United States
Environmental Protection
Agency
Office °f TransP°rtaticin EPA420-B-05-008
and Air Quality August 2005
Guidance for Creating Annual
On-Road Mobile Source
Emission Inventories for PM2.5
Nonattainment Areas for Use in
SIPs and Conformity
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EPA420-B-05-008
August 2005
Guidance for Creating Annual On-Road Mobile Source
Emission Inventories for PM2.5 Nonattainment Areas
for Use in SIPs and Conformity
Transportation & Regional Programs Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
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Background Information
The purpose of this document is to provide areas that are nonattainment or maintenance for the
annual PM2 5 national ambient air quality standard ("standard") with guidance on developing
annual PM2 5 on-road motor vehicle emissions estimates to meet state air quality implementation
plan (SIP) and transportation conformity requirements.
For previous and existing air quality standards (e.g., 1-hour ozone, 8-hour ozone, PM10 and
carbon monoxide (CO)), areas typically have been required to examine a typical summer or
winter day because areas were violating a standard established for a time period of 24 hours or
less. As a result, these areas have developed on-road motor vehicle SIP inventories, motor
vehicle emissions budgets ("budgets"), and regional emissions analyses1 for transportation
conformity determinations using modeling inputs and parameters that were specific to a typical
day within a particular season. However, all areas currently designated nonattainment for PM2 5
are violating the annual standard for this pollutant. In order to be consistent with this standard,
these areas must develop annual emission inventories for the purpose of developing SIP budgets
and demonstrating transportation conformity. This guidance provides information on how areas
should fulfill these requirements.
JThe process for generating on-road motor vehicle emissions estimates for conformity
purposes is commonly referred to as a "regional emissions analysis" in conformity documents.
However this term could be confused with the process of creating an inventory for a SIP. To
avoid that confusion, we will refer to a "regional emissions analysis" for transportation
conformity as a "regional conformity analysis" in this document.
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Table of Contents
1. Q. What effect does this guidance have on ozone, CO, and PM10 SIPs and regional
conformity analyses? 1
2. Q. What are on-road motor vehicle emission inventories, budgets and regional
conformity analyses? 1
3. Q. What pollutants and precursors are covered by this guidance? 2
4. Q. Does this guidance create new requirements? 3_
5. Q. What emissions models should be used to develop SIP inventories and regional
conformity analyses for direct PM2 5 and PM2 5 precursors? 3
6. Q. What issues should state and local air quality agencies and transportation agencies
consider when creating annual emissions inventories with MOBILE6.2 for SIPs and
regional conformity analyses? 4
7. Q. What options do areas have to develop annual PM2 5 and PM2 5 precursor SIP
inventories and regional conformity analyses with MOBILE6.2? 5.
8. Q. How do emission factors for direct PM2 5 and for PM2 5 precursors vary with changes
in external commands? 8
9. Q. What other requirements apply when calculating regional emissions for transportation
conformity? 9
10. Q. For areas currently using network based travel models, does travel demand modeling
need to be done for each season or month? 9
11. Q. Prior to the development of the PM2 5 SIP, can simpler methods be used for regional
conformity analyses? K)
12. Q. Once the SIP budget is developed, should the same methods be used for regional
conformity analyses? H
13. Q. What is the National Mobile Inventory Model (NMEVI) and how can it be used to
determine annual emissions inventories? H
14. Q. Who can I contact if I have further questions about developing annual PM25 SIP
emissions inventories and budgets, and regional conformity analyses? H
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1. Q. What effect does this guidance have on ozone, CO, and PM10 SIPs and regional
conformity analyses?
A. This guidance applies to SIPs and regional conformity analyses for PM2 5
nonattainment and maintenance areas that need to develop annual PM2 5 inventories, such
as areas that are violating the annual PM25 standard. Ozone, CO, and PM10 SIPs and
regional conformity analyses should continue to be based on inventories for a typical
summer day or winter day, as applicable, using appropriate MOBILE6.2 input conditions
and vehicle miles traveled (VMT). Areas that need to develop inventories for the 24-hour
PM2 5 standard should follow existing guidance for creating daily emission inventories.
