Procedures for Emission Inventory Preparation - Vol IV: Mobile Sources
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ACKNOWLEDGMENT
Several people in EPA's Emission Planning and Strategies
Division have contributed to this document. Their names and
contributions are listed below.
Chapter 1 Natalie Dobie Introduction
Chapter 2 Natalie Dobie Overview
Chapter 3 Terry Newell MOBILE4.1
Natalie Dobie Vehicle Miles Traveled
Chapter 4 Greg Janssen Nonroad Sources
Joe Somers
Chapter 5 Richard Wilcox Aircraft
Chapter 6 Peter Okurowski Locomotives
NOTICE
The Procedures For Emission Inventory Preparation consists of
these five volumes:
Volume I - Emission Inventory Fundamentals
Volume II - Point Sources
Volume III - Area Sources
Volume IV - Mobile Sources
Volume V - Bibliography
Volume I is a guide to the managerial and technical aspects of
the emission inventory. It outlines the information sources
available, methods of estimating emissions, data validation and
quality assurance techniques, as well as procedures to maintain and
update the inventory. Also included are a detailed analysis of the
manpower and resources required to derive each component of an
emission inventory and a comprehensive glossary.
Volume II discusses point sources identification, data
collection, emissions calculation, and data presentation. It
establishes standardized methods and procedures to develop a point
source data base.
Volume III outlines the methods of collecting and handling
emission data from sources too small and/or too numerous to be
surveyed individually. Collectively, these sources are known as
area sources. Procedures are presented to identify area source
categories. Important reference material that can be used to
determine the activity levels associated with area source
categories are also listed. Finally, emission factors, emission
calculations, pollutant allocation and projection techniques, and
methods of data presentation are included to assist in the
preparation and maintenance of the area source emission
inventories.
Volume IV presents an overview of the mobile source category
as a whole and identifies specific methods that can be used to
identify and inventory sources, estimate emissions, and establish
and maintain a useful, current mobile source emissions inventory.
Volume V presents an extensive listing of currently available
reference material designed to assist in the development of an
emission inventory. A concise abstract is provided for each
reference cited, outlining the pertinent emission inventory
information.
These volumes are intended to present emission inventory
procedures and techniques applicable to state and local air
programs. Please forward comments and suggestions for improvement
to the U. S. Environmental Protection Agency, Monitoring And
Reports Branch (MD- 14), Research Triangle Park, North Carolina
27711.
Other U.S. EPA emission inventory procedures publications include:
Emission Inventory Requirements For Ozone State Implementation
Plans, EPA-450/4-91-010, U. S. Environmental Protection Agency,
Office of Air Quality Planning and Standards, Research Triangle
Park, North Carolina, March 1991.
Procedures for the Preparation of Emission Inventories for Carbon
Monoxide and Precursors of Ozone, Volume 1: General Guidance for
Stationary Sources EPA-450/4-91-016, U. S. Environmental Protection
Agency, Office of Air Quality Planning and Standards, Research,
Triangle Park, North Carolina, May 1991.
Procedures for the Preparation of Emission Inventories for Carbon
Monoxide and Precursors of Ozone, Volume II: Emission Inventory
Requirements for Photochemical Air Quality Simulation Models, EPA-
45014-9-014, U. S. Environmental Protection Agency, Office of Air
Quality Planning and Standards, Research Triangle Park, North
Carolina, May 1991.
Emission Inventory Requirements for Carbon Monoxide State
Implementation Plans, EPA-450/4-91-011, U. S. Environmental
Protection Agency, Office of Air Quality Planning and Standards,
Research Triangle Park, North Carolina, March 1991.
Example Documentation Report For 1990 Base Year Ozone and Carbon
Monoxide State Implementation Plan Emission Inventories, EPA-450/4-
92-007, U. S. Environmental Protection Agency, Office of Air
Quality Planning and Standards, Research Triangle Park, North
Carolina, March 1992.
AIRS Facility Subsystem Source Classification Codes (SCCs) and
Emission Factor Listing for Criteria Pollutants, EPA-450/4-90-003,
U. S. Environmental Protection Agency, Office of Air Quality
Planning and Standards, Research Triangle Park, North Carolina,
March 1990. Revised edition to be issued Summer 1992.
Guidance for the Preparation of Quality Assurance Plans O3/CO SIP
Emission Inventories, EPA-450/4-88-023, U. S. Environmental
Protection Agency, Office of Air Quality Planning And Standards,
Research Triangle Park, North Carolina, December 1988.
Quality Review Guidelines For 1990 Base Year Emission Inventories,
EPA450/4-91-022, U. S. Environmental Protection Agency, Office of
Air Quality Planning and Standards, Research Triangle Park, North
Carolina, September 1991.
SIP Air Emission Inventory Management System (SAMS) Version 4.1 and
SAMS User's Guide, U. S. Environmental Protection Agency, Office of
Air Quality Planning and Standards, Research Triangle Park, North
Carolina, September 1991.
User's Guide to MOBILE4.1 (Mobile Source Emission Factor Model),
EPA-AA-TEB-91-01, U. S. Environmental Protection Agency, Office of
Mobile Sources, Ann Arbor, Michigan, July 1991.
Procedures for Estimating and Applying Rule Effectiveness in Post-
1987 Base Year Emission Inventories for Ozone and Carbon Monoxide
State Implementation Plans, U. S. Environmental Protection Agency,
Office of Air Quality Planning and Standards, Research Triangle
Park, North Carolina, June 1989.
Surface Impoundment Modeling System (SIMS) Version 2.0 User's
Manual, EPA-450/4-90-019a, U. S. Environmental Protection Agency,
Research Triangle Park, North Carolina, September 1990.
Background Document for Surface Impoundment Modeling System (SIMS)
Version 2.0, EPA-450/4-90-019b, U. S. Environmental Protection
Agency, Research Triangle Park, North Carolina, September 1990.
TABLE OF CONTENTS
Page
1.0 INTRODUCTION 1
2.0 OVERVIEW OF THE MOBILE SOURCE CATEGORY 2
2.1 INDIVIDUAL MOBILE SOURCE CATEGORIES 2
2.1.1 Highway Vehicles 2
2.1.2 Nonroad Sources 3
2.1.3 Aircraft 3
2.1.4 Locomotives 4
3.0 EMISSIONS FROM HIGHWAY VEHICLES 5
3.1 GUIDANCE ON THE USE OF MOBILE4.1 VS MOBILE5 FOR THE 1990
BASE YEAR INVENTORY AND OTHER INVENTORIES 5
3.2 MOBILE SOURCE EMISSION ESTIMATION PROCESS 6
3.2.1 Overview of Factors Influencing Motor Vehicle
Emission Inventories 6
3.2.1.1 Vehicle Fleet Activity 7
3.2.1.2 Emission Factors 7
3.2.1.3 Fleet Characteristics 7
3.2.1.4 Fuel Characteristics 7
3.2.1.5 Correction Factors 8
3.2.1.6 Control Programs 8
3.2.2 Overview of MOBILE4.1 Input Requirements 8
3.2.2.1 Fleet Characteristics 9
3.2.2.1.1 VMT Mix 9
3.2.2.1.2 Annual Mileage Accumulation Rates 9
3.2.2.1.3 Registration Distributions 10
3.2.2.2 Fuel Specifications 10
3.2.2.2.1 RVP 10
3.2.2.3 Correction Factors 10
3.2.2.3.1 Speed 10
3.2.2.3.2 Temperature 10
3.2.2.3.3 Operating Modes 11
3.2.2.3.4 Minor Correction Factors 11
3.2.2.4 Tampering and Misfueling 12
3.2.2.5 Control Programs 12
3.2.2.5.1 Refueling Emissions 12
3.2.2.5.2 Inspection and Maintenance Programs 12
3.2.2.5.3 Anti-Tampering Programs (ATPs) 12
3.3 GUIDANCE ON SELECTING MOBILE4.1 INPUTS 13
3.3.1 Emission Factors 13
3.3.1.1 Region 14
3.3.1.2 Calendar Year 14
TABLE OF CONTENTS
Page
3.3.2 Fleet Characteristics 15
3.3.2.1 Vehicle Miles Traveled Mix by Vehicle Type 15
3.3.2.2 Annual Mileage Accumulation Rates and
Registration Distributions by Vehicle
Type and Age 16
3.3.2.3 Trip Length Distribution 18
3.3.2.4 Diesel Sales Fractions 20
3.3.3 RVP Determination 22
3.3.3.1 EPA-Provided 1990 RVP Estimates 26
3.3.3.2 "Period 1" RVP and "Period 2" RVP 26
3.3.3.3 Interpolation 27
3.3.3.4 Inputs for Future Year RVP 27
3.3.3.4.1 Future Summer RVP 27
3.3.3.4.2 Future Winter RVP 28
3.3.4 Oxygenated Fuels 29
3.3.5 Correction Factors 30
3.3.5.1 Speed 30
3.3.5.2 Temperature 34
3.3.5.3 Operating Modes 38
3.3.5.4 Additional Correction Factors for Light-
Duty Gasoline-Fueled Vehicle Types 40
3.3.6 Control Programs 43
3.3.6.1 Refueling Emissions 43
3.3.6.2 Inspection and Maintenance Programs 45
3.3.6.2.1 I/M 47
3.3.6.2.2 Start Year 47
3.3.6.2.3 Stringency 47
3.3.6.2.4 First Model Year 47
3.3.6.2.5 Last Model Year 48
3.3.6.2.6 Waiver Rates 48
3.3.6.2.7 Compliance Rate 49
3.3.6.2.8 Inspection Frequency 50
3.3.6.2.9 Vehicle Classes 51
3.3.6.2.10 I/M Test Types 51
3.3.6.2.11 Alternate I/M Credits 53
3.3.6.2.12 Centralized Programs 53
3.3.6.2.13 Decentralized Programs (Manual) 53
3.3.6.2.14 Computerized Inspection 54
3.3.6.2.15 Tech I-II and Tech IV+ 55
3.3.6.3 Anti-Tampering Programs 55
3.3.6.3.1 ATP 57
3.3.6.3.2 Tampering and Misfueling 57
3.3.6.3.3 Air Pump Inspection 57
TABLE OF CONTENTS
Page
3.3.6.3.4 Catalyst Inspection 57
3.3.6.3.5 Fuel Inlet Restrictor Inspection 58
3.3.6.3.6 Tailpipe Lead Detection Test 58
3.3.6.3.7 EGR Inspection 59
3.3.6.3.8 Evaporative Control System 59
3.3.6.3.9 PCV Inspection 60
3.3.6.3.10 Gas Cap Inspection 60
3.3.6.3.11 Tampering Rates 60
3.4 VEHICLE MILES TRAVELED 62
3.4.1 Highway Performance Monitoring System 62
3.4.1.1 Role of the HPMS in SIP Development 62
3.4.1.2 Overview of HPMS 63
3.4.1.3 Consistency Between HPMS and SIP VMT 65
3.4.1.3.1 Expansion Factors 65
3.4.1.3.1.1 Non-Attainment Area the Same As
the Federal Aid Urbanized Area 65
3.4.1.3.1.2 Non-Attainment Area Inside of
the Federal Aid Urbanized Area 66
3.4.1.3.1.3 Non-Attainment Area Outside of
the Federal Aid Urbanized Area 66
3.4.1.3.1.4 Non-Attainment Area and Federal
Aid Urbanized Area Crossover 67
3.4.1.3.2 Local Functional System 67
3.4.1.3.3 Seasonal Adjustment 68
3.4.1.3.4 Daily Adjustment 68
3.4.1.4 Allocating VMT to Time of Day 68
3.4.1.5 Allocating VMT to Functional Systems 69
3.4.1.6 Estimating VMT in Rural and Small Urban
Areas 69
3.4.1.6.1 Apportionment of Statewide VMT-
Recommended Method 72
3.4.1.6.2 Apportionment of Statewide VMT-
Alternative Methods 74
3.4.1.6.2.1 Motor Vehicle
Registrations 74
3.4.1.6.2.2 Population 74
3.4.1.6.2.3 Fuel Sales 75
3.4.2 Travel Demand Network Models 78
3.4.2.1 Role of Transportation Models in SIP
Development 78
3.4.2.2 Background. 78
3.4.2.3 Overview of Network Models 79
3.4.2.3.1 Level of Service 81
3.4.2.3.2 Physical Attributes 83
3.4.2.3.3 Locational Link Attributes 83
3.4.2.3.4 Trip Generation 86
TABLE OF CONTENTS
Page
3.4.2.3.5 Trip Distribution 86
3.4.2.3.6 Modal Split 86
3.4.2.3.7 Traffic Assignment 86
3.4.2.3.8 Feedback 87
3.4.2.4 Consistency Between Transportation Model
VMT and HPMS 87
3.4.2.4.1 Non-Attainment Area the Same As the
Network Model Area 88
3.4.2.4.2 Non-Attainment Area Inside of the
Network Model Area 89
3.4.2.4.3 Non-Attainment Area Outside of the
Network Model Area 89
3.4.2.4.4 Non-Attainment Area and Network Model
Area Crossover 90
