EXAMPLES subdirectory (which is located in
the RUN directory) as well as in the RUN directory on an equal level with the
executable file?
Yes. The external data files and input file(s) may be located anywhere, as long as they are
properly referenced (including the proper DOS path). However, if no path is supplied,
MOBILE6 will expect that the data file is located in the same directory as the application.
When constructing command input files, it will be easier for users if the external data
files they intend to use are located in the same directory as the application. Since users
may inadvertently make changes to the external data files and example input files when
creating custom versions, it is also wise to keep an unaltered copy of the example input
files external data files in the EXAMPLES directory for future reference.
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Files / External
Q. Do the terms external data input file and alternate external input file refer to the
same entity?
Yes. External data input files are used to substitute local values for the default values
contained in MOBILE6. As a result, these external files are sometimes referred to as
"alternate" data files, since they contain alternate values to be used in place of the default
values.
Input
Input File / Structure
Q. Would it be correct and helpful to say that the beginning of a new run data section
restores all defaults?
It is actually not true that all defaults get restored at this point. Header level defaults are
not restored.
Q. What is the difference between a "Run" and a "Scenario"?
"Run" refers to a group of scenarios which share certain parameters. For example, the
scenarios within a run can be a series of calendar years or a series of different average
speeds for a single calendar year. MOBILE6 produces emission result reports for each
scenario. This arrangement was intended to provide maximum flexibility while reducing
the amount of input that is required for each scenario.
Input/Commands
Q. Where can we find a sample template for the users to begin developing our own
input files?
The "Table of Required MOBILE6 Input Records" (Section 2.5 of the User Guide)
provides one starting point. It shows the minimal set of commands in an order which is
acceptable to the program. Also the EXAMPLES.IN file distributed with MOBILE might
be a good starting point. Starting with a minimal input file, commands can be added one
at a time.
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For commands which require an external data file, example files containing the
MOBILE6 default information are provided. These have the .DEF extension. These can
be used as "templates" for external data files.
Input/ Problems
Q. If a MOBILE6 input file does not process, what are some common errors to look
for?
Here are some common errors and approaches to try:
1) There may be too many spaces (or not enough spaces) between the command text and
the colon character, which is intended to mark column 20 (i.e., the twentieth character on
the line). You will need to keep track of which column you are in for each line, either by
using a fixed width font such as Courier or by counting characters (including spaces as
characters). A suggestion is to use a text editor such as the NOTEPAD.EXE application
that comes with MS Windows, which automatically uses a fixed width font.
2) In general, your input may be too complex. Until you get more experience, try creating
an input file with the minimum number of inputs. Then expand your inputs to include
more parameters of interest, one at a time.
3) You may have forgotten the END OF RUN command.
4) You may have a TAB in your input file. TAB characters are not allowed.
Input Data
Input Data / Hourly Temperatures
Q. For hourly temperatures (User Guide, Sections 2.8.6.4 and Appendix B Table 4), it's
not altogether clear exactly what temps are required. Suppose we're using the 24
hourly temperature observations reported by a nearby National Weather Service
[NWS] site. Temperatures are generally reported on the hour (in NWS practice),
such that the 8 AM temperature (actually measured a few minutes before 8 AM)
might be 75 degrees. Is that the number we should put in the input for 8 AM?
We are using the observed temperatures at the beginning of each hour not the average
temperature during the hour. For more information on temperature, see the MOBILE6
Technical Guidance document (available on the MOBILE6 Web site).
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Q. What about Daylight Savings Time and hourly temperature inputs? This question
may be important when using MOBILE6 outputs as inputs to ozone models, since
ozone models use Standard time. Do you mean Daylight time (Standard time plus 1
hour), such as prevails during the summer over most of the US, or do you mean
actual Standard time? Normally, NWS historical data are reported as at Standard
time—if it's in Daylight time, the report will usually say so.
The difference between Standard and Daylight time may also be important when users put
in fractions of VMT by hour of the day (User Guide, Section 2.8.8.2.b) and similar inputs.
Both the activity data and the temperature data assume that the times (i.e., hours of the
day) are "clock time" which in the summer is usually Daylight Savings Time. However,
practically speaking, shifting the hourly temperatures by 30 to 60 minutes should make
little difference. The most important thing is to make sure the inputs are all consistent
with each other and with the models that generate MOBILE inputs and process MOBILE
results.
Q. Are the temperature limits for the HOURLY TEMPERATURES command the
same as for Min/Max Temperatures?
Yes.
Input Data / Fuel Parameters
Q. MOBILE6 requires the user to enter a nominal fuel RVP; however, the (descriptive)
output shows both a nominal RVP and a weathered RVP. What is the difference?
How has this changed since MOBILES?
The Reid Vapor Pressure (RVP) of the fuel is a measure (in units of pounds per square
inch) of the volatility of the fuel.
The "Nominal RVP" represents the volatility of the fuel when it is first pumped into the
vehicle. The most volatile components of this fuel begin to evaporate shortly after the fuel
is dispensed, resulting in a lower volatility for the fuel remaining in the vehicle's gas tank.
The "Weathered RVP" is the RVP of this usually less volatile "tank" fuel. This use of
"Weathered RVP" in MOBILE6 is identical to its use in the previous version of MOBILE
(i.e., MOBILES).
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Q. Are there any user inputs in MOBILE6 for diesel fuel that I need, such as diesel fuel
sulfur level, ULSD, or cetane number?
The sulfur content of the diesel fuel is a required input when you estimate particulate
matter (PM) or toxics (HAPs) emissions. You would then use the command:
DIESEL SULFUR : 340
Assuming (of course) the sulfur content of the fuel were 340 ppm. (See Section 2.8.10.3
of the MOBILE6.2 User Guide.) That is the only command for fuel properties of diesel
fuel in MOBILE6.
Q. When I use the OXYGENATED FUELS command the output does not match the
input numbers. What is wrong?
You are correct. MOBILE6 is ignoring the OXYGENATED FUELS command because
you also specified an RFG program using the FUEL PROGRAM command. (See "Tips"
in Section 2.8.10.4 of the Users Guide.) In this case, the oxygenated fuel parameters of
your RFG program take precedence over the OXYGENATED FUELS command.
Q. I am trying to calculate the individual benefits of Phase II RFG and NLEV
programs in year 2002. While results for RFG benefits looked OK, a NOx disbenefit
(i.e., increase) was observed in the case of NLEVs. What is the reason behind an
increase in NOx emission factors instead of an expected decrease when "NLEV" is
used?
You are seeing the effect of gasoline sulfur on NLEV vehicles. NLEVs are designed to
work with low sulfur fuel and do not tolerate high sulfur levels well. As a result, NLEV
vehicles run on high sulfur fuel often exhibit higher emission levels than Tier 1 vehicles
run on the same fuel, even though their emission standards are tighter. This disbenefit
will disappear once low sulfur fuels (required by the Tier 2 rules) become available.
