Emissions Inventory Guidance for Ozone & PM NAAQS Implementation and Regional Haze Regulations

9/29/2016 DRAFT

DRAFT

Emissions Inventory Guidance for 

Implementation of Ozone and Particulate 

Matter National Ambient Air Quality 

Standards (NAAQS) and Regional Haze 

Regulations
Emissions Inventory Guidance for Ozone & PM NAAQS Implementation and Regional Haze Regulations

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Table of Contents

List of Tables ................................................................................................................................................. v

Acronyms and Abbreviations Used in this Guidance................................................................................... vi

1 Introduction ..........................................................................................................................................1

1.1 Purpose .........................................................................................................................................1

1.2 Other Inventory Guidance and Resources....................................................................................2

1.3 Summary of Relevant Statutory Provisions ..................................................................................4

1.4 Summary of Guidance Contents...................................................................................................7

2 SIP Inventory Overview.........................................................................................................................8

2.1 Recommended Steps for Building the Inventory Parts of a SIP....................................................8

2.2 Discuss the Inventory Approval Process with EPA......................................................................10

2.3 Planning and Modeling Inventory Components.........................................................................12

2.3.1 Ozone SIP Inventory Components......................................................................................13

2.3.2 Annual and 24-Hour PM2.5 SIP Inventory Components ......................................................14

2.3.3 Regional Haze SIP Inventory Components..........................................................................15

2.4 Other Relevant Federal Emissions Reporting Requirements......................................................18

2.5 Emissions Inventory Definitions..................................................................................................19

2.5.1 Source Category/Data Category .........................................................................................19

2.5.2 Types of Emissions: Annual, Seasonal, Actual, Potential, and Allowable ...........................20

2.5.3 Duration and Timing of Emissions ......................................................................................21

2.5.4 Data Elements.....................................................................................................................21

2.5.5 Throughput and Activity .....................................................................................................22

2.5.6 Emission Factors..................................................................................................................22

2.5.7 Emissions Reporting Data Elements ...................................................................................23

2.5.8 Point Sources ......................................................................................................................25

2.5.9 Nonpoint Sources................................................................................................................26

2.5.10 On-road Mobile Sources.....................................................................................................26

2.5.11 Nonroad Mobile Sources ....................................................................................................27

2.5.12 Events..................................................................................................................................28

2.5.13 Biogenic Sources.................................................................................................................28

2.5.14 Inventory Codes..................................................................................................................28
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2.5.15 Point Source Facility Details................................................................................................30

2.5.16 Alternative IDs.....................................................................................................................31

2.5.17 Location Definitions............................................................................................................32

2.5.18 Control Data Elements........................................................................................................33

2.5.19 EIS Sectors, Tiers, and Facility Type ....................................................................................34

2.5.20 Other Terms........................................................................................................................34

3 SIP Inventory Requirements and Recommendations.........................................................................35

3.1 Inventory Component Timing .....................................................................................................35

3.2 Inventory Preparation Plan.........................................................................................................36

3.3 Base Year Inventory ....................................................................................................................39

3.4 Rate of Progress and Reasonable Further Progress Baseline NAA Inventory.............................40

3.4.1 Ozone ROP/RFP Requirements and Timing ........................................................................40

3.4.2 Ozone ROP/RFP Emissions Guidance..................................................................................41

3.4.3 PM2.5 RFP Requirements and Timing ..................................................................................42

3.4.4 PM2.5 RFP Emissions Guidance ............................................................................................42

3.5 Periodic Inventories....................................................................................................................43

3.5.1 Ozone Periodic Inventory....................................................................................................43

3.5.2 Regional Haze Recent Year Inventory.................................................................................44

3.6 Emissions Statement Requirement for SIPs................................................................................45

3.7 Maintenance Inventories............................................................................................................46

3.8 Projected Inventories..................................................................................................................46

3.8.1 Attainment Year Projected Inventories..............................................................................48

3.8.2 Rate of Progress and Reasonable Further Progress Emissions Reductions........................49

3.8.3 Regional haze emissions reductions...................................................................................50

3.9 Public Hearings............................................................................................................................50

3.10 Submit Inventory Parts of SIP to the EPA ...................................................................................51

3.10.1 Regulatory Requirements in Addition to Implementation Rules........................................52

3.10.2 How to Submit Inventory Parts of a SIP..............................................................................53

3.10.3 What to Submit...................................................................................................................54

4 Developing Current Emissions inventories.........................................................................................59

4.1 Inventory Years...........................................................................................................................59

4.1.1 Ozone baseline year............................................................................................................59
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4.1.2 PM2.5....................................................................................................................................60

4.1.3 Regional Haze......................................................................................................................61

4.1.4 Considerations for choosing an alternative base year........................................................62

4.2 Pollutants and Pollutant Precursors to Include in Inventories...................................................62

4.2.1 Condensable PM emissions ................................................................................................63

4.2.2 Regulatory Definition of VOC..............................................................................................65

4.2.3 Treatment of Methane .......................................................................................................65

4.3 Identification of Priority Sources................................................................................................66

4.4 Spatial Extent ..............................................................................................................................69

4.4.1 Nonattainment Areas with Partial Counties.......................................................................69

4.4.2 Point Source Locations Needed for Modeling of Traditional Nonpoint Sources................71

4.4.3 Shapefile-based Inventories................................................................................................71

4.4.4 Link-based On-road Mobile Emissions and Travel Demand Models ..................................71

4.5 Temporal Basis............................................................................................................................72

4.5.1 Temporal Considerations for Ozone SIPs............................................................................72

4.5.2 Temporal Considerations for PM2.5 SIPs .............................................................................73

4.6 Emission Factors for SIP Emissions Inventories..........................................................................74

4.7 Inventory Data Codes..................................................................................................................76

4.7.1 Codes to Use for Nonattainment Area Inventories............................................................76

4.7.2 How to Request New Codes or Code Retirements.............................................................76

4.8 Estimation of Base Year Emissions..............................................................................................77

4.8.1 Stationary Point ..................................................................................................................77

4.8.2 Stationary Nonpoint............................................................................................................82

4.8.3 On-road Mobile...................................................................................................................85

4.8.4 Nonroad Mobile Equipment ...............................................................................................86

4.8.5 Other Nonroad Mobile........................................................................................................87

4.8.6 Biogenic and Geogenic Sources..........................................................................................95

4.8.7 Wildland Fires .....................................................................................................................98

4.9 Quality Assurance .....................................................................................................................101

4.9.1 QA Reference Materials and Planning ..............................................................................102

4.9.2 Types of Problems in Emissions Inventories.....................................................................102

4.9.3 Checks Available through the EPA’s Emissions Inventory System....................................103
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4.9.4 Inventory Comparisons.....................................................................................................104

4.9.5 Checking Data Codes.........................................................................................................105

4.9.6 Point Versus Nonpoint and Reconciliation .......................................................................105

4.9.7 Ranking Techniques..........................................................................................................106

4.9.8 Outlier Analyses................................................................................................................107

4.9.9 Using Maps to QA Nonpoint Emissions ............................................................................108

4.9.10 Expected Pollutants ..........................................................................................................108

4.9.11 Facility and Release Point Locations.................................................................................109

4.9.12 Release Point Parameters.................................................................................................110

4.9.13 Biogenic QA/QC.................................................................................................................111

4.9.14 Timing of QA and Follow-up .............................................................................................111

5 Developing Projected Emissions Inventories....................................................................................113

5.1 Projected Emissions Overview..................................................................................................113

5.1.1 Readily Available Projections Information........................................................................113

5.1.2 Background Materials.......................................................................................................113

5.2 Identify and Prioritize Key Sectors and Regions for Projections...............................................115

5.3 Collect Available Data and Models for Each Sector Needing Projections ................................115

5.3.1 Electric Generating Units..................................................................................................116

5.3.2 Non-EGU Stationary Sources ............................................................................................121

5.3.3 On-Road Mobile Sources ..................................................................................................122

5.3.4 Nonroad Mobile Equipment .............................................................................................123

5.3.5 Other Nonroad Mobile Sources........................................................................................123

5.3.6 Other Projection Resources..............................................................................................124

5.4 Evaluate and Refine Data for Projections in Inventory Region.................................................127

5.5 QA for Projected Inventories....................................................................................................128

6 References ........................................................................................................................................129
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List of Tables

Table 1: List of EPA estimation resources for emissions inventories ...........................................................2

Table 2: Steps for building the inventory parts of a SIP................................................................................9

Table 3: Recommended items to include in inventory information provided to the EPA..........................12

Table 4: Ozone SIP emissions inventory components................................................................................13

Table 5: PM2.5 SIP emissions inventory components..................................................................................14

Table 6: Regional haze SIP emissions inventory components....................................................................17

Table 7: Point source triennial reporting thresholds (potential to emit) for criteria pollutants in 

the Air Emissions Reporting Rule .................................................................................................26

Table 8: Timing of emissions inventory components.................................................................................35

Table 9: Checklist of elements to consider for including in Inventory Preparation Plans..........................37

Table 10: Conditions for AERR submittal to meet ozone periodic inventory requirement........................44

Table 11: Suggested Elements of an Emissions inventory SIP Document ..................................................55

Table 12: Summaries to include in the background ...................................................................................56

Table 13: Summaries to include in the point sources section ....................................................................57

Table 14: Summaries to include in the nonpoint and mobile sources sections.........................................57

Table 15: Source types expected to include condensable PM ...................................................................63

Table 16: Hierarchy of emission factors for use in point source emissions inventories ............................75

Table 17: List of nonpoint categories, pollutants, and available approaches and tools.............................83

Table 18: Aircraft SCCs and Data Categories in EPA Estimates...................................................................88

Table 19: Commercial marine SCCs and emission types in EPA emissions.................................................91

Table 20: Commercial marine vessel recent regulations and impacted pollutants ...................................91

Table 21: Current EPA methods for commercial marine vessel emissions. ...............................................92

Table 22: Locomotive SCCs.........................................................................................................................94
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Acronyms and Abbreviations Used in this Guidance

AEO Annual Energy Outlook

AERR Air Emissions Reporting Rule

APU Auxiliary power unit

BEIS Biogenic Emissions Inventory System

BELD3 Biogenic Emissions Land Use Database, version 3

AT Air Taxis

C1 Category 1 (marine vessel)

C2 Category 2 (marine vessel)

C3 Category 3 (marine vessel)

CAA Clean Air Act

CAMD Clean Air Markets Division

CAP Criteria air pollutant

CAS Chemical Abstracts Service

CEMS Continuous Emissions Monitoring System(s)

CMV Commercial marine vessel

CNG Compressed natural gas

CO Carbon monoxide

CO2 Carbon dioxide

DARS Data Attribute Rating System

DOE Department of Energy

ECA-IMO Emissions Control Area-International Maritime Organization

EDD Effective date of designation

EDMS Emissions and Dispersion Modeling System

EE Energy efficiency

EGU Electric Generating Unit

EIA Energy Information Administration

EIIP Emissions inventory Improvement Program

EIS Emissions Inventory System

EMCH Emissions Modeling Clearinghouse

EPA Environmental Protection Agency

EPMM Electric Power Market Model

ERTAC Eastern Regional Technical Advisory Committee

FAA Federal Aviation Administration

FHWA Federal Highway Administration

FIPS Federal Information Processing Standards

FRS Federal Registry Service

GA General Aviation

GHG Greenhouse gas

GIS Geographic Information Systems

GSE Ground support equipment
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HAP Hazardous air pollutant

Hg Mercury

HCl Hydrochloric acid

IPM Integrated Planning Model

IPP Inventory Preparation Plan

LPG Liquified petroleum gas

LTO Landing and takeoff

MARKAL Market Allocation (model)

MEGAN Model of Emissions of Gases and Aerosols from Nature

MIMS Multimedia Integrated Modeling System

MOVES Motor Vehicle Emissions System

NAAQS National Ambient Air Quality Standards

NAA Nonattainment area

NAICS North American Industry Classification System

NATA National Air Toxics Assessment

NEEDS National Electric Energy Data

NEI National Emissions Inventory

NEMS National Energy Modeling System

NESHAP National Emission Standard for Hazardous Air Pollutants

NH3 Ammonia

NLDN National Lightning Detection Network

NMIM National Mobile Inventory Model

NONROAD Nonroad emissions model

NMHC Non-methane hydrocarbon

NOX Nitrogen Oxides

O3 Ozone

OAP Office of Atmospheric Programs

OAQPS Office of Air Quality Planning and Standards

OAR Office of Air and Radiation

OECA Office of Enforcement and Compliance Assurance

ORIS Office of Regulatory Information Systems

OTAQ Office of Transportation Air Quality

OTR Ozone Transport Region

PM Particulate matter

PM2.5 Particulate matter with diameter 2.5 microns and less

PM10 Particulate matter with diameter 10 microns and less

QAPP Quality Assurance Project Plan

RE Renewable energy

RICE Reciprocating Internal Combustion Engines

RFP Reasonable Further Progress

ROP Rate of Progress

SCC Source classification code
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SIP State implementation plan

SMOKE Sparse Matrix Operator Kernel Emissions

SO2 Sulfur Dioxide

SSM Startup, Shutdown, and Malfunction

STEEM Ship Traffic, Energy and Environmental Model

TAF Terminal Area Forecast

TCA 1,1,1-trichloroethane

TDM Travel demand model

TIP Tribal Implementation Plan

TOG Total organic gases

TRI Toxics Release Inventory

U.S. United States

USDA U.S. Department of Agriculture

VMT Vehicle miles traveled

WFU Wildland fire use

WLF Wildland fire

WRF Weather Research and Forecasting

VOC Volatile organic compounds
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1 Introduction

1.1 Purpose

This document provides guidance on how to develop emissions inventories to meet state 

implementation plan (SIP) requirements for complying with the 8-hour ozone national ambient air 

quality standard (NAAQS), the 24-hour and annual fine particulate matter (PM2.5) NAAQS, and the 

regional haze regulations. It is intended for use by the United States (U.S.) Environmental Protection 

Agency (EPA) Regional offices; state, local, and tribal air quality management authorities; and the 

general public. This guidance is designed to implement national policy on these emissions inventories as 

embodied in the Clean Air Act (CAA), the Ozone SIP Requirements Rule for the 2008 ozone NAAQS (also 

called the Ozone Implementation Rule)1, the PM2.5 SIP Requirements Rule2, and the Regional Haze 

Regulations.

3

This document does not substitute for provisions or regulations of the CAA enumerated above, nor is it a 

regulation itself. As the term “guidance” suggests, it provides recommendations on how to implement 

the inventory requirements. Thus, it does not impose binding, enforceable requirements on any party, 

nor does it assure that the EPA will approve all instances of its application, as the guidance may not 

apply to a particular situation based upon the circumstances.

The EPA and state, local, and tribal decision makers retain the discretion to adopt approaches on a caseby-case basis that differ from this guidance where appropriate. Final decisions by the EPA regarding a 

particular SIP demonstration will only be made based on the statute and regulations, and will only be 

made following a final submission and after notice and opportunity for public review and comment. 

Interested parties are free to raise questions and objections about the appropriateness of the 

application of this guidance to a particular situation; the EPA will, and state, local, and tribal air agencies

should, consider whether or not the recommendations in this guidance are appropriate in that situation.

Through the provisions of the Tribal Authority Rule, tribes are not required to develop emissions 

inventories. If tribes choose to develop an emissions inventory that may become part of a SIP or Tribal 

Implementation Plan (TIP) inventory covered by this guidance, the tribes are encouraged to follow the 

provisions of this guidance. Notwithstanding the possible development by local and tribal air agencies, 

inventories prepared for NAAQS implementation will be called “SIPs” in the remainder of this document.

 

1

Implementation of the 2008 National Ambient Air Quality Standards for Ozone: State Implementation Plan Requirements; 

(40 CFR 51 Subpart AA; see also https://www.epa.gov/ozone-pollution/implementation-2008-national-ambient-air-qualitystandards-naaqs-ozone-state).

2 Fine Particulate Matter National Ambient Air Quality Standards: State Implementation Plan Requirements, (40 CFR 51 

Subpart Z; see also https://www.epa.gov/pm-pollution/implementation-national-ambient-air-quality-standards-naaqs-fineparticulate-matter).

3 Regional Haze Regulations (40 CFR 51 Subpart P) proposed to be amended by Protection of Visibility: Amendments to 

Requirements for State Plans; Proposed Rule (81 FR 26942, May 4, 2016). See also https://www.epa.gov/visibility/visibilityregulatory-actions. 
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1.2 Other Inventory Guidance and Resources

This document is a guide for state, local, and tribal air agencies (called “air agencies” in this guidance) 

for preparing and submitting their emissions inventories for the 8-hour ozone NAAQS, 24-hour and 

annual PM2.5 NAAQS, and for the regional haze program. It is not a procedures document that 

comprehensively covers the methods for compiling and documenting emissions inventories. 

Furthermore, the predecessors to this guidance have historically relied on separate guidance documents 

to add information for various specific purposes. In Section 2.3, we list the various inventory 

components needed for 8-hour ozone, PM2.5, and regional haze implementation and identify the 

associated sections of this guidance and other relevant guidance materials.