3.4.2.5 Local Functional System 90
3.4.2.6 Seasonal Adjustment 91
3.4.2.7 Daily Adjustment 91
3.4.2.8 Allocating VMT to Time of Day 91
3.4.2.9 Allocating VMT to Functional Systems 91
3.4.3 Exception to the Use of HPMS VMT 92
Appendix 3-A 94
4.0 EMISSIONS FROM NONROAD, SOURCES 98
4.1 Introduction 98
4.2 Inventory Options Under This Guidance 99
4.2.1 Options for Areas With EPA Provided
Inventories 99
4.2.2 Options For Areas With EPA Provided
Inventories 101
4.2.3 Options For Areas Without EPA Provided
Inventories 102
4.3 Explanation of EPA Provided Inventory 102
4.3.1 Derivation of AMS Inputs 103
4.3.2 AMS Inputs 105
4.4 General Methodology Used In Deriving Emission Inventories
For 33 Areas 107
4.4.1 Explanation of Methodologies to Distribute
Equipment Within Each Category Type at the
County Level 108
4.4.2 Explanation of Methodologies For Distributing
Equipment Within Each Category at the Sub-
County Level 112
4.4.3 Seasonal Adjustment Methodology 113
4.5 New York Non-Attainment Area Example 115
Appendix 4-A 117
Appendix 4-B 124
Appendix 4-C 132
Appendix 4-D 135
TABLE OF CONTENTS
Page
5.0 EMISSIONS FROM AIRCRAFT 137
5.1 OVERVIEW OF THE INVENTORY METHODOLOGY 137
5.1.1 Factors Affecting Emissions 138
5.1.1.1 Aircraft Categorization 138
5.1.1.2 Pollutant Emissions 139
5.1.1.3 Aircraft Engines 140
5.1.1.4 Operating Modes 140
5.2 INVENTORY METHODOLOGY 144
5.2.1 Airport Selection 144
5.2.2 Mixing Height Determination 145
5.2.3 Activity and Emissions for Commercial
Aircraft 149
5.2.4 Activity and Emissions for General Aviation
Aviation and Air Taxi Aircraft 173
5.2.4.1 Aircraft-Specific Procedure 173
5.2.4.2 Alternative, Fleet-Average Procedure
176
5.2.5 Activity and Emissions for Military Aircraft
178
5.3 VARIATIONS TO THE INVENTORY CALCULATION PROCEDURE 190
5.3.1 Variability of Activity - Daily and Seasonal
190
5.3.2 Operational Activity that Affects Aircraft
Emissions 191
5.3.2.1 Reduced Engine Taxiing 191
5.3.2.2 Derated Take-off 192
5.3.3 Particulate Emissions 192
5.4 OTHER EMISSION SOURCES 192
5.4.1 Auxiliary Power Units 192
5.4.2 Evaporative Emissions 197
5.5 EFFECT OF FUTURE CHANGES TO THE FLEET 197
5.6 CONVERTING FROM TOTAL HYDROCARBONS (THC) TO VOLATILE
ORGANIC COMPOUNDS (VOC) 198
5.6.1 Commercial and Military Conversions 198
5.6.2 General Aviation and Air Taxi Conversions
199
6.0 EMISSIONS FROM LOCOMOTIVES 200
6.1 OVERVIEW OF RECOMMENDED INVENTORY METHODOLOGY 202
6.2 RECOMMENDED METHODS 202
6.2.1 Class I Line Haul Locomotives 202
6.2.1.1 Fuel Consumption 202
6.2.1.2 Emission Factors: 204
6.2.2 Class II and III Line Haul Locomotives 205
6.2.2.1 Fuel Consumption 205
6.2.2.2 Emission Factors 206
6.2.3 Yard Operations 206
6.2.3.1 Number of Yard Locomotives 206
6.2.3.2 Emissions Per Yard Locomotive 206
TABLE OF CONTENTS
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6.3 TAILORING METHODS 208
6.3.1 Locomotive Roster Tailoring Method 208
6.3.1.1 Identify the locomotives in the area
208
6.3.1.2 Determine the engine type 209
6.3.1.3 Sum the total of the conversions 209
6.3.1.4 Calculate the new fleet average emission
factors 210
6.3.1.5 Multiply the new emission factors by fuel
consumption 210
6.3.2 Duty Cycle Tailoring Method 210
6.3.3 SO2 Tailoring Method 212
6.4 ALTERNATIVE METHOD 213
6.5 RE-ENGINED LOCOMOTIVES 213
6.6 CONVERTING FROM TOTAL HYDROCARBONS (THC) TO VOLATILE
ORGANIC COMPOUNDS (VOC) 213
Appendix 6-1 215
Appendix 6-2 216
Appendix 6-3 217
Appendix 6-4 219
Appendix 6-5 222
Appendix 6-6 223
Appendix 6-7 225
Appendix 6-8 227
1.0 INTRODUCTION
A fundamental requirement in the effort to control pollution
in any form is to quantify the emissions being released. This is
necessary to understand the relationships between emissions and the
ambient concentrations that result, and to develop appropriate
policies and methods to ensure that ambient pollutant
concentrations remain within acceptable limits.
Specific air pollution requirements are set forth in Title 40,
Code Of Federal Regulations, Part 51.321 (40 CFR 51), and in the
Clean Air Act as amended, for the development and maintenance of
ongoing programs to inventory specific pollutant emissions. States
are required by 40 CFR 51 to prepare and submit annual reports to
the U.S. Environmental Protection Agency (EPA) regarding the
emissions of particulate matter, sulfur oxides, carbon monoxide,
nitrogen oxides, and volatile organics from point sources within
their boundaries. The amendments to the Clean Air Act require the
development of "...comprehensive, accurate, and current..."
inventories from all sources of each pollutant for every non-
attainment area, in conjunction with the preparation of revised
State Implementation Plans (SIPs). EPA recognizes that a
significant effort will continue to be needed to develop and
maintain emission inventories to meet the requirements for both
technical analysis and administrative reporting.
To assist the states in meeting the requirements for emission
inventory development, a five volume series has been prepared that
describes in detail many of the technical aspects of the inventory
process. This document is the fourth volume in the series, and it
focuses on mobile sources. Specifically, this document presents
specific methods that can be used to identify sources, estimate
emissions, and establish and maintain a useful, current mobile
source emissions inventory. Special attention has been given to
preparing the 1990 SIP inventories.
Following this introductory chapter, Chapter 2 gives an
overview of the mobile source category. Chapters 3 through 6
present specific methods that should be used to derive emission
estimates for each of the primary mobile source subcategories.
Chapter 1 - Introduction
Chapter 2 - Overview
Chapter 3 - Highway Vehicles
Chapter 4 - Nonroad Sources
Chapter 5 - Aircraft
Chapter 6 - Locomotives
2.0 OVERVIEW OF THE MOBILE SOURCE CATEGORY
An inventory of pollutant emission sources should classify
sources into two major categories - point sources and area sources.
The point source category is described in detail in Volume II of
this series. The area source category is described in detail in
Volume III of this series. Mobile sources are a subcategory within
the area source category of pollutant emission sources. However,
the procedures for preparing and maintaining an inventory of
emissions from mobile sources are presented herein, as a separate
document in this series, because the inventorying procedures are
different from those for other area source subcategories and
because the mobile source emissions inventory represents a major
portion of the total emissions of volatile organics (VOC), nitrogen
oxides (NOx ), and carbon monoxide (CO).
The mobile sources for which inventory and emission
calculation procedures are presented in this document are highway
vehicles, nonroad mobile sources, aircraft, and locomotives.
Recreational marine equipment and commercial marine vessels are
discussed in the nonroad mobile source section. The procedures
describe how to calculate tailpipe emissions and emissions from the
fuel carried on the vehicle (evaporative VOC emissions) for these
four mobile source categories. The emissions that result from tire
wear and travel over roads or other surfaces should be calculated
from the procedures in Volume III of this series and are
specifically excluded from consideration in this document.
2.1 INDIVIDUAL MOBILE SOURCE CATEGORIES
2.1.1 Highway Vehicles
Highway vehicles include all vehicles registered to use the
public roadways. The predominant emissions source in this category
is the automobile, although trucks and buses are also significant
sources of emissions.
The total highway vehicle population can be characterized by
eight individual vehicle type categories:
- Light duty gasoline powered vehicles (LDGV);
- Light duty gasoline powered trucks, from 0 to 6000 lb.
gross vehicle weight (LDGT1);
- Light duty gasoline powered trucks, from 6001 to 8500 lb.
gross vehicle weight (LDGT2);
- Heavy duty gasoline powered vehicles (HDGV);
- Light duty diesel powered vehicles, from 0 to 6000 lb. gross
vehicle weight (LDDV);
- Light duty diesel powered trucks (LDDT);
- Heavy duty diesel powered vehicles (HDDV);
- Motorcycles (MC).
2
Numerous characteristics for each vehicle type are necessary
before emissions can be calculated. These characteristics include,
among others, model year, the age distribution of vehicles within
the class, annual mileage by vehicle age, and average speed.
Chapter 3 of this document presents detailed procedures for
identifying and using these and other key characteristics.
2.1.2 Nonroad Sources
This mobile source category includes a diverse set of source
types. The movement of sources in this category occurs on surfaces
other than the public highways. Nonroad vehicles can be classified
into ten categories:
- Lawn and Garden Equipment,
- Industrial Equipment,
- Airport Service Equipment,
- Construction Equipment,
- Recreational Equipment,
- Agricultural Equipment,
- Recreational Marine Equipment,
- Logging Equipment,
- Light Commercial Equipment,
- Commercial Marine Vessels.
These categories are difficult to inventory, since few data
are available to determine either their activity levels or
operating characteristics. Chapter 4 of this document provides
procedures for inventorying and estimating emissions from these
categories.
2.1.3 Aircraft
Aircraft include all types of aircraft, whether civilian,
commercial, or military. Emissions from idling, taxiing, and during
landings and takeoffs are included. Landing and takeoff cycle
(LTO) emissions are those that occur between ground level and an
attitude of about 3000 feet. Aircraft emissions above 3000 feet
need not be included in either the base year emission inventory or
in the modeling inventory.
The larger civil and commercial airports with continuously
manned control towers maintain records of LTO cycles by type of
aircraft as part of their standard operating procedure. Smaller
airports also maintain these records to the extent that their
control towers are manned or landing fees are recorded. Difficulty
may be encountered in obtaining data on military aircraft
operations at military airports.
EPA has compiled a complete set of emission factors for
different types of aircraft operating in the different modes (idle,
taxi, LTO). Chapter 5 of this document provides instruction on how
to calculate emissions from this mobile source category.
3
2.1.4 Locomotives
Locomotives include all fossil fuel fired locomotive engines
operated on railways.
The quantity of fuel used by locomotives and the size, in
horsepower, of the locomotives are
necessary to calculate emissions from this source. This
information is discussed in Chapter 6.
0
3.0 EMISSIONS FROM HIGHWAY VEHICLES
In most urban areas, highway vehicles represent the
largest single source of carbon
monoxide (CO) emissions and contribute significantly to the
area's production of volatile
organic compounds (VOC), sulfur oxides (SO2) and oxides of
nitrogen (NOx ).