Q. Will MOBILE6 allow me to change "x" percent of certain vehicle classes to run on
ethanol?
You can control the "market share" of oxygenated fuels in MOBILE6 using the
OXYGENATED FUELS command (or the OXYGENATE command if air toxics are
needed). The market share controls the fraction of all vehicles which use an oxygenated
fuel. Although you cannot restrict the use of oxygenated fuels to specific vehicle classes,
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you can always run the model multiple times, where each run uses a different fuel
assumption and then retain only certain vehicle class results from that run.
Q. I am using MOBILE6.2 to estimate toxic emissions. The Users Guide indicates that I
must use the OXYGENATE command to do this. Since the local gasoline is not
oxygenated, I specified a market share of zero for each of the four oxygenates.
However, MOBILE6.2 generated the following error message:
"M 9 Error:
Inconsistent Alcohol Blend input encountered"
What am I doing wrong?
With the OXYGENATE command, you are required to enter (as you did), for each of
four (4) oxygenates, both the oxygenate content in the fuel (by volume percentage) and
the market share.
It seems to make sense that if the market share is zero then the oxygenate content is
irrelevant. However, as a hold over from the MOBILES code, when the market share is
zero the oxygenate content must also be zero. Therefore, if you change all four of the
contents to zero, then your input file should run.
Input Data / Time of Day
Q. Does the data entry for VMT by Hour start at 6 a.m.?
Yes. As with the other MOBILE6 commands, hour starts at 6:00 a.m.
Input Data / Air Conditioning
Q. Will we be able to put in a percent AC use in order to see what effect [i.e., increase
in emissions] will occur with and without AC use?
Not directly. Air conditioning activity is controlled by user inputs of temperature,
sun/cloud parameters, and humidity. As a practical matter, however, input of a low
temperature, e.g. below 70 degrees, will eliminate any effects of air conditioning usage
from the emission estimations.
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Input Data / Altitude
Q. "ALTITUDE" is a parameter that won't change for an area from one scenario to
another, but the documentation requires this to be in the scenario section. Does it
need to be repeated with each scenario record? Can it be moved to the RUN section
to apply to all scenarios?
For high altitude areas, the ALTITUDE input does need to be input for each scenario.
However, EPA can see that making "ALTITUDE" a run level command would be more
convenient since it's not likely to change and we have added that to our list of changes
desired for future versions of the model.
Q. Is there a recommended division for which altitudes use "Low" and above which
altitude you use "High"? We have traditionally used the "High Altitude" option
even though at 4500 feet we are below the 5500 foot threshold defined in the
documentation. What is the appropriate altitude parameter for us?
See the MOBILE6 Technical Guidance document (available on the MOBILE6 Web site).
Input Data / I/M Programs
Q. How do I model a traditional I/M program that transitions to an OBD I/M
program?
You need to model two I/M programs. You can refer to the User Guide "Tip" for the I/M
MODEL YEARS command.
Q. It is said that the files ASMDATA.D and TECH12.D are required if the user wants
to model the effects of I/M programs. Is modeling the effects of I/M programs a
standard feature of the MOBILE6 model and must those two files be accessible by
the command input files?
Since modeling the effects of I/M programs is a standard feature of the MOBILE6 model,
it makes sense to consider these two files (ASMDATA.D and TECH12.D) as files that
are necessary to use the MOBILE6 application. If I/M programs are not described in the
command input files, then these two files are not used.
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Q. Can MOBILE6 be used to model I/M programs based on remote sensing and "clean
screening" as well as the ability to change the ASM cutpoints?
No. These capabilities were in EPAis original plan for MOBILE6, but are quite
complicated to model and were eventually dropped for lack of resources. They will be
considered in future versions of the model.
Q. For an IM240 program, why do you have to use both the I/M CUTPOINTS and I/M
STRINGENCY commands? Shouldn't the cutpoint command replace the stringency
input?
This is an unfortunate artifact of MOBILES. We have added this to the list of possible
changes for future versions of MOBILE6.
Q. With respect to I/M cutpoints in MOBILE6:
a. Will the MOBILE6 "package" include a sample or typical I/M CUTPOINTS
table CUTPOINT.D?
b. I'd like to suggest that these sample "external" files (like CUTPOINT.D) contain
comments which explain their purpose and where additional information can be
found.
The MOBILE6 package contains CUTPOINT.D and example files for most of the input
commands which require an external data file. These files are examples only and do not
necessarily represent "typical" inputs, unless they have the file extension .DEF indicating
that they are intended to replicate the default data contained in MOBILE6. They are
commented to aid users wishing to modify them for their own use.
Q. In MOBILES the user specified the program start year as the first year that an I/M
program was in place, regardless of whether it had changed over the years. This was
necessary because of the way in which the tampering offset was handled. Does this
still hold for MOBILE6, or is it OK to set the program start year to the actual year
of the program when multiple programs are implemented?
In order to obtain the full effect of tampering deterrence and to correctly model an
uninterrupted I/M program, users will still have to enter the program start year as the first
year that an I/M program was in place (not just the current version of the I/M program).
(This is explained in more detail in the Users Guide.)
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Q. In the I/M program portion of the users' guide for MOBILE6, there is a frequent
mention of a column number. I would like to know if the values entered in the I/M
descriptive file would have to be repeated in the columns specified and where they
exist.
The I/M DESC FILE command allows you to describe the I/M program in an "external"
data file rather than within the command input file. You would enter the I/M DESC FILE
command in the command input file along with the file name (and path, if necessary) to
locate the file containing the I/M program description. No other I/M command (other than
the I/M DESC FILE command) needs to be located in the command input file. All of the
other I/M commands which describe the program (i.e., I/M Program, I/M MODEL
YEARS, etc.) can be located in the external file. All of the commands located in the
external data file must be in the same format that they would appear if you had entered
them in the command input file.
Q. In the "Grace Period" field, is the number of years specified, deducted from the last
model year entered in the "I/M Model Year" field or is it considered from the
current year? Example: In calendar year 2001, if the model years subject to the I/M
Program are 1967 -1996, and if the Grace Period is set to 5 years, is 5 deducted
from 1996 to set the effective coverage up to the model year 1991 or is it deducted
from 2001 to set the effective coverage to the model year 1996?
In the case of the I/M GRACE PERIOD command, age 1 refers to the current model year
(same as the evaluation calendar year). So, if the grace period is set to 5 in calendar year
2001, then there will be no I/M benefit from model years 1997 through 2001. The grace
period calculations are independent of the I/M MODEL YEAR command values. So, in
your example, where only 1967 through 1996 model years are included in the I/M
program, setting the grace period to 5 in calendar year 2001 will have no effect on I/M
benefits.
Input Data / Mileage Accumulation Rates
Q. Are comments allowed before each block of data in an external file used with the
MILE ACCUM RATE command?