2. Q. What are on-road motor vehicle emission inventories, budgets and regional
conformity analyses?
A. An on-road motor vehicle emission inventory represents the total amount of emissions
of a particular pollutant or precursor that is emitted by cars, trucks, buses, and
motorcycles in a given area for a given point in time. The emissions reductions from on-
road motor vehicle control measures are also accounted for in the SIP inventory. When
developing an attainment demonstration, reasonable further progress (RFP) plan, or
maintenance plan, areas are required to develop emission inventories for all source
categories (e.g., point, area, on-road motor vehicle and off-road sources) for specific
years. For some of these years, the on-road motor vehicle emission inventory may also
serve as the SIP budget that is used to demonstrate transportation conformity. A budget
provides a limit or ceiling on the amount of emissions transportation sources can produce
in a given area that is consistent with attainment, RFP or maintenance.
The transportation conformity rule (40 CFR parts 51 and 93), requires areas to
demonstrate that projected emissions from the planned transportation system do not
exceed the budgets established in the applicable SIP. Prior to an adequate or approved
SIP budget, 40 CFR 93.109(i)(2) and 93.119(e) provide interim emissions tests that also
require a regional conformity analysis. For PM2 5 areas that need to do conformity for the
annual PM2 5 standard, regional conformity analyses should also represent total annual
emissions for given years as required by 40 CFR 93.118 and 93.119.
In simplest terms, emissions estimates are created by multiplying emissions factors for a
given pollutant or precursor by the total number of vehicle miles traveled (VMT) in a
given area for a given year. This document provides guidance on how annual emissions
estimates should be developed for inventories, budgets, and regional conformity analyses
for SIPs and conformity purposes.
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Q. What pollutants and precursors are covered by this guidance?
A. This guidance is applicable to the estimation of annual SIP and conformity inventories
of direct PM2 5 from motor vehicle tailpipe emissions, emissions from motor vehicle
brake and tire wear, and re-entrained road dust and construction dust from highway or
transit projects. This guidance would also apply, as applicable, to the estimation of annual
inventories of applicable PM25 precursors, i.e., volatile organic compounds (VOCs),
nitrogen oxides (NOx), sulfur oxides (SOx), and ammonia. EPA's future PM25
implementation rule will address when SIP inventories and budgets are established for
PM25 precursors. Requirements for inclusion of precursors in transportation conformity
analyses are addressed in a May 6, 2005, final rule (70 FR 24280) and are specified at 40
CFR 93.102(b)(iv)-(v) and 93.119 (f)(9)-(10).
Consistent with the May 6, 2005, final rule, if on-road motor vehicle emissions of one or
more PM2 5 precursors are determined through the SIP development process to be
significant contributors to an area's PM2 5 nonattainment problem, an emissions budget for
each significant precursor must be established in the SIP. Alternatively, a PM2 5 SIP
would not establish a motor vehicle emissions budget for precursors that are determined
to be insignificant through the SIP development process, and regional emissions analyses
for insignificant precursors would not be required for subsequent conformity
determinations. See the May 6, 2005, final rule for more information on the requirements
for addressing PM2 5 precursors in transportation conformity.
PM25 areas must also address re-entrained road dust in their conformity analyses, if a SIP
establishes an adequate or approved PM2 5 budget that includes re-entrained road dust.
Prior to adequate or approved budgets, areas must include road dust in conformity
analyses only if EPA or the State air agency finds road dust to be significant.
Requirements for inclusion of road dust in transportation conformity analyses can be
found at 40 CFR 93.102(b)(3) and 93.119(f)(8).
Construction-related fugitive dust is not required to be included in any PM2 5 conformity
determinations before a SIP is submitted. As described in the conformity rule (40 CFR
93.122(f)), construction dust is not required to be considered in the conformity process
unless the PM25 SIP identifies it as a significant contributor to the nonattainment area's
PM25 problem. Areas that are contemplating making this type of determination need to
include specific information in their SIPs in order to facilitate future conformity
determinations. The inventories should clearly identify how much of the regional
construction dust is attributable to highway and transit construction, as opposed to other
construction activities. If the SIP is to identify construction dust emissions as a significant
contributor, the highway and transit construction dust emissions need to be included and
identified as such in the direct PM2 5 on-road motor vehicle emissions budget. In addition,
the regional conformity analysis would account for the level of construction activity, the
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fugitive PM2 5 control measures in the SIP (if there are any), and the dust producing
capacity of the proposed construction activities (November 5, 2003, 68 FR 62711).