Emission estimates for highway vehicles are usually based on
the combination of two fundamental measures of activity: travel and
the average rate of pollutants emitted in the course of travel.
Both measures reflect complex patterns of behavior.
The Environmental Protection Agency and the Department of
Transportation Federal Highway Administration (FHWA) have developed
a series of tools/models to estimate the rate of emissions produced
by vehicles per mile of travel and the amount of travel itself.
The knowledge base and disciplines required to understand and
operate these models are distinct, as are their audiences. This
distinction generally ensures that environmental analysts have
little appreciation for the accuracy of the travel estimates
produced by transportation analysts and vice versa.
The purpose of this chapter is to provide guidance for
preparing the highway vehicle portion of mobile source emission
inventories, particularly those associated with the development of
State Implementation Plans (SIPs) for ozone (03) and CO. The
accuracy of the inventory will be no better than the accuracy of
the estimates of either the emission rates or vehicle miles
traveled (VMT).
This chapter responds to concerns that little effort has been
devoted to the
development of accurate projections of travel within non-attainment
areas, that projections of attainment dates have been based on
dated information and that highway vehicles are responsible for a
greater portion of the emissions inventory than recent estimates
have suggested. Because of these concerns, the earlier guidance on
the use of available travel estimates has been carefully reviewed
and updated.
3.1 GUIDANCE ON THE USE OF MOBILE4.1 VS MOBILE5 FOR THE 1990
BASE YEAR INVENTORY AND OTHER INVENTORIES
At the time of this writing, MOBILE4.1 is EPA's current
emission factor model. MOBILE5 will be available within months of
the publication of this document. EPA will accept 1990 base year
emission inventories prepared with either MOBILE4.1 or MOBILE5
emission factors. The November 15, 1992 submittal date for
inventories will apply no matter which version of the model is
used.
Since MOBILE5 will incorporate the new vehicle standards for
VOC and NOx mandated by the Clean Air Act (CAA), estimates of
those emissions for years after 1990 will be significantly
different than those estimated by MOBILE4.1. Therefore, ozone non-
attainment areas should submit projections using MOBILE5. However,
the 1990 highway vehicle emissions inventories should be
recalculated as soon as possible after November 15, 1992 using
MOBILE5 so that all required inventories are consistent. CO
5
non-attainment areas may use MOBILE4.1 for the November 15, 1992
projection submittal, and if they do, recalculation of the 1990
inventory is not necessary.
The SIP Attainment/Reasonable Further Progress Demonstration,
including projection year inventories, should use MOBILE5 for ozone
non-attainment areas.' In addition, if the base year inventory was
originally developed using MOBILE4.1, it should be recalculated
using MOBILE5 and resubmitted.'
For CO non-attainment areas, the base year and projection year
inventories may be developed using either MOBILE4.1 or MOBILE5.
Submissions after the November 15, 1992 submission should use
MOBILE5. Such submissions may be voluntary or due to bump up or
other provisions of the Clean Air Act.
The release of MOBILE5 will be accompanied by a supplement to
this document explaining the differences between MOBILE4.1 and
MOBILE5 and the additional inputs contained in MOBILE5.
3.2 MOBILE SOURCE EMISSION ESTIMATION PROCESS
3.2.1 Overview of Factors Influencing Motor Vehicle
Emission Inventories
Many complex processes govern the formation of pollutants in
motor vehicles. The EPA and the California Air Resources Board
(CARB) maintain large data collection program to quantify the rate
at which pollutants are emitted by individual categories of motor
vehicles. Both organizations have used this information to develop
models that help analysts in estimating motor vehicle contributions
to the local emissions inventory. These models, commonly known as
emission factor models, are designed to account for the effect of
numerous vehicle parameters on the volume of pollutants emitted.
The current EPA model is called MOBILE4. 1.
The primary components of an emission factor model include the
base emission factors, characterization of the vehicle fleet, fuel
characteristics, vehicle operating conditions and the effect of
local ambient conditions, the effect of alternative I/M programs
and the effect of tampering and misfueling. None of these factors
is static: technology is continually evolving, leading to changing
in-use emission performance. Changes in fuel prices and economic
conditions lead to changes in vehicle sales and travel patterns. A
substantial effort is required to accurately quantify these factors
and to stay current with the influence of all of these factors on
vehicular emission levels.
___________________________
1 The SIP Attainment/Reasonable Further Progress and
projection year inventories are due on either November 15, 1993 or
November 15, 1994, depending upon the non-attainment
classification.
2 EPA may set a date prior to November, 1993 for submission
of draft projection and recalculated base year inventories, similar
to the current requirement to submit the draft base year inventory
no later than May, 1992.
6
3.2. 1.1 Vehicle Fleet Activity
It is standard practice in preparing highway vehicle emission
inventories to express vehicle activity in terms of vehicle miles
traveled, and the emission factors in units of grams per mile of
travel. Actually, vehicles also emit hydrocarbons while
stationary. Estimates of emission-producing activities that do not
involve travel are built into MOBILE4.1. These non-moving emissions
are spread over estimated miles of travel by vehicles of a
particular age and output as an equivalent per mile emission
factor. Therefore, EPA will accept VMT as the measure of local
vehicle activity for all inventories required under the Clean Air
Act.3
VMT can be estimated in several possible ways. Direct
observation via traffic counts (usually at a sample of roadway
points with statistical expansion to represent the universe of all
roadways in the area) and highway/transit network models are the
more preferred approaches. EPA does not recommend reliance on fuel
sales data, owner reports, or periodic odometer surveys as
substitutes. The two recommended methods are discussed in Section
3.4.
3.2.1.2 Emission Factors
Emission rates are computed from test measurements of in-use
vehicles at various odometer readings designed to capture two
fundamental processes: the baseline emission rate and the
deterioration that takes place as the vehicle ages. Linear
regressions are performed on the data to quantify the level of
pollutants emitted by each model year's vehicles. The results are
commonly referred to as the intercept, or zero-mile (ZM), emission
rate and the slope, or deterioration rate (DR), that occurs over
each 10,000 mile interval.
3.2.1.3 Fleet Characteristics
The emission factors quantify the performance of individual
model year vehicle fleets by vehicle type. The age distribution,
the rate of mileage accumulation and the mix of travel experienced
by each vehicular category can significantly alter the fleet
average emission rate. While the emission factor models employ
national average distributions for each of the factors, local input
is allowed, often encouraged, and, for some inputs, required.
Differences between local and national average distributions can
alter the emissions contributions of the individual vehicle
categories.
3.2.1.4 Fuel Characteristics
Emission test measurements are conducted on a standardized
test fuel known as Indolene. The characteristics of this fuel are
well defined and ensure that test results are repeatable. Since
consumers cannot purchase Indolene at their local service stations
and
___________________________
3 VMT must usually be disaggregated such that each subset of
it can be reasonably represented by a single emission factor
determined by one set of inputs of the types described below. EPA
also accepts the trip-based activity methods described in this
document.
7
differences between the volatility of local fuels and Indolene can
influence the level of both evaporative and tailpipe pollutants,
MOBILE4.1 requires local input of fuel volatility.
3.2.1.5 Correction Factors
To ensure the repeatability of measurements, standardized test
conditions have been specified for each vehicle category. They
include driving cycle, temperature, humidity, vehicle load, and the
distribution of starting conditions. Since not all vehicle trips
match these test conditions, a series of correction factors has
been developed to allow the emission factor model to account for
differences.
3.2.1.6 Control Programs
Emission factors are based on the performance of vehicles
independent of any local control programs such as I/M, anti-
tampering and Stage II refueling. Each of these programs is
designed to reduce the level of pollutants emitted by vehicles
operating under in-use conditions. Further, differences in program
designs can have a significant impact on their effectiveness in
reducing emissions. Therefore, it is important to specify
correctly program parameters in order to estimate correctly their
effect on vehicular emissions.
3.2.2 Overview of MOBILE4.1 Input Requirements
MOBILE4.1, EPA's emission factor model, computes separate
emission estimates for eight vehicle categories:
- Light-duty gasoline-powered vehicles (LDGV), i.e.,
passenger cars;
- Light-duty diesel-powered vehicles (LDDV), i.e., diesel-
powered passenger cars;
- Light-duty gasoline-powered trucks, type 1 (LDGT1), i.e.,
pickup trucks and vans that have a gross vehicle weight
(GVW) of 0 - 6000 pounds;
- Light-duty gasoline-powered trucks, type 2, (LDGT2),
i.e., pickup trucks, vans, and other, small trucks that
have a GVW of 6001 - 8500 pounds;
- Light-duty diesel-powered trucks, types 1 & 2 (LDDT);
- Heavy-duty gasoline-powered trucks (HDGV), i.e., all
vehicles with a GVW greater than 8,500 pounds, powered by
gasoline engines;
- Heavy-duty diesel-powered vehicles (HDDV), i.e., all
diesel powered trucks with a GVW greater than 8,500
pounds; and
- Motorcycles (MC).
There are large differences in the emission characteristics of
the vehicles represented by these categories; therefore, it is
important that estimates of local or regional emission rates
incorporate the distribution of VMT by vehicle type.
The emission factors produced by MOBILE4.1 are derived from
measurements conducted under standardized test conditions. For
light-duty vehicles, the standard set of test conditions is
referred to as the Federal Test Procedure (FTP). It involves the
simulated
8
operation of a vehicle over a specific driving cycle, the Urban
Driving Cycle, under controlled operating and environmental
conditions, during which emissions are measured in three sequences.
The Urban Driving Cycle represents an average trip over an urban
network that includes travel on local and arterial streets, major
arterials, and expressways. The basic test conditions include:
- Ambient temperature range of 68'F to 86'F;
- Absolute humidity adjusted to 75 grains of water per
pound of dry air;
- Average speed of 19.6 mph with 18 percent idle operation;
- Average percent of VMT` in cold start operation of 20.6
percent;
- Average percent of VMT in hot start operation of 27.3
percent;
- Average percent of VMT` in stabilized operation of 52.1
percent; and
- Average trip length of 7.5 miles.
In order to understand fully the derivation of emission
factors and the influence of these conditions on emission levels,
refer to Chapter 2 of the MOBILE4.1 User's Guide. A condensation
of that material is included in Section 3.3 of this report.
MOBILE4.1 inputs can be altered to reflect city-specific
conditions. A brief review of each of the primary options is
presented below. They are not organized as they are in the
MOBILE4.1 User's Guide, but rather in the order in which they will
be discussed in more detail later in this chapter.
3.2.2.1 Fleet Characteristics
3.2.2.1.1 VMT Mix
The distribution of travel across the eight vehicle categories
determines how the individual emission factors are weighted to
produce a composite emission factor for the entire highway vehicle
fleet. The LDGVs generally comprise over 50 percent of the travel
recorded in any area of the country and, therefore, tend to be the
dominant source of highway emissions. (HDDVs are an important
source of NOx emissions.) MOBILE4.1 will calculate the VMT mix
based on national data characterizing registration distributions,
annual mileage accumulation rates by age, diesel sales fractions,
and vehicle counts. These values may not, however, be
representative of certain areas, such as western states where
pickup trucks form a larger share of the vehicle population or
rural areas where a broader distribution of vehicles exists.
3.2.2.1.2 Annual Mileage Accumulation Rates
The primary effect of the rate of mileage accumulation by age
(in combination with registration data) is to determine the
relative weighting of each model year's contribution to the average
emission factor computed for each vehicle category. MOBILE4.1
provides the option of using a national average value or inputting
data characterizing local conditions. The rate of mileage
accumulation may be different from national average conditions in
both rural and urban areas at either end of the economic spectrum.
9
3.2.2.1.3 Registration Distributions
These are used in concert with mileage accumulation rates to
determine the relative weighting of each model year's contribution
to the average emission factor for each vehicle category.
MOBILE4.1 provides the option of using national average values or
inputting data characterizing local registrations. The areas most
likely to be distinct from national average values are rural areas,
areas in which cars do not rust out and urban areas at either end
of the economic spectrum.