In addition to this guidance, the “Guidance on the Use of Models and Other Analyses for Demonstrating 

Attainment for Air Quality Goals for Ozone, PM2.5, and Regional Haze” (http://www3.epa.gov/ttn/

scram/guidance_sip.htm), or “Modeling Guidance”, covers the creation of emissions inventories for use 

in air quality modeling. These modeling inventories differ from the inventories covered in this document 

in that they usually have a larger spatial extent (i.e., a multi-state area as opposed to just the 

nonattainment area or individual state) and require extra processing steps to obtain the spatial and 

temporal resolution of the air quality model input species. To the extent the Modeling Guidance covers 

future-year emissions, efforts have been made to harmonize the future-year guidance in that Guidance

with the information included in Section 5 of this document.

In accordance with 40 CFR Subpart A Section 51.5(b), states are urged to use applicable, state-of-the art 

techniques for estimating emissions. A number of inventory development resources are available from 

the EPA for use by air agencies for estimating base year emissions inventories. Table 1 provides these 

references and a brief description of how they can be used. More information on how these resources 

can be applied is provided in Sections 4 and 5.

Table 1: List of EPA estimation resources for emissions inventories

Resource and website Description 

National Emissions Inventory (NEI)

(www.epa.gov/air-emissionsinventories)

Summaries of the latest NEI data and documentation 

are available from links provided at this website. More 

detailed data are available to air agencies from the 

Emissions Inventory System (EIS).

Emission Factors used for the National 

Emissions Inventory (NEI)λ

(www.epa.gov/air-emissionsinventories/national-emissionsinventoryl)

The National Emissions Inventory (NEI) program 

provides emissions estimation tools (that include 

emission factors) and point-source emission factors 

based on EPA rule development efforts. In many cases, 

these factors are more recent than those in WebFire 

and AP-42, and agencies are encouraged to rely on 

these where available. 

Emissions Inventory System

(www.epa.gov/air-emissionsinventories/emission-inventory-systemeis-gateway)

The EIS allows for summaries and extraction of 

detailed data for use by air agencies. The EIS allows 

both state-submitted as well as the composite EPAstate data used for the NEI to be queried and 
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Resource and website Description 

downloaded. Training on reports is also available, and 

Appendix A provides an EIS quick reference for county 

and facility reports.

Local-scale emissions inventory

development (www.epa.gov/airemissions-inventories/local-scaleemission-inventory-development)

In 2010, the EPA completed a project to assess the 

building of multi-pollutant inventories at the scale of 

nonattainment areas. A project description and the 

final report are available; the latter documents the 

recommended actions for state and local agencies that 

want to develop local-scale emissions inventories, 

including SIP planning inventories.

Electric Generating Unit emissions data 

from Continuous Emissions Monitoring 

Systems (CEMS)

∆

(ampd.epa.gov/ampd)

Carbon dioxide (CO2), nitrogen oxides (NOX), sulfur 

dioxide (SO2), and heat input data from Electric 

Generating Units (EGUs) collected by the EPA. Heat 

input data can be used to estimate hourly emissions of 

other pollutants using emission factors.

Emissions Inventory Improvement Program 

(EIIP) Documentation, Volumes 1-9.

(www.epa.gov/air-emissionsinventories/emission-inventoryimprovement-program-eiip)

Emissions estimation procedures for point, area 

(nonpoint) and mobile sources. While much of the 

information included in these reports is still relevant, 

the EIIP documentation for point and nonpoint sources 

has some outdated parts, and the sections below 

indicate some of those limitations and alternative 

resources. The EIIP documentation for mobile sources 

(volume 4) is out of date because it refers to previous 

versions of EPA emissions models. Use more current 

resources, listed below for mobile sources and fires. 

While EIIP is also out of date for biogenic sources, 

these sources are relevant only for modeling 

inventories and not for the inventories covered by this 

guidance.

Emission Factors – WebFire 

(www3.epa.gov/ttn/chief/webfire/) and 

AP-42 (www3.epa.gov/ttn/chief/ap42)

Emission factors are used to estimate emissions, 

generally by multiplying by some activity value for that 

emissions activity, where no better method of 

emissions quantification or estimation is available. AP42 is not an appropriate source of emission factors for 

mobile sources other than road dust.

General Principles for the 5-Year Regional 

Haze Progress Reports for the Initial 

Regional Haze State Implementation 

Plans 

(https://www.epa.gov/visibility/generalprinciples-5-year-regional-hazeprogress-reports) 

This document has been developed by the EPA for the 

EPA Regional offices and states in preparing and 

reviewing the 5-year progress reports for the initial 

Regional Haze SIPs. This document summarizes and 

clarifies the requirements for the first 5-year reports, 

including emissions inventory aspects.
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Resource and website Description 

Motor Vehicle Emissions System (MOVES)

(www3.epa.gov/otaq/

models/moves/)

The model, technical documentation, and guidance on 

the use and application of the MOVES model for onroad and nonroad mobile sources.

Mobile sources Policy and Guidance

(www3.epa.gov/otaq/

stateresources/policy/)

Webpage that provides state and local agencies with 

direction on how to implement strategies for 

controlling emissions from mobile sources. Includes 

links to guidance documents that describe process for 

securing SIP air quality credits for voluntary and 

innovative emission reduction programs. See also link 

to nonroad documentation now available from the 

MOVES page.

Transportation Conformity

(www3.epa.gov/otaq/

stateresources/transconf/)

General information, policy and technical guidance 

about transportation conformity.

λ From this main web page, obtain factors using the link to the latest available NEI or NEI Plan.

∆ Select “Data Sets and Published Reports”.

Many other resources from other organizations exist. Where specific methods or tools from non-EPA 

organizations are included in this documentation, this document includes relevant references for those 

materials to help readers find these additional sources of information.

In addition to base year emissions, emissions projections are needed for a variety of reasons, including 

redesignation maintenance plans, the attainment projected inventory for a nonattainment area (NAA), 

air quality modeling for attainment plans, and projecting reasonable progress goals for visibility 

improvement as part of Regional Haze SIPs. In Section 5, this guidance provides information on methods 

for emissions projections. The EPA provides emissions projections data and documentation as part of its 

modeling platforms that are available at the Emissions Modeling Clearinghouse

(https://www.epa.gov/air-emissions-modeling). On-road and nonroad mobile source projections are 

covered in the mobile regulatory documentation associated with MOVES

(https://www3.epa.gov/otaq/models/moves/). Additional nonroad mobile projections information (for 

aircraft, commercial marine vessels, and locomotives) are included as part of this documentation in the 

nonroad mobile sources part of Section 5.

1.3 Summary of Relevant Statutory Provisions

Numerous sections of the CAA apply to the implementation of emissions inventory requirements. This 

section provides a brief synopsis of some of those sections that may be relevant for reference in later 

sections of this guidance. All of the sections below are a part of CAA Title I – Programs and Activities.

 Section 110(a) describes the basic requirements for SIPs, which provide for 

implementation, maintenance, and enforcement of the NAAQS. 

o Section 110(a)(1) specifies parameters for infrastructure SIPs.

General
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o Section 110(a)(2) specifies a “notice and public hearing”4 process is needed.

o Section 110(a)(2)(F) specifies that infrastructure SIPs shall require, as may 

be prescribed by the EPA Administrator, “periodic reports on the nature and 

amounts of emissions and emissions-related data” from stationary sources.

o Section 110(a)(2)(K) requires infrastructure SIPs to provide for air quality 

modeling as determined necessary by the Administrator and allows the EPA 

to require the submission upon request of data related to that air quality 

modeling.

o Section 110(a)(2)(J) addresses requirements related to Part C of the CAA, 

related to prevention of significant deterioration and visibility protection.

Readers should refer to the “Guidance on Infrastructure State Implementation Plan 

(SIP) Elements under Clean Air Act Sections 110(a)(1) and 110(a)(2); September 

2013; www3.epa.gov/airquality/urbanair/sipstatus/infrastructure.html.

General

 Section 110(p) requires states to submit “reports” according to the Administrator’s 

schedule, “relating to emissions reductions, vehicle miles traveled, congestion 

levels, or any other information the Administrator may deem necessary to assess 

the development effectiveness, need for revision, or implementation of any plan or 

plan revision” for a SIP.

General

 Sections 169A and 169B focus on visibility protection for Federal class I areas and 

require the EPA to promulgate regulations establishing requirements for 

implementation plans to assure reasonable progress towards improving visibility. To 

do this, the EPA has codified the regional haze program requirements in 40 CFR

51.308 and 51.309.

Regional 

Haze

 Section 172(c)(3) in Part D, Subpart 1 describes NAA SIP requirements for all criteria 

air pollutants (CAPs) – carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), 

ozone, particulate matter (PM), and SO2 – that, unless superseded by subsequent 

NAAQS-specific subparts, apply to SIPs for NAAs submitted to the EPA. This section 

includes a requirement that such SIPs include a current inventory of actual 

emissions in a NAA. It allows the EPA Administrator to require periodic updates 

needed to determine if the general requirements of nonattainment SIPs (i.e., 

Subpart 1) are met.

General

 Section 175A describes maintenance plans, which are SIP revisions that provide for 

the maintenance for 10 years of any primary NAAQS for which an area has been 

redesignated to attainment and may include additional control measures necessary 

General

 

4 Public hearings are explained at 40 CFR 51.102(a).
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to ensure this maintenance. Updated emissions inventories are to be included in a 

maintenance plan.

 Section 182(a)-(f) provide SIP inventory requirements for all areas designated as 

nonattainment for the ozone NAAQS.

o Section 182(a) includes SIP requirements for ozone NAAs classified as 

Marginal and above.

 Section 182(a)(1) requires a current and comprehensive inventory of 

actual emissions of all sources of ozone precursors to be included in a 

SIP.

 Section 182(a)(3)(A) requires periodic inventories to be provided by a 

state to EPA, until an area is redesignated to attainment, no later than 

the end of each 3-year period after the submission of the inventory for 

182(a)(1).

 Section 182(a)(3)(B) requires areas to include in their SIP a requirement 

for stationary sources of NOX and volatile organic compounds (VOC) to 

provide the state with an annual statement showing actual emissions of 

NOX and VOC.

o Section 182(b) includes SIP requirements for ozone NAAs classified as 

Moderate and above.

 Section 182(b)(1)(A) requires a rate of progress (ROP) demonstration, 

and describes requirements for annual reductions in VOC and NOX

emissions as necessary to attain the ozone NAAQS. 

 Section 182(b)(1)(B) requires a baseline emissions inventory for NOX

and VOC from all anthropogenic sources in the NAA to be included in 

ozone SIPs for purposes of ROP/reasonable further progress (RFP) 

demonstrations. 

o Section 182(c) includes SIP requirements for ozone NAAs classified as 

Serious and above. This section defines major sources for areas classified as 

Serious as sources that emit or have the potential to emit at least 50 tons of 

VOC per year.

 Section 182(c)(2)(A) requires an attainment demonstration based on 

photochemical modeling. A modeling inventory is needed for the 

attainment demonstration analysis. 

 Section 182(c)(2)(B) requires a RFP demonstration, and describes 

requirements for a timeline of VOC emissions reductions. RFP 

demonstrations must include baseline emissions for comparison against

the emissions reductions. 

o Section 182(d) defines major sources for areas classified as Severe as 

sources that emit or have the potential to emit at least 25 tons of VOC per 

year.

Ozone
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o Section 182(e) defines major sources for areas classified as Extreme as 

sources that emit or have the potential to emit at least 10 tons of VOC per 

year.

o Section 182(f) provides that all major source CAA requirements for VOC also 

apply for NOX unless a waiver is received.

 Section 182(j)(1) requires states in a multi-state ozone NAA to take all reasonable 

steps to coordinate, substantively and procedurally, the revisions and 

implementation of SIPs for a multi-state ozone NAA.

 Section 184(b)(2) defines major sources for areas within the ozone transport region 

(OTR) as any stationary source that emits, or has the potential to emit, 50 tons per 

year of VOC. Per the requirement for Section 182(f), this definition also applies to 

sources of NOx.

Ozone

 Section 189(a)(2) in Part D, Subpart 4 provides a different and earlier schedule (18 

months rather than 3 years) for PM SIPs than provided for in the general provision 

in Part D, Subpart 1. PM2.5

 Section 189(b)(3) (also Part D, Subpart 4) defines major sources in Serious PM NAAs 

as sources that emit 70 tons per year of PM10. Inventories for PM SIPs for Serious 

NAAs should establish the inventory of major point sources using the lower 

threshold. This section of the CAA has been determined through the courts to be 

the appropriate governing section for PM2.5 implementation as well as PM10. 

PM2.5

It is important to note the distinction between a requirement in the CAA that an inventory be a SIP 

submission or included in a SIP submission, a requirement that a state’s SIP provide for the preparation 

and submission to EPA of an inventory or other emissions information, and a requirement that a SIP 

demonstrate some fact (such that a NAAQS will be attained by a future date) that can only be 

established by using sound emission inventory information in an appropriate manner. This distinction is 

important because if an inventory is a required SIP submission, then certain procedural requirements 

apply. Inventories that are SIP submissions, or included in SIP submissions, must comply with all 

procedural requirements that are applicable to any other SIP submission including but not necessarily 

limited to those specified CAA section 110 and 40 CFR 51.102, 103 and Appendix V. This document 

clarifies which emission inventories are required SIP submissions and which are not.

1.4 Summary of Guidance Contents

This guidance document will define parts of and considerations for emissions inventories to meet SIP 

requirements for complying with the 8-hour ozone NAAQS, the PM2.5 NAAQS (24-hour and annual), and 

the regional haze regulations. Section 2 provides an overview of the creation, documentation, and 

submission of the planning inventories. Section 3 has subsections for each of the separate inventory 

requirements or recommendations, with details provided for all requirements except the base year and 
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projected emissions inventories. Sections 4 and 5 provide the detailed guidance on developing base year 

emissions inventories and projected emissions inventories, respectively.

2 SIP Inventory Overview

There are two categories of emissions inventories associated with SIP development: planning

inventories and modeling inventories. Modeling inventories are used for modeled attainment 

demonstrations and setting reasonable progress goals for visibility protection. Planning inventories are 

needed for ozone and PM2.5 SIPs to quantify nonattainment area emissions, and they are used as a part 

of RFP analysis and transportation conformity. The primary focus of this guidance is the planning 

inventories, whereas the modeling inventories are primarily addressed in the modeling guidance that is 

provided separately in the Modeling Guidance document (see Section 1.2).

Generally, the EPA recommends that the planning inventories and modeling inventories be consistent, 

though there are exceptions. The exceptions are noted where they arise throughout this guidance and 

the Modeling Guidance; however, the EPA generally encourages air agencies to minimize inconsistencies

where possible. 

The many aspects of planning inventories are covered more thoroughly in Section 3. These aspects 

include the timing for submitting the various inventory components, the inventory preparation plan

(IPP), the base year NAA inventory, the RFP baseline inventory, the ozone periodic inventory, the ozone 

emissions statement requirement for SIPs, the maintenance inventories, the attainment projected 

inventory for the NAA, public hearings, and inventory approval by the EPA.

In this section, we provide an overview of the creation, documentation, and submission of the planning 

inventories. Section 2.1 lists the various steps associated with building the planning inventory parts of a 

SIP and identifies the relevant sections of this document that address those steps. The discussions 

between air agencies and the EPA regarding the inventory approval process is described in Section 2.2. 

In Section 2.3, we list all of the required inventory components, including modeling inventories that are 

needed for ozone, PM2.5, or regional haze, but otherwise defer to the Modeling Guidance for the factors 

for air agencies to consider in building those inventories. In Sections 2.4 and 2.5, we describe other 

federal emissions reporting requirements that are relevant to NAAQS implementation and provide 

definitions of key terms needed to understand, develop, and submit emissions inventories for SIPs. 

2.1 Recommended Steps for Building the Inventory Parts of a SIP

Throughout this document, we describe many technical components that could be included in

inventories in a SIP for ozone, PM2.5, or regional haze implementation. Before air agencies start to create 

those components, it is important to recognize that there is a process associated with building them. 

This section describes the process agencies should undertake for a successful inventory development 

and submittal effort. 

The inventory development process involves both required and optional steps. Some optional steps may 

be taken by the air agency if it believes it is in its best interest to do so, and optional steps can also 
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become critical to a particular SIP development process and identified as such by an EPA Regional office. 

Thus, a fundamental part of the process is establishing and maintaining open lines of communication 

between the air agency and the respective Regional office. The Regional office should be viewed by the 

air agency as the typical starting point for questions related to planning and modeling inventory 

development, and the Regional office staff will direct questions needing EPA headquarters input as they 

arise.

A key aspect of this guidance is identifying opportunities for states to leverage existing work to help 

them meet the implementation requirements. This starts in Section 2.4 with a description of how work 

done in support of the NEI can be helpful for the SIP inventory work. 

Table 2 provides a list of steps for completing the planning inventory and related parts of a SIP. The list is 

provided in groups of steps that occur in phases presented roughly in chronological order. The second 

column of the table indicates the section(s) of this document that give more information on the 

particular step. States and Regional offices should look for opportunities to be efficient where possible 

to reduce workload. For example:

 an Inventory Preparation Plan for an air agency that has prepared emission inventory SIPs for a 

given NAAQS may not be needed, could reference a past plan, or could need fewer details;

 combining documentation from separate steps into a single document where that makes sense;

and

 referencing externally provided inventory documentation where externally prepared emissions 

data have been used in the SIP inventory is acceptable (in this case, the air agency would just 

explain why the externally prepared data are applicable for the SIP inventory).