Yes, as shown in the MILEDAT.D example file.
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Input Data / Natural Gas Vehicle (NGV) Fractions
Q. Do you have to provide data for all 28 vehicle types when using the NGV
FRACTION command?
Yes. One way to deal with this is to construct a "template" file of all zeros from the
NGVFR.D file provided, and then to change the fractions for the vehicle types you want.
Input Data / Starts per Day
Q. Relative to the STARTS PER DAY command, what exactly is a "trip"? Does your
definition of an engine start refer to a round trip? I ask because your example uses
1.0 for weekday starts which says to me that the average driver goes some place and
does not come back.
A trip in MOBILE6 is defined by whatever travel occurs from when the engine is turned
on to when it is turned off. You have correctly interpreted the example file, but these files
are not intended to demonstrate the full range of the program or even to be realistic. They
are simply examples of valid input data to the model.
Input Data / Barometric Pressure
Q. Now that MOBILE6.2 has an input for barometric pressure, can I use this
parameter to determine the effects of changes in altitude?
No, the new BAROMETRIC PRES command ambient pressure value is only used to
convert hourly relative humidity values provided by the user with the RELATIVE
HUMIDITY command to absolute humidity units. The absolute humidity values are
needed to calculate the NOx humidity corrections. No other adjustments use the
barometric pressure value. The effects of changes in altitude on emissions can only be
modeled using the ALTITUDE command.
Output
Q. Can the same run produce both a descriptive output and a database output?
Yes. If the DATABASE OUTPUT command is used, and nothing is done to suppress the
descriptive output, then both outputs will be produced.
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Q. In the descriptive output, is there a way for MOBILE6 to output the emission rates
for all exhaust, all evaporative (evap), and exhaust+evap as three separate values,
without the full detail? Is it true that with either the "EXPAND EVAP" or the
"EXPAND EXHAUST" command, you still get composite emissions, plus the extra
detail?
There is no way to show the emissions rates for all evap, all exhaust, and exhaust plus
evap, without using both the EXPAND EVAP and the EXPAND EXHAUST commands.
Composite emissions are always reported.
Q. The use of 6 am as the starting point, as discussed in the User Guide Section 2.6.6.3,
may cause confusion. Does the hourly output also start there, such that emissions
are reported over parts of two days?
The hourly output does also start at 6 AM, as explained in Chapter 3 of the User's Guide.
This may be somewhat confusing, but is at least consistent.
Q. DATABASE FACILITIES - If this command is used, do you still get an all facilities
output?
This command has NO effect on the aggregated output and should not be used with the
aggregated database command. It should only be used with the daily or hourly database
output formats. "All facilities" results do not exist in these output formats. Instead, every
database output record includes a value of FTYPE. A particular value of FTYPE, namely
"NONE," is used to report emission types, such as hot soak losses which do not occur on
roadways.
Q. DATABASE VEHICLES and AGGREGATED OUTPUT - Can you use these
commands to get an "all vehicles" output?
These commands ONLY affect the database output. The database output never includes
"all vehicle" results. To get "all vehicle" results, you must either use the descriptive
output, or externally manipulate the aggregated database output to sum emission factors
across vehicle classes.
NOTE: The "all vehicle" emissions should be computed as a weighted sum across vehicle
classes, with weighting by VMT fractions ("VMT").
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Q. Will there be a way to get hourly emission factors that are aggregated by age and
emission type? Section 3.1.3 of the User Guide implies that we will only be able to
aggregate *daily* emissions by age, roadway and emission type.
While MOBILE6 does not directly produce this output, it is possible to derive it from the
hourly database output.
We realized early in the MOBILE6 process that we would not be able to produce a result
output format that would meet every need. Instead of expanding the number of output
options, we now have a "database" output which is designed to include everything that
should be needed to create a variety of output formats by aggregating the database output
results.
Output / Alternate Units
Q. Are all emissions always reported in g/mile, including non-running emissions? If the
output is always in g/mile, how can a user get a "per start" (or "per trip" or
whatever) emission mass, or total start emissions mass, in a logically consistent way?
MOBILE6 does always report emissions in terms of grams per mile per vehicle. In the
database output results also are reported in grams per time period per vehicle. While
MOBILE6 never reports in units such as "per start" or "per trip," the user can get "per
start" results by using the database output of emissions and the number of starts to
calculate the desired result. (See also the questions under the "Emissions / Start
Emissions" heading.)
Q. Why does MOBILE6 calculate emission rates (grams per mile) instead of
inventories (tons per day)?
MOBILE6 database output is intended to facilitate user calculation of a wide variety of
alternate output results. With emissions in grams per mile, users can easily apply the
results to their own VMT per day estimates to calculate inventory results.
Q. How can I best convert MOBILE output from grams per mile (g/mi) to other units
such as "tons per million VMT," pounds per thousand VMT, and grams per vehicle
kilometer traveled?
As your question makes clear, there are a lot of output options that people might find
useful. The database output lets you produce results in an electronic form that can be
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opened in a spreadsheet. Then you can easily apply the VMT estimates and units
conversion of your choice.
Q. I need to calculate refueling loss emissions in grams per gallon units. With
MOBILESb I set HCFLAO3. How do I do this in MOBILE6?
MOBILE6 will produce estimates of refueling emissions in units of grams per mile.
Multiplying those values by fuel economy (in units of miles per gallon) will give you
refueling emissions in terms of grams per gallon. Thus, the question is how to obtain the
fuel economy estimates.
If you are working with the database output, the fuel economy estimates are provided.
If you are working with the descriptive output, you can direct MOBILE6 to print out the
fuel economy values by using the POLLUTANTS command and specifying CO2. If you
are interested in only refueling emissions, you would need to specify only the CO2 and
HC.
NOTE: The POLLUTANTS command should be added to the "header" section of your
input file. (See Section 2.8.3.1 of the User Guide.)
Output / Evaporative Emissions
Q. Is composite VOC assumed to be exhaust VOC only? Are evaporative emissions
included? Do I need to subtract evaporative emissions from VOC to avoid double
counting?
As in MOBILES output, the composite VOC output, which is always displayed in the
descriptive output, contains both exhaust and evaporative emissions. The EXPAND
EXHAUST and EXPAND EVAPORATIVE commands add more detail but do not affect
the composite VOC output values. If you need evaporative and exhaust VOC emissions
separately, you will need to either subtract the total evaporative emissions (obtained by
using the "EXPAND EVAP" command) from the composite VOC values or expand the
exhaust emissions (using the "EXPAND EXHAUST" command) and subtract the total
exhaust VOC value.
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Output / Vehicle Types
Q. The MOBILE6 User Guide sometimes categorizes vehicles into 16 vehicle categories,
yet Table 1.2.3 lists 28 vehicle classifications. What is the proper (best) breakdown
for registration data input?