3.2.2.2 Fuel Specifications
3.2.2.2.1 RVP
Evaporative and, to a lesser extent, exhaust emissions vary
with fuel volatility. EPA's new vehicle certification program and
much of its in-use vehicle testing program use gasoline with a fuel
volatility (RVP) of 9.0 psi. In recent years much of the country
has been supplied with gasoline of higher volatility. MOBILE4.1
adjusts estimated emission factors to account for the effects of
volatility. No national average value for this variable is
available in MOBILE4.1; one must supply this input.
3.2.2.3 Correction Factors
3.2.2.3.1 Speed
Emission factors are very sensitive to the average speed that
is assumed. In general, emissions tend to increase as average
speeds decrease from the 19.6 mph average FTP speed. MOBILE4.1
does not assume an average speed; rather it requires that an
estimate of the speed experienced by vehicles operating in the area
and roadway segment or collection of interest be specified.
MOBILE4.1 adjusts the emission factors for speeds other than 19.6
mph through the use of speed correction factors. These
multiplicative adjustments to the base emission factors tend to
follow a non-linear relationship that increases the emission levels
as speeds decline from 19.6 mph and increase beyond 48 mph.4
3.2.2.3.2 Temperature
Emissions from mobile sources are significantly influenced by
the ambient temperatures under which they are operating.
Temperature has an effect on both the exhaust and the evaporative
emission levels. MOBILE4.1 deals with these effects separately.
In general, exhaust emissions are at a minimum at the temperature
specified for the FTP (75'F), with emissions increasing as
temperature either increases or decreases from that value. No
ambient temperature is assumed by MOBILE4.1. One must be provided
as an input to the model.
___________________________
4 The speed correction factors in MOBILE5 may be
significantly revised at speeds above 48 mph.
10
3.2.2.3.3 Operating Modes
Emission factors based on FTP measurements are collected for
three separate segments, usually referred to as bags because the
vehicle exhaust is collected in three separate teflon bags, each
with differing emissions performance. The three bags correspond to
the following modes of operation: cold start, hot stabilized, and
hot start. Bag 1, the cold start mode, reflects conditions
experienced at the beginning of a trip when the engine and the
emission control system begin operation at ambient temperature and
are not performing at optimum levels (i.e., the catalyst is cold
and has not reached the "light off" temperature needed to
efficiently control emissions coming from the engine) until part
way through the trip.5 The hot start mode, Bag 3, reflects the
condition of an engine that has been restarted after being turned
off for 10 minutes and, therefore, has not cooled to ambient
conditions. Under this circumstance the engine and catalyst are
warm and, although not at peak operating efficiency when started,
still have significantly improved emissions performance relative to
the cold start mode. Bag 2, the hot stabilized mode, reflects the
condition of the engine when the vehicle has been in continuous
operation long enough for all systems to have attained stable
operating temperatures. The proportion of VMT accumulated in cold
and hot start modes must be specified based on the conditions in
the area to be modeled. Specifications must be made for catalyst
and non-catalyst vehicles separately.
3.2.2.3.4 Minor Correction Factors
This category has been added to cover the effects of four
special correction factors that are available:
- Air conditioning;
- Extra vehicle loading;
- Trailer towing;
- NOx humidity.
These factors are designed to account for the effect of
unusual vehicle operating conditions relative to those experienced
in the FTP. Generally, it is difficult to quantify the extent of
these vehicle operating parameters, and their effect on emission
factors tends to be small. Therefore, EPA recommends that few
resources be expended to develop the inputs needed. The effect of
the NOx humidity correction factor is also slight, and, unless
NOx is of particular concern, little effort should be devoted to
its use.
5 "Bag 1" is usually a mix of cold and warmed operation,
since, except under very cold ambient conditions, the 505 seconds
of driving represented by this bag constitutes a longer period than
is needed for the engine and catalyst to get warm.
11
3.2-2.4 Tampering and Misfueling
The basic emission factors in MOBILE4.1 receive an adjustment
to account for estimates of vehicle tampering rates as a function
of accumulated mileage for each gasoline-fueled vehicle category
and eight categories of tampering (e.g., air pump disablement,
misfueling, etc.). These rates are combined with offsets (the
increase in emissions that results from the given type of
tampering) and added to the non-tampered emission factors. Options
are available to input local tampering rates. The use of local
information must be supported by an approved survey. If locally
developed information is not available, a national average rate
will be used by MOBILE4.1.
3.2.2.5 Control Programs
3.2.2.5.1 Refueling Emissions
The refueling of gasoline-fueled vehicles results in the
displacement of fuel vapor from the vehicle fuel tank to the
atmosphere.
There are two basic approaches to the control of vehicle
refueling emissions, generally referred to as "Stage III" (at the
pump) and "onboard" (in the vehicle) vapor recovery systems.
MOBILE4.1 can model refueling emissions with no controls as well as
with either or both of the control options.
3.2.2.5.2 Inspection and Maintenance Programs
Many areas of the country have implemented I/M programs as a
means of further reducing mobile source air pollution. MOBILE4.1
can model the impact of an operating I/M program on the calculated
emission factors. There is no average national I/M program; local
inputs must be supplied. Details are given in Section 3.3.6.2 of
this document and in the MOBILE4.1 User's Guide.
3.2.2.5.3 Anti-Tampering Programs (ATPs)
Some areas of the country have implemented these programs to
reduce the frequency and related emission impacts of emission
control system tampering. MOBILE4.1 allows the effects of such a
program on the calculated emission factors to be estimated. Due to
the wide variation in the characteristics of ATPs and the lack of a
national program, there is no national average estimate of ATP
parameters. Details of the required inputs are given in Section
3.3.6.3 of this document and in the MOBILE4.1 User's Guide.
12
3.3 GUIDANCE ON SELECTING MOBILE4.1 INPUTS6
MOBILE4.1 may be used to develop highway vehicle emission
factors and emission inventories for use in the State
Implementation Plan process.7 The proper version of MOBILE4.1 to
use in preparing SIP inventories is the one dated November 4, 1991.
Older versions should be discarded or erased.8
This section contains EPA's recommendations and suggestions
with regard to determining appropriate MOBILE4.1 inputs. However,
for many inputs there is no single correct answer or recommendation
that is best for every local area. For those using MOBILE4.1 for
SIP-related modeling purposes, it is important that the appropriate
EPA Regional Office personnel be kept involved in decisions
concerning questionable or controversial assumptions in the
MOBILE4.1 modeling and inventory development process.
3.3.1 Emission Factors
Description
The basic emission rates (BERs) used in MOBILE4.1 are
expressed as linear equations and consist of a zero-mile level and
one or two deterioration rates.9 There are different BER
equations in MOBILE4.1 for each vehicle type/pollutant/model year
group, with the model year groups defined on the basis of
applicable emission standards and emission control technologies
used.
Although MOBILE4.1 provides the capability to change the BER
equations, the BERs in MOBILE4.1 accurately reflect all promulgated
emission standards as of late 1990, and no locality-specific
changes to these equations are warranted for use in developing
emission factors or inventories for calendar years through 1992.
Specifically, no need exists for modification of the BERs in
MOBILE4.1 in order to develop emission factors for the development
of base year 1990 emission inventories by the states in response to
the requirements of the Clean Air Act.
___________________________
6 This section is in part a condensation of material that
appears in the User's Guide to MOBILE4.1, Chapter 2. It is not a
substitute for the User's Guide. You are advised to obtain and
thoroughly read the User's Guide before running the model. It is
available from the National Technical Information Service (NTIS),
5285 Port Royal Road, Springfield, VA 22161 (703/487-4650). The
NTIS accession number is PB91-228759.
7 Highway vehicle emission factors and emission inventories
for non-attainment areas in California may be developed using the
FISAFAC model.
8 While future year inventories are discussed in this
document, MOBILE4.1 should not be used for projecting VOC or NOx
emissions beyond January 1, 1994, since it does not reflect new
standards that begin to have an effect after that date. MOBILE4.1
may be used for CO inventory projections. MOBILE5 will be released
in final form in August, 1992 and will allow VOC and NOx
projections.
9 A deterioration rate is the gram per mile increase in
emissions per 10,000 miles accumulated mileage.
13
Guidance
No need exists for modifying the BERs in MOBILE4.1 in order to
develop VOC or NOx emission factors for any calendar year through
1992 inclusive,10 or to develop CO emission factors for any
calendar year through 2020.
3.3. 1.1 Region
Description
MOBILE4.1 provides two options for region: low-altitude and
high-altitude. Low-altitude emission factors are based on
conditions representative of approximately 500 feet above mean sea
level (+500 ft MSL), and high-altitude factors are based on
conditions representative of approximately +5500 ft MSL.
MOBILE4.1, like MOBILE4, does not calculate California emission
factors. There have been no revisions to this variable or how it
is input to the model since the release of MOBILE4.
Guidance
For the majority of MOBILE4.1 applications, low-altitude is
the appropriate choice. For those areas designated as high-
altitude by EPA for mobile source regulatory purposes, generally
those counties that lie "substantially" above +4000 ft MSL, high-
altitude should be selected.11
3.3.1.2 Calendar Year
Description
The value used for calendar year in MOBILE4.1 defines the year
(as of January 1) for which emission factors are to be calculated.
It is frequently referred to as the calendar year of evaluation.
MOBILE4.1 has the ability to model emission factors for the years
1960 through 2020 inclusive. There have been no revisions to this
variable or how it is input to the model since the release of
MOBILE4.
___________________________
10 EPA expects to update the model to version 5.0 to
incorporate all of the requirements of the November 1990 CAA in
time for states to project mobile source HC and NOx emissions and
demonstrate attainment of the National Ambient Air Quality Standard
for ozone.
11 A list of those counties EPA has designated as high-
altitude appears in 86.088-30, paragraphs (a)(5)(ii) and (iv),
Code of Federal Regulations.
14
Guidance
The 1990 base year SIP inventories represent emissions during
a typical day in the pollutant season, most commonly summer for
ozone and winter for CO. Thus, base year VOC inventories should be
based on interpolation of the calendar year 1990 and 1991 MOBILE4.1
emission factors.12, 13, 14
CO SIP inventories should be based on emission factors from
January 1990 regardless of the three-month period for which CO is
being modeled.
Similar instructions apply to the development of Reasonable
Further Progress (RFP) inventories. For modeling of specific
episode days, the best results will be obtained by interpolating
exactly to the day being modeled. In attainment demonstrations, it
is acceptable to account for fleet turnover through November 15th
of the year being modeled.
3.3.2 Fleet Characteristics
3.3.2.1 Vehicle Miles Traveled Mix by Vehicle Type
Description
The vehicle miles traveled mix specifies the fraction of total
highway VMT that is accumulated by each of the eight regulated
vehicle types. The VMT mix is used in MOBILE4.1 only to calculate
the composite (all vehicle) emission factor for a given scenario on
the basis of the model's eight vehicle class-specific emission
factors.
___________________________
12 For example, if most exceedances of the ozone National
Ambient Air Quality Ozone Standard occur during the months of June,
July, and August, then the appropriate base year emission factor is
the average of the January 1, 1990 and January 1, 1991 emission
factors.
13 Since the accuracy gained by interpolating for typical
summer days other than July 1st is minimal and since the AIRSAMS
mainframe version of MOBILE4.1 for VOC and NOx inventories
automatically generates July I emission factors, EPA will accept
1990 VOC and NOx emissions estimates based on July 1st emission
factors. Areas preparing draft 1990 inventories may select an
input of January 1, 1990 for their ozone season inventory and note
this prominently in the documentation. However, the inventory must
be switched to a July 1, 1990 basis for the final submission to
EPA.
14 January 1st and July 1st evaluations only differ in that
the July 1st vehicle fleet is composed of more of the latest model
year vehicles and fewer of the 25th and older model year vehicles,
as a result of new car sales and scrappage of older vehicles
between January I and June 30. The January I vs. July I choice is
independent of all temperature and other vehicle operating
conditions, which should represent the appropriate pollutant
season.
15
MOBILE4.1 calculates a typical urban area VMT mix based on
national data characterizing model-year-specific registration
distributions and annual mileage accumulation rates by age for each
vehicle and fuel type,15 the fraction of travel by each vehicle
type that occurs in typical urban areas, and the total number of
vehicles of each vehicle type.