4. Q. Does this guidance create new requirements?
A. No, this guidance is based on the existing Clean Air Act (CAA) and associated
regulations and does not create any new requirements. It merely explains how to fulfill
current SIP and conformity requirements for developing PM2 5 emission inventories and
budgets.
The statutory provisions and EPA regulations described in this document contain legally
binding requirements. This document is not a substitute for those provisions or
regulations, nor is it a regulation itself. Thus, it does not impose legally binding
requirements on EPA, states, or the regulated community, and may not apply to a
particular situation based upon the circumstances. EPA retains the discretion to adopt
approaches on a case-by-case basis that may differ from this guidance, but still comply
with the statute and SIP and conformity regulations. Any decisions regarding a particular
SIP and conformity determination will be made based on the statute and regulations. This
guidance may be revised periodically without public notice.
5. Q. What emissions models should be used to develop SIP inventories and regional
conformity analyses for direct PM2 5 and PM2 5 precursors?
A. For states other than California2, MOBILE6.2 is currently EPA's approved emission
factor model for estimating direct PM2 5 emissions from on-road vehicle exhaust and
brake and tire wear, and for PM2 5 precursor emissions from vehicle exhaust and
evaporative emissions. For all states, including California, the methods for estimating re-
entrained road dust emissions from cars, trucks, buses, and motorcycles on paved and
unpaved roads are incorporated in Chapter 13 of AP-42. These are EPA's approved
methods for estimating road dust emissions. However, alternative methods may be used if
such methods are approved by EPA and announced in the Federal Register. The use of
MOBILE6.2 and AP-42, including discussion of alternatives to AP-42, in SIPs and
2State and local agencies developing SIPs and conformity analyses for California should
consult with EPA Region 9 for information on the current version of EMFAC approved for use
in California and for information of how to create annual inventories using EMFAC. However,
the general concepts in this document for accounting for variation during the year should be
followed when creating annual inventories with EMFAC for the PM2 5 annual standard.
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transportation conformity analyses is described in detail in a separate guidance
document.3
Chapter 13 of AP-42 includes information on the variability of re-entrained road dust
emissions based on environmental conditions, including factors that vary based on time
of year. The AP-42 method can be used as described in Chapter 13 to develop annual re-
entrained road dust inventories. EPA plans on issuing separate guidance on how to apply
adjustments to estimated road dust emissions to reflect the true impact of re-entrained
road dust on regional air quality in SIPs and regional conformity analyses. The remainder
of this document addresses the use of MOBILE6.2 to calculate annual inventories for
direct PM2 5 emissions from vehicle exhaust and brake and tire wear, and for applicable
PM25 precursor emissions from vehicle exhaust and evaporative emissions.
EPA has made available the National Mobile Inventory Model (NMEVI), which
incorporates MOBILE6.2 as well as a database of local activity information and a post-
processing system that can produce annual emission inventories. NMEVI is an additional
tool that can be used for inventory development, although its use is not required. Question
13 of this document discusses the option to use NMIM in SIP development and regional
conformity analyses.
6. Q. What issues should state and local air quality agencies and transportation agencies
consider when creating annual emissions inventories with MOBILE6.2 for SIPs and
regional conformity analyses?