MOBILE6 calculates emission factors for 28 individual vehicle categories. However,
many of these categories form gasoline/diesel pairs (e.g., light-duty gasoline-fueled
vehicles / light-duty diesel-fueled vehicles). Registration data are often available only for
these 16 composite (i.e., paired) vehicle categories. Thus, when registration data are
entered into MOBILE, they are in the form of these 16 composite vehicle categories.
MOBILE uses these data plus the diesel sales fractions to calculate (internally) the vehicle
distributions for all 28 individual vehicle categories
Appendix B of the MOBILE6 User Guide defines the 16 composite registration
categories , 14 composite vehicle types used for diesel sales fractions, and the full 28
individual vehicle class categories.
Q. If I change only the VMT fractions (which should only affect the resultant "All
Veh" composite EFs, not individual vehicle type EFs), why does the output file
indicate changes for the LDDT and the LDGT(all) vehicle categories?
You are partially correct. Changing the VMT fractions will not affect the emission factors
for the individual vehicle classes. However, the classes "LDDT" and "LDGT" are
composite (not individual) classes of two and four individual LDT classes, respectively
and, therefore, will change with changing VMT fractions. Similar situations exist with the
busses and heavy-duty truck classes.
Output / Messages (Error / Warning)
Q. Why do I always get a warning message regarding no sales for HDGVSb? The
default input data does not include any of these vehicles.
If a user supplied their own inputs which caused no sales for some other vehicle category,
one would want to see such a warning.
Output / Spreadsheet
Q. How do I obtain just the exhaust HC emissions using the spreadsheet output?
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The Spreadsheet output was designed to report the same numbers as the descriptive
output. Since the descriptive output reports the HC results as the sum of the evaporative
and exhaust HC emissions, so does the spreadsheet output. To obtain the exhaust (only)
HC, subtract the sum of the evaporative components from the total HC emissions.
Output / Aggregating
Q. Can Mobile6 produce TRAVEL WEIGHTED EMISSION RATES BY MODEL
YEAR (as in MobileS)? Can Mobile6 CONSOLIDATE emissions by VEHICLE
CLASSES in the output?
MOBILE6 can generate emission rates by model year and vehicle class, using the
DATABASE OUTPUT command option. This additional output, however, is in an
disaggregate form and must be aggregated either by hand (such as in a spreadsheet) or
electronically (using database software). This aggregation process must use VMT
weighted sums, rather than simple sums. All of the fields that you may need to make any
sort of custom aggregation report are available in the database output. MOBILE6 does do
some simple aggregations, such as DAILY OUTPUT, which aggregates over the hours of
the day, and AGGREGATE OUTPUT, which aggregates over the model years, roadway
types and emission types.
Q. I have had extreme difficulty in aggregating some of the database output
information. How do you calculate emissions for each facility type and vehicle type?
There are two solutions to your problem.
1) You can restrict the output of the model to a single facility type using the VMT
BY FACILITY command. For example, if you wanted freeway emissions, you
would enter fractions for freeway and freeway ramp VMT and set the
arterial/collector and local fractions to zero. This will require more MOBILE6
runs, (separately for each roadway type) rather than processing the full database
output. The AGGREGATED OUTPUT command could still be used to create
aggregated machine readable output using this approach. If you are uncomfortable
using database or statistical applications, I would strongly urge you to consider
this option.
2) If you plan to regularly produce the same type of output repeatedly, you could
create a database application (using applications such as FoxPro or MS Access)
which will do the appropriate aggregation of the database output. This approach
would require acquiring the necessary skilled workers (perhaps using a contractor)
to create the database application and would require knowing the proper way to
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aggregate the values. But once the application was available, you would be able to
run MOBILE6, then run the database application to create the custom output
anytime you needed. We anticipate that such custom applications might be needed
and hope that as States (and others) created these tools, that they will share them
with other MOBILE6 users. The needs of States vary so widely that it is not
possible for EPA to create custom output for every possible situation.
Q. I am going to do some MOBILE6 runs to try and estimate annual emissions for each
vehicle per facility type. In order to minimize having to aggregate data in the
database output, I plan to do runs for one facility type at a time and then use the
descriptive output. I am using external data files for VMT for each facility, and
specifying that 100 percent of the travel is in that facility type. Why does this need to
be done? I want to be sure my results are reflecting real life type emissions where
the VMT is actually split amongst the different facility types, even though I am only
looking at one of the facilities.
Remember that MOBILE6 only reports emission *rates*, not tons. The emission rate for
each roadway is unaffected by whether more or less travel occurs on other roadway types.
Only when you combine the rates (to get an overall emission rate) do you need to know
how much of the total travel (VMT) occurs on each roadway type. This is the VMT
distribution by facility. You don't get tons until you multiply the emission rate (in grams
per mile) times the appropriate miles traveled.
If you want to demonstrate this, you can run MOBILE6 four times (once for each
roadway class, setting the VMT BY FACILITY command input file to 100 percent of the
roadway type of interest) and then again setting the VMT by facility to a fixed
distribution for every hour. The weighted sum (using the VMT by facility distribution
values) of the individual roadway runs and the single MOBILE6 run using the VMT by
facility should be the same. It is the VMT by facility distribution weighting that allows
MOBILE6 to report a single emission rate (in grams per mile) that reflects all roadway
types. But this distribution is not needed if you intend to combine the roadway types
outside of MOBILE6.
MOBILES Versus MOBILES
Q. Do I need to understand MOBILES in order to use MOBILE6?
No. It is not necessary to understand MOBILES to understand and use MOBILE6. The
MOBILE6 User Guide does not assume that readers are already familiar with MOBILES.
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If, for whatever reason, people need to understand MOBILES, it has its own User Guide,
which is EPA report number EPA-AA-AQAB-94-01. Chapter 5 of the MOBILE6 User's
Guide explains how to translate MOBILES input files to MOBILE6.
MOBILES v. MOBILES / MOBILES Function Like MOBILES
Q. Is it possible to use MOBILE6 with only the 8 vehicle types familiar to MOBILES?
How is this done?
MOBILE6 descriptive output has the familiar eight vehicle types as output. However,
users must address all of the 28 MOBILE6 individual vehicle types when entering local
data. The MOBILE6 database output uses all 28 vehicle classes but users can manipulate
the database externally to aggregate to the eight MOBILES types.
Q. Can the user model multiple calendar year scenarios under a single set of input
parameters similar to MOBILES?
Yes. As in MOBILES, there are parameters that can be entered once for all scenarios
(where each scenario represents a calendar year) in the run section of a MOBILE6 input
(similar to the "one time data" section in MOBILES). There are also, however, parameters
that must be entered for each scenario.
Distributions
Q. How do the REG DIST, MILE ACCUM RATE, AND VMT FRACTIONS inputs
interact in the model? Are there potential problems with conflicts between these
inputs? For example, if I use local information for VMT FRACTIONS and the
defaults for the other two inputs, am I presenting the model with contradictory
information?