For SIP-related highway vehicle emission inventory development
in moderate and above non-attainment areas, EPA expects states to
develop and use their own specific estimates of VMT by vehicle type
and highway functional system.16 VMT fractions based on local
estimates of VMT by vehicle type should be used as input to
MOBILE4.1.17
Each VMT mix supplied as input must consist of a set of eight
fractional values, representing the fraction of total mobile source
VMT accumulated by each of the eight vehicle types. All values
must be between zero and one, and the eight values must sum to 1.0.
There have been no revisions to how alternate VMT mixes are
supplied to the program as input data since the release of MOBILE4.
Guidance
Techniques for calculating estimated VMT by vehicle type (and
thus, total VW and the VMT mix fractions) from available data
sources are described in Chapter 6 of the report, "Techniques for
Estimating MOBILE2 Variables.18 Metropolitan Planning
Organizations (MPOs) and state Departments of Transportation (DOTs)
should also be consulted. Information from these agencies can be
used to determine the proportion of passenger vehicles and light
duty trucks relative to heavy duty trucks by time of day and
facility class. These two groups of vehicles can then be allocated
into the eight MOBILE4.1 vehicle classes using the national default
mix within each group provided by MOBILE4.1.
3.3.2.2 Annual Mileage Accumulation Rates and Registration
Distributions by Vehicle Type and Age
Description
MOBILE4.1's emission factor calculations rely in part on
travel fractions for vehicles of each given age within a vehicle
type, which in turn are based on estimates of the average annual
mileage accumulation by age (first year to 25th-and-greater years
of operation) for
___________________________
15 Total HDDV registrations and annual mileage accumulations
are also distributed within the model by truck weight class.
16 Highway functional systems are commonly designated as the
interstate system, other freeways and expressways, other principal
arterials, minor arterials, and collectors.
17 Ozone and carbon monoxide non-attainment areas classified
as marginal, submarginal, or transitional may use the MOBILE4.1
default VMT mix if no local estimate is readily available.
18 Techniques for Estimating MOBILE2 Variables" and
"Additional Techniques for Estimating MOBILE2 Variables," Energy
and Environmental Analysis, Inc. for EPA (EPA Contract Number 68-
03-2888).
16
each of the eight vehicle types, and the registration distributions
by age (age 0-1 to age 24 and 25+) for each vehicle type, except
motorcycles, for which annual mileage accumulation rates and
registration distributions are only provided for the first to 12th-
and-later years of operation (ages 0-1 to 11 and 12+). For all
vehicles except motorcycles, this represents an increase in detail
from the 20 years of operation used in MOBILE4.19
To use locality-specific annual mileage accumulation rates by
age, a total of 200 input values is required: the estimated annual
mileage accumulated by vehicles of each of the eight types for each
of 25 ages.
To use locality-specific registration distributions by age, a
total of 200 input values is also required. For each vehicle type,
a set of 25 values is required to represent the fraction of all
vehicles of the given type that are of a given age.
If both annual mileage accumulation rates by age and
registration distributions by age are supplied, the annual mileage
accumulation rate corresponding to any vehicle type/age combination
accounting for a non-zero fraction of registrations must be
positive. That is, if vehicles of a certain type and age are
registered, then they are assumed to be driven.20
If locality-specific mileage accumulation rates and/or
registration distributions by age are not used, the information in
MOBILE4.1 is used for all calendar years evaluated.
Guidance
Mileage Accumulation
EPA recommends, the use of the national annual mileage
accumulation rates by age that are included in MOBILE4. 1. Most
local sources of data on mileage accumulation rates by age are
subject to sampling bias or data entry errors, and the use of such
data should be approached with caution. States that wish to use
alternate mileage accumulation rates in their development of
highway vehicle emission inventories in response to the
requirements of the new CAA should obtain prior approval from their
EPA Regional Office before using such rates in their emission
factor modeling.20
___________________________
19 MOBILE4.0 modeled vehicles from age 0 through age 19 with
the 20th year representing all vehicles 20 years and older.
20 MOBILE4.1 will issue an error warning if vehicles of a
certain type and age are registered but do not accumulate mileage.
A warning will also be issued if there are no vehicles of a certain
type and age yet the mileage accumulation distribution includes a
positive value for that category.
21 If local annual mileage accumulation rates are used, they
normally should not change from one evaluation year to the next.
17
Registration Distributions
EPA recommends and encourages the use of actual locality-
specific 1990 registration distributions by age to develop base
year SIP emission inventories.22, 23 Local registration
distributions are particularly appropriate for those inventory
areas where there are significant differences from the national
average. The exception to the use of local data would be in those
areas that have relatively few local HDDV registrations, but that
experience significant interstate trucking activity. Such areas
may want to retain and use the MOBILE4.1 national registration
distributions.
EPA will issue at a later date additional guidance on how 1990
registration distributions by age can be adjusted to reflect future
years. This guidance will provide a mathematical routine that
preserves the average age of the fleet in 1990, while retaining the
general shape of the local distribution for 1990 and earlier model
years.24, 25
Methods for estimating the annual mileage accumulation rates
by age and the registration distributions by vehicle type and age
are presented in Chapters 2 and 3, respectively, of the report
Techniques for Estimating MOBILE2 Variables.26
3.3.2.3 Trip Length Distribution
Description
Running loss emissions are a form of evaporative volatile
organic compound (VOC) emissions that occur while the vehicle is
being operated. Running loss emissions are different from
"traditional" evaporative emissions that occur after the vehicle
has been driven(hot soak evaporative emissions) and while it is
parked during periods of rising ambient temperatures (diurnal
evaporative emissions). MOBILE4 was the first version of the
emission factor model to account for these emissions. In MOBILE4.1
estimates of running loss emissions have been extensively revised.
___________________________
22 Marginal, sub-marginal, and transitional non-attainment
areas may use the MOBILE4.1 distributions for all vehicle types, if
local distributions are not available.
23 Registration distributions by age may be developed from
data available through state motor vehicle registration records,
either directly or commercially through R.L Polk Company.
24 Effectively, the routine will apply a scrappage curve to
the existing 1990 registration distribution. The result will be
that the pattern of high and low vehicle sales will propagate down
the Registration distribution as vehicles age with successive
evaluation years.
25 EPA will not accept a locally developed registration
distribution that implies that the average age of the vehicle fleet
is becoming younger in the future than is reflected in the
registration distribution used for the base year unless the state
provides adequate justification for the new distribution .
26 "Techniques for Estimating MOBILE2 Variables" and
"Additional Techniques for Estimating MOBILE2 Variables," Energy
and Environmental Analysis, Inc. for EPA (EPA Contract Number 68-
03-2888).
18
EPA has determined through its running loss emission test
programs that the level of running loss emissions depends on
several variables: average vehicle speed, ambient temperature, fuel
volatility, and the length of the trip.27 Test data show that for
any given set of conditions (average speed, ambient temperature,
and fuel volatility), running loss emissions are zero to negligible
at first, but increase significantly as trip duration lengthens.
In MOBILE4, running loss emissions were modeled as direct
functions of the input temperature and volatility; average speed
and trip duration were held constant within the model to values
representative of typical urban area traffic patterns. In
MOBILE4.1 running loss emissions are modeled as a direct function
of the input temperature, fuel volatility, average speed and trip
duration.
The input data record of the VMT-weighted trip duration
distribution must list the fraction of all travel (VMT) being
accumulated over the time period that the emission factors apply:
- Under 10 minutes
- 11 to 20 minutes
- 21 to 30 minutes
- 31 to 40 minutes
- 41 to 50 minutes
- 51 minutes and longer
Note that the first value should be the fraction of VMT that
occurs in trips that end within 10 minutes of their start, not the
fraction of VMT that occurs within 10 minutes of trip start for
longer trips. The other values are defined similarly. Note also
that the running loss emission factor that is calculated by
MOBILE4.1 is a fleet and area-wide average that applies to all of
the VMT in all of the trips for each vehicle type. Any geographic
disaggregation by VMT density will be only approximate. Situations
more heavily affected by emission rates at the end of trips, such
as a central business district in the morning rush hour, are more
complex to model. EPA staff should be consulted in such cases.
If this option of specifying trip length distributions is not
selected, then MOBILE4.1 will calculate the running loss emission
factors on the basis of the typical trip duration included in the
model.
Guidance
Since reliable local data on the distribution of trip
durations is often unavailable, EPA will accept the use of the
model's typical distributions for the estimation of running loss
VOC emission factors for the 1990 emission inventory. Where the
transportation modeling process
___________________________
27 "Length of trip" as used here refers to the duration of
the trip (how long, in minutes, the vehicle has been traveling),
not on the distance traveled in the trip (how far the vehicle has
been driven).
19
can produce reliable inputs for trip duration, use of such inputs
will produce a more accurate estimate of the benefits attributable
to SIP measures which shorten average trip lengths without
eliminating entire trips.28
Most SIP inventories will be constructed by adding together
emission estimates for several functional classifications of
roadway. EPA recommends that one area-wide trip length
distribution be used for all roadway classifications, due to the
complexity of trying to develop separate distributions.
3.3.2.4 Diesel Sales Fractions
Description
Sales of diesel powered light-duty vehicles and trucks
underwent a surge in the late 1970's and early 1980's, peaking at
5.9% of LDV sales in the 1981 model year, and at 9.3% of LDT sales
in the 1982 model year. Since then diesel sales have fallen
precipitously, to virtually zero for LDVs29 and to about 0.2% of
LDTs since the 1988 model year. While MOBILE4 contained forecasts
of increasing diesel sales for both LDVs and LDTs through the early
1990's, MOBILE4.1 assumes a more limited and slower increase from
the current, very low diesel sales rates. MOBILE4.1 assumes that
future LDV diesel sales never exceed 0.3% and that future LDT sales
never exceed 2.15%.
MOBILE4.1, like earlier versions of the model, uses a single
set of registration distributions by age and annual mileage
accumulation rates to describe all LDVs, and another set to
describe all LDTs. This is due in part to the fact that it is
nearly impossible to develop such information for gas and diesel
LDVs and LDTs separately, and in part since there is little
evidence to suggest that typical use patterns and mileage
accumulation rates are different for gas and diesel LDVs and for
gas and diesel LDTs.
Diesel sales fractions represent the share of all sales30 in
a given model year that are diesel-fueled vehicles. The use of
model-year-specific diesel sales fractions allows MOBILE4.1 to
internally split the LDVs and LDTs into gas and diesel sub-
categories, which have distinctly different emission rates.
If vehicle registration data that distinguish between gas and
diesel LDVs and gas and diesel LDTs exist, it is possible to input
local diesel sales fractions by model year. These data must be
supplied for every calendar year of evaluation; since they apply to
vehicles of
___________________________
28 The use of trip length distributions other than those
included in MOBILE4.1 should be adequately documented in the SIP.
29 LDV diesel sales accounted for less than 0.05% of total
LDV sales in the 1988-1990 model years.
30 Diesel sale fractions apply only to LDVs and LDTs. Heavy
duty gasoline and diesel vehicles are treated separately within the
MOBILE models.
20
Ages 1, 2, 3, ..., to 25-and-older, different sets of fractions are
required for each calendar year. Including this information will
create a more accurate highway mobile source emission inventory
estimate.
For each scenario, the fractions of LDV and of LDT sales that
were diesel for each model year from the calendar year of
evaluation back to 25 model years ago must be entered as a model
input. For example, if the calendar year of evaluation is 1990,
then diesel sales fractions for model year 1990, 1989, 1988, ...,
1967, and 1966-and-older LDVs and LDTs must be provided. If two
different scenarios are being run, both for calendar year 1990 but
with other differences, then the same set of diesel fractions would
have to be supplied again as part of the second scenario. If a
scenario with calendar year 1995 was also being run, then the
diesel sales fractions would represent model year 1995, 1994,...,
1972, and 1971-and-older vehicles. The same values would be used
for the model years in common to the two sets of sales fractions,
but the five oldest model year values would not be used in the
second sequence to make room for the five most recent model years
sales fractions.