Table 2: Steps for building the inventory parts of a SIP

Inventory-related SIP development step Document 

section

Planning

 Review this guidance. All

 Become familiar with inventory development resources.

1.1

1.2

 During development of the Inventory Preparation Plan, clarify and discuss the

inventory approval process that your Regional office will use, including all 

requirements, reporting formats and elements to be included, timing, and 

technical documentation contents.

2.2

(general)

3.1 (timing)

 Understand required and potentially required inventory components for the SIP. 

Determine which inventories must be created for your SIP. 2.3

 Understand inventory definitions and data requirements. 2.5

 Understand timing requirements of submitted components. 3.1

 Prepare and finalize the Inventory Preparation Plan. 3.2

 Prepare a quality assurance project plan (QAPP) 3.2
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Inventory-related SIP development step Document 

section

 Understand requirements and choose key inventory features, including inventory 

year(s), the pollutants and precursors to be included, and priority emissions 

sources.

4

Creating Inventories

 Assess what inventories already exist that can be used as a starting point.

2.4

3.3

 Create base-year planning inventories. 4

 Quality assure and refine base year inventory data in accordance with the IPP 

and QAPP.

0

 Create projected planning emission inventories. 5

 Quality assure and refine projected planning emission inventories in accordance 

with the IPP and QAPP. 5.5

Preparing Documentation

 Review requirements with respective Regional office in light of the particular 

challenges associated with a given SIP development process, such a first 

nonattainment SIP development for a state, area, or air agency staff.

2.2

 Prepare documentation. 3.10

Public Hearings 3.9

Submit to the EPA for Approval 3.10

2.2 Discuss the Inventory Approval Process with EPA

Early in the SIP planning process, air agencies should discuss the emissions inventory approval process 

for that specific SIP with their respective EPA Regional office. These discussions should address the

tangible items (e.g., reports, data files, summary spreadsheets) that will be delivered to the EPA as

necessary to support the EPA’s review and approval.

A common source of confusion is whether inventories must undergo public review. Inventories that are 

SIP submissions, or are included in SIP submissions, must comply with all procedural requirements that 

are applicable to any other SIP submission including but not necessarily limited to those specified in CAA 

section 110 and 40 CFR 51.102, 103 and Appendix V. Details on the public review issue are addressed in 

more detail in Section 3.9.

As new issues arise during the SIP development process, air agencies or the EPA Regional offices may 

need to reassess the previously discussed inventory approval process. Both agencies should be aware of 

possible changes that are needed as the inventory development process occurs, and either party should 

raise concerns about the need for discussion and possible revision of the process. Details of the process 

and documentation can be modified through additional discussions as the need arises. As the inventory 

development process works far better when lines of communication remain open, the EPA encourages 

periodic coordination meetings or check-ins during the inventory development periods.

Table 3 provides a list of tangible reports, summaries, and data files that could be included as part of the 

inventory efforts for SIPs. Some of these items are needed to meet statutory and/or regulatory 
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requirements, and these are listed in Tables 4 through 6 below. The form of reports, summaries, and 

data files to be included should be clarified with the Regional office at the start of the inventory 

development process for SIPs. States should discuss and confirm with the EPA Regional office the items 

to be included in the SIP to avoid inadvertent omission of items required for approval of the inventory 

parts of the SIP. While certain recommended items in Table 3 are not specifically required by the 

regulations, the EPA review of the required elements requires supporting documentation so that the 

EPA can assess whether the approaches used for developing inventories are technically sufficient and 

meet the requirements to use appropriate methods and current, accurate information.

One notable and important difference between regional haze and the other two programs (ozone 

attainment SIPs and PM2.5 attainment SIPs) is that there is no requirement to submit detailed emissions 

inventory data as an element of a required Regional Haze SIP. While the regional haze program expects 

that inventories will be used to accomplish the required elements of the Regional Haze SIP and the 

regional haze regulations require the SIP to provide for the preparation of emission inventories for 

particular timeframes,

5

the detailed emissions inventory data are not submitted as a SIP (or SIP revision) 

like the data must be for ozone and PM2.5. The recommended items in Table 3, however, will allow the 

EPA to make a determination whether the emissions information used in Regional Haze analysis is 

sufficient for the purposes of the SIP.

Following receipt of an emissions inventory from air agencies, the EPA will determine if it is part of a SIP 

submission or not. If an emissions inventory is provided to the EPA as a component of a SIP submittal, 

then EPA will process that SIP submittal, including the emissions inventory component, in accordance 

with applicable laws and regulations (FN: CAA section 110 and 40 CFR 51.102, 103 and Appendix V). As 

an example, emissions inventories required by CAA § 182(a)(1) are to be provided to EPA as a SIP 

submission (FN: See 80 FR 12264,12291). Conversely, if an emissions inventory is provided to the EPA 

outside of a SIP submittal, then EPA will review and process that inventory as necessary.

 

5

See 40 CFR 51.308(d)(4)(v) (Regional Haze SIPs must “provide for” statewide emissions inventories of pollutants 

that are reasonably anticipated to cause or contribute to visibility impairment in any mandatory Class I Federal area.). 

The proposed amendments to the Regional Haze Rule would similarly require state’s Regional Haze SIPs for the 

second and subsequent implementation periods to provide for such emissions inventories. Protection of Visibility: 

Amendments to Requirements for State Plans; Proposed Rule (81 FR 26942, May 4, 2016). 
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Table 3: Recommended items to include in inventory information provided to the EPA

Inventory Item Ozone PM2.5

Regional 

Haze

Planning Inventories

Inventory Preparation Plan X X X

Emissions Inventory Report, with explanations of methods, quality 

assurance and emissions summaries. All inventories can be 

included in such a report, including base year and projected 

inventories. The inventories to be included depend on the 

NAAQS, the nonattainment classification, and the stage of 

implementation (initial SIP, redesignation, etc.).

X X

Emissions Inventory Report, with explanations of methods, quality 

assurance and emissions summaries. All inventories can be 

included in such a report, including base year, most recent 

year, and projected inventories.

X

Tables or spreadsheets of emissions summaries included in the 

inventory report

X X X

Detailed emissions inventory files, databases, or spreadsheets X X

Modeling Inventories

Modeling Demonstration Report – The emissions information can 

be a section in a larger report or could have its own report. 

The content should include methods, quality assurance, and 

emissions summaries for the modeling inventories, 

including projected inventories. A statewide emissions 

summary is needed, and this can be included in the 

planning inventory SIP for convenience.

X X X

Tables or spreadsheet summaries of emissions inventory used for 

modeling X X X

Detailed emissions inventory files, databases, or spreadsheets X X

Detailed data about emissions controls used to model X X

Other

Emission Statement SIP showing the state’s program to require 

sources to submit emissions statements annually X

2.3 Planning and Modeling Inventory Components

The ozone, PM2.5, or regional haze implementation rules have specific inventory requirements for SIPs to 

support attainment, maintenance, and/or reasonable progress demonstrations. Additionally, there are 

possible inventory details that could be required or advisable for various reasons. One major distinction 

between the inventory requirements of the ozone and PM2.5 rules versus the proposed Regional Haze 

Rule is that the former two rules require inventories for NAAs, while the latter expects that states 

develop statewide inventories. This section provides a list for each of the three rules for the required 

and possible inventory components of SIPs, with additional details for the types of planning inventories.
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2.3.1 Ozone SIP Inventory Components

The Ozone Implementation Rule (40 CFR part 51, Subpart AA) provides the regulatory requirements for 

inventories of actual emissions that support ozone NAAQS implementation by air agencies. In Table 4

below, we list the inventory components that could be part of preparing an ozone SIP and identify the 

governing portion of the CAA for required SIP elements, rule citation and deadline for required SIP 

elements and the applicable guidance in which they are addressed. The deadlines are expressed in 

months after the effective date of designation (EDD) of the area.

States and other air agencies that perform modeled attainment demonstrations should also refer to the 

Modeling Guidance (http://www3.epa.gov/ttn/scram/guidance_sip.htm), which includes a section on 

emissions inventories for modeling. As part of that guidance, emissions inventory summaries are 

described for inclusion in technical documentation about the modeled attainment demonstration. To 

assist the public when it comments on the SIP submittal and to facilitate EPA review of the SIP 

submissions, these summaries of modeling inventories can be submitted as part of the documentation 

provided with the emissions inventory SIP, but are not required SIP elements. 

Table 4: Ozone SIP emissions inventory components

Inventory Component Required by CAA? 

(CAA Section)

Regulatory Citation 

(Due Date)

Guidance Sections and/or 

Document(s)

Inventory Preparation Plan This document, Section 3.2

Base year inventory for the 

NAA

Yes

182(a)(1)

40 CFR 51.1115(a)

(24 months after EDD)

This document, Section 3.3

and Section 4

ROP/RFP baseline NAA 

inventory (part of the 

ROP/RFP plan(s); for 

Moderate and above 

areas)

Moderate & 

above:

182(b)(1)(B)

Serious & above: 

182(c)(2)(B)

40 CFR 51.915

40 CFR 51.1110(b)

(36 months after EDD)

This document, Section 3.4

and Section 4

Periodic NAA inventory Yes

182(a)(3)

40 CFR 51.915

40 CFR 51.1115(b)

(No later than the end 

of each 3-year period 

after required base 

year submission)

This document, Section 3.5

and Section 4

Attainment projected 

inventory for the NAA+

Summary required by

40 CFR 51.114 (b)

This document, Section 3.8

and Section 5

Maintenance NAA inventory Yes‡

175A(a)

This document, Section 3.7

and Section 4

Source Emission Statements 

requirement for SIPs

Yes

182(a)(3)(B)

40 CFR 51.915

(24 months after EDD) This document, Section 3.6

Inventories for modeled 

attainment 

demonstrations (base and 

projected attainment 

year)

Yes∆

182(c)(2)(A)

40 CFR 51.908

40 CFR 51.1108

(Timing depends on 

nonattainment 

classification)

See the Modeling Guidance. 

Summaries of inventories 

(base and projected 

attainment year) are 

encouraged to be included 

in technical documentation.
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Inventory Component Required by CAA? 

(CAA Section)

Regulatory Citation 

(Due Date)

Guidance Sections and/or 

Document(s)

Public hearing requirement 

for SIP inventories

Yes

110(a)(2)

40 CFR 51.102 This document, Section 3.9

Planning inventory 

documentation This document, Section 3.10

+ If this inventory is the mechanism by which the ROP/RFP plan requirements will show the emissions reductions required for 

ROP/RFP, then this inventory or its summary would necessarily become part of the ROP/RFP plan(s). Motor vehicle emissions 

budgets for purposes of transportation conformity are the on-road portion of the projected emissions associated with the 

ROP/RFP plan.

‡ Maintenance NAA inventories are only associated with maintenance SIPs rather than attainment demonstration SIPs.

∆ While modeling inventories are not an explicit requirement of the CAA, it is presumed here that states would not be able to 

perform credible modeled attainment demonstrations without modeling inventories.

2.3.2 Annual and 24-Hour PM2.5 SIP Inventory Components

The PM2.5 SIP Requirements Rule (40 CFR part 51, Subpart Z) provides the regulatory requirements for 

inventories of actual emissions that support PM2.5 NAAQS implementation by state, local, and tribal

agencies. In Table 5 below, we list the inventory components that could be part of a PM2.5 SIP and 

provide a map to the governing portion of the CAA, regulations, and the applicable guidance.

For areas that perform modeled attainment demonstrations, please also refer to the Modeling 

Guidance, which includes a section on emissions inventories for modeling. As part of that guidance, 

summaries are described for statewide inventories. For convenience, these summaries can be included 

in the emissions inventory SIP. Areas that do not perform modeled attainment demonstrations would 

not need to provide the statewide inventories for PM2.5.

Table 5: PM2.5 SIP emissions inventory components

Inventory Component

Required by 

CAA? 

(CAA Section)

Regulatory Citation 

(Due Date)

Guidance Sections 

and/or Document(s)

Inventory Preparation Plan This document, Section 

3.2

Base year inventory for the NAA Yes

172(c)(3)

Moderate

40 CFR 51.1008(a)(1)α

(18 months after EDD)

This document, Section 

3.3 and Section 4

Attainment projected inventory 

for the NAA

Moderate: 

40 CFR 51.1008(a)(2)α

(18 months after EDD)

This document, Section 

3.8 and Section 5

Maintenance NAA inventory Yes+

175A(a)

This document, Section 

3.7 and Section 4

Inventories for modeled 

attainment demonstrations 

(base year and projected 

attainment year)

Yes∆

189(a)(1) for 

Moderate 

areas and 

189(b)(1) for 

Serious areas

40 CFR 51.1001 and 

Appendix W

(18 months after EDD)

Modeling Guidance. 

Summaries of statewide 

inventories (base and 

projected attainment 

year) are encouraged to 
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Inventory Component

Required by 

CAA? 

(CAA Section)

Regulatory Citation 

(Due Date)

Guidance Sections 

and/or Document(s)

be included in technical 

documentation.

Public hearing requirement for 

SIP inventories

Yes

110(a)(2) 40 CFR 51.102 This document, Section 3.9

Planning inventory 

documentation

This document, Section 

3.10

α If an area is reclassified as Serious, the applicable requirements refer back to this subsection from 40 CFR 51.1008(b). If a 

Serious area has failed to meet the PM2.5 NAAQS by the Serious area attainment date, then see 40 CFR 51.1008(c), which still 

refers back to the Moderate area requirements and has a 12 month deadline.

+ Maintenance NAA inventories are only associated with maintenance SIPs rather than attainment demonstration SIPs.

∆ While modeling inventories are not an explicit requirement of the CAA, it is presumed here that states would not be able to 

perform credible modeling attainment demonstrations without modeling inventories.

2.3.3 Regional Haze SIP Inventory Components

The Regional Haze Rule (40 CFR part 51, Subpart P) has significantly different inventory requirements 

from ozone and PM2.5 implementation rules. One reason for this difference is the time horizon of the 

regional haze program. The program consists of implementation periods that are referred to as the first 

planning period, second planning period and so forth. There are parts of regional haze implementation 

plans that have been completed via the SIP for the first implementation period (ending in 2018) and may 

not need to be revisited in SIPs for later implementation periods, but for completeness we also explain 

these here. Another difference is that the Regional Haze Rule requires the use of statewide planning 

inventories rather than inventories for only a smaller NAA. Finally, the most important difference is that 

there is no requirement that emission inventories be submitted as a SIP revision. While the inventories 

are needed to support the SIP and the SIP must provide for the specified emission inventory 

information, the detailed statewide inventories need not be submitted.

As noted above, the EPA proposed changes to the Regional Haze Rule in May 2016.6If finalized, these 

changes would apply only for the second and subsequent planning periods; requirements for the first 

planning period are governed by the existing Regional Haze Rule. This section briefly discusses the 

existing requirements related to emission inventories for the first planning period, and then goes on to 

address the same requirements as proposed for the second and subsequent planning periods.

For the first planning period, the Regional Haze Rule in 40 CFR 51.308(d)(3)(iii) required a SIP submission 

to include the technical basis on which the State was relying to determine its apportionment of emission 

reduction obligations necessary to achieve reasonable progress towards natural visibility conditions, 

including identification of the “baseline emissions inventory” used for such technical analysis. The rule 

indicated that the baseline emissions inventory year for the first planning period is presumed to be the 

most recent year of the consolidated periodic emissions inventory (as defined in Subpart A and 

commonly called the NEI). As a result, this “baseline” inventory used in the SIPs reflects a year from the 

2000-2004, for which the 2002 was the applicable NEI year. For the first planning period, most states 

 

6 Protection of Visibility: Amendments to Requirements for State Plans; Proposed Rule (81 FR 26942, May 4, 2016).
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also used a year from this period as the base year for their regional air quality modeling (states that 

were members of the Lake Michigan Air Directors Consortium (LADCO) also used 2005 in their air quality 

modeling).

In addition, 40 CFR 51.308(d)(4)(v) required that the plan provide for a statewide inventory of emissions 

for a “baseline” year, for the most recent year in which data are available (“recent year”), and estimates 

of future projected emissions. This section made it clear that the inventories associated with the 

planning effort needed to be “statewide.” This document does not explain in detail the emission 

inventory requirements for SIPs for the first implementation period, which have already been submitted 

and acted upon by the EPA.

The regional haze rule at present also includes a requirement for 5-year progress reports in 40 CFR 

51.308(g). The initial 5-year progress report (for the first planning period) is required to be a SIP 

revision.

7 Because many of the first of these required 5-year progress report SIP revisions have already 

been submitted and the remaining progress reports should be far along in preparation as of the date of 

this document, this document does not address the first-required progress reports. For guidance on 

these first required 5-year progress reports, readers should use the separate guidance document 

“General Principles for the 5-Year Regional Haze Progress Reports for the Initial Regional Haze State 

Implementation Plans.”

The EPA has proposed to revise the Regional Haze Rule so that the next required progress report would 

be due on January 31, 2025. Thereafter, progress reports would be due midway between “10-year” SIP 

revisions, with due dates of July 31 of 2033, 2043, etc. Under the proposed revisions, these future 

periodic progress reports would not be required to be SIP revisions. Section 308 (g)(4), as it would be 

amended under the proposed revisions, requires states to analyze the change in emissions over the 

period since the previous plan revision (i.e., the SIP revisions due in 2021, 2028, and every 10 years 

thereafter). The requirement sets out that “[e]missions changes should be identified by type of source 

or activity,” and this guidance provides recommendations for what emissions summaries these reports 

should include to accomplish that objective. In addition, the proposed Regional Haze rule revisions 

specify that the analysis of historical emissions must extend at least through the most recent year for 

which the state has submitted emission inventory information to the EPA in compliance with the 

triennial reporting requirement of the Air Emissions Reporting Rule (AERR), 40 CFR part 51, Subpart A. 