Disregarding the VMT FRACTIONS input for a moment, the way this information
interacts in the model is as follows: For a given calendar year MOBILE6 contains vehicle
count information for the 16 combined vehicle classes. A default registration distribution
allocates these vehicle counts to the 25 most recent model years; the diesel fraction
information then splits the 16 combined vehicle classes into the 28 individual vehicle
classes for all 25 model years; and finally the mileage accumulation information is
applied to that 28 x 25 matrix.
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User input of REG DIST, MILE ACCUM RATE, and DIESEL FRACTION information
presents no special problems. The user input information is just used instead of the
default information in the process described above.
Note that the user is not allowed to supply new vehicle count information as such.
If VMT FRACTION information is input, which is done in terms of the 16 combined
vehicle classes, MOBILE6 internally adjusts its vehicle counts to avoid the problem with
"contradictory information" that you are concerned about.
However, it is possible for contradictory vehicle activity information to be input that
MOBILE6 does not check for. It would be contradictory for example to input low mileage
accumulation rates in combination with a high numbers of starts per day and high
percentages of mileage being accumulated on long trips.
Distributions / VMT
Q. Am I correct in understanding that the VMT and speed profiles used in M6 will
generate a single emission rate for the total daily VMT for the year being modeled,
and that the rate will reflect all of the speed variations during the course of a day?
Yes.
Q. VMT profiles (by speed, hour, and facility) change over time. Can multiple years be
combined in the same file, or is it necessary to create a separate file for each year
that is profiled?
You will need a separate VMT profile for each calendar year.
Q. What is the default VMT mix for MOBILE6 for ALL years - 19xx to 2050? This
question refers to the VMT FRACTION record of VMT accumulation data by the
16 vehicle types that is coded directly into the scenario section of the model, not to
the mileage accumulation rate data by model year (MILE ACCUM RATE).
The default VMT mix is calculated by MOBILE6 and changes by calendar year. A table
of the VMT mix can be created using the AGGREGATED OUTPUT command, which
generates a database output that includes the VMT by vehicle class.
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Q. Does the EPA default VMT mix change over time? In particular, does EPA
anticipate a higher percentage of trucks in the future?
The default VMT distribution by vehicle class will change over time, since the default
values are computed from the annual mileage accumulation rates, registration
distributions, diesel sales fractions and the vehicle counts by calendar year. The vehicle
counts also change over time. The default data do anticipate a higher percentage of trucks
in the future.
Q. Our local traffic data show that VMT mix (vehicle types) varies greatly by facility
class. To model this with M5 we have defined a different local area VMT for each
facility class. How would this be done with MOBILE6? Would it be necessary to run
three scenarios for each calendar year - one for freeways & ramps, one for arterials,
and one for locals - apply the corresponding VMT mix, and adjust the VMT profiles
accordingly? Or would we have to develop a single composite VMT mix from the
three facility classes, weighted by the relative traffic volume of each class?
Technically, you could do either. However, a third option might be better. You can use
the VMT BY FACILITY command to enter different distributions for each of the 28
vehicle classes.
Q. Does the (national) default VMT mix change by road type?
While it seems common sense that HDVs are more common on freeways than on local
roads, we lacked the data necessary to determine national default VMTs by both vehicle
type and roadway type. Our approach was to have the defaults NOT vary by roadway
type, but to give the users the capability to vary their VMT mix by roadway type, using
the VMT BY FACILITY command.
Q. The User Guide explanation of the VMT by Facility states that the distribution
fractions are for "freeway, arterial, local, and ramp". Are collectors included as
arterials or as locals?
As arterials. The User Guide provides more detail in Section 1.2.5 which introduces
"Roadway Classifications."
Q. The VMT BY FACILITY command is expressed in terms of hours of the day. So
what does VMT BY HOUR do? Would you ever want to use both? Do these inputs
have an impact on CO and NOx emissions, or just HC?
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VMT BY HOUR takes daily VMT and distributes it by hour. VMT BY FACILITY
distributes each hour's VMT among the facility types. We expect most users will use
both. The inputs affect all pollutants because VMT BY HOUR determines the relevant
hourly temperature and VMT BY FACILITY allocates emissions among different speed
curves.
Q. Can MOBILE6 show me what would happen if people switched their mode of
transportation to walking or the bus?
All results from MOBILE6 are in terms of grams per mile of travel per vehicle.
Inventories (in tons) are determined by multiplying the grams per mile times the total
vehicular travel during the time period of interest. If walking reduces the total miles of
vehiclular travel needed, then this adjustment is made outside the MOBILE6 model.
However, it may be necessary to reduce the fraction of all miles traveled by passenger
cars to account for the reduction in total miles driven in relation to other vehicle classes.
For this same reason, it may be necessary to increase the fraction of vehicle miles traveled
(VMT) for buses to account for the increase in the number of buses utilized to carry the
additional passengers. Changes in the distribution of VMT between vehicle classes is
done using the VMT FRACTIONS command. Additional changes may be needed in the
number of trips per day per vehicle (STARTS PER DAY command), the distribution of
VMT by hour of the day (VMT BY HOUR command), the engine soak time (START
DIST command and the SOAK DISTRIBUTION command) and perhaps even the
roadway speeds (SPEED VMT command) to account for the change in the congestion
levels on roadways. I would suggest that you use an activity "model" that properly
accounts for the interaction between all of these activities, even if it is just a spreadsheet.
Distributions / Speed
Q. I do not understand the MOBILE6 assumptions about speeds and roadway types.
Since MOBILES did not have roadway type as integral to the outputs, it would be
helpful to get more information on this. It wasn't clear to me what speed fractions
are, or what is meant by the phrase "allocate VMT by speed."
MOBILE6 computes emissions on four roadway (or "facility") types. These are listed in
Section 1.2.5 of the MOBILE6 User Guide. Local roadways and (freeway) ramps have
default average speeds that cannot be changed. Freeways (excluding ramps) and arterials
have a distribution of average speeds that the user can change with the "SPEED VMT"
command.
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Basically, there are fourteen average speed bins, where each bin represents a range of
average speeds. The values in the fourteen bins represent the distribution of those average
speeds by the vehicle miles traveled at those speeds. In a traffic network model, each link
usually has a distance (miles), load (number of vehicles) and average speed. The distance
times the number of vehicles is the vehicle miles traveled (VMT). The average speed
determines into which bin the VMT is placed. Once all of the VMT is placed, the sum of
the VMT in each bin is divided by the sum of all VMT in all bins to give a distribution.
The sum of the distribution values should be one.
Q. Why is there no speed distribution input for ramps and local class facilities? What is
the assumed speed profile for these roads?
The document, " Development of Speed Correction Cycles," (M6.SPD.001) found on our
web site (www.epa.gov/otaq/m6.htm) discusses how the local roadway and freeway ramp
cycles were developed, the details of the cycle profiles, and why these two roadway types
do not have a dependence on average speed. There are several reasons. The most critical
was that developing a speed dependence would have required additional vehicle testing.