A. State and local agencies need to consider whether MOBILE6.2 inputs or VMT vary
during the year enough to affect PM2 5 annual emissions estimates. MOBILE6.2 is
designed to allow users to estimate motor vehicle emissions based on specific input
conditions that include month of evaluation (i.e., January or July), environmental factors
(e.g., temperature, humidity), fleet characteristics (e.g., age distribution of fleet,
distribution of VMT by vehicle class), activity measures (e.g., speed distributions,
distribution of VMT by roadway type), and fuel characteristics (e.g., gasoline RVP, sulfur
content). Some of these input conditions will vary based on time of year. For some
pollutants, these seasonal variations for certain input conditions will result in different
emissions estimates (these variations are discussed in more detail in Question 8). In
addition to the input conditions that affect MOBILE6.2 emission factors, VMT may also
vary by time of year. These differences in emission factors and VMT by time of year need
to be considered in the development of annual inventories.
3"Policy Guidance on the Use of MOBILE6.2 and the December 2003 AP-42 Method for
Re-Entrained Road Dust for SIP Development and Transportation Conformity", memorandum
from Margo Oge and Steve Page to EPA Regional Air Division Directors, February 24, 2004,
which can be found at: www.epa.gov/otaa/models/mobile6/mobil6.2 letter.pdf.
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The key question in the development of annual PM25 emissions estimates for SIPs and
conformity is how much temporal disaggregation of input data is needed to produce
annual emissions inventories that properly reflect local conditions. If, as a result of local
conditions, MOBILE6.2 emissions factors vary significantly over the course of the year,
state air quality agencies and transportation agencies may need to do multiple
MOBILE6.2 runs with different input conditions to properly develop SIP inventories and
regional conformity analyses. State and local air quality and transportation agencies
should work together with EPA and the U.S. Department of Transportation, via the
interagency consultation process, to determine the appropriate inputs and number of
MOBILE6.2 runs needed to produce accurate annual inventories in a given nonattainment
or maintenance area. During the interagency consultation process, air quality and
transportation agencies should take into account the needs and capabilities of the air
quality modeling tools that will be used to develop the SIP, the availability of seasonal or
monthly VMT and MOBILE6.2 input data, and the seasonal or monthly variability of that
data. Depending on the variability of input conditions and the effect that variability has on
emissions, state and local air quality and transportation agencies in consultation with EPA
and DOT may determine for some areas that a single set of MOBILE6.2 runs is
appropriate, or alternatively, that multiple sets of runs using seasonal or monthly
conditions are necessary.
7. Q. What options do areas have to develop annual PM2 5 and PM2 5 precursor SIP
inventories and regional conformity analyses with MOBILE6.2?
A. Depending on variability in local input conditions and on the impact of that variability
on the overall inventory, states may choose from a range of options for the degree of
temporal disaggregation used when creating annual inventories for SIPs and regional
conformity analyses. To determine how much temporal disaggregation is appropriate in a
given area, states may choose to calculate simplified annual emission inventories using
the different approaches (i.e., run MOBILE6.2 using representative annual and seasonal
inputs) and compare the results. Through this exercise, states may find that the
differences between these methodologies are insignificant and further emissions analyses
can be performed using a less detailed process. The interagency consultation process
should be used to determine which approach is most appropriate for a given PM2 5
nonattainment or maintenance area. This process should include consultation among state
and local transportation and air quality agencies, as well as EPA and the U.S. Department
of Transportation. Whichever approach is chosen, that approach should be used
consistently throughout the analysis for a given pollutant or precursor. For example, if the
2002 base year annual inventory used in the attainment SIP is based on an analysis using
input assumptions broken down for four seasons, the attainment year inventory used in
that SIP should also be based on input assumptions for four seasons.
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The following are some examples of methodologies that could be used. Other approaches
may also be appropriate as determined through the interagency consultation process.
Developing Annual Inventories and Regional Conformity Analyses Using Annual
Average MOBILE6.2 Inputs and a Single Set of MOBILE6.2 Runs
Single-Run Approach: This methodology would involve a single set of modeling runs
with MOBILE6.2 for each year or scenario using only annual average inputs for all
MOBILE6.2 input parameters and for VMT. EPA believes that in some areas this
methodology may be appropriate because some input conditions may not vary
significantly by time of year in a particular area. For example, distribution of VMT by
vehicle class may be fairly constant in most areas. In addition, some input conditions may
vary without affecting the emissions estimates for some pollutants generated by
MOBILE6.2. For example, direct PM2 5 emissions in MOBILE6.2 are insensitive to
differences in ambient temperature and humidity (these cases are described in more detail
in Question 10).