For data (such as EGU data) that are submitted directly to the EPA and then made available by EPA, the 

data used must be the most recent year for which the EPA has provided state-level summary data or an 

internet-based tool by which the State may obtain such a summary. Further, section (g)(5) as amended 

would require an assessment of any significant changes in anthropogenic emissions within or outside 

the state that have occurred since the previous SIP revision.

Section 40 CFR 51.308(f) of the Regional Haze Rule requires 10-year SIP revisions for the second and 

subsequent implementation periods; EPA proposed revisions to the requirements for SIP revisions in 

 

7 On May 4, 2016, the US EPA proposed amendments to the regulatory requirements for the Regional Haze SIPs that included a 

proposed change such that the 5 year progress reports would no longer be a SIP revision.
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May 2016. As it would be revised, section 308(f)(2) specifies that the SIP revision include a long-term 

strategy and section (f)(2)(iv) requires that the state document the technical basis for its determinations 

of the emission reductions from anthropogenic sources that are necessary for achieving reasonable 

progress towards natural visibility conditions. The proposed wording in this section requires that a state 

“identify the baseline emissions inventory on which its strategies are based.” In the context of this 

particular provision (308(f)(2)(iv)), the phrase “baseline emissions inventory” refers to source emissions 

corresponding to the baseline of current on-the-books emission control measures, which is the starting 

point for consideration of any additional measures that are necessary to achieve reasonable progress. It 

does not refer to the 2000-2004 “baseline period” that appears in other parts of section 308. In the 

proposed section 308(f)(2)(iv), the baseline emissions inventory year is further explained to mean the 

most recent inventory year submitted to the EPA under the triennial reporting requirements of AERR or 

another year if justified. Thus, the term “baseline emission inventory” in this section could be the same 

as the “recent year” inventory. 

In the proposed rule revision, section 308(f)(6)(v) mirrors a part of section 308(d)(4) by specifying that SIP 

revisions for the second and subsequent planning periods must provide for a statewide inventory of 

emissions for a “baseline year,” for the most recent year in which data are available (“recent year”), and 

estimates of projected emissions. In this context, “baseline year” refers to a year in the 2000-2004 period. 

A state that has submitted statewide inventory information in its SIP revision for the first implementation 

period has demonstrated that its SIP does provide for the creation of this “baseline year” inventory. In its 

SIP revision for the second and subsequent implementation periods, a state will need to describe its 

process for preparing, or providing essential information for the collective preparation of, a statewide 

emission inventory for the latest “recent year” and a projection inventory for the last year of the 

implementation period.

Table 6 summaries a list of the Regional Haze SIP emissions inventory components, including the 

appropriate references to the CAA, the regulations, and the relevant sections of this guidance.

Table 6: Regional haze SIP emissions inventory components

Inventory Component Required?

(CAA Section)

Regulatory Citation 

(Due Date)

Guidance Sections 

and/or Document(s)

Inventory Preparation Plan This document, Section 

3.2

Baseline statewide inventory

Recent year statewide inventory

Projected statewide inventory

(For the first planning period)

Yes

169A(b)+

40 CFR 

51.308(d)(4)(v)

(completed)

Previously completed as 

part of initial Regional 

Haze SIP for the first 

planning period.

5-year (or “periodic” as proposed 

in the revised Regional Haze 

Rule) progress report, recent 

year statewide inventory

Yes

169A(b)+

40 CFR 51.308(g)(4) 

and (5)

(see guidance noted 

above)

This document, Section 

3.5.2 and Section 3.10.3

10-year SIP revisions,

baseline statewide inventory ∆

Yes

169A(b)+

40 CFR 

51.308(f)(6)(v)

(varied dates)

May refer to the 

emissions from the first 

planning period
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Inventory Component Required?

(CAA Section)

Regulatory Citation 

(Due Date)

Guidance Sections 

and/or Document(s)

10-year SIP revisions,

recent year statewide 

inventory ∆

Yes

169A(b)+

40 CFR 

51.308(f)(6)(v)

(varied dates)

This document, Section 

3.5.2 and Section 4

10-year SIP revisions,

projected statewide inventory∆

Yes

169A(b)+

40 CFR 

51.308(f)(6)(v)

(varied dates)

This document, Section 

3.8 and Section 5

Documentation This document, Section 

3.10

+ CAA 169A(b) provides the authority to provide guidelines to the states on appropriate techniques and methods to implement 

the visibility protection goals in the CAA. This has been done through regulations as codified in 40 CFR 51.308 and 51.309.

∆ If modeling is done to support the 10-year SIP revision, these are modeling inventories.

2.4 Other Relevant Federal Emissions Reporting Requirements

The Air Emissions Reporting Rule (AERR), 40 CFR part 51, Subpart A, requires states to provide to the 

EPA annual statewide emissions inventory data, regardless of whether that state has nonattainment 

areas. This regulation requires reporting of actual emissions of CO, NOX, VOC, SO2, PM2.5, particulate 

matter with diameter ten microns and less (PM10), and ammonia (NH3). For PM2.5, the AERR requires 

states to report primary (i.e., “direct”) PM2.5, which is defined as the sum of filterable PM2.5 and 

condensable PM. The agencies are required to report the filterable PM2.5 and condensable PM 

separately where they are present from the source (see Section 4.2.1). States may delegate the 

reporting requirements to local air agencies. Tribes can also submit these data to the NEI, but are not 

required to do so. These data are used by the EPA’s Office of Air and Radiation’s (OAR) Office of Air 

Quality Planning and Standards (OAQPS) to build the NEI, and a complete version that includes all 

sources of emissions (with minor exceptions) is released every 3 years.

The AERR includes both triennial and annual statewide reporting requirements, with more extensive 

reporting requirements for triennial inventory years. Triennial years for the NEI include 2011, 2014, and 

every 3rd year thereafter. For the interim annual inventories, reporting is limited to emissions data from 

only the larger point sources (Type “A” sources), as defined by Appendix A of 40 CFR part 51, Subpart A. 

For the triennial inventories, lower point source thresholds are given in Appendix A, consistent with the 

definition of major sources in 40 CFR part 70. All other sources of emissions must be reported as 

nonpoint or mobile sources on a county basis in the triennial submissions. Additionally, lower point 

source thresholds may apply to ozone, PM2.5, PM10, and CO NAAs, depending on the area’s 

nonattainment classification.

The AERR requirements are relevant for SIP planning inventory purposes in several ways. First, the AERR 

and its associated EIS (data system) provide the appropriate data elements (fields to be reported) for 

inventories for electronic transfer to the EPA. Second, AERR-based inventories are often used as a 

starting point for SIP inventories. Third, in some cases, AERR submissions can meet some of the 

inventory requirements described in this document. Finally, the resources provided by the EPA for states 

to meet their AERR reporting obligations, such as emissions inventory tools and models, associated 
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documentation, emission factors, new emissions methods, and quality assurance approaches, can assist 

inventory developers when making inventories for SIP planning and modeling purposes.

As a point of clarification, air agencies must meet all relevant planning inventory SIP requirements 

regardless of the availability or use of the EPA data associated with the AERR/NEI process. While the EPA

resources can be helpful to states in meeting those requirements, the SIP requirements and the 

associated public review and Regional office approval process are independent requirements that must 

be met. It would not be sufficient to merely point to the EPA’s publication of the NEI as having met the 

SIP emissions inventory requirements. However, it may be sufficient for a state to submit as a SIP 

revision an inventory that contains some or all of the same data as in the NEI, with sufficient justification

and using the applicable public comment process.

2.5 Emissions Inventory Definitions

Emissions inventory work requires knowledge of many terms and codes. In this section, we define the 

terms relevant to inventories that are used throughout this document. This section also references 

external materials needed for obtaining definitions of all valid fields that can be used for building 

inventories, which are based on the tables used to submit data to the EPA for the AERR. Many 

definitions relevant for emissions inventories are defined in the CAA or codified in the AERR and the 

implementation rules. These are included here for a convenient reference in a single location since

these terms are used throughout this document. Without these definitions, the meaning of the 

subsequent sections would be less clear. The sample below is not intended to represent a 

comprehensive list of inventory related definitions.

2.5.1 Source Category/Data Category

Traditionally, the term “source category” has been used to identify the major types of emissions 

inventory groupings: stationary point sources, stationary area (or nonpoint) sources8, on-road mobile 

sources, nonroad mobile sources. In addition, fires and biogenic sources are sometimes included as 

additional source categories.

In practice, some inventories have started including large moveable facilities, such as asphalt plants, in 

the point sources inventory without associating them with a specific county. Additionally, some mobile 

sources such as locomotives and commercial marine vessels (CMVs) are treated as nonpoint data (i.e., 

the formatting of it) for the purpose of reporting these data to the EPA via the EIS (see also Section 4.8.5

as well as the EIS information referenced in Table 1). Similarly, because of the data format and their 

physical nature (i.e., a discrete geographic location), rail yards and airports are treated as point data in 

the EIS.

With the advent of the EIS, the preferred term for indicating subsets of inventory data has become “data 

category” rather than “source category” because the groupings have much more to do with how the 

 

8 Over time, stationary area sources have been renamed “nonpoint” sources to avoid confusion with the air toxics 

regulatory term “area sources,” which are often sources that meet the criteria pollutant definition of point 

sources. Likewise, the term “stationary” is often ignored in common use for both the point source and nonpoint 

labels.
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data are created and the data structure than the features of the actual emissions sources included in the 

category. For example, locomotives and commercial marine vessels are included in the nonpoint data 

category for inventories in the EIS, but in fact are nonroad mobile sources. Additionally, the EIS ushered 

in a new data category called “events,” which are typically wildfires or prescribed fires (agricultural fires 

are part of the nonpoint data category), but in principle could include other significant, short-duration, 

emissions events.

9

2.5.2 Types of Emissions: Annual, Seasonal, Actual, Potential, and Allowable

Various terms are used in this and other documents to reference various types of emissions. Here, we 

provide a brief list of definitions associated with various emissions types.

Actual emissions means the emissions of a pollutant from an affected source determined by taking into 

account actual emission rates associated with normal source operation and actual or representative 

production rates (i.e., capacity utilization and hours of operation) (40 CFR 51.491). This is in contrast 

with potential emissions or allowable emissions. These actual emissions should include emissions of a 

pollutant that occur during periods of startup, shutdown, and malfunction (see Section 4.8.1). In 

addition, please see Section 4.1 for a description of selecting a year for creating actual emissions 

estimates.

Allowable emissions means the emissions of a pollutant from an affected source determined by taking 

into account the most stringent of all applicable SIP emissions limits and the level of emissions 

consistent with source compliance with all federal requirements related to attainment and maintenance 

of the NAAQS and the production rate associated with the maximum rated capacity and hours of 

operation (unless the source is subject to federally enforceable limits that restrict the operating rate, or 

hours of operation, or both) (40 CFR 51.491).

Annual emissions are defined in the AERR (40 CFR 51.50) as actual emissions for a plant, point, or 

process that are measured or calculated to represent a calendar year. This means that annual emissions 

should include all emissions of the relevant sources at a facility or from a nonpoint or mobile process 

occurring during a calendar year. In general, unless otherwise specified, annual emissions means actual 

emissions, as defined above.

Average season day emissions means the sum of all emissions during the applicable season divided by 

the number of days in that season. This definition is used in the context of inventories developed for 

SIPs associated with the 24-hour PM2.5 NAAQS.

Potential to emit means the maximum capacity of a stationary source to emit a pollutant under its 

physical and operational design (40 CFR 51.301). Also called “potential emissions,” 40 CFR 51.301 also 

defines this term to specify that any physical or operational limitation on the capacity of the source to 

emit a pollutant including air pollution control equipment and restrictions on hours of operation or on 

 

9 While the EIS is not currently configured to accept event data other than for fires, this means that the idea of an 

“event” could be expanded to other types of events that are not currently inventoried because of the difficult 

nature and non-routine nature of these types of emissions.
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the type or amount of material combusted, stored, or processed, shall be treated as part of its design if 

the limitation or the effect it would have on emissions is federally enforceable. Secondary emissions do 

not count in determining the potential to emit of a stationary source.

As described in “Potential to Emit: A Guide for Small Businesses” (EPA-456/B-98-003), potential to emit 

considers the equipment design and can also consider certain controls and limitations on the operation 

of the equipment. Potential emissions can be expressed as potential controlled emissions, which are the

pollutant emissions while a source is operating at the maximum design capacity, a schedule of 8760 

hours per year, and considering the efficiency of the control equipment. In contrast, potential 

uncontrolled emissions are simply the pollutant emissions while the source is operating at the maximum 

design capacity and a schedule of 8760 hours per year without factoring in controls.

Startup, Shutdown, and Malfunction (SSM) emissions are emissions that occur during startup, 

shutdown, or malfunctions at industrial facilities. As a factual matter, the emissions rate during periods 

of SSM can be much larger than the emissions rates during periods of normal operating procedure (that 

is, not during periods of SSM). However, applicable emissions limitations apply at all times, including 

during periods of SSM.10 More information about the startup and shutdown emissions in the SIP 

inventory context is described in Sections 4.3 and 4.8.1.

Ozone season day emissions means an average day’s emissions for a typical ozone season work 

weekday. The state shall select, subject to EPA approval, the particular month(s) in the ozone season 

and the day(s) in the work week to be represented, considering the conditions assumed in the 

development of RFP plans and/or emissions budgets for transportation conformity.

2.5.3 Duration and Timing of Emissions

Inventory start date means the first day of the inventory period. This can be used for compiling and 

reporting monthly or other shorter period emissions, including events.

Inventory end date means the last day of the inventory period. This can be used for compiling and 

reporting monthly or other shorter period emissions, including events.

Inventory year or Emissions year means the year for which emissions estimates are reported (40 CFR 

51.50).

See also “Emission Type” in codes Section 2.5.14.

2.5.4 Data Elements

The term data elements means parts of a data file that contain individual items of data. These are also 

called fields or variables. Examples of key data elements associated with emissions inventories include 

emissions; activity; emission factors; state/county identifiers; facility, unit, and process identifiers; plant 

 

10 On May 22, 2015, the EPA issued a final action to ensure that states have plans in place that are fully consistent 

with CAA and court decisions concerning SSM operations. This action included a “SIP call” that directs 36 states 

to correct specific SSM provisions in their SIPs. The SIP submission deadline for the states subject to this action is 

November 22, 2016. See http://www3.epa.gov/airquality/urbanair/sipstatus/emissions.html for more 

information.
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names; control measure codes; and process codes. This list is just a sample and is not intended to 

represent a comprehensive list of required codes (see also Section 2.5.7).

Note that emissions data categories (Section 2.5.1) are not a data element per se, but rather emissions 

inventory submissions usually are broken out into separate datasets for each data category.

2.5.5 Throughput and Activity

Throughput means a measurable factor or parameter that relates directly or indirectly to the emissions 

of an air pollution source during the period for which emissions are reported (40 CFR 51.50). Throughput 

is also called activity. Depending on the source category, activity information may refer to the amount of 

fuel combusted, raw material processed, product manufactured, or material handled or processed. It 

may also refer to population, employment, miles travelled, or number of existing units. Activity 

throughput is typically the value that is multiplied against an emission factor to generate an emissions 

estimate. Consequently, it is important to coordinate activity used for a given emissions source with the 

available emission factors for that source.

11

There are some special types of activities always used for certain categories. Examples for mobile 

sources include:

Vehicle Miles Traveled (VMT): The number of miles travelled within a year or as an average daily value 

for all vehicles within a group (e.g., vehicle type, fuel type, and county). See also MOVES Technical 

Guidance (http://wwws.epa.gov/otaq/models/moves/index.htm#sip).

Vehicle population: The number of vehicles of a certain type and fuel usage within a group (such as a 

county).

Equipment population: The number of nonroad equipment units of a certain type and fuel usage within 

a group (such as a county).

2.5.6 Emission Factors

An emissions factor is a representative value that attempts to relate the quantity of a pollutant released 

to the atmosphere with an activity associated with the release of that pollutant. These factors are 

usually expressed as the weight of pollutant divided by a unit weight, volume, distance, or duration of 

the activity emitting the pollutant (e.g., kilograms of particulate emitted per megagram of coal burned). 

Such factors facilitate estimation of emissions from various sources of air pollution. In most cases, these 

factors are simply averages of all available data of acceptable quality, and are generally assumed to be 

representative of long-term averages for all facilities in the source category (i.e., a population average). 

Emission factors have long been the fundamental key to developing emissions inventories for air quality 

NAAQS implementation. More information on emission factors is available from the Clearinghouse for 

Inventories and Emissions Factors (https://www.epa.gov/chief/) on the EPA website.

 

11 Some air agencies have raised the concern that throughput at industrial facilities is considered confidential 

business information (CBI), however, this field is required by the AERR. Since this field is “emission data” 

required by the AERR, it cannot be protected as CBI under CAA section 114(c).
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The general equation for emissions estimation is:

E = A × EF × (1 − ER⁄100)

where:

E = emissions,

A = activity total,

EF = emission factor, and

ER = overall emission reduction efficiency percent.