The 50 diesel sales fractions, 25 each for LDVs and LDTs, must
be specified as fractions. For example, if in a given area the
1990 model year had diesel sales of 1.1% of LDVs and 1.8% of LDTs,
the diesel sales fractions are 0.011 and 0.018 respectively. The
values are supplied in pairs: The first two values on the first
record are the diesel sales fractions for one year old LDVs and
LDTs;31 the second two values are the sales fractions for two
year old LDVs and LDTs, and so on, with the last two values on the
third record being the sales fractions for LDVs and LDTs 25 years
and older.
Guidance
This option has been provided in MOBILE4.1 for two reasons.
First, some users performing highway vehicle emission factor
modeling may have access to vehicle registration data, or data from
other sources, enabling them to characterize diesel sales of LDVs
and LDTs in the area being modeled. Particularly if these sales
fractions differ significantly from those included in MOBILE4.1, it
will enhance the accuracy of the emission factors and inventory to
use those sales fractions as model input. Second, as can be seen
by the sharp rise and equally sharp fall of diesel sales in the
late 1970's and early 1980's, it is extremely difficult to forecast
diesel sales fractions for future model years. This provision will
allow modelers to account for future increases in diesel sales, if
such increases occur.32
___________________________
31 A vehicle is assumed to be one year old if the model year
of that vehicle is the same as the evaluation year. Thus, a 1990
model year vehicle is assumed to be one year old in 1990.
Similarly, a 1989 model year vehicle is assumed to be two years old
in 1990.
32 EPA does not envision any circumstances in which a state
or locality should substitute its own projection of future diesel
sales for that built into MOBILE4.1.
21
3.3.3 RVP Determination
Description
The basic emission rates that underlie the emission factor
calculations are developed from vehicles tested at FTP conditions,
including a fuel volatility of 9 psi Reid Vapor Pressure (RVP).
For other fuel volatility levels, MOBILE4.1 adjusts the emission
factors for exhaust and evaporative emissions as well as for
running loss, resting loss, and refueling loss emissions.
Vehicle emission rates increase as the volatility of the fuel
increases, for temperatures between 45'F and 75'F and for RVP
values between 9.0 and 11.7 psi. This effect is most pronounced at
higher RVP levels and at higher ambient temperatures. Since there
is a significant interaction effect between RVP and temperature, it
is important that RVP and temperature inputs to MOBILE4.1 be
consistent. That is, RVP and temperature should be chosen in such
a way that they represent the same time period.33 In general, use
July 1990 RVP levels to estimate VOC and CO emissions during the
ozone non-attainment season . Use January 1990 RVP levels to
estimate CO emissions during the CO non-attainment season.
Guidance
Gasoline survey data should be used to determine historical
RVP, if quality-assured survey data are available. The survey
samples should be drawn at the pump, not "upstream" of the pump at
a refinery or fuel distribution terminal.34, 35, 36
___________________________
33 High RVP fuel is used in the winter months to facilitate
vehicle starting. If the same high RVP fuel were used in the
summer, a vehicle could experience vapor lock and stall.
34 EPA will also accept the use of RVP determined from
either of two regularly published gasoline volatility surveys, one
performed by oil companies and compiled by the National Institute
for Petroleum and Energy Research (NEPER) and the other sponsored
by the Motor Vehicle Manufacturers' Association (MVMA) and
conducted by the Southwest Research Institute (SwRI). Since the
NIPER survey is not city-specific, the MVMA survey is the preferred
choice.
35 A third survey is sponsored by a consortium of oil
companies, the American Petroleum Institute (API), and is also
conducted by Southwest Research Institute. This survey includes
more cities and sampling months, but the data from it axe
proprietary.
36 A final possible source of RVP data is the sampling done
by some states to enforce their state RVP limits. However, before
using this approach, it should be discussed with EPA to determine
if RVP data collected for enforcement purposes are suitable for
determining average RVP for inventory purposes.
22
Procedure for Determining RVP Using the MVMA Survey
Obtain the appropriate edition of the MVMA National Gasoline
Survey, published semi-annually37 by the Motor Vehicle
Manufacturers' Association. Ordering and price information is
available from:
Motor Vehicle Manufacturers' Association
300 New Center Building
Detroit, NE 48202
Phone (313) 872-4311
Use the summer MVMA survey to estimate VOC and CO emissions
during the ozone non-attainment season. Use the winter MVMA survey
to estimate CO emissions during the CO non-attainment season.
If the average RVP for a specific city is desired and that
city is included in the MVMA survey, use the RVP for that city.38
Find the average RVP value(s) for the city or cities selected
from the summary table that appears near the end of the MVMA
survey. The average RVP for regular unleaded gasoline is provided
for all cities; the average RVP for premium and/or mid-grade
unleaded and/or regular leaded gasoline is also provided for many
cities. Ignore RVP values for any ethanol blends that may also be
listed.39
Calculate the overall average RVP from the averages supplied
for different grades of gasoline as follows:
___________________________
37 The data reflected in MVMA National Gasoline Survey are
generally collected as of January 15th and
July 15th.
38 If no city from the inventory area is included in the
MVMA survey, use the RVP for a city that is both geographically
close to the city with the largest population within the inventory
area and that was subject to the same EPA, state, or ASTM
volatility limit at the time. If fuel distribution patterns are
known, give preference to a survey city with the same distribution
system.
If the RVP for outlying areas of a state is desired (for
example, to complete the inventory for the fringes of an Airshed
modeling domain) and a city within that state is included in the
MVMA survey, use the RVP for that city. If two or more cities in
that state are included in the MVMA survey, average the RVPs from
those cities. If no city within the state is included in the MVMA
survey, use the RVP for a city that is both geographically close to
the state and that was subject to the same EPA, state, or ASTM
volatility limit at the time. If fuel distribution patterns are
known, give preference to a survey city with the same distribution
system.
39 As the use (and market share) of regular leaded fuel
continues to decline, survey values for mid-grade unleaded are
replacing those for regular leaded.
23
- If only the average RVP for regular unleaded gasoline is
provided, use that value;
- If the average RVP for regular unleaded and one of the other
fuel grades (premium unleaded, mid-grade unleaded, or regular
leaded) is provided, weight the values using the local sales
mix, if known, or at 75 percent regular unleaded and 25
percent of the other grade for which the RVP is provided,
according to equation 3-1.
Average RVP = 0.75 - (average RVP of regular unleaded) +
0.25 - (average RVP of premium unleaded or mid-grade
unleaded or regular leaded)
(3-1)
- If the average RVP is provided for three fuel grades, weight
the values, using the local sales mix, if known, or at 50
percent regular unleaded, 25 percent premium unleaded, and 25
percent mid-grade unleaded or regular leaded, according to
equation 3-2.
Average RVP = 0.50 - (average RVP of regular unleaded) +
0.25 - (average RVP of premium unleaded) +
0.25 - (average RVP of mid-grade unleaded or regular
leaded)
(3-2)
If the average RVP is provided for all four fuel grades,
weight the values using the local sales mix, if known, or at 50
percent regular unleaded, 20 percent premium unleaded, 20 percent
mid-grade unleaded, and 10 percent regular leaded, according to
equation 3-3.
Average RVP = 0.50 - (average RVP of regular unleaded) +
0.20 - (average RVP of premium unleaded) +
0.20 - (average RVP of mid-grade unleaded) +
0.10 - (average RVP of regular leaded)
(3-3)
The RVP thus calculated is used as the value of historical RVP in
MOBILE4.1.
Procedure for Determination of RVP Using the NIPER Survey
Obtain the appropriate edition of the report Motor Gasolines,
published semi-annually by NIPER. Samples for the summer survey
are taken in June, July, and August. Samples for the winter survey
are taken in December, January, and February.
24
The cost per report is $60, and it is available from:
Cheryl L. Dickson
National Institute for Petroleum and Energy Research
P. 0. Box 2128
Bartlesville, OK 74005
Phone (918) 336-2400
Use the summer NIPER survey to estimate VOC and CO emissions
during the ozone non-attainment season. Use the winter NIPER
survey to estimate CO emissions during the CO non-attainment
season.
The NIPER survey divides the country into seventeen districts,
which are described in a table and illustrated on a map of the U.S.
Use the district in which the inventory area is located. If the
RVP for an entire state is desired and that state lies entirely
within one district, use that district. If the state lies within
two or more districts, average the RVPs from the districts within
which the state lies.
Table 4 of the NIPER survey presents the average RVP of three
grades of gasoline for each district: regular unleaded, regular
leaded, and premium unleaded.40
Determine the overall average RVP of gasoline in a district by
weighting these three values by the local sales mix, or, in the
absence of local data, by an assumed sales mix of 50 percent
regular unleaded, 25 percent premium unleaded and 25 percent
regular leaded according to equation 3-4.
Average RVP = 0.50 - (average RVP of regular unleaded) +
0.25 - (average RVP of premium unleaded) +
0.25 - (average RVP of regular leaded)
(3-4)
The calculated RVP (or the average of the calculated RVPs, if
the area for which the RVP is being determined resides within two
or more districts) is then used as the value of historical RVP in
MOBILE4.1.
Procedure for Determining RVP from Applicable RVP Limit
For an area without its own survey data and for which it is
not possible to use a city or district for which survey data exist,
RVP can be determined from the applicable RVP limit41 adjusted by
either a non-compliance margin or a compliance safety margin.
Where ASTM
___________________________
40 Ignore RVP values for any ethanol blends that may be
listed.
41 Historically, fuel volatility was subject to voluntary
limits according to ASTM Standard D439, "Standard Specification for
Automotive Gasoline." More recently, fuel volatility has been
subject to federal and/or state regulatory requirements.
25
limits were the applicable limit, average RVP often exceeded the
ASTM limit (noncompliance margin) by an amount that varied with the
year and the ASTM limit itself. On the other hand, gasoline
regulated by EPA or state RVP limits usually has had an average RVP
below the EPA or state ceiling.
To estimate RVP using the "limit" approach, apply the
historical margin to the applicable limit according to equation 3-
5.
RVP = Applicable Limit + Margin
(3-5)
where
Applicable Limit = Federal or state regulatory limit, or, if
none applies, ASTM standard for state and
month for which an inventory is being
estimated;
Margin = Non-Compliance Margin, if average RVP is greater
than the applicable limit, or Compliance Safety
Margin, if average RVP is less than the applicable
limit.42
3.3.3.1 EPA-Provided 1990 RVP Estimates
For states that do not wish to apply one of the above methods
themselves, EPA will post the 1990 RVPs recommended for use in
modeling mobile source HC, CO, and NOx emissions on a typical
summer day and the RVPs recommended for use in modeling mobile
source CO emissions on a typical winter day on the Chief Bulletin
Board System maintained by the Office of Air Quality Planning and
Standards.
3.3.3.2 "Period 1" RVP and "Period 2" RVP43
MOBILE4.1 requires two RVP inputs, one for "period I" and one
for "period 2". The purpose of having two RVP inputs is to allow a
step change in fuel volatility as of a specific calendar year.44
___________________________
42 The non-compliance margin is always positive; the
compliance safety margin is always negative.
43 "Period 1" RVP was called base or pre-control RVP in
MOBILE4; "period 2" RVP was called in-use RVP in MOBILE4.
44 MOBILE4.1 assumes this change to occur as of January 1 of
the specified calendar year.
26
The value to be used for the "period 1" RVP is the average in-
use RVP of gasoline in either the time before a volatility control
program took effect or the years preceding a change in the
controlled RVP level such as will take effect for most areas in
1992 when EPA's Phase I volatility control limits are superseded by
the Phase II volatility control limit.45 Period 1 RVP can be
between 7.0 psi and 15.2 psi inclusive. "Period 2" RVF can be
between 6.5 and 15.2 psi inclusive. The earliest allowed "period
2" start year is 1989.
There have been no revisions in the input of these two
variables since the release of MOBILE4; only the names of these
variables have been changed.