States should determine which input conditions do not vary significantly during the year.
For these conditions, annual average input values may suffice. If local conditions are such
that there is no significant variation in emissions derived from MOBILE6.2 based on time
of year, state and local air quality and transportation agencies, in consultation with EPA
and DOT, may choose to base annual SIP inventories and regional conformity analyses on
MOBILE6.2 runs based on a single set of inputs and using total annual VMT. For this
approach, the evaluation month in MOBILE6.2 should be July of the calendar year being
evaluated.
Developing Annual Inventories and Regional Analyses Using Seasonal or Monthly
Average MOBILE6.2 Inputs and Two or More Sets of MOBILE6.2 Runs
In some cases, variations in input conditions at different times of the year may result in
significant differences in MOBILE6.2 emission factors. In some areas, there may also be
significant differences in VMT at different times of the year. In these areas, developing
inventories based on seasonal average input conditions may be necessary. Depending on
the temporal variability of input data for a given area, from two to twelve sets of
modeling runs with MOBILE6.2 may be used for each year or scenario. Some possible
approaches are included below, but this does not include a comprehensive list of options.
State and local air quality and transportation agencies, in consultation with EPA and
DOT, should choose the approach that best suits local conditions.
Two-Season Approach: This approach uses winter and summer input conditions to
develop inventories based on two sets of MOBILE6.2 runs. This approach assumes that
each set of input conditions can be used to model six months of the year. The "winter"
inventory would be based on average input conditions for the coldest months of the year
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and the "summer" inventory would be based on average input conditions for the hottest
months of the year. Annual VMT would be apportioned as appropriate to the winter or
summer runs. If VMT does not vary significantly by season, half of the annual VMT
could be apportioned to each of the two sets of MOBILE6.2 runs to create winter and
summer inventories. If VMT is significantly different between the "winter" and
"summer" seasons, then the VMT should be apportioned based on those differences.
Under this approach, the total annual inventory for an area would be the sum of the
"winter" and "summer" inventories. For this approach, January should be used as the
input for evaluation month in MOBILE6.2 for the "winter" inventory and July should be
used for the "summer" inventory. Because the evaluation month input in MOBILE6.2 can
also affect fuel parameters, MOBILE6.2 users should take care to ensure that model
inputs for fuel parameters are set to properly represent the season modeled.
Four-Season Approach: This approach bases the total annual inventory on four sets of
seasonal input conditions and four sets of MOBILE6.2 runs: winter, spring, summer, and
fall. This approach assumes that four sets of inputs are used, one for each of the four
seasons. VMT would be apportioned appropriately for each of these seasonal periods. If
VMT does not vary significantly by season, one quarter of the annual VMT would be
apportioned to each of the seasonal inventories. If VMT is significantly different between
the seasons, then the VMT should be apportioned based on those differences.
The total annual inventory for an area would be the sum of the four seasonal inventories.
MOBILE6.2 only has two input options for evaluation month (January and July). January
should be used as the input for evaluation month in MOBILE6.2 for the winter inventory
and July should be used for the summer inventory. For the spring inventory, July should
be used as the input for evaluation month in MOBILE6.2, while January of the following
year should be used as the input for the fall inventory. Because the evaluation month
input in MOBILE6.2 can also affect fuel parameters, MOBILE6.2 users should take care
to ensure that model inputs for fuel parameters are set to properly represent the season
modeled.
Monthly Approach: Another available approach for developing annual inventories and
regional conformity analyses would involve twelve sets of MOBILE6.2 modeling runs
using monthly average input conditions and VMT. As a result, this methodology is more
resource intensive than the previous approaches. States should note that this is the
approach that is used to create the 2002 National Emission Inventory (NET) that some
areas may use as their 2002 base year inventory for SIP purposes. For detailed guidance
on how to set the evaluation month in MOBILE6.2 to prepare monthly inventories for
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calculation of annual inventories, see Section 2.2 of EPA's "Technical Guidance on the
Use of MOBILE6.2 for Emission Inventory Preparation"4.