The ER term is the combination of the relevant percents related to emissions controls and rules that 

reduce emissions, as listed in Section 2.5.18.

Emission factors are not limited to factors that are only represent broad national industry averages 

published by the EPA. Emission factors can also be stack-, process-, unit-, or facility-specific, depending 

on the basis of the source test information. Whether for a single facility or a group of facilities of the 

same type, it is still considered an emission factor for the purposes of this guidance.

2.5.7 Emissions Reporting Data Elements

The Ozone Implementation Rule and PM2.5 SIP Requirements Rule refer to the AERR and this guidance 

for information on reporting data elements. Since the AERR is focused on emissions submitted for the 

annual NEI inventory, it only includes definitions for those data and fields that are required by the NEI 

program. For ozone SIP inventories, ozone season day emissions are required, but are optional for 

reporting in accordance with the AERR. For PM SIP inventories, either or both annual or average season 

day emissions can be included (depending on the circumstances), but the average season day emissions 

are optional for reporting for AERR purposes.

The many required and optional data elements for emissions inventories are described in Excel files 

called Staging Requirements. These files are comprehensive for all of the required and optional fields 

that can be submitted to the EPA for SIP inventories, not only the submissions for the NEI. The shared 

field list for NEI and SIP inventories helps air agencies by promoting clarity and familiarity through 

common data element definitions across multiple inventory requirements. The descriptions provided in 

those Excel files are very limited, so this document provides additional information for the most 

important of these fields. These spreadsheets are available at the Emissions Inventory Training website 

at https://www.epa.gov/air-emissions-inventories/emission-inventory-training using the links for 

“Training Materials” listed for each training on the page. Within the Zip file provided for the “Training 

Materials” links, an Excel file is available that includes “staging requirements” and lists the data 

elements and their definitions.

Although many fields in the Staging Requirements are optional for AERR reporting purposes, some of 

these fields can be critical for inventory development for SIPs and even the NEI. By “optional,” we mean 

those fields that are not explicitly required by the AERR or that are not enforced during electronic 

reporting as needing to be provided to successfully submit the data to the EPA. The list below identifies
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fields that are “optional” but that are strongly recommended to be included in all emissions inventories, 

documentation, and reporting as applicable for the sources in the state or NAA. For example, control 

parameters are optional only because emissions controls do not exist on every unit and process in an 

inventory, thus the EPA cannot enforce this during electronic reporting as a required field. However, the 

control information is required by the AERR when it exists at the emissions source and should be 

provided for SIP inventories.

Unit Description: This field describes the unit in words. It can be invaluable in determining incorrect 

source classification codes (SCCs).

Unit design capacity: Unit design capacity means a measure of the size of a point source, based on the 

reported maximum continuous throughput or output capacity of the unit (40 CFR 51.50). Useful for 

boilers, this field is needed when trying to understand the cost per ton of pollutant emissions reduced 

associated with controls on boilers. The cost per ton is generally higher for smaller boilers. The design 

capacity can be critical in SIP planning for insuring that good decisions are made about controls on 

stationary boilers and other units. While required by the AERR, the EIS does not enforce this field as 

many states do not have this information available. By including these data in SIP inventory 

development, states can help to further improve the data available to them for decision making as well 

as for consideration by the EPA.

Control measure: This field is a unique code for the type of control device or operational measure (e.g., 

wet scrubber, flaring, process change, and ban) used to reduce emissions (see 40 CFR 51.50). When 

assessing whether an emissions source can be further controlled to contribute emissions reductions 

needed for attainment, it is imperative to know whether there is a control measure included on a unit or 

process. The type of measure is also needed to understand whether other controls can be added. The 

associated data elements such as control efficiency, capture efficiency, rule effectiveness, and rule 

penetration are all useful as well for the same reasons. These latter control parameters are also needed 

when calculating potential controlled emissions.

Alternative facility names or identifiers: This field can be very useful in matching the facility with other 

databases, such as the Toxics Release Inventory, the Federal Registry System, or the National Electric 

Energy Data System (NEEDS) database. If air agencies change their agency IDs (facility, unit, process, and 

release point) from one year to the next, this table can help prevent creating duplicates in the EIS and 

other databases.

EIS IDs: While the EIS Facility ID, Unit ID, Process ID, and Release Point ID (see Section 2.5.15) are not 

required by the AERR or for SIP inventories, it is the standard by which the EPA compiles and reports all 

point source inventories. The EPA reports and modeling all use these EIS IDs, and so to the extent air 

agency databases can use these IDs in their data systems, it helps facilitate matching across the many 

purposes for inventories.

Release Point Geographic Coordinates: As described in Section 2.5.17, the EIS and inventories generally 

can have location coordinates (i.e., latitude/longitude) for a facility or at individual release point

locations. 
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2.5.8 Point Sources

As defined by the AERR in 40 CFR 51.50, point sources are large, stationary (non-mobile), identifiable 

sources of emissions that release pollutants into the atmosphere. A point source is a facility that is a 

major source under 40 CFR part 70 for one or more of the pollutants for which reporting is required by 

40 CFR 51.15 (a)(1). This does not include the emissions of hazardous air pollutants, which are not 

considered in determining whether a source is a point source for ozone, PM2.5, and regional haze 

emissions inventory development and reporting. These point sources can be associated with a single 

point or group of points in space. Examples of important point source emissions categories include 

power plants, industrial boilers, petroleum refineries, cement plants, and other industrial plants.

According to the ozone and PM2.5 Implementation Rules, the AERR governs the emissions level above 

which sources are inventoried as point sources. While the AERR also requires data from state agencies 

for use in the NEI, the AERR emissions point source thresholds have been coordinated with the various 

SIP implementation rules and definitions of major sources and, therefore, they apply to all inventories 

provided to the EPA. The AERR bases the emissions level on a potential to emit emissions size threshold. 

Table 7 provides the potential to emit point source reporting thresholds included in the AERR. For 

evaluating the potential to emit threshold, it is appropriate to use potential controlled emissions for 

controlled sources. As shown in Table 7, special requirements apply to NAA sources, where even lower 

thresholds may apply depending upon the severity of the NAA classification.

Air agencies may include point sources at even lower levels in their NEI or SIP planning and modeling

inventories. The AERR thresholds simply provide a minimum requirement. Depending on the nature of 

the nonattainment problem and the need to characterize smaller sources as point sources, a state may 

want to consider and the EPA Regional offices may encourage lower thresholds so air agencies can 

produce effective SIPs.

Type A source means a large point source with potential annual emissions of any CAP or CAP precursor 

greater than or equal to the corresponding emission threshold listed in the AERR. These sources must be 

reported by states every year and have higher reporting thresholds. The thresholds are not repeated in 

the table below because they are not relevant for the definition of point sources for SIP inventory 

purposes, which follow the Type B thresholds for triennial reporting as shown below. See Appendix A of 

40 CFR part 51, Subpart A for the Type A source thresholds.

Type B source means a point source smaller than a Type A source with potential annual emissions of any 

CAP or CAP precursor greater than or equal to the relevant Type B emission threshold listed in the AERR 

and in this section. Type B point sources are required by the AERR to be included in a state or local 

inventory submission for point sources in the triennial years and are applicable to SIP planning and 

modeling inventories as well.
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Table 7: Point source triennial reporting thresholds (potential to emit) for criteria 

pollutants in the Air Emissions Reporting Rule

Pollutant

Triennial NEI thresholds: potential to emit (tons/yr)

Type B sources (default) NAA sources

1 SO2................. ≥ 100 ≥ 100

2 SO2................. PM2.5 (Serious) ≥ 70

3 VOC or NOx.... ≥ 100 ≥ 100

4 VOC or NOx.... ................................. O3 (Moderate within OTR) ≥ 50+

5 VOC or NOx.... ................................. O3 (Serious) ≥ 50

6 VOC or NOx.... ................................. O3 (Severe) ≥ 25

7 VOC or NOx.... ................................. O3 (Extreme) ≥ 10

8 VOC or NOx.... ................................. PM2.5 (Serious) ≥ 70

9 VOC................ ................................. O3 (within OTR) ≥ 50+

10 CO .................. ≥ 1000 ≥ 1000+

11 CO .................. ................................. CO (all areas) ≥ 100

12 Pb................... ≥ 0.5* ≥ 0.5

13 PM10 ............... ≥ 100 ≥ 100

14 PM10 ............... ................................. PM10 (Serious) ≥ 70

15 PM2.5 .............. ≥ 100 ≥ 100

16 PM2.5 .............. ................................. PM2.5 (Serious) ≥ 70

17 NH3................. ≥ 100 ≥ 100

18 NH3................. ................................. PM2.5 (Serious) ≥ 70

+ This threshold for ozone nonattainment areas is incorrectly listed in the AERR as 100 tpy. 

This is not consistent with other regulations. The EPA is working to make this correction in 

the AERR.

* The lead threshold is in actual emissions rather than potential to emit to be consistent with 

the lead monitoring rule requirements.

2.5.9 Nonpoint Sources

Nonpoint sources are also called “area sources”.

12 These sources collectively represent individual 

sources of emissions that have not been inventoried as specific point or mobile sources. These individual 

sources treated collectively as nonpoint sources are typically too small, numerous, or difficult to 

inventory using the methods for the other classes of sources. Examples of key nonpoint sources include 

residential combustion, institutional and commercial boilers, agricultural burning, industrial and nonindustrial solvents and graphic arts, degreasing, most agricultural sources, consumer products, 

architectural coatings, and sources of dust including roads and construction.

2.5.10 On-road Mobile Sources

On-road mobile sources are also called “highway mobile sources.” These sources are the motor vehicles 

(e.g., automobiles, buses, trucks) traveling on local and highway roads. A motor vehicle is any selfpropelled vehicle used to carry people or property on a street or highway (40 CFR 51.50). On-road 

mobile sources should be estimated by the latest recommended on-road mobile source models. 

 

12 This data category type is different from the air toxics definitional of “area sources” as defined by CAA section 

112(a)(2).
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Currently, that means the MOVES model (http://www3.epa.gov/otaq/models/moves/) for all states but 

California, and the EMFAC13 model (http://www.arb.ca.gov/msei/modeling.htm) for California.

2.5.11 Nonroad Mobile Sources

Nonroad mobile sources are also called “off-highway” mobile sources. These are defined as a nonroad 

engine or nonroad vehicle. As per 40 CFR 51.50, a nonroad engine is an internal combustion engine 

(including the fuel system) that is not used in a [on-road] motor vehicle or a vehicle used solely for 

competition, or that is not affected by sections 111 or 202 of the CAA. Also defined by 40 CFR 51.50, a

nonroad vehicle (rather than engine) is a vehicle that is run by a nonroad engine and that is not a [onroad] motor vehicle or a vehicle used solely for competition. Examples of nonroad mobile sources 

include aircraft, airport ground support equipment, locomotives, CMVs, construction equipment

powered by an internal combustion engine, agricultural equipment powered by an internal combustion 

engine, and lawn and garden engines and equipment. 

There are a variety of tools for estimating emissions from nonroad mobile sources. The EPA develops 

and maintains approaches for aircraft, locomotives, and CMVs, which are documented as part of the NEI 

program (for the latest approaches, refer to the latest NEI documentation available at 

http://www.epa.gov/ttn/chief/eiinformation.html). 

Starting with the MOVES2014a version of MOVES, the MOVES model should be used for nonroad source 

emissions. MOVES2014a includes features not previously available in MOVES that simplify processing of 

emissions output and includes updated fuel input files that result in small changes in emission results. In 

addition, the prior tools available for nonroad emissions estimations (NONROAD2008 and NMIM2008)

may no longer work with current operating systems, and the EPA cannot continue to provide technical 

support for these models. Therefore, the EPA recommends that for modeling nonroad emissions, 

MOVES2014a be used for all new SIP development, although state and local agencies that have already 

completed significant work with MOVES2014, NONROAD2008, or NMIM2008 can continue to do so to 

allow for timely submission of the SIP. As a general matter, air agencies should review and possibly 

update and customize the MOVES model inputs to give better emissions estimates for their state. 

Unlike models for onroad emissions, the EPA does not specifically “approve” nonroad models. However, 

use of alternative models can be justified as part of a SIP submission. For example, California has 

previously developed and used a state-specific approach for estimating its nonroad source emissions for 

SIP purposes, called OFFROAD2007 

(http://www.arb.ca.gov/msei/offroad/downloads/models/offroad2007_1215_exe.zip). This model has

now been replaced by category-specific methods for many categories (see 

http://www.arb.ca.gov/msei/categories.htm#offroad_motor_vehicles). As described by the website, the 

OFFROAD2007 model is still the default approach for categories not listed with newer methods on the 

website. Any use of such an alternative approach is subject to review by the EPA as part of the review of 

 

13 The EMFAC model name is not an acronym. The documentation and website for EMFAC do not include a further 

explanation of the name.
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the SIP, and thus states are encouraged to coordinate with Regional offices on any use of alternative 

models. 

2.5.12 Events

Starting with the 2008 NEI, the EIS introduced a new data category called “Events,” which are discrete 

and large emissions sources that individually occur over a short period of time, but which can 

collectively result in a significant amount of emissions. Example events captured by this data category 

include naturally occurring wildfire and a managed prescribed burns. In EIS, these data are stored as 

day-specific events that can span many days for a given fire/event. More information on events data 

and its unique nature is available in Section 4.8.7.

2.5.13 Biogenic Sources

For the purposes of creating emissions inventories, biogenic emissions come from natural sources. They 

need to be accounted for in photochemical grid models, as many types of biogenic emissions are 

widespread and ubiquitous contributors to atmospheric chemistry leading to ozone and PM formation. 

In the NEI, only the emissions from vegetation and soils are included, but other relevant sources include 

volcanic emissions, lightning, and sea salt spray.

Biogenic emissions from vegetation and soils are computed using models that utilize spatial information 

on vegetation and land use. The models also use environmental conditions of temperature and solar 

radiation to apply emission factors. The model inputs are typically horizontally allocated (gridded), and 

the outputs are gridded biogenic emissions, which can then be summed for use in planning inventories 

and speciated and utilized as input to photochemical grid models. As described in Section 4.8.6, a variety 

of models are available to estimate the emissions from vegetation and soils.

2.5.14 Inventory Codes

The list provided below is a sample of the codes that can be used in building inventories; it is not a 

comprehensive list of codes that should be used in building inventories. The EIS is the repository for the 

most updated valid codes used for building inventories. Dozens of tables of codes are available to EIS 

users from the “Reporting Code Tables” link on the EIS Gateway. For others who do not have access to 

the EIS (e.g., contractors not working for a state, international users, and the general public), the EPA

periodically posts Microsoft ® Access ® compilations of these codes at the Emissions Inventory System 

Gateway under the “EIS Code Tables (including SCCs)” link.

Source classification codes: An SCC means a process-level code that describes the equipment and/or 

operation [or practice] that is emitting pollutants (40 CFR 51.50).

14 These codes are 8 digits for point 

sources and 10 digits for nonpoint and mobile sources. The codes have multi-level descriptions, which 

are included in the EIS codes table. These codes are also used in the emission factors online database 

WebFire (https://www.epa.gov/electronic-reporting-air-emissions/webfire). The EPA also provides a 

web service with the most current list of codes, available at https://ofmpub.epa.gov/sccwebservices/.

 

14 Several air agencies have raised the concern that SCC at industrial facilities is considered CBI. However, this field is “emission 

data” required by the AERR and thus cannot be provided as CBI under CAA section 114(c).
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North American Industry Classification System (NAICS) codes: The NAICS codes are U.S. Department of 

Commerce's codes for businesses by products or services and have replaced Standard Industrial 

Classification codes. They are assigned to facilities in inventories to identify the primary industrial 

activity of the facility.

Federal Information Processing Standards (FIPS) state and county codes: Federal Information Processing

Standards (FIPS) codes means the system of unique identifiers maintained by the National Institute of 

Standards and Technology, which are used to identify counties and county equivalents (e.g., parishes, 

boroughs) of the U.S. and its territories (e.g., American Samoa, Guam, Northern Mariana Islands, Puerto 

Rico, and US Virgin Islands). In the EIS Codes Tables, additional codes have been added to support 

offshore regions and sources, such as asphalt plants, that do not remain in a single county during the 

entire year.

Tribal codes: A three-digit code that represents the American Indian Tribe or Alaskan Native entity. The 

EIS Table also identifies which EPA region is associated with the tribe. These codes are used instead of 

FIPS state and county codes when reporting emissions from tribal areas.

Pollutant codes: Pollutant code means a unique code for each reported pollutant assigned by the 

reporting format specified by the EPA for each inventory year (40 CFR 51.50). All of the criteria 

pollutants, except for lead, have codes that describe them: CO, NOX, VOC, SO2, NH3, PM10-PRI, PM10-

FIL, PM25-PRI, PM25-FIL, and PM-CON. Lead and the hazardous air pollutants (HAPs) use numerical 

codes15 to represent them, for example elemental lead is 7439921 and elemental mercury is 7439976.

Control measure codes: Control measure means the name of the type of control device (e.g., wet 

scrubber, flare, baghouse) or practice (e.g., product substitution). The EIS Code Tables use a three-digit

numeric control measure code to represent each of the different types of controls, and identifies each 

as a device or a practice. The control device type codes were updated with additional codes in 2013. 