3.3.3.3 Interpolation
If emission factors are being calculated on a month-by-month
basis, or if the consecutive three-month period with the highest
frequency of NAAQS exceedance days occurring in the inventory area
is some period other than June, July, and August for ozone modeling
or November, December, and January for carbon monoxide modeling,
the RVP appropriate to each of the specific months being modeled
should be used. The July RVP value may be used for the entire
period of the EPA RVP control program (May through September). For
periods other than the period of EPA's control program, it is not
correct to average RVP values from different months or seasons
together, and it may be incorrect to use RVP from a time period
other than that used to determine the temperatures input to
MOBILE4.1.46, 47, 48
3.3.3.4 Inputs for Future Year RVP
3.3.3.4.1 Future Summer RVP
All parts of the United States will be subject to more
stringent EPA summer RVP limits beginning in 1992, with the highest
limit being 9 psi. The 1992 summer survey data may not be
available in time to prepare draft or final inventories for 1996
and beyond. Also,
___________________________
45 To model the effects of the Federal volatility control
program promulgated by EPA, in which volatility is limited in the
summer months (May - September), see the relevant Federal Register
notices (54 FR 11868, March 22, 1989; 55 FR 23658, June 11, 1990),
or contact an EPA Regional Office to determine the applicable RVP
limits for a specific State and month. The interim (Phase I)
controls were in effect during 1989, 1990, and 1991, and the final
(Phase H) controls took effect during 1992.
46 The 1990 base year SIP inventories represent emissions
during a typical day in the pollutant season, most commonly summer
for ozone and winter for CO. The procedure for choosing typical
day temperatures is described in section 3.3.5.2.
47 For attainment demonstrations, states should use
temperature and RVP values that reflect the conditions of the
specific episodes being modeled.
48 MOBILE4.1 does not model effects of RVP on emissions at
temperatures of less than 45' F (T Q, nor does it model effects of
RVP greater than an in-tank (weathered) level of 11.7 psi. Under
summer temperature conditions an in-tank RVP of 11.7 psi
corresponds to a dispensed fuel RVP of approximately 12.5 psi.
27
since 1992 is the first year of the "Phase II" volatility
regulation, it may not be representative of long-term RVP levels.
ASTM Class A and B areas will have a limit of 7.8 psi. To forecast
future RVP, each area should determine its compliance safety margin
in 1990 and/or 1991 relative to the 1990 EPA or state limit
(ranging from 9.0 to 10.5). This margin should be subtracted from
the future EPA or state limit, if that limit is 9.0. Areas without
1990 or 1991 survey data should subtract a default compliance
safety margin of 0.3 psi. Areas with a 1990/91 limit of 10.5 psi
and a safety margin significantly greater than 0.3 (which may have
been the result of distribution of fuel intended to comply with a
9.0 limit in nearby areas, or which may be the result of other
unique circumstances) should consult with EPA. It may be
appropriate to use the 0.3 psi default, rather than the 1990/91
compliance safety margin for future years. Areas with a 9.0 limit
in 1990/91 which also observed a safety margin significantly in
excess of 0.3 psi should also consult with EPA as to the
representativeness of the surveys involved.
Areas with a future summer RVP limit of 7.8 should, in
general, not assume a safety margin, since RVP reductions below 7.8
psi are more costly than those below 9.0 psi, and loss of RVP
between refinery and pump will be lower. However, if an area
subject to the 7.8 psi limit in 1992 determines that a safety
margin does exist in 1992 based on quality assured survey data, it
may request that EPA review 1992 summer MVMA and/or API survey data
from several cities to support its claim.
3.3.3.4.2 Future Winter RVP
There are no plans for EPA to establish winter RVP limits. If
there are no state standards for future winter RVP or if they are
the same as the state limit in 1990/91, the 1990/91 winter RVP
input should be used for future years.
If a state is tightening its winter RVP limit from a limit
actively enforced in 1990/91, the 1990/91 RVP compliance safety
margin should be applied to the future limit. If no margin was
applied in the 1990/91 inventory, none should be applied for the
future year.
If a state is establishing a winter RVP limit where no limit
or only an ASTM limit (not backed by state law including active
enforcement) applied in 1990/91, the safety margin relative to the
new limit should be the limit calculated from the 1990/91 (or more
recent) summer survey.49
___________________________
49 Alternatively, the default margin of 0.3 psi may be
assumed.
28
3.3.4 Oxygenated Fuels
Description
MOBILE4.1 can model the effects of two types of oxygenated
fuels, gasoline/alcohol blends and gasoline/ether blends, on
exhaust carbon monoxide (CO) emissions50 provided that the
following information is input:
- Ether blend market share (as a fraction);
- Average oxygen content of ether blend fuels (percent
weight, expressed as a fraction);
- Alcohol blend market share (as a fraction);
- Average oxygen content of alcohol blend fuels (percent
weight, expressed as a fraction);
- RVP waiver. (If oxygenated fuels must meet the same RVP
limits as gasoline, this indicator is set to 1; if such
fuels have been granted a 1.0 psi waiver, this indicator
is set to 2.51)
Guidance
Areas that are known to have significant market
penetration52, 53 of ether blends and/or alcohol blends should
characterize the relative market shares and oxygen content of these
fuel blends and account for them in their mobile source emission
inventory.
EPA should be contacted for assistance in modeling the effects
of oxygenated fuels if any of the following situations apply:
___________________________
50 Reductions in exhaust CO emissions are estimated for
gasoline-fueled vehicle types (LDGV, LDGT1, LDGT2, HDGV, and MC).
No effects on exhaust VOC or NOx emission factors or on any of the
evaporative components of VOC emissions are currently modeled with
the exception that, if the oxygenated fuels have a higher
volatility than base gasoline in an area, exhaust and evaporative
emissions will be increased to reflect the increased volatility of
the oxygenated fuels. MOBILE5 will contain adjustments for exhaust
HC.
51 Gasoline/ether blends axe assumed to have the same RVP as
gasoline, indicated by the regular RVP value input.
52 If, together, ethanol blends account for less than 2% of
total gasoline sales within an inventory area, and if there is no
mandatory or locally endorsed voluntary program for ether blends,
oxygenated fuels need not be explicitly modeled for the 1990 base
year inventory. Market shares for ethanol blends are readily
available by state.
53 There have been no recent significant sales of other
oxygenated blend types (e.g., gasoline/methanol).
29
` - the fuels available in an area include blends containing
both ether(s) and alcohol(s) in the same fuel ;
- an RVP waiver greater than 1.0 psi is applicable to
oxygenated fuels in an area;
- no RVP waiver is in effect, but the volatility of base
gasoline is currently below the regulated limit (in this
situation, the practical effect may be same as if a
waiver were in effect);
- if two or more types of alcohol blends are marketed under
different RVP waiver treatment (for example,
gasoline/methanol blends might not be given the same
waiver as gasoline/ethanol blends).
3.3.5 Correction Factors
3.3.5.1 Speed
Description
There is considerable variation in vehicle emission factors as
average vehicle speed changes.54 In general, however, exhaust
emissions are at a minimum at about 48 mph.55 All emission rates
(VOC, CO, NOx ) display very high emissions at very low speeds,
with emissions decreasing (sharply at first and then more slowly)
as average speed increases, until minimum emissions are reached at
around 48 mph. Above 48 mph, further increases in speed result in
increased emissions.
MOBILE4.1 will calculate emission factors for average speeds
of 2.5 to 65.0 mph, in increments of 0.1 mph.56 One average speed
may be used for all vehicles, or a different average speed may be
used for each vehicle type.
Guidance
Selection of vehicle speeds is a difficult and complex
process. Although it is appropriate for some purposes to use an
average speed for all vehicle trips and vehicle types within urban
areas as a whole, such an approach is not suitable for SEP
inventory preparation. Instead, VMT should be left disaggregated
into subsets that have roughly equal speed, with separate VOC, CO,
and NOx emission factors for each subset.57 At a minimum, speeds
should be estimated separately by roadway functional class.
___________________________
54 The speed correction factors in MOBILE4.1 are
substantially revised from those in MOBILE4.
55 The average speed of the Highway Fuel Economy (HFE) test
cycle is approximately 48 mph.
56 The maximum average vehicle speed allowed in MOBILE4.0
was 55 mph.
57 Since emissions are a non-linear function of speed, with
significant curvature at low and high speeds, total daily area-wide
emissions are to some degree incorrectly estimated if VMT "events"
occurring at significantly different speeds are averaged together.
30
Travel Demand Network Approach
The recommended approach to estimating speeds is to post-
process the output from a local travel demand network model.58, 59
Two documents that provide guidance on speed estimation for areas
using network models are: Highway Vehicle Speed Estimation
Procedures for Use in Emissions Inventories and A Study of Highway
Vehicle Emission Inventory Procedures in Selected Urban Areas.60
The primary purpose of speed within a transportation planning
model is to allocate travel across the network. It is used
primarily as a measure of impedance to travel rather than as a
prediction of accurate travel times.
The report, Highway Vehicle Speed Estimation Procedures for
Use in Emissions Inventories, focuses on speed estimation methods
that are extensions of traffic assignment procedures. The basic
method presented takes the link-specific traffic estimates provided
as an output by a UTPS-type highway assignment model and calculates
speeds based on the estimated highway volume-to-capacity ratios and
a set of speed formulas that are more specific to different road
types than the formula built into most assignment models.61 As
such, this method is not as simple as using the direct traffic
assignment output without modifications.62
A second approach to estimating speeds using travel demand
network models is to use the output from traffic assignment
directly.63 If the network model assigns traffic to links on the
basis
___________________________
58 Travel demand models that do not meet the performance and
validation requirements for use in forecasting VMT growth may
nevertheless be suitable for deriving speed estimates. However,
reasonable efforts and success in validating the model are still
required.
59 Once link-by-link speeds are determined from speed
formulas, the results may be aggregated into functional classes.
60 Both documents were prepared for EPA by Cambridge
Systematics, Inc. Much of the information contained in the
remainder of this section was taken directly from these documents.
61 In most metropolitan areas, transportation planners
calibrate their highway assignment models to replicate observed
volume levels, treating highway speeds only as tools to obtain good
volume estimates rather than as critical outputs in their own
right. In many cities, assignment-predicted speeds are too high to
match actual conditions; in some cities, they are too low.
62 For those urban areas that can demonstrate that their
assignment-predicted link speeds closely match observed speed data
and/or speeds estimated using the Federal Highway Administration's
Highway Capacity Manual, the assignment-predicted link speeds may
be used directly in vehicle emissions inventories.
63 If the year for which the inventory is being calculated
is not the same as one of the years for which the network model as
been run, speeds may be interpolated between chronologically
adjacent network model runs.
31
of a capacity restraint algorithm, then the associated link speeds
are likely to be more accurate than if another type of assignment
methodology is used.64, 65 However, the unique manner in which
the traffic assignment algorithm manipulates speed for a particular
link does not necessarily provide an accurate estimate of speed for
that link but rather provides a value that optimizes the traffic
assignment over the entire congested network.
Highway Performance Monitoring System (HPMS) Roadway Classification
Approach
Post-processing with better speed formulas is often combined
with a direct link to the emission factor model, and link speeds
are either used directly as MOBILE inputs66 or grouped into ranges
based on the speed at which VMT occurs on each link.67
One way to further reduce the number of MOBILE4.1 runs is to
use FHWA's Highway Performance Monitoring System (HPMS) roadway
classification scheme to group portions of VMT by the functional
classification of the roadways on which they occur. This results
in 12 subsets of VMT.68 Within each subset, speed is weighted by
VMT to calculate an average speed and emission factor.
This disaggregation of VMT by functional system avoids most of
the undesirable VMT averaging that might otherwise cause errors in
the emission inventory. Further accuracy improvements can be
obtained by dividing the day into separate time periods so that
congested VMT and free-flowing VMT are not mixed. While two
periods are the minimum split to get more homogeneity in vehicle
speeds, more than two periods are possible. Each functional system
can, for example, be characterized by four average speeds during
distinct periods of the day: a morning peak period, a mid-day non-
peak period, an afternoon peak period, and a late evening/night
non-peak period.69, 70 Under this approach separate MOBILE4.1
emission factors are calculated for each
___________________________
64 The capacity restraint method is a common type of traffic
assignment algorithm. It is based on the inverse relationship
between speed and congestion. It attempts to model congested
speeds during peak conditions for all facility types. As
congestion increases, vehicle operating speeds decrease. The
capacity restraint methodology is used as the default formula in
many urban areas' traffic assignment models.