States that wish to use this approach may also want to consider using the NMEVI model to
reduce the amount of data processing needed. See Question 13 below for more
information on the use of NMEVI in developing annual inventories and regional emissions
analyses.
Q. How do emission factors for direct PM2 5 and for PM2 5 precursors vary with changes
in external commands?
A. MOBILE6.2 uses different algorithms to estimate emissions from different pollutants.
Inputs that contribute to seasonal variability in emissions for some pollutants may not
result in variability for others. As a result, state and local agencies may be able to use
simpler approaches for some pollutants than for others.
Direct PM2;. SOx. and Ammonia
MOBILE6.2 uses simple algorithms to estimate direct PM2 5 emissions and SOx, and
ammonia precursor emissions. In general, emissions of these pollutants and precursors do
not vary, or vary only by small amounts, for most of the input conditions in MOBILE6.2,
including key commands such as temperature, humidity, vehicle speed, and roadway type.
However, emission factors for direct PM2 5 emissions and SOx and ammonia precursor
emissions are affected by the following MOBILE6.2 input options:
Registration (age) distribution
• Diesel sales fractions
Annual mileage accumulation rates
• Distribution of VMT by vehicle class
Input options that affect gasoline and diesel fuel sulfur content
Of these input options, registration distribution, diesel sales fractions, and annual mileage
accumulation rates should not change based on time of year. If the remaining parameters
do not vary significantly by time of year, a single set of MOBILE6.2 runs, using July as
the evaluation month, may be sufficient to develop annual inventories for SIPs and
regional conformity analyses for direct PM2 5, SOx, and ammonia.
^'Technical Guidance on the Use of MOBILE6.2 for Emission Inventory Preparation",
Office of Transportation and Air Quality, US EPA, August 2004, EPA420-R-04-013, which can
be found at www.epa.gov/otaq/m6.htm.
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NOx and VOC
MOBILE6.2 emissions estimates of NOx and VOC precursor emissions are affected by
temperature and humidity, fleet characteristics (e.g., age distribution of fleet, distribution
of VMT by vehicle class), activity measures (e.g., speed distributions, distribution of
VMT by roadway type), and fuel characteristics (e.g., gasoline RVP, sulfur content).
When evaluating which approach to use for generating annual inventories for NOx and
VOC, states should first consider which of the input conditions actually vary significantly
based on time of year. States can then test whether those variations are likely to result in
significant differences in emissions throughout different times of the year, as appropriate.
Based on an analysis of various parameters, states may conclude that simpler methods
(i.e., the annual or seasonal methods) can be used to develop direct PM2 5 inventories than
are needed for PM2 5 precursors such as NOx and VOC. The interagency consultation
process should be used to determine if the use of different approaches for direct PM25 and
for PM2 5 precursors would ease the resource burden of developing SIPs and conformity
analyses while maintaining credible results.
9. Q. What other requirements apply when calculating regional emissions for transportation
conformity?
A. Section 93.122 of the transportation conformity rule contains requirements for
estimating VMT and inclusion of control measures in regional conformity analyses.
Section 93.122(b) of the transportation conformity rule requires that serious, severe and
extreme ozone nonattainment areas and serious CO nonattainment areas use network
based travel models to perform regional conformity analyses. There is no similar
requirement to use network based travel models for PM2 5 nonattainment areas. However,
PM2 5 areas that are currently using network based travel models must continue to use
them when calculating annual emission inventories, per Section 93.122(d). Areas without
a network based travel model may use other appropriate methods for estimating VMT
consistent with best professional practice and Section 93.122(d) of the conformity
regulation. In addition, sections 93.110 and 93.111 require the latest planning
assumptions and emission models to be used in all conformity analyses.
10. Q. For areas currently using network based travel models, does travel demand modeling
need to be done for each season or month?