Instead, we decided to focus our testing on the freeway and arterial/collector cycle testing.
The default daily average speeds for each roadway type are given in the MOBILE6 User
Guide (Section 5.3.4).
Q. Can I trick the model into giving me Local Facility emission rates by using
Arterial/Collector Facilities at a low average speed?
It is technically possible to model arterial/collector facilities at low speeds. See the
MOBILE6 Technical Guidance document (available on the MOBILE6 Web site) for
information on correctly assigning facility type to local roadways.
Q. In Chapter 5 of the User Guide, the section headed Average Speed for Freeways
states: "When modeling freeways exclusively, if current estimates of average speed
do not include freeway ramp travel, then the current speed estimate should be
directly used for the SPEED VMT command input. The speed may need to be
distributed among the average speed bins..."
Does this mean that, assuming my freeway average speeds do not include freeway
ramp travel, I could use Table 3 in Appendix F, the Arterial/Collector Average
Speed table, to convert MOBILES average freeway speeds to MOBILE6 freeway
speeds?
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Yes. If your freeway distribution of average speeds does not include ramps, then you can
use the arterial table.
Q. Is it possible to model a single average speed using MOBILE6 the same way that it
was used in MOBILES?
Yes, use the AVERAGE SPEED command.
However it is important to note that even a single average speed represents a trip-length
average of many cars traveling over a driving schedule, not the instantaneous speed of a
single vehicle. Like MOBILES, MOBILE6 is not really designed for micro-scale
modeling.
Q. Is the "SPEED VMT" command the only way to change roadway speeds? This is
very detailed, requiring over 600 data entries.
The AVERAGE SPEED command can be used in some cases. When using the SPEED
VMT command, the best approach is to modify an existing external input file for this
command rather than building one from scratch.
Distributions / Soak Time
Q. Do the various SOAK inputs affect CO and NOx emissions, or just HC?
SOAK DISTRIBUTION is used to allocate hot and cold starts, so it affects all exhaust
start emissions (HC, CO and NOx). HOT SOAK ACTIVITY and DIURN SOAK
ACTIVITY affect hot soak and diurnal evaporative emissions (HC only).
Distributions / Diesel Sales
Q. To calculate the diesel sales fraction for LDDTs, the User Guide asks you to
consider the total sales of LDTs for both gas and diesel. Does that mean we should
include LDGT2s as well?
MOBILE6 needs *four* sets of diesel sales fractions for light-duty trucks, one for each of
the four weight categories:
o LDT1 (0-6000 Ibs GVWR, 0-3750 Ibs LVW)
o LDT2 (0-6000 Ibs GVWR, 3751-5750 Ibs LVW)
o LDT3 (6001-8500 Ibs GVWR, 0-3750 Ibs LVW)
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o LDT4 (6001-8500 Ibs GVWR, 3751-5750 Ibs LVW)
So, you need to know the total sales for all trucks (gasoline and diesel) for each model
year within a weight category. Then, you need to determine for each model year what the
fraction of total sales were diesels.
Ideally, you would have the necessary vehicle counts to do this. However, in calculating
our national default diesel sales fractions, we could not find sales data split by loaded
vehicle weight (LVW), so our diesel sales fractions are the same for LDT1 and LDT2 and
are the same for LDT3 and LDT4. We expect this will be true for many users.
Q. Please clarify the last paragraph in the Users Guide's discussion of "Diesel Sales
Fractions" (Section 5.3.2).
That paragraph references Appendix E which provides the default MOBILE6 diesel sales
fractions by model year. In copying these values to a command input file, the user must
begin with the scenario calendar year and select values for 25 vehicle ages (i.e., model
year). The values in that appendix are identical to the values in example file,
DIESFRAC.DEF, which is provided as part of the distribution of the MOBILE6
program."
The default diesel sales fractions are associated to model years, not ages. Consequently, if
you plan to model several different calendar years (each year being a separate scenario),
you would need to create a different set of diesel sales fractions (i.e., a separate external
file) for each calendar year, if the internal default values are not used.
The reference (in Section 5.3.2) to having sales fractions repeated refers to the fact that
the default diesel sales fractions used in MOBILE6 for model years before 1972 are the
same as for the 1972 model year (repeated) and those for model years newer than 1996
are the same as for the 1996 model year.
You can test your understanding of how this works by entering the default values as a
user input. You must shift the numbers for each calendar year in order that the diesel sales
fraction for each model years aligns with the appropriate age for that calendar year. You
will know that you did it right if the VMT fractions are identical to the case where you do
not use the DIESEL FRACTIONS command.
Distributions / Vehicle Mix
Q. In MOBILES, vehicle mix was used *only* to create aggregated emission factors
across all vehicle types. Is this still true?
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Yes, though such aggregations do not need to involve all vehicle types. They are, for
example, used to estimate the LDGT (all) emissions in the descriptive output. Also, the
command "vehicle mix" was the term used in MOBILES. In MOBILE6, the command is
"VMT FRACTIONS."
Distributions / Registration Data
Q. The technical guidance and the MOBILE6 User Guide Chapter 5.0 both say to use
the table in Appendix D to weight together MOBILES based gasoline and diesel
heavy duty registration distributions to get MOBILE6 heavy duty age distributions.
However, some areas may have additional local information about the populations
of gasoline and diesel heavy duty vehicles, in addition to the MOBILES heavy duty
age distributions. Should areas use their local populations of gasoline and diesel
heavy duty vehicles or use the values in Appendix D instead?
Areas * should* use their local populations of gasoline and diesel heavy duty vehicles to
weight together the age distributions, if such information is available. The values in
Appendix D should only be used if better (local) information is not available, which
would be true if the user only had the MOBILES input files
Q. Since I am modeling a typical summer day inventory, I am running MOBILE6 with
the evaluation month being July. When I obtain the age distribution of the fleet
from the Department of Finance and Administration, do I obtain January fleet ages
and then MOBILE6 will automatically grow the fleet the 6 months, or do I obtain
July fleet ages and MOBILE6? I am confused how the model does grow the fleet, if
it does. Or, should I obtain both January and July fleet ages and use the
appropriate data depending on the evaluation month?
In order to override the default distributions in MOBILE6, you need to use the REG DIST
command along with an external data file. That file contains (for each vehicle class) the
25 age fractions, representing the fraction of vehicles of that age in that composite vehicle
class in July. MOBILE6 will use these fractions directly if a July evaluation month is
requested or will convert them to January if the user requests that evaluation month.
Therefore, you need only obtain/determine the fleet distributions for the month of July
(regardless of which month the model is evaluating).
Emissions
Emissions / At Single Average Speed
Q. Can MOBILE6 be used to model a single average speed?
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Yes, there are two approaches to modeling a single average speed in MOBILE6. Note,
neither of these methods produce an "instantaneous" or "steady-state" speed, rather the
result is an average over a driving cycle.