These codes are required when control devices are present because they are critical to understanding 

the nature of the emissions source and the possibility of additional emissions reductions to achieve 

attainment.

Emission calculation method: This field is the code describing how the emissions for a pollutant were 

calculated (e.g., by stack test, continuous emissions monitor, EPA emission factor, etc.) (40 CFR 51.50). 

This is a required field for inventories because it allows the EPA to get some sense of the quality of the 

emissions estimate. Comment fields can also be used to further explain the origin of the emissions 

estimate.

Reporting period type: This field is the code describing the time period covered by the emissions 

reported (i.e., annual, 5-month ozone season (previously used for NOx SIP call), ozone season day, or

average season day) (40 CFR 51.50). In concert with the release of the final PM SIP Requirements Rule, a 

 

15 These numerical codes are the unique numerical identifiers assigned and maintained by the Chemical Abstracts 

Service (CAS) and are often referred to as CAS numbers or CAS registry numbers. The CAS numbers often appear 

with dashes embedded in the string of numbers, but these are dropped for most inventory reporting purposes.
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new code (“ASD”) has been updated in EIS to support submissions to EIS of average season day 

emissions used for SIPs addressing the 24-hour PM2.5 standard. This field is also called the emission type

code in some references.

2.5.15 Point Source Facility Details

See also Section 2.5.7, which points to the spreadsheets available that list all required and optional 

emissions inventory data elements.

Facility Inventory means the data tables of all facility information included in the EIS, but could also be 

used to refer to similar data tables for any inventory storage system. The facility inventory is the 

complete list of facilities, units, processes, and stacks associated with all point sources. It is designed to 

be the latest repository of facility information and needs updating only when changes occur to facilities. 

The facility inventory concept was intended to allow facility data to be constant except where real 

changes occur, allowing easier building and submitting of inventories and encouraging the use of 

consistent facility IDs, unit ID, process ID, and stack IDs from one inventory year to the next.

Physical address means the location address (street address or other physical location description), 

locality name, state, and a postal zip code of the facility. This is the physical location where the 

emissions occur, not the corporate headquarters or a mailing address (40 CFR 51.50).

Site name (also called Facility Site Name) means the name of the facility emitting emissions that are 

included in the inventory (40 CFR 51.50). 

EIS ID means any facility, unit, process, or release point (stack) ID generated by the EIS and intended to 

remain constant for the life of that facility, unit, process, or stack across inventory years. The proper 

names of the EIS IDs in the EIS system are EIS Facility Site Identifier, EIS Emission Unit Identifier, EIS 

Emission Process Identifier, and EIS Release Point Identifier.

Agency ID means any facility, unit, process, or release point (stack) ID generated by an agency reporting 

emissions to the EPA. These IDs are used in the air agency data systems and may not match the EIS IDs 

described above. The EPA tracks both the EIS IDs and agency IDs within the EIS. Using consistent agency 

IDs from year to year is highly encouraged, as it allows the EPA to more readily see which facilities are 

being reported for the AERR requirements. Using inconsistent agency IDs can result in duplicative 

facilities, units, processes, and release points within the EPA data systems.

Facility ID code (also called “Facility site identifier)” means the unique codes for a plant or facility 

treated as a point source, containing one or more pollutant-emitting units (40 CFR 51.50). The EPA's 

reporting format for a given inventory year may require several facility ID codes to ensure proper 

matching between databases (e.g., the state's own current and most recent facility ID codes, the EPAassigned facility ID codes, and the ORIS (Department of Energy) ID code if applicable).

Unit ID code means a unique code for the unit that generates emissions, typically a physical piece of 

equipment or a closely related set of such equipment (40 CFR 51.50). An example of a closely related set 

of equipment is a boiler and its control equipment. The EPA's reporting format for a given inventory year 
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may require multiple unit ID codes to ensure proper matching between databases (e.g., the state's own 

current and most recent unit ID codes, the EPA-assigned unit ID codes if any, and the ORIS (Department 

of Energy) ID code if applicable).

Process ID code is also called the “emission process identifier” and it means a unique code for the 

process generating the emissions, typically a description of a process (40 CFR 51.50).

Release Point ID code means a unique code for the point where emissions from one or more processes 

release into the atmosphere (40 CFR 51.50). Traditionally called a Stack ID, but this was changed with EIS 

to acknowledge that many emissions releases do not come from stacks, such as fugitive emissions.

Release point type code means the code for physical configuration of the release point (40 CFR 51.50). 

The physical configurations include fugitive, vertical, horizontal, goose neck, vertical with rain cap, and 

downward-facing vent. The first two types are the most common. The release point type is particularly 

important for dispersion modeling needed for permitting, risk modeling, and other purposes.

Release Point Diameter Measure (or Stack diameter) means the inner physical diameter of a stack (40 

CFR 51.50). This should be measured at the exit release point of the emissions, and the expected 

reported units are feet.

Release Point Height Measure (or Stack height) means physical height of a stack above the surrounding 

terrain (40 CFR 51.50). Expected reported units are feet.

Release Point Exit Gas Flow Rate Measure (or Exit gas flow rate) means the numeric value of the flow 

rate of a stack gas (40 CFR 51.50). Various units are permitted by the EIS.

Release Point Exit Gas Temperature Measure (or Exit gas temperature) means the numeric value of the 

temperature of an exit gas stream in degrees Fahrenheit (40 CFR 51.50). 

Release Point Exit Gas Velocity Measure (or Exit gas velocity) means the numeric value of the velocity of 

an exit gas stream (40 CFR 51.50). Various units are permitted by the EIS.

2.5.16 Alternative IDs

In addition to the primary facility, unit, process, and release point IDs, alternative IDs can be very useful 

for matching information in a given inventory to other databases. In the case of the EIS, it stores not 

only the EIS unique IDs, but also the air agency IDs. Other IDs are stored as well, including the following:

EIA Plant Code (a.k.a ORIS Plant Code). This plant ID is defined by the U.S. Department of Energy (DOE) 

Energy Information Administration (EIA) and used in its EIA-767 form. It has previously been defined as 

the Office of Regulatory Information System (ORIS) code, though that term is no long used by DOE. The 

EPA’s Office of Atmospheric Programs’ (OAP) Clean Air Markets Division (CAMD) uses whatever code 

was originally assigned to the facility as the tracking code for compiling data for its emissions markets in 

support of the acid rain program and NOX trading, in particular the CEMS data. 
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CAMD Unit ID. In addition to the EIA facility IDs, CAMD uses unit IDs for storing the emissions markets 

data as well. The EIS stores the unit codes used by CAMD as an alternative ID called “EPACAMD,” and 

the values are usually based on the CAMD hourly CEMS data reports. These have historically also been 

referred to as ORIS Boiler or ORIS Unit IDs, however, those references are no longer valid.

IPM/NEEDS ID. The EPA Integrated Planning Model (IPM) is used to forecast future-year electricity 

demand and emissions. Underlying this model is the National Electric Energy Data System (NEEDS)

database that contains the generation unit records used to construct the “model” plants that represent 

existing and planned/committed units in EPA modeling applications of IPM. This database mostly uses 

IDs that are often the same as the EIA Plant Codes and the CAMD Unit IDs, but this database has more 

units than are included in the hourly CEMS data. In the EIS, the EPA stores these alternative IDs as 

“EPAIPM” IDs.

Toxics Release Inventory (TRI) ID is used by the EPA’s Office of Environmental Information to build the 

Toxics Release Inventory from facility-submitted facility-total emissions to air, water, and land. This 

database includes NH3, which is why it can be relevant to the NAAQS for PM2.5 implementation. In the 

EIS, the EPA stores these alternative IDs as “EPATRI” IDs.

Federal Registry Service (FRS) ID is used by the EPA’s Office of Environmental Information to link many 

EPA databases with facility information (see https://www.epa.gov/enviro/facility-registry-service-frs). 

2.5.17 Location Definitions

Geographic locations are critical in point source inventories for identifying the location of the emissions. 

In the EIS, locations can be provided for both the facility center and for individual release points. Fugitive 

release points have additional parameters that can be specified, such as the orientation of the release, 

and these parameters are listed in the spreadsheets available that list all required and optional 

emissions inventory data elements, referenced in Section 2.5.7.

Latitude Measure (or just latitude) is the measure of the angular distance on a meridian north or south 

of the equator. Reported as decimal degrees.

Longitude Measure (or just longitude) is the measure of the angular distance on a meridian east or west 

of the prime meridian. Reported as decimal degrees.

Horizontal Accuracy Measure is the measure of accuracy (in meters) of the latitude/longitude 

coordinates. 

In addition, refer to https://www.epa.gov/geospatial/latitudelongitude-data-standard for more 

information on the proper usage of the following code set.

Horizontal Collection Method Code is the method used to determine the latitude/longitude coordinates 

for a point on the earth. For example, 001 is address matching-house number, and 004 is address

matching nearest intersection.
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Horizontal Reference Datum Code represents the reference datum used to determine the 

latitude/longitude coordinates. Options are: 001 (North American Datum of 1927), 002 (North American 

Datum of 1983), and 003 (World Geodetic System of 1984).

Geographic Reference Point Code represents the place for which geographic coordinates were 

established. Code value should be 106 (e.g., point where substance is released). 

2.5.18 Control Data Elements

Control efficiency means the efficiency by which a control device or measure reduces emissions for a 

particular pollutant. For emissions reporting in accordance with the AERR, this data element applies for 

both point and nonpoint sources and is called the “Percent Control Measures Reduction Efficiency.” For 

point sources, the data element is included in the “Control Pollutant Table” of the facility staging tables.

For nonpoint sources, the data element is included in the “Control Pollutant Table” of the nonpoint 

staging tables. In the EIS, it is a percent and should be reported as a value greater than or equal to 0.1 

and less than or equal to 99.9 (the EIS allows for one decimal place only).

Control Capture efficiency means the percentage of an exhaust gas stream actually collected for routing 

to a set of control devices (40 CFR 51.50). It is sometimes combined within the value for the control 

efficiency, but unlike control efficiency, it is not pollutant-specific. For emissions reporting in accordance 

with the AERR, this data element is for point sources only, and it is called “Percent Control Capture

Efficiency” and is included in the “Control Approach Table” of the facility staging tables. It is a percent 

and should be reported as a value greater than or equal to 0.1 and less than or equal to 99.9 (the EIS

allows for one decimal place only).

Control Effectiveness means the percentage of time or activity throughput that a control approach is 

operating as designed, including the capture and reduction devices. This percentage accounts for the 

fact that controls typically are not 100 percent effective because of equipment downtime, upsets and 

decreases in control efficiencies (40 CFR 51.50). 

Control effectiveness is an estimate of the portion of the reporting period's activity for which the overall 

control system or measure (including both capture and control measures) were operating as designed 

(regardless of whether the control measure is due to rule or voluntary). It applies to both point and 

nonpoint sources and is not pollutant-specific. For point source emissions reporting in accordance with 

the AERR, this data element is called “Percent Control Approach Effectiveness” and is included in the 

“Control Approach Table” of the facility staging tables. For nonpoint source emissions reporting in 

accordance with the AERR, this data element is also called “Percent Control Approach Effectiveness” and 

is included in the “Control Approach Table” of the nonpoint staging tables. In the EIS, it is a percent and 

should be reported as a value greater than or equal to 0.1 and less than or equal to 99.9 (the EIS allows 

for one decimal place only). For nonpoint, see also “rule effectiveness,” below.

Rule penetration means the percentage of a nonpoint source category activity that is covered by the 

reported control measures (40 CFR 51.50). For emissions reporting in accordance with the AERR, this 

data element is called “Percent Control Approach Penetration” and is included in the “Control Approach 
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Table” of the nonpoint staging tables. In the EIS, it is a percent and should be reported as a value greater 

than or equal to 0.1 and less than or equal to 99.9 (the EIS allows for one decimal place only).

Rule effectiveness means a rating of how well a regulatory program achieves all possible emissions 

reductions. It is analogous to the “control effectiveness” above and, for emissions reporting in 

accordance with the AERR, this information is reported using the EIS Control Effectiveness field for 

nonpoint (see above). Like control effectiveness, it should reflect the assumption that controls typically 

are not 100 percent effective because of equipment downtime, upsets, decreases in control efficiencies, 

noncompliance, and other deficiencies in emission control. Rule effectiveness adjusts the control 

efficiency from what could be realized under ideal conditions to what is actually emitted in practice due 

to less than ideal conditions. 

2.5.19 EIS Sectors, Tiers, and Facility Type

Because emissions inventories are large and detailed, most reports about emissions inventories include 

summaries of the contents of those emissions. Some summaries use the data categories described 

above (point, nonpoint, on-road, nonroad, events, and biogenic). For other summaries, that level of 

detail is not enough. The EIS SCC tables provide three other mechanisms for summaries, all of which 

depend on SCC.

First, the SCCs have a four-part description, and the first three levels of this description could be used as 

a summary category. Second, the SCCs are assigned to “Tiers” using the fields “Tier 1 Description,” “Tier 

2 Description,” and “Tier 3 Description.” These Tiers have been used traditionally as part of many EPA 

reports and trends for criteria emissions. While wildfires are included in the “Miscellaneous” Tier 1, 

wildfires are sometimes separately broken out from Miscellaneous because of their natural origin and 

because they can be a large contributor for some pollutants. Finally, the EIS Sectors can be used to 

summarize emissions (this is the field “Sector” in the EIS SCC code table). The EIS Sector was added with 

the creation of the EIS and allocates emissions to about 60 categories with multi-pollutant inventories in 

mind. Whereas the Tiers are more useful for criteria pollutants, the EIS Sectors work better for the 

combination of HAPs and CAPs. The EIS Sectors can also be further aggregated to provide custom 

summaries; one example of this is presented in the report “2008 National Emissions Inventory: Review, 

Analysis and Highlights” (EPA-454/R-13-005, May 2013).

2.5.20 Other Terms

High electricity demand day (HEDD) means an ozone season day when demand for electricity is high. A 

study of the Northeast Corridor16 air quality has indicated that NOX emissions from EGUs during HEDD 

periods may be significantly greater than emissions that occur on an average ozone season day.

Distributed generation means the use of small power generators sited at or near the point of energy 

consumption. This usually occurs during periods of peak energy demand or emergency loss of the 

normal EGU generating capacity. Distributed generation sources of electricity can include rooftop solar 

 

16 “High Electric Demand Day and Air Quality in the Northeast.” White Paper Prepared by the Northeast States for 

Coordinated Air Use Management. June 5, 2006. Available at: http://www.nescaum.org/documents/highelectric-demand-day-and-air-quality-in-the-northeast/final-white-paper-hi-electric-demand-day-06052006.pdf.
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or wind sources, biomass combustion, as well as backup fossil fueled generators or other engines. The 

combustion sources of distributed generation can cause emissions that could be of interest for SIP 

planning purposes.

RFP projected emissions means the estimated emissions for direct PM2.5 and PM2.5 plan precursors by 

sector for the years in which quantitative milestones are due for a nonattainment area.

3 SIP Inventory Requirements and Recommendations

As described in Section 2, there are many required and potentially required inventory efforts needed as 

part of a SIP development effort. This section covers the fundamental attributes of planning inventories 

and their preparation. The topics covered include the timing of the various inventory components

(Section 3.1), the inventory preparation plan (Section 3.2), the base year inventories (Section 3.3), the 

ROP/RFP baseline NAA inventory (Section 3.4), the ozone periodic inventory (Section 3.5), the ozone 

emissions statement requirement for SIPs (Section 3.6), maintenance inventories (Section 3.7), the 

projected inventories (Section 3.8), public hearings (Section 3.9), and submitting the inventory to the 

EPA for approval (Section 3.10). The details of inventory preparation for base year inventories and 

projected inventories are left for Sections 4 and 5, respectively.

3.1 Inventory Component Timing

Table 8 provides the timing requirements and recommendations (for optional items) for the inventory 

components of SIPs. Where this table is inconsistent with the ozone, PM2.5, or regional haze regulatory 

text that is applicable at the time a SIP is prepared and reviewed, the regulations have precedence over 

the information provided here.

Table 8: Timing of emissions inventory components

Inventory Component Required: Timing

Inventory Preparation Plan Prior to preparation of emissions inventories covered by 

the SIP.

Base year NAA inventory 

Ozone Within 2 years of the effective date of designations

PM2.5

Within 18 months from the date of a nonattainment 

designation.

ROP/RFP baseline NAA

inventory

Ozone

Moderate & above: Within 3 years of the effective date 

of designations.

Serious & above: Additional plan within 4 years.

PM2.5

Within 18 months from the date of a nonattainment 

designation.

Periodic NAA inventory Ozone The end of each 3-year period after SIP submittal until 

the NAA is redesignated to attainment.

Source Emission Statements 

requirement for SIPs Ozone Within 2 years of the effective date of designations.

Base case statewide inventory Ozone and 

PM2.5

+

With modeled attainment demonstration (if required, 

for convenience, can be included with the inventory SIP).
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Inventory Component Required: Timing

Attainment projected 

inventory for the NAA

Ozone: 

summary 

only

For Ozone, With SIP and/or ROP/RFP plan. On-road 

portion can become the motor vehicle emissions 

budget.

PM2.5

Within 18 months from the date of a nonattainment 

designation.