A. In some areas, variations in VMT or other vehicle activity inputs over the course of
the year may not have a significant effect on MOBILE6.2 emissions estimates for direct
PM2 5 or PM2 5 precursors. In such cases, a single travel demand modeling run would be
sufficient to generate an annual VMT estimate or any other activity inputs derived from
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the travel model. Annual VMT estimates would then be divided appropriately according
to the level of temporal disaggregation used for the emissions estimation as described in
Question 7.
State and local air quality and transportation agencies, in consultation with EPA and
DOT, should determine whether significant seasonal variations in the output of network
based travel models is expected and whether these variations would have a significant
impact on PM2 5 emissions estimates. The interagency consultation process should be
used to determine the most appropriate method for estimating VMT and identifying the
appropriate source for existing VMT data.
11. Q. Prior to the development of the PM2 5 SIP, can simpler methods be used for regional
conformity analyses?
A. Yes. EPA expects that the most thorough analysis to determine the appropriate
methods to be used for developing annual inventories will occur during the development
of the SIP, taking into account the needs and capabilities of air quality modeling tools and
the limitations of available data. Prior to the development of the SIP, state and local air
quality and transportation agencies may not have all of the information they need to
determine how much temporal disaggregation is necessary to adequately account for
variation in emissions during the year. State and local agencies may also need additional
time to collect data on a seasonal or monthly basis if that data is needed. For these
reasons, state and local air quality and transportation agencies may, through the
interagency consultation process, decide to use simplified methods for regional
conformity analyses done prior to an adequate or approved SIP budget. For example,
through the interagency consultation process, state and local agencies may choose to base
regional conformity analyses used in interim conformity tests (build-no-greater-than-no-
build or no-greater-than-2002 tests) on average annual inputs and a single set of
MOBILE6.2 runs even while they are working on developing an RFP, attainment, or
maintenance SIP using a more complicated approach.
Whatever approach is selected, the latest planning assumptions, latest emissions model,
and appropriate methods for estimating travel and speeds must be used as required by
Sections 93.110, 93.111, and 93.122 of the conformity rule. Also, the approach that is
selected for the interim emissions tests should be used consistently when completing a
conformity test. Whether a submitted or draft 2002 SIP inventory or some other inventory
determined through the interagency consultation process is used, the regional conformity
analysis for the baseline year test should be based on the same approach that was used to
develop the baseline inventory for conformity purposes. For example, if the two-season
approach is used to develop the 2002 baseline year for conformity purposes, the same
two-season approach should be used for the regional conformity analysis. See the
preamble of the July 1, 2004 conformity rule (FR 40015 left column) for more
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information on considerations for the 2002 baseline test. Similarly, the same approach
should be used to develop the build and no-build scenarios under the build-no-greater-
than-no-build test.
12. Q. Once the SEP budget is developed, should the same methods be used for regional
conformity analyses?
A. Yes. Regional conformity analyses should be based on the same approach used to
develop the direct PM2 5 and any PM2 5 precursor budgets established in the applicable
SIP. For example, if the NOx SIP budget was determined using average seasonal inputs
in MOBILE6.2 for winter, spring, summer, and fall, the same approach should be used
for regional conformity analyses based on that budget. State and local air quality and
transportation agencies should use the interagency consultation process while developing
the approach used for the SIP budgets to consider the impact this will have on data
collection, modeling, and analysis needs for future regional conformity analyses.
13. Q. What is the National Mobile Inventory Model (NMEVI) and how can it be used to
determine annual emissions inventories?
A. NMEVI is a graphical user interface that contains the MOBILE6.2 and NONROAD5
models and a database of county-level input information, the National County Database
(NCD). NMEVI produces monthly inventories by source classification code (SCC) and
county. When using NMEVI, users can simply select the year, months, and county or
counties they wish to evaluate. Since NMEVI includes county-level information, it will
automatically write MOBILE6.2 input files, run MOBELE6.2 and multiply the emission
factors by VMT to produce emission inventories for each county for each month.6 NMEVI
also provides a post-processing module that will aggregate the months into an annual
inventory and produce tab-delineated ASCII output that can be read into database or
spreadsheet software applications.