One approach is to use the speed distribution input using the instructions in Chapter 5.
A second (and less complicated) approach is to use the AVERAGE SPEED command.
This command still uses the same 14 speed bins, so it interpolates to get the specific
speed value. This method automates the approach described in Chapter 5 (and referenced
in the previous paragraph). Therefore, these two approaches will produce identical
results.
Emissions / At Idle
Q. How can MOBILE6 be used to estimate vehicle idle emission rates?
We did not include the calculation of idle emission rates as such in MOBILE6. See the
MOBILE6 Technical Guidance document (available on the MOBILE6 Web site) for
information on how to estimate idle emission rates.
Emissions / Start Emissions
Q. Can MOBILE6 be used to estimate start emissions in grams per start?
Yes. They can be calculated from the database output. The values in the database output
for GM_DAY (for daily) or GM_HOUR (for hourly) are the grams independent of miles
(VMT). The STARTS column indicates, for that row, the number of engine starts
assumed for that unit of time (daily or hourly). Grams per engine start would be the grams
value divided by the number of engine starts. This calculation can be done in a
spreadsheet.
However, MOBILE6 does not generate a grams per start estimate for heavy-duty vehicles
or buses. This is NOT because heavy-duty vehicles do not have engine start emissions,
but because we were not able to split the emission rates for the heavy-duty vehicles into
engine start and running portions. This means that any aggregation of engine start
emissions can only apply to motorcycles and to light-duty vehicles and trucks. (See also
the questions under the "Output /Alternate Units" heading.)
Q. When I used the database output to calculate start emission in units of "grams per
start" [see preceding question], I found that the value varied significantly hour by
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hour (3.83 g/start - 1.39 g/start). Is this due to the increased fractions of hot starts?
If so, where does the model indicate the cold and hot operating mode fractions?
How do MOBILE6 model users input locally drawn cold and hot operating mode
fractions? All the previous MOBILE model series provided this optional input,
"operating mode fractions."
The grams per engine start emissions vary because of the change in both the distribution
of engine soak times and the change in ambient temperature by hour. The operating mode
fractions used in MOBILES have been dropped in favor of a combination of the
distribution of soak times (entered using the SOAK DISTRIBUTION command) and the
starts per day (entered using the STARTS PER DAY command). This allows a more
precise determination of the effects of changes in the engine soak time than the 10 minute
(hot start) versus 12 hour (cold start) capability reflected in the operating mode approach
used in MOBILES. Converting operating mode information from a MOBILES input for
use in MOBILE6 is discussed in Chapter 5 of the MOBILE6 User Guide.
Q. Can MOBILE6 output hot starts and cold starts separately?
MOBILE6 calculates engine start emissions at *all* soak times from 1 to 720 minutes.
You can see the emissions for a specific soak time by setting the SOAK DISTRIBUTION
to 100 percent at the soak time of interest.
Q. MOBILE6 provides exhaust start emissions separately from the running emissions;
however, the start emissions are dispersed throughout the entire time period (i.e.,
hour or day) and the database output table shows the start emissions rate per
vehicle mile. If the trip length (VMT distribution either from the model default or
user supplied input) is the variable distributing the start emissions per VMT, it may
cause serious consequences for the ozone conformity analysis for rural non-
attainment areas. For instance, external to external (E-E) trip VMT (usually
interstate freeways) contributes a large portion of regional VMT total, the equal
distribution of non-existent start emissions to E-E VMT would overestimate
regional total emissions.
The engine start emissions from MOBILE6 are also reported in the database output in
units of grams per hour. This can be used to distribute the emissions from engine starts
spatially (i.e., rural versus urban), instead of by vehicle miles traveled. Note, MOBILES
always reported engine start emissions as grams per mile.
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Q. In the descriptive format output, start emissions are reported ONLY at the vehicle
type level. It would be very helpful for modelers if start emissions are reported for
"All Vehicles."
While MOBILE6 introduces the reporting of start emissions, it only calculates start
emissions separately for light-duty vehicles and motorcycles. It cannot calculate them for
"all vehicles", because they are not calculated for heavy-duty vehicles. We believe that
MOBILE6 is an improvement in this area relative to MOBILES, which did not calculate
separate start emissions at all.
Q. Why does the DATABASE OUTPUT provide the split between start and running
emissions differently than the DESCRIPTIVE OUTPUT?
We believe that the arrangement we adopted is the best compromise, given the two
different output formats and the limitations of MOBILE6.
The problem is that MOBILE6 is only able to break exhaust emissions into start
emissions and running emissions for light-duty vehicles and motorcycles. So in the
descriptive output, which is rather "loose" and can contain additional information, we
chose to display a "composite" emissions result for the heavy-duty vehicle classes and to
leave blank their "start" and their "running" emissions. This is not possible in the database
output, however, because there is no place in its database scheme for "composite" results.
Every row in the database output must have a value of ETYPE and we feel that, all things
considered, there should not be an ETYPE for "Composite" because it would overlap
with "Start" and "Running". So in the database output we report the total exhaust
emissions of heavy-duty vehicles as "Running." Admittedly this is not ideal, but, on
balance, we feel it is preferable to having a "Composite" value of ETYPE.
Emissions / Refueling Emissions
Q. How will the benefits of stage II refueling emissions be replaced by those of
Onboard Refueling Vapor Recovery (ORVR) systems. Are there any ideas as to how
to model ORVR penetration? Also is there a projection as to when the effects of
stage II will be totally replaced by those shown by ORVR systems?
MOBILE6 handles ORVR systems the same way as ORVR was handled in MOBILES.
We have proposed no changes in the methods for calculation of refueling emissions or the
effects of ORVR systems on refueling emissions.
Refueling emissions are basically the average displaced grams (5.26) per gallon of
dispensed fuel, plus a small amount of grams (0.31) for spillage. The ORVR effectiveness
is assumed to be 98 percent on the portion of refueling emissions that does not include
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spillage. However, ORVR is also assumed to reduce spillage by 50 percent. Stage 2
controls only affect the portion of the fleet which does not have ORVR systems. No
additional benefits are assumed from Stage 2 for vehicles with ORVR systems.
Tampering with the evaporative control canister is assumed to make the ORVR system
ineffective in reducing refueling emissions, however, these emissions will be affected by
Stage 2 controls.
The effects of ORVR technology is phased in over several model years. For passenger
cars, 40, 80, and 100 percent of 1998, 1999, and 2000 and newer model year cars
respectively will have ORVR. For light duty trucks up to 6000 pounds gross vehicle
weight, 40, 80, and 100 percent of 2001, 2002, and 2003 and newer model year trucks
will have ORVR. For light duty trucks between 6001 and 8500 pounds gross vehicle
weight, 40, 80, and 100 percent of 2004, 2005, and 2006 and newer model year trucks
will have ORVR. All gasoline fueled trucks and busses over 8,500 pounds GVWR are
assumed not to use ORVR technology.