65 A potential problem with the use of any single function
is that it may not account well for the variations in traffic
operating conditions across all types of links, especially on very
congested links. A single formula may be unable to accurately
estimate speed for facility types having very different operating
characteristics. A more appropriate procedure would include
separate equations for estimating speed for each facility class for
each condition, i.e., peak versus off-peak travel.
66 In this case, emission factors are developed for each
highway link and multiplied by the VMT on each link to calculate
link-specific vehicle emissions.
67 Typically, a range would be one or two mph. If this
approach is used, emission factors are calculated for the midpoint
of each speed range and multiplied by the associated VMT.
68 FHWA designates roadway segments separately within urban
and rural areas into six functional classes each.
69 AIRS/AMS is set up according to this approach, with up to
12 roadway classifications and one to four time periods within a
day.
32
period based on the speeds and temperatures prevailing during the
period.71 This approach has most of the advantages of link-
specific hour-by-hour modeling, but requires fewer MOBILE4.1
runs.72
Estimating emissions for separate time periods within the day
also requires that particular attention be paid to the treatment of
the temperature inputs to MOBILE4.1. The sum of the emissions
within the four periods should be logically consistent (except for
the effect of the speeds) with that which would result from using
the 24-hour approach. In order to achieve this consistency, the
24-hour minimum and maximum temperatures should be used to
determine diurnal evaporative emissions for each of the time
period-specific MOBILE4.1 runs.
Ambient temperature, on the other hand, should be set to the
VMT-weighted average temperature of the period in question.73 For
example, if the night period extends from 7 pm to 6 am, the
temperature for each of the hours occurring during the night should
be weighted by the percent of night period VMT in each hour.74, 75
Highway Performance System National Estimates76
If no network model is available, and in marginal and sub-
marginal non-attainment areas, the national speed estimates listed
in Table 3-1 may be used. Individual areas may be able to obtain
comparable but locally specific speed estimates through their state
DOT or FHWA division office. These speeds are calculated from HPMS
traffic counts and site-specific speed formulas and are not actual
speed observations.
___________________________
70 The start and end times of periods should be locally
determined to reasonably separate higher from lower speed traffic
periods.
71 Inputs other than Speed and temperature might also differ
by functional system and time of day. The hot/cold mix of vehicle
operation is one example of such an input.
72 This approach is most worthwhile when a significant
portion of the highway network gets much more congested during part
of the day, with considerable VMT in both the congested and non-
congested periods.
73 The TEMFLG control flag should be set to accomplish this.
See the User's Guide to MOBILE4. 1, section 2.1.14.
74 The recommended method of apportioning daily VMT to
specific hours is to use the state's continuous monitors available
within the FAUA. If no such monitors exist within the inventory
area, then the state may rely on other continuous monitors located
in areas similar in geographic, land use and demographic
characteristics, or on those areas' final Airshed Emission
Preprocessor profiles.
75 Hour-by-hour temperatures should be determined from the
10-worst-days method used to determine the minimum and maximum
temperatures for inventory purposes. See section 3.3.5.2.
76 SOURCE: Federal Highway Administration, Highway
Performance Monitoring System, Impact Analysis for 1989 Base Year.
33
Table 3-1
Geographic Roadway Autos, Vans All
Area Functional Classification Pickups Trucks
Rural Interstates 57.3 43.6
Other principal arterials 45.4 36.0
Minor arterials 39.9 33.3
Major collectors 35.1 29.8
Minor collectors 30.5 24.4
Urban Interstates 46.3 39.0
Other freeways and expressways 43.3 36.5
Other principal arterials 18.9 16.0
Minor arterials 19.6 19.6
Collectors 19.6 16.4
Consistency Over Time
Speed estimates for years other than 1990 must be logically
related to the 1990 methodology and estimates, with no arbitrary or
unsupported assumptions of speed changes.
3.3.5.2 Temperature
Description
The basic emission rates that underlie the emission factor
calculations are developed from emission data from vehicles tested
at FTP conditions, including an ambient temperature of 75'F (24'C).
MOBILE4.1 uses temperature correction factors to correct the
emission factors for other temperatures.
MOBILE4.1 provides temperature correction factors for
temperatures in the range of 0'F (-18'C) to 110'F (43'C). If a
temperature below 0'F is entered, a warning message is issued, and
0'F is used in the calculations. Similarly, if a temperature above
110'F is entered, a warning message is also issued, and 110'F is
used in the calculations.
The temperature used to adjust the exhaust emission factors
for all three pollutants, the hot soak component of evaporative
emissions, refueling emissions, and resting loss and running loss
emissions can be calculated on the basis of the input minimum and
maximum daily temperatures.77 Alternatively, the model can use a
single temperature that represents ambient conditions at a
particular time.
___________________________
77 The maximum temperature must not be less than the minimum
temperature.
34
However, even if a single temperature is used as the basis of
the temperature correction factors for all exhaust emissions, hot
soak evaporative emissions, refueling emissions, and resting loss
and running loss emissions,78 minimum and maximum daily
temperatures will still be used to calculate the diurnal component
of evaporative emissions.
Minimum and Maximum Daily Temperatures
Minimum and maximum daily temperatures are used directly in
MOBILE4.1 to calculate the diurnal portion of evaporative VOC
emissions79 and to estimate the temperature of dispensed fuel for
use in the calculating refueling emissions. Unless overridden80
the temperatures used in calculating temperature corrections for
exhaust VOC, CO, and NOx emissions, the hot soak portion of
evaporative emissions, and resting loss and running loss VOC
emissions are also calculated by MOBILE4.1 based on the minimum and
maximum temperatures entered as input to the model.
Since the basic exhaust emission rates for VOC, CO, and NOx
are based on the standard test temperature of 75'F (24'C),
MOBILE4.1 also adjusts these rates for other temperatures. Using
the minimum and maximum daily temperatures and a representative
profile of temperature versus time of day, MOBILE4.1 first
calculates i temperature for each pollutant representing average
emissions over the course of the day and then adjusts the exhaust
emission factors for temperature effects accordingly.81
Hot soak emissions at FTP conditions are based on a
temperature of 82'F (28'C). Again using the minimum and maximum
temperatures, MOBILE4.1 calculates a temperature by which to adjust
hot soak emissions.
___________________________
78 Logically, the single temperature used to represent a
typical day must be between a typical day's minimum and maximum
temperatures.
79 Diurnal emissions are most frequently measured for a
temperature range of 68-86'F (20-30'C). However, MOBILE4.1 adjusts
diurnal emission rates for the minimum and maximum temperatures
provided as input based on special EPA testing over additional
temperature ranges.
80 A single ambient temperature can also be used to
determine the temperature corrections for exhaust VOC, CO, and NOx
emissions, hot soak evaporative emissions, dispensed fuel
temperature in the refueling emissions calculations, and resting
loss and running loss emissions, through the choice of a value for
the control flag TEMFLG (see the "User's Guide to MOBILE4.1,"
section 2.1.14). This approach is not recommended unless modeling a
short time period, such as an hour. Refueling emissions should
always be modeled using the "full day" approach; hourly
temperatures should not be used. Diurnal emissions can only be
modeled directly in MOBILE4.1 using the "full day" approach, since
the algorithm used is inaccurate over the very small temperature
rises (1' to 5'F) typical of a single hour.
81 The algorithm used in MOBILE4.1 to determine temperatures
for correcting emissions on the basis of the input minimum and
maximum temperatures takes into account both the typical 24-hour
diurnal temperature profile for a day having the specified minimum
and maximum, and the typical distribution of travel over the course
of 24 hours. Thus, the emission factors calculated in this way are
appropriately weighted for trips, vehicle miles traveled, and
emissions at different temperatures and result in factors that can
be multiplied by total daily VMT when total daily emissions are the
desired result.
35
Resting loss and running loss VOC emissions are also dependent
on temperature. As in the cases of exhaust and hot soak emissions,
MOBILE4.1 calculates appropriate average temperatures for
estimating resting loss and running loss emissions, weighted to
account for differing emission levels at different temperatures in
the range of the minimum and maximum daily temperatures and
differing travel fractions over the course of a day. Restrictions
on these temperatures are: the maximum temperature must be greater
than or equal to the minimum temperature, and the ambient
temperature should be between the minimum and maximum (minimum ó
ambient ó maximum).
There have been no revisions to this variable's use or input
data format requirements since the release of MOBILE4.
Guidance
EPA recommends that the minimum and maximum daily temperatures
be used to determine the temperatures for corrections to the
emission factors, if daily average, rather than hour-by-hour,
emissions are to be estimated.82, 83
Minimum and maximum temperatures are normally calculated from
the most recent three-year period for which validated ozone and/or
CO monitoring data exists at the time the emission inventory is
due. For 1990 inventories, the period to be used for temperature
determination should be 1988-1990.84
___________________________
82 If hourly diurnal emissions are required for
photochemical or other models, an acceptable approach is to first
calculate the daily diurnal emissions, then allocate to specific
hours in proportion to the temperature rise per hour. For example,
if during the modeling day the temperature increases from 60-84'F
within the 7 a.m. to 5 p.m. ten-hour period, and if during the 1-2
p.m hour temperature increases from 76-80'F then, assuming the
total diurnal emission factor is 3 g/vehicle, the emission factor
for the 1-2 p.m. hour is 0.5 g/vehicle. The diurnal emission
factor for hours other than 5 am. to 3 p.m. is zero. Other
reasonable methods may also be acceptable. States wishing to use
another method should consult with EPA staff.
Hourly Emission Factor = Hour-Specific Temperature
Increase/(Maximum Daily Temperature - Minimum Daily Temperature) -
Total Diurnal Emission Factor
(3-6)
83 For CO modeling inventories, the recommended temperature
is the average of the 8-hour high concentration period temperatures
rather than the minimum and maximum temperatures used to calculate
a typical winter day inventory:
Click HERE for graphic.
84 The temperatures used in the 1990 inventory must also be
used for all projection inventories.
36
Procedure
Determine the consecutive three-month period with the highest
frequency of NAAQS exceedance days occurring in the inventory
area.85 The same consecutive three-month period applies for each
year, with a total of nine months used to determine temperature.
If the months containing the highest frequency of exceedances are
not consecutive, or if two or more sets of consecutive months have
the same frequency, use the months of June, July, and August for
ozone modeling and the months of November, December, and January
for carbon monoxide modeling.
Next, list the 10 highest concentrations86 that occurred in
the inventory area during those nine months and the dates of those
concentrations.87
The ten highest ozone concentrations for each site in a county
and the dates on which they occurred are contained in the
Aerometric Information Retrieval System (AIRS) AMP440/Maximum
Values Report. Eight-hour average CO concentrations and the dates
on which they occurred can be found in the AIRS AMP350 raw data
report. The AMP355/Standard Report contains the CO values that
exceed the NAAQS. These reports are available from EPA's National
Air Data Branch. Be sure to specify the year(s) and counties of
interest and indicate that the request is for preparation of a SIP
emission inventory to avoid being charged the normal processing
fee. To obtain copies of these reports, contact Tom Link, U.S. EPA
Office of Air Quality Planning and Standards, at (919) 541-5456.
Determine the maximum and minimum temperatures for each of the
10 days for the area being inventoried. This information is
contained in the Local Climatological Data Monthly Summary for the
inventory area and is available from:
National Climatic Data Center
Federal Building
Asheville, NC 28801-2696
Telephone: (704) 259-0682
Maximum and minimum daily temperatures are located in columns
2 and 3, respectively, on page 1 of the Summary.
___________________________
85 Consider the three-year period as a whole when making
this determination.
86 The 10 highest concentrations need not all be
exceedences.
87 There are four exceptions to selecting the 10 highest
concentrations: 1) More than 10 concentrations may be needed to
identify 10 unique dates. 2) If the 10th ranked concentration level
occurs on more than one day, all of those days should be included
in the tempera