5Because it incorporates MOBILE6.2 and NONROAD, NMEVI can be used to generate
emissions inventories for both on-road motor vehicles (cars, trucks, buses, and motorcycles) and
off-road equipment (agricultural and construction equipment, lawn and garden equipment, and
off-road recreational vehicles among others) for SEP purposes. Because transportation conformity
applies only to on-road motor vehicles, only the on-road portion of an inventory generated using
NMEVI would be used to generate SEP budgets and regional conformity analyses.
6 "EPA's National Mobile Inventory Model (NMEVI), A Consolidated Emissions
Modeling System for MOBILE6 and NONROAD". H. Michaels, et al. U.S. EPA.
www.epa.gov/otaq/models/nmim/420r05003.pdf
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NMIM is not considered a new model and does not start a new conformity grace period
pursuant to 40 CFR 93.111. Because NMIM incorporates MOBILE6.2, it may be used to
generate emissions inventories for SIPs and regional conformity analyses. NMIM may
provide an easier way for states to develop annual inventories because it is designed to
create annual inventories based on monthly inputs. However, before using NMIM, state
and local air quality and transportation agencies should work together with EPA and DOT
to determine whether NMEVI is appropriate given local conditions and modeling methods
and to determine what modifications, if any, are needed to the NMEVI database to
accurately model current local conditions.
The use of NMEVI is not required for SEPs or regional conformity analyses. Some areas
may choose not to use NMEVI simply because it does not provide a significant resource
advantage compared to pre- and post-processing methods already being used. State and
local agencies should carefully review the NMEVI documentation before deciding whether
to use it. NMEVI has some limitations in some applications and, as a result, the use of
NMEVI may not be appropriate in all areas. For example, some areas may already be using
more sophisticated methods for pre- and post-processing input and emissions data than
NMEVI can accommodate. In that case, state and local agencies should not use NMEVI.
States have provided information for the NCD as part of the National Emissions
Inventory (NEI) development process. However, given the NEI cycle, this may not be the
most recent or best available information at the time a state initiates modeling as required
in the latest planning assumptions provisions of the conformity rules (40 CFR 93.110).
For SIPs and regional conformity analyses, state and local agencies should review the
information in the NCD to verify that it is still accurate and up-to-date. Where more
current information is available, the database must be modified to incorporate the most
recent data to meet latest planning assumptions requirements for SEPs and conformity.
(EPA encourages states to separately submit updates to the NCD so that the most accurate
database is available for both national and local inventory development). The NCD works
at the county level and will need to be modified to account for areas containing partial
counties, if necessary. The interagency consultation process should be used to evaluate
whether the use of NMEVI is appropriate in a given area, and to evaluate what changes are
needed in the NMEVI database.
State and local agencies should take special care to ensure that VMT data used in NMEVI
is derived appropriately. Areas required to use VMT data from travel demand models
need to make sure that the appropriate VMT estimates are incorporated into the NCD.
One limitation of the NCD is that it includes VMT data for only select years but not
necessarily for the years that need to be evaluated for SEP or transportation conformity
purposes. Therefore, even if the VMT data in the NCD are correct for a specific year,
areas wishing to use NMEVI may need to calculate and enter the necessary VMT inputs
for other years into the NCD. Areas should also evaluate the speed assumptions in the
NCD and revise them as needed to reflect current local estimates.
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The current version of NMIM is available at www.epa.gov/otaq/nmim.htm . EPA is
currently working on updates to the NONROAD portion of NMEVI and to the NCD and
expects to release a revised version of NMEVI later in 2005, which will be posted on the
website and notice sent out through our list-server.
14. Q. Who can I contact if I have further questions about developing annual PM25 SEP
emissions inventories and budgets, and regional conformity analyses?
A. For specific questions about a particular nonattainment or maintenance area, please
contact the SEP or transportation conformity staff person responsible for your state at the
appropriate EPA regional office. A listing of regional offices, the states they cover, and
contact information for EPA regional conformity staff can be found at the following
website: www.epa.gov/otaq/transp/conform/contacts.htm.
General questions about this guidance can be directed to Gary Dolce at EPA's Office of
Transportation and Air Quality, dolce.gary@epa.gov or 734-214-4414.
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