The 2004 FID vehicle regulations extended the effects of ORVR to heavy-duty vehicles,
ORVR is required on all complete HD gasoline vehicles up to 10,000 pounds. The
standard is 0.20 grams per gallon of fuel dispensed. The phase-in is 80 percent in 2005
and 100 percent in 2006, although there will probably be 40 percent "optional"
compliance in 2004.
With all of these phase-in periods, the full fleet effect of ORVR will not be in place until
2030, assuming all vehicles over 25 years old are negligible.
Q. I am trying to estimate the benefits of Stage II using MOBILE6 for analysis year
2007.1 added the command "STAGE II REFUELING" to my run and the results
show a reduction (benefit) in VOC emissions. My question is: Of that VOC benefit,
what can be attributed to Stage II and what to ORVR?
Since the MOBILE6 results (without the STAGE II REFUELING command) already
include the effects of ORVR, the entire reduction in VOC is attributed to Stage II. (If you
need to know the effect of ORVR on refueling emissions you would need to use the NO
CLEAN AIR ACT command to turn off ORVR and observe the effect on refueling
emissions.)
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Emissions / Heavy-Duty / Consent Decrees
Q. Does MOBILE6 include the assumption that there will be an early 2002
introduction of cleaner HDDV engines (2004 standards)?
The model contains the latest version of the defeat device, pull ahead, and rebuild
information that was available as of March 2001. For more information, see report
M6.HDE.003 on the MOBILE Web site.
Q. What does Mobile6 include for the 2005 and 2006 years after the Consent Decree
requirements end and the 2007 standards begin? Is it assumed that NOx emissions
from HDDV remain constant or the current worst-case that emissions would
increase because the Consent Decrees end?
MOBILE6 models no defeat device effects for MY 2005 and 2006. Emissions for those
model years are estimated using regular emission factors based on the 2004 standards.
Q. If DOJ/EPA agree to any delays or changes to the Consent Decrees, will EPA or the
states have to modify Mobile 6 to reflect the assumptions put into the model that
reflect the current decrees?
We expect that any changes would have a small impact on emissions and would not
require a change to the model. If a change did have a large impact on emissions, EPA
would provide guidance and/or modeling tools for properly calculating emission
estimates.
Emissions / Carbon Dioxide (CO2) Emissions
Q. What is the difference between "atmospheric" and "tailpipe" CO2? What is
reported by MOBILE6.2? How should these values be used?
MOBILE6 reports "atmospheric" CO2 as determined by a carbon mass balance equation.
This includes not only the emissions of CO2 formed during combustion (tailpipe CO2)
but also the CO2 formed later in the atmosphere by the natural oxidation of the vehicles'
HC and CO emissions. (See Report Number M6.GHG.001, "Updating Fuel Economy
Estimates in MOBILE6.2".) Current guidelines issued by the Intergovernmental Panel on
Climate Change (IPCC) suggest that estimates of carbon dioxide from petroleum
products such as the CO2 estimates reported by MOBILE6 should be decreased by one
percent (1%) to account for carbon that is never oxidized (i.e., particulate matter). This
has not been done in MOBILE6 to avoid double-counting and to allow the user maximum
flexibility in selecting a reduction factor.
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Note that the estimates for CO2 differ from all of the other pollutant values in MOBILE6
which are estimates of what is produced by the vehicle in evaporation and combustion
processes and do not account for later chemical reactions in the atmosphere.
Emissions / Natural Gas Vehicle (NGV) Emissions
Q. I ran scenarios comparing 100 percent new LDGVs to 100 percent new NGVs. I was
surprised that NOx emissions for NGVs exceed LDGVs NOx after 2005. Is this
correct or was TIER2 or some other correction omitted from the NGVs rates?
This is correct. NGV NOx emission rates are higher than those of Tier2 vehicles after
2005.
Modeling / For Sensitivity Testing
Q. It would be helpful to know the effects of user input compared to defaults ~ How
large a change in emissions can be expected when the user changes hourly
temperatures, clouds, sunset and rise, etc.?
We have performed sensitivity testing on MOBILE6 and compiled the results into a
technical report entitled "Sensitivity Analysis of MOBILE6.0" (EPA Report Number
EPA420-R-02-035). This report can be found on (and downloaded from) our Web site
that contains the documentation for MOBILE6 at:
www.epa.gov/otaq/models/mobile6/m6tech.htm.
Miscellaneous
Misc / Programming
Q. The two-valued inputs still use "1" (for "off") and "2" (for "on"). Why not use the
more intuitive "1" for on and "0" for off?
Historically the MOBILE has used the "l"/"2" values for on and off. Because MOBILE6
uses much of the MOBILES code, we continued to use the old system.
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Q. The discussion on the Anti-Tampering option states that a "1" must be entered after
the GAS BUS toggle. What is this for?
This number originally represented the Program Type (as in MOBILES). Program Type is
no longer a parameter of this command in MOBILE6, but the input for it was not
removed.
Q. Is it necessary to know FORTRAN to use the model?
MOBILE6 was written using the computer language FORTRAN90. We then compiled it
(to run on Windows or DOS PCs) to create the executable program Mobile62.exe.
Therefore, if you plan to simply run MOBILE6 on a PC with DOS or Windows, you need
to do nothing in FORTRAN, simply run that executable.
If you plan on modifying the program itself (i.e., the source code) or running it on some
other platform (e.g., mainframe computer, Macintosh, Unix, etc.), then you will need
some knowledge of FORTRAN. (See Chapter 4 of the Users Guide.) This is not trivial.
No knowledge of FORTRAN is necessary to input data. See:
— thefilenamedReadme3.txt
— the example files in the "Examples" folder within the "Run" folder
— Chapter 2 of the Users Guide.
Misc / Documentation
Q. We think there is a need to present and discuss some of the important underlying
technical assumptions supporting the validity of MOBILE6. Is there a separate
document that does this? A single report summarizing the major workings of the
model and perhaps exploring some of the issues regarding use of local inputs would
be very useful. We realize that there are a series of technical reports that address
some or all of these assumptions, but we are thinking here of something not quite so
technical, but summarizing the workings of the most important technical features. It
would explore the concepts and limitations of such models, including examples of
appropriate and inappropriate applications.
We have a number of documents available on MOBILE6. The most technical information
is in the individual technical reports. More general information is on our Web site in the
form of presentations given at various meetings. While there may not be a single
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document that addresses all your questions, you should be able to find the information
you need without too much trouble. (The MOBILE6 Web site is located at:
www. epa. gov/otaq/m6 .htm)
Q. Have you considered using hypertext in developing your documentation? It would
make for easier cross-referencing between sections (although you do a good job of
indicating where related additional information is available).
We are considering this option for future versions of our documentation.
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