Projected statewide inventory

(summaries)

Ozone and 

PM2.5

+

With modeled attainment demonstration (for 

convenience, can be included with the inventory SIP).

Baseline statewide inventory

(summaries) Haze

Previously completed for initial Regional Haze SIPs.

Subsequent progress reports and SIP revisions may refer 

to this inventory.

Recent statewide inventory 

(summaries) Haze

Needed for the progress reports that are due every 5 

years (or periodically) from submittal of initial Regional 

Haze SIP.∆ Also needed to support Regional Haze SIPs for 

second and subsequent implementation periods, due 

every 10 years beginning on July 31, 2018 α

Projected statewide inventory 

for 2018 (summaries) Haze

Previously completed to support first Regional Haze SIP 

(due by December 17, 2007)

Projected statewide inventory 

for 2028, 2038, etc. 

(summaries)

Haze

To support Regional Haze SIPs for second and 

subsequent implementation periods, due every 10 years 

beginning on July 31, 2018 α.

Public hearing Ozone and 

PM2.5

Prior to submittal of inventory for approval by the EPA.

Inventory documentation All With associated SIP.

Maintenance NAA inventory Ozone and 

PM2.5

After attainment, with a redesignation SIP.

+ Only required in association with modeled attainment demonstration. If the inventory needed for the modeled attainment 

demonstration is not the entire state, then a “statewide” inventory would not need to be provided, but rather the emissions 

from the part of the state included in the modeling domain could be provided instead.

∆ On May 4, 2016, the EPA proposed to revise the timing of and requirements for the 5-year progress report requirement so 

that it is not considered a SIP revision, while the 10-year requirement would remain a SIP revision (see 

https://www.epa.gov/visibility/proposed-rulemaking-amendments-regulatory-requirements-state-regional-haze-plans).

α

In the same proposed revision noted in the previous footnote, the EPA proposed to revise the requirements for submitting 

SIPs that include an extension of the due date to July 31, 2021. Subsequent interim progress report submission deadlines 

have been proposed to be adjusted so that second and subsequent progress reports would be due by January 31, 2025, July 

31, 2033, and every 10 years thereafter.

3.2 Inventory Preparation Plan

Air agencies can use IPPs as a planning tool to guide inventory preparation and to help ensure that 

emission estimates are of a high quality and consistent with CAA requirements. In accordance with 40 

CFR 51.5(b), states are urged to use applicable, state-of-the art techniques for estimating emissions. In 

some cases, the EPA Regional offices will have a preset list of expectations for air agencies to include in 

their SIP and will provide this information to the agency at the start of the SIP planning process. In these 

cases, an IPP may not be needed, especially for air agencies with prior SIP development experience for a 

particular NAAQS.
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The IPPs are an opportunity for state and local agencies to tell their EPA Regional office how they plan to 

compile their emissions inventories and allow the EPA to provide feedback prior to the agency spending 

resources in preparing their inventory. They can also allow the EPA Regional offices to coordinate to 

maintain consistency across attainment plans where that is advisable. The IPPs can be especially 

valuable for agencies with new nonattainment problems, less experienced inventory staff or for 

agencies otherwise needing additional feedback on inventory development.

If used, IPPs precede the development of inventories; therefore, the timing of IPPs is early in the SIP 

planning process. The EPA recommends that air planning agencies coordinate with their Regional office

on the need for a plan, the timing of delivery of the plan, the response time back from the EPA on the 

plan, and any other IPP approval process details. Agencies should recognize that time may also be 

needed in their planning schedule to address the EPA comments on the plan, while the EPA Regional 

offices will attempt to provide as timely feedback as possible. 

A high-quality IPP could be a strong foundation for the emissions inventories used in planning and/or 

attainment modeling, which are themselves critical pieces of SIP development; therefore, the IPP should 

include enough details for the EPA to properly assess what the agency plans to do. The content can 

include a number of key elements, for which a checklist is provided in Table 9. The actual elements to 

include in a specific IPP can depend on a number of factors, depending on the needs of the state or local 

agency as well as the EPA for understanding the inventory plans.

Table 9: Checklist of elements to consider for including in Inventory Preparation Plans

IPP Element

Important for 

a High Quality 

IPP?

Inventory objectives – the purpose and scope of the inventory Y

The inventory year(s) and rationale for selecting them Y

The pollutants to be included, including the expected definition of VOC (for 

inventories needing VOC) Y

For PM2.5, the temporal basis expected to be used for the SIP emissions, which can 

include annual, average season day, or both (see Section 4.5)

Y

A list of inventories to be included with the SIP, including the types (e.g., reports, 

data summaries) and formats of information (i.e., electronic data files and 

their expected formats, electronic media used for transfer), that the agency 

intends to include in their SIP

Y

Plans for (or results of) identifying the most important source sectors to the 

nonattainment problem Y

The source categories to be included, the emissions level(s) for defining point 

sources, and if available, the expected priority for these Y

The plans for compiling the inventory, including any other known inventories and 

how the agency plans to assess and improve upon what is already available

A summary of expected approach differences among different inventories that could 

be the same (e.g., between a base year NAA inventory and a base year 

modeling inventory)
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IPP Element

Important for 

a High Quality 

IPP?

For projected inventories:

The drivers for future-year emissions growth and reductions Y

+

The source categories to be forecast Y

+

The expected approaches for forecasting each source category Y

+

A plan for public review

Important sources of emissions or other inventory uncertainties

Quality assurance steps to be taken Y

A schedule for inventory preparation, review, and submission to the EPA, including 

any plans to submit pieces of the inventory separately and in accordance with 

regulatory timeline requirements

Y

+

If a projected inventory is required.

In addition to Table 9, other inventory planning resources may be of use for agency planning purposes. 

During the preparation of their IPPs, state and local agencies are referred to Volume I of the EIIP 

guidance, available at http://www.epa.gov/ttn/chief/eiip/techreport/volume01, which discusses 

emissions inventory planning and development. While somewhat out of date, the EIIP volumes still can 

help guide new inventory developers, especially with basic information about creating emissions. Users 

of that document should defer to this guidance where newer information is provided, such as updated 

versions of mobile models. For stationary point sources, a good resource is EIIP Volume II, Chapter 1, 

Section 3. For nonpoint sources, EIIP Volume III, Chapter 1, Section 2 gives nonpoint-specific planning 

information. The mobile source volumes for EIIP are more out of date and the EPA Office of 

Transportation and Air Quality (OTAQ) mobile source guidance is preferable (see Table 1) .

A quality assurance plan should always be a part of the IPP. In cases where a more formal QAPP is 

developed, the QAPP will satisfy the need for an IPP quality assurance plan. In some cases, the EPA 

Regional offices will include a QAPP or other quality assurance requirement as part of their CAA section 

105 grant to a state that typically includes funding for emissions inventory development. Each Regional 

office manages its own quality assurance plan. Information about EPA quality assurance specifications 

and tools as well as a map that links to each Regional office’s quality assurance project are available at 

https://www.epa.gov/quality. 

For additional information on quality assurance planning, Chapter 2 of Volume VI, “Quality Assurance 

Procedures and DARS software” (https://www.epa.gov/air-emissions-inventories/volume-6-qualityassurance-procedures-and-dars-software) provides some general guidance for quality assurance and 

some specific approaches to uncertainty evaluation. Other quality assurance resources include Section 0

of this guidance, and quality assurance steps described in the latest NEI Technical Support 

Documentation available at the latest NEI documentation page link on the main NEI web page. Where 

air agencies are relying on their NEI submission as a starting point for meeting any SIP inventory 

requirements, a QAPP developed for that air agency’s NEI submission may offer a starting point for the 

SIP inventory QAPP.
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3.3 Base Year Inventory

The purpose of the base year inventory is to provide a current and comprehensive data source for 

emissions contributing to the air quality problem addressed by the SIP. In the case of ozone and PM2.5, 

the inventory requirement is for the NAA, whereas for regional haze, the inventory requirement is for a 

statewide inventory. As listed in Table 4 and Table 5, the ozone or PM2.5 SIPs can include statewide 

inventories as part of the modeled attainment demonstration, and these can be included as separate 

inventory submissions included with the NAA inventory.

The general SIP provisions in CAA section 172(c)(3) explains that base year inventories should include a 

“comprehensive, accurate, current inventory of actual emissions from all sources” in the applicable area. 

Details associated with preparing a base year inventory are provided in Section 4, including selection of 

year(s), temporal extent, spatial extent, pollutants to include, approaches for developing emissions, data 

codes, and quality assurance. Additionally, CAA section 182(a)(1) provides a specific base year inventory 

requirement for ozone SIPs. PM2.5 SIPs generally rely on the general SIP provisions in CAA 172(c)(3), 

though some PM-specific elements of CAA section 189 impact the timing and other aspects of PM2.5 SIP 

inventories. Other requirements for SIP inventories are found in the SIP implementation rules as listed in 

Table 4.

The base year inventory is the starting point from which the other SIP-related inventories are derived. 

These other inventories should relate to the base year inventory with regard to the emissions sources 

included, pollutants, temporal aspect, and other key aspects that allow the SIP to be a coherent and 

connected plan across the various inventories. For planning purposes, the base year inventory provides 

a way for decision makers to consider the sources of emissions that contribute relevant pollutants and 

consider emissions reductions strategies needed for the SIP.

Section 4.1 covers the choice of inventory years for the base year inventory. The choice of the inventory 

year for the planning is a critical one that significantly impacts the workload of inventory staff and 

greatly depends on availability of data and timing of other inventory efforts required by the EPA.

Section 4.2 covers the pollutants to be included in the base year inventory. These pollutants differ 

depending on the nature of the air quality problem for a given SIP. Where feasible, the EPA encourages 

planning agencies to include precursors for all ongoing nonattainment problems in a state to promote 

efficiency in preparing multiple base year inventories, and to allow for the consideration of emissions 

reduction strategies that improve air quality across multiple air quality concerns and nonattainment 

issues. In addition, states may wish to include emissions of HAPs, which can have co-benefit reductions 

associated with strategies aimed at reducing ozone, PM2.5, and/or regional haze.

The spatial and temporal extent of the base year inventories are also an important inventory 

characteristic. These topics are covered fully in Sections 4.4 and 4.5. Each of the implementation rules 

for ozone, PM2.5, or regional haze has different considerations and requirements associated with the 

planning inventories. For example, ozone NAA inventories must include ozone season day emissions, the 

PM2.5 NAA inventories can be either annual or season day (depending on the nature of the PM2.5

nonattainment problem), and the regional haze inventories are only annual.
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The Ozone Implementation Rule and PM2.5 SIP Requirements Rule require inventories for the NAA. 

These inventories are called the base year inventory for the NAA. These inventories may need to be 

updated through multiple iterations of a SIP when they are used a baseline inventories for assessing 

progress for ROP/RFP. For regional haze, the inventory is commonly called the baseline inventory. For 

clarity here, we also call the regional haze inventory the baseline statewide inventory to distinguish it 

from a NAA inventory. 

3.4 Rate of Progress and Reasonable Further Progress Baseline NAA 

Inventory

RFP is a concept included in the CAA under Part D, Title I to assure that states take steps to ensure that 

the air quality in NAAs makes steady and incremental progress toward attaining air quality standards. 

The ROP/RFP baseline NAA inventory is necessary for ozone NAAs designated as Moderate and above. 

This baseline inventory is part of a plan or plans that meet the RFP requirements as described by the 

CAA and the Ozone Implementation Rule, and summarized here. The PM2.5 SIP Requirements Rule also 

includes requirements for RFP as described below. Though there are specific considerations related to 

ROP and RFP, the anthropogenic portion of the base year inventory for the NAA also serves as the 

base ROP/RFP baseline NAA inventory for both ozone and PM2.5 SIP development.

3.4.1 Ozone ROP/RFP Requirements and Timing

CAA section 182(b)(1)(A) requires Moderate and above NAAs to submit a plan to ensure a 15 percent

VOC reduction (accounting for growth) by 6 years after the NAA designation, which is the Moderate area 

maximum attainment date. The 15 percent reduction requirement is commonly called “rate of progress”

or ROP. According to the Ozone Implementation Rule, air agencies must submit this plan within 3 years 

after the area’s nonattainment designation. This timing aligns with the Moderate NAA deadline for 

submitting attainment demonstration SIPs.

In addition, CAA section 182(c)(2)(B) requires each Serious and above ozone NAA to submit a plan to 

achieve at least a further annual 3 percent VOC reduction. Additionally, CAA section 182(c)(2)(C) allows 

for substitution of NOX for VOC emissions reductions in the plan under certain conditions. This 

requirement is also called “annual RFP.” As described by the Ozone Implementation Rule, the annual 

RFP requirement applies for any remaining 3-year periods out to the attainment date beyond the first 6

years. According to the Ozone Implementation Rule, air agencies with NAAs classified as Serious and 

higher must submit this plan within 4 years after the area’s nonattainment designation. This timing 

aligns with the Serious and higher areas’ deadline for submitting attainment demonstration SIPs; 

however, these NAAs still must submit the plan for ROP within 3 years.

States maintain the flexibility to submit plans early to provide more time for implementation of their SIP 

control measures. The EPA recommends that states complete their ROP/RFP plans as expeditiously as 

practicable after designation to provide as much time as possible for sources to implement the emission 

reductions.

The associated terminology for the various plans and inventories can be confusing, and so the 

terminology used in this guidance is described here. In this guidance, we use “ROP/RFP plan” to refer
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generally to all plans (i.e., those addressing just the 15 percent ROP requirement for Moderate areas as 

well as plans additionally addressing the annual RFP requirement). For Serious and above areas, since 

the timing allows that two separate plans for ROP and annual RFP could be submitted (i.e., at 3 years 

and 4 years after designations, respectively), it may also be necessary to refer to such plans separately. 

Consequently, this guidance uses “ROP/RFP plan(s)” to allow for the idea that multiple plans could be 

submitted. Lastly, in the case where the individual plans must be referenced, they will be called “ROP 

plan” and “RFP plan.”

3.4.2 Ozone ROP/RFP Emissions Guidance

The same ROP/RFP baseline NAA inventory is used for all ROP/RFP plan(s), regardless of whether they 

address the ROP requirement (for Moderate areas) or both ROP and annual RFP (for Serious and above). 

As implied by the name, these inventories include emissions data just for the NAA region and not other 

areas in a state. The purpose of the ROP/RFP baseline NAA inventory is to provide a reference point to 

determine the emissions reductions that must be achieved for the 15 percent VOC (or NOX) reduction 

and the subsequent 3 percent reductions for Serious areas and above out to the attainment date. This 

requirement stems from CAA requirements.

For ozone, VOC and NOX should be included in the ROP/RFP baseline NAA inventory. Further, the EPA 

expects that the year of the ROP/RFP baseline NAA inventory will be the same year used for the base 

year inventory for the NAA.

For ozone, the ROP/RFP baseline NAA inventory emissions should be provided using the same base year 

as is used for base year inventory for the nonattainment area. In most cases, the ROP/RFP baseline 

inventory should also have the same emissions values as are in the base year inventory for the NAA. One 

key exception is that biogenic emissions should not be included in the ROP/RFP baseline. 

The EPA also expects that the spatial extent of the ROP/RFP baseline NAA inventory will also match the 

base year NAA inventory so that they are both either “whole county” inventories or both consistently 

“partial county” inventories. Section 4.4.1 provides more information on partial county inventories.

In the event that updated ROP/RFP plan(s) are needed after initial submittal to meet the requirements 

of the Ozone Implementation Rule, the ROP/RFP baseline NAA inventory may also need to be updated.

This can occur with significant methods changes, such as updated mobile models. For example, after an 

initial ROP/RFP plan has been submitted as part of a SIP, a new mobile model may become available that 

alters the emissions estimates. It would be inconsistent to compare estimates from a new mobile model 

at the end of a 6-year period to estimates using an old model at the beginning of that 6-year period, and 

so the EPA could be unable to determine whether a 15 percent reduction (or subsequent 3 percent 

reductions) can be (or has been) achieved by an ROP/RFP plan. Thus, an updated ROP/RFP baseline NAA 

inventory may need to be created using the updated mobile model to appropriately meet the 

requirements of the Ozone Implementation Rule.

Additional ROP/RFP emissions reductions guidance is provided in Section 3.8.2.
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3.4.3 PM2.5 RFP Requirements and Timing

Section 172 of the CAA addresses the PM2.5 attainment plan provisions in general. In addition, Section 

189(c) of the CAA requires that PM2.5 SIPs contain quantitative milestones, which are to be achieved 

every 3 years until the area is redesignated attainment and which demonstrate reasonable further 

progress. For Moderate areas, these quantitative milestone years would be no later than 4.5 and 7.5 

years after designation of the area. For Serious areas, these quantitative milestone years are no later 

than 7.5 and 10.5 years after designation of the area. Most (if not all) Serious areas will have first been 

Moderate areas, and thus could also be subject to the quantitative milestone at 4.5 years as well.

In the PM2.5 SIP Requirements Rule, one required element of an RFP plan is RFP projected emissions by 

sector and precursor pollutant for each applicable quantitative milestone year. In addition, the 

emissions reductions must show that the implementation schedule (of reductions) achieves either (1) 

generally linear progress toward the projected attainment date; or (2) stepwise progress toward the 

projected attainment date. The rule also holds out the possibility that a state could develop an 

approvable RFP plan even if emissions of one or more PM2.5 plan precursors were not decreasing; 

however, additional analysis would need to be done (as described in the rule).

3.4.4 PM2.5 RFP Emissions Guidance

For PM2.5 SIPs, all plan precursors must be included in the RFP emissions provided as part of the RFP 

plan. Further, the EPA expects that the year of the RFP baseline emissions will be the same year used for

the base year inventory for the NAA.

Unlike other emissions inventory requirements, the PM2.5 RFP requirement does not specify that 

detailed projected emissions inventory data must be developed and submitted to the EPA. The 

emissions requirement for most sectors is for “RFP projected emissions for direct PM2.5 and all PM2.5 plan 

precursors for each applicable milestone year,” which does not indicate a detailed emissions inventory.

17

In addition, the PM2.5 SIP Requirements Rule preamble specifically indicates emissions provided by 

sector and pollutant are expected. It would be sufficient to include a table that has rows for each sector 

and columns for the baseline and projected emissions of all relevant pollutants. 

States may be able to develop their RFP projected emissions in some cases with less effort than it would 

take to do a detailed emissions inventory projection. One approach for the calculating sector/pollutant 

emissions for the milestone years would be to rely on the projected attainment inventory for the NAA 

and use linear interpolation from the baseline year to the milestone year(s). To take this approach, 

states should explain why the linear assumption makes sense for a particular sector. For example, 

residential work combustion emissions are projected to the future in part through phase-in rate 

assumptions for newer and lower emitting woodstoves. Using this sort of “% adoption per year”

approach would support using a linear rate of change assumption such that the whole residential wood 

combustion sector could be interpolated between the baseline year and the projected attainment year.

 

17 For the purposes of motor vehicle emissions budgets for transportation conformity (as required by 40 CFR part 

93), an inventory of onroad mobile source emissions in the NAA is required (see 40 CFR 51.1012(a)(2)).
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3.5 Periodic Inventories

The Ozone Implementation Rule requires a periodic inventory for the NAA, and the Regional Haze Rule 

requires a “recent year” statewide inventory. The ozone periodic inventories should be consistent with 

the base year inventory in the temporal extent (seasonal, annual, or both), spatial extent (statewide, 

NAA counties, partial counties), and sources inventoried. In some cases for ozone, it may also be 

advisable to have the emissions inventory methods of the periodic inventory consistent with the base 

year inventory, but this decision should be made in consultation with the relevant EPA Regional office.

Regional offices may consider additional relaxation of the consistency recommendation above in specific 

instances. For regional haze, the “recent year” inventory must always be statewide. There is no 

expectation that the recent year inventories for regional haze will use consistent methodologies as were 

used for the baseline inventory for a year in the range of 2002-2004 submitted as part of the first 

Regional Haze SIP revision. Rather, the most recent emission estimation methods would be expected to 

be used.

3.5.1 Ozone Periodic Inventory

For ozone, the requirement in 182(a)(3)(A) is that the periodic inventory be submitted no later than

each 3-year period after the submission of the base year NAA inventory. This language provides the 

flexibility to air agencies to submit sooner than 3 years, but not later than 3 years. If any other current 

NAA inventory is being submitted to the EPA for any reason, including ROP/RFP plans, then states 

should consider also using that data to meet the periodic inventory requirement. Also, if the air agency 

comes into attainment and receives a Clean Data Determination prior to the 3-year period, or if a 

maintenance plan (with a maintenance inventory) is being submitted prior to or in sync with this 3-year 

timetable, then the these actions are very likely to make it unnecessary for the air agency to submit a 

separate ozone periodic inventory.

Since the periodic inventory requirement in the CAA does not specify the degree of detail required for 

the periodic inventory, the EPA has defined the necessary components through the AERR. Thus, whether 

the inventory is submitted as a separate submission or through the AERR, it would need to be consistent 

with requirements of the AERR in order to meet the statutory periodic inventory requirements.

The periodic inventory is a check-in with the air agency on how the emissions are trending, and had 

previously been considered to not have “regulatory significance” or been treated as a SIP submission. 

This is no longer EPA’s recommended approach. In accordance with the CAA, periodic inventories are a 

revision to the inventory portion of the ozone SIP and are thus subject to CAA section 110 and 40 CFR 

51.102, 103, and Appendix V. For discussion of public hearings, please see Section 3.9 of this guidance 

document.

To meet the periodic inventory requirement for ozone, the periodic inventories must include ozone 

season day emissions values. Since this emissions type is allowed to be submitted to the EPA through EIS 

for the AERR, it is possible that the AERR submission could meet the periodic inventory requirement if 

the conditions listed in Table 10 apply. As with all SIP requirements, air agencies should coordinate with 

the Regional office regarding what is appropriate for a particular NAA.
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Table 10: Conditions for AERR submittal to meet ozone periodic inventory requirement

Condition Comment

Public hearing requirement

A public hearing has been offered and either no party requested a 

public hearing or a public hearing has been held. Evidence should 

be provided that this requirement has been met.

Ozone season day 

emissions must be 

submitted for the NAA 

counties

If ozone season day emissions cannot be extracted from the EIS (as 

is the case in 2014), using an AERR submission to meet the periodic 

inventory requirement is only acceptable if approved by the 

Regional office.

Emissions are submitted for 

mobile sources.

Ongoing review of the AERR may result in requiring mobile model 

inputs and making emissions data optional. If this occurs, the SIP 

periodic inventory must still include emissions. Thus, emissions 

would need to be submitted for the NAA counties regardless of 

any AERR requirements to be able to meet the periodic ozone 

inventory requirement.

Complete – all sectors

Using the AERR to meet the periodic inventory requirement is only 

possible during triennial years (see Section 2.4) because the EIS 

only accepts nonpoint and mobile data during these years, and 

these data categories must be included to be complete.

Submitted within 3 years 

since the base year 

inventory.

It is acceptable to be less than 3 years, after which time the 

triennial submissions would be every three years.

Partial county approach 

approved by Regional office

The EIS does not support partial counties. In some cases, it may be 

acceptable to use the entire county emissions for the periodic 

submission.

Notification If the AERR approach is approved, the Regional office should be 

notified in writing when the submission has been completed. 

Documentation

Must be created. Submission is at the request of the EPA Regional 

office. The degree of documentation for periodic inventories can 

be less than that of other inventories because of the lack of 

regulatory significance.

3.5.2 Regional Haze Recent Year Inventory

For the Regional Haze Rule, a recent year inventory is needed to support the analyses required with 

each 5-year (or “periodic,” as proposed) progress report or 10-year SIP revision. The inventory is not 

submitted with the SIP or progress report, but emissions summaries are expected as part of the analysis 

provided with the SIP. The inventory to be used is a complete statewide annual inventory as was the 

initial baseline inventory (included in the original Regional Haze SIP). See Table 1 above for the latest 

reference materials with guidance on the Regional Haze Progress Reports. Since the regional haze 

requirement is for inventories to be statewide and cover emissions over an annual period, the spatial 

and temporal aspects better match the AERR reporting requirements as compared to the NAA 

inventories required for ozone and PM2.5 implementation. In addition, since the recent year inventory is 

not an element of the SIP that must be submitted itself, there is not a public hearing requirement for the 

inventories used for regional haze. Thus, the AERR inventories are an appropriate starting point for
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meeting the requirement for the recent year inventory for regional haze. If better information becomes 

available for particular source sectors at a time when it can be incorporated into a state’s long-term 

strategy development, it should be.

The inventory approach for the recent year inventories needed for 5-year (or “periodic,” as proposed) 

progress reports and 10-year SIP revisions is relatively straightforward. The following list provides 

considerations for states in developing their inventories: 

 The rule prescribes that the inventory year is the same as triennial AERR year that has most 

recently been submitted by the state to the EPA, with additional special considerations

o For the recent year emissions inventory for a 10-year SIP revision, states can use 

another inventory year if adequately justified (see 40 CFR 51.308(f)(2)(iv)). One example 

of using another year would be to update the EGU portion of the inventory to use more 

current data that is available because of CEMs. There may also be better information on 

a particular source sector in a special study that collected more refined source-specific 

information but for a different year.

o For the 5-year (or “periodic,” as proposed) progress reports, 40 CFR 51.308(g)(4) 

specifically requires that any newer data reported directly to the EPA (like the CEM data) 

be used if summaries of the data are available from the EPA. 

 The inventory can be the same as the NEI or can be revised from the NEI. The proposed revision 

to the rule does not say that the inventory must be the NEI inventory, rather, it says that the 

year must be the same year.

 States must identify the recent inventory in their plan, but the inventory does not need to be 

submitted to the EPA.

 States may rely on EPA-provided parts of the NEI to support the recent year inventory data 

needed for their plan.

3.6 Emissions Statement Requirement for SIPs

For ozone NAAs, states should develop emission reporting programs for VOC and NOX sources in 

accordance with CAA section 182(a)(3)(B). The required state program and associated regulation defines

how states obtain emissions data directly from facilities and report it to the EPA as part of SIP 

inventories and for the AERR. This is called an Emission Statement regulation, and it outlines how 

facilities must report emissions and facility activity data to an air agency. Once the air agency develops 

the Emission Statement regulation, the regulation should be sent to the EPA as a SIP for approval by the 

deadline as specified in Table 8. As an element of a SIP, the public hearing requirements apply to the 

Emission Statement SIP. Air agencies must ensure that their emissions reporting program, promulgated 

in accordance with CAA section 182(a)(3)(B), complies with all applicable statutory and regulatory 

requirements on a continuing basis.
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To properly implement the emissions reporting requirements, Emission Statement regulations should be 

coordinated carefully with the AERR and any associated guidance and specifications, the Modeling 

Guidance, and this guidance. The required and optional data elements needed for inventories 

(described in Section 2.5.7) should be included as fields for reporting to the EPA. Where the AERR/EIS 

has required fields, those fields should be required by the Emission Statement regulations. In addition, 

the Emission Statement regulations should require ozone season emissions in accordance with the final 

Ozone Implementation Rule.

Additional details on developing emission statement regulations can be found in a dated, but still 

relevant guidance document titled, “Guidance on the Implementation of an Emission Statement 

Program (DRAFT),” available at https://www.epa.gov/air-emissions-inventories/implementationemission-statement-program. Where data elements to be collected are mentioned, users of that 

guidance should defer to the paragraph above, explaining that data elements should be coordinated 

with AERR/EIS requirements and the recommendations in this guidance for optional fields.

3.7 Maintenance Inventories

Maintenance inventories reflect the emissions conditions associated with a NAAQS redesignation to

attainment following an initial area designation of nonattainment. Thus, the attainment year inventory 

for the maintenance demonstration should reflect emissions during the first year for which monitoring

data showed compliance with the standard on which the state is basing their maintenance 

demonstration. Otherwise, all characteristics of the attainment year inventory are the same as the base 

year inventory submitted with the implementation plan prior to attainment. An attainment year

maintenance inventory is a component of a maintenance plan and is, therefore, submitted to the EPA as 

a SIP revision. Thus all guidance in this document for completeness, temporal extent, spatial extent, 

public hearings, documentation, and summaries applies to these inventories.

The on-road portion of the inventory for the relevant pollutants and precursors for the last year of the 

maintenance plan would be the motor vehicle emissions budget(s) required for maintenance areas by 40 

CFR 93.118(b). States may elect to also establish motor vehicle emissions budgets for the interim year or 

years for which they have submitted inventories or for the attainment year of the maintenance plan. 

Those decisions should be made through the interagency consultation process that is required by CAA 

section 176(c)(4)(D) and 40 CFR 93.105.

Specific additional guidance for maintenance plan SIPs is also available in the memorandum titled,

“Maintenance Plan Guidance Document for Certain 8-hour Ozone Areas Under Section 110(a)(1) of 

Clean Air Act” (available at http://www.epa.gov/ttn/caaa/t1pgm.html).

3.8 Projected Inventories

The purpose of the projected inventories is to provide an emissions estimate for a year beyond the base 

year. These inventories are also called future-year inventories. Projected inventories will reflect the 

effects of control measures needed to reduce emissions and bring nonattainment areas and states into 

compliance with a NAAQS and regional haze rules. 
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For ozone and PM2.5 implementation, the projected inventories are used for modeled attainment 

demonstrations, ROP/RFP plans, and in maintenance plans to show that the attainment year inventory 

will not be exceeded in future years. For the modeled attainment demonstration, the projected 

inventory represents the emissions needed to attain a NAAQS (according to the modeling) in a future 

year. For the ozone ROP/RFP plan(s), expected percent reductions of pollutants on specific timetables 

need to be shown. Those the ROP/RFP plan(s) require only the “expected reductions” and not projected 

inventories, calculating expected reductions involves estimation of emissions reductions, which can 

result in a projected inventory that shows the remaining emissions after those reductions have 

occurred.

For regional haze, the proposed rule revisions of section 308(f)(6)(v) include the requirement for states 

SIPs to “provide for” a statewide inventory of estimates of future projected emissions. The purpose of 

this projected inventory is to address the SIP requirement to focus on emission reduction measures that 

are designed to achieve reasonable further progress by 2028 by reducing emissions that lead to regional 

haze.

In addition for regional haze, the proposed rule revisions, if adopted, would eliminate an existing 

requirement for projecting emissions. The existing section 308(g)(4) of the regional haze regulations 

(regarding progress reports in between SIP revisions) states that the analysis tracking the change in 

emissions over the past 5 years must be based on the most recent updated emissions inventory, with 

estimates projected forward as necessary and appropriate to account for emissions changes during the 

applicable 5-year period. The proposed revisions of section 308(g) would remove mention of projecting 

emissions. The proposed tracking analysis would encompass emissions up to the year of the most recent 

NEI submission for all sources, and for relevant sources up to the most recent year for which the EPA has 

provided a summary of emission information submitted by sources directly to the EPA.

Additional relevant provisions of 40 CFR part 51 must be considered for ozone, PM2.5, and regional haze 

implementation purposes. Subpart G of 40 CFR part 51 provides some foundational regulatory aspects 

of the control strategy development needed for creating projected inventories, including what SIPs must 

include with regard to control strategies and their associated inventories. In particular, 40 CFR 51.114 

“Emissions data and projections” reads:

(a) Except for lead, each plan must contain a detailed inventory of emissions from point and area 

sources. Lead requirements are specified in §51.117. The inventory must be based upon 

measured emissions or, where measured emissions are not available, documented emission 

factors.

(b) Each plan must contain a summary of emission levels projected to result from application of the 

new control strategy.

(c) Each plan must identify the sources of the data used in the projection of emissions.

In addition, 40 CFR 51.116 “Data availability” reads:
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(a) The State must retain all detailed data and calculations used in the preparation of each plan or 

each plan revision, and make them available for public inspection and submit them to the 

Administrator at his request. 

(b) The detailed data and calculations used in the preparation of plan revisions are not considered a 

part of the plan.

(c) Each plan must provide for public availability of emission data reported by source owners or 

operators or otherwise obtained by a State or local agency. Such emission data must be 

correlated with applicable emission limitations or other measures. As used in this paragraph, 

correlated means presented in such a manner as to show the relationship between measured or 

estimated amounts of emissions and the amounts of such emissions allowable under the 

applicable emission limitations or other measures.

Thus, states should keep these requirements in mind as they develop their projected inventories so that 

these required SIP elements can be included. 

3.8.1 Attainment Year Projected Inventories

As listed in Table 4, there are two possible attainment year projection inventories. The first is the 

attainment projected inventory for the NAA. The second is the projected inventory for the modeled 

attainment demonstration.

For ozone SIPs, as described in Section 3.8.2, one possible use of the attainment projected inventory for 

the NAA is for showing emissions reductions associated with an ROP/RFP plan. While it is conceivably 

possible to estimate the emissions reductions (rather than future year emissions inventories) required 

by the ROP/RFP plan, the most common way to estimate the reductions is by creating future-year 

inventories and then calculating the difference between the ROP/RFP baseline and the attainment 

projected inventories for the NAA.

For PM2.5 SIPs, the attainment projected inventory for the NAA is a required element of the SIP. The 

detailed requirements for various aspects of this inventory are included in the regulatory text at 40 CFR 

51.1008(a)(2) for Moderate areas and 40 CFR 51.1008(b)(2) for Serious areas. For the attainment 

projected inventory for the NAA, a key difficulty can be calculating projected emissions for partial 

counties, which would not necessarily be available from the modeling inventory. It may be possible to 

use the gridding aspect of modeled inventories to estimate emissions in partial counties for nonpoint 

and mobile sources. This issue is addressed more completely in Section 4.4.1.

In addition, the on-road mobile portion of the attainment projected inventory for the NAA can serve as

the motor vehicle emissions budgets for the ozone and PM2.5NAAQS. Specific requirements for motor 

vehicle emissions budgets for ozone (NOx and VOCs) and PM2.5 (direct PM2.5, NOx and possibly VOCs, 

SO2 and ammonia) can be found in 40 CFR 93.102(b). Since these budgets are required for 

transportation conformity purposes by 40 CFR 93.118(b), such inventories provide a convenient and 

consistent way to meet that requirement. In this case, the on-road inventory should be created in 
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accordance with transportation conformity guidance

(http:/www.epa.gov/otaq/stateresources/transconf/).

Creating projected inventories includes a large number of data sources and requires estimates for 

stationary point, nonpoint, on-road mobile, and nonroad mobile sources. In addition, some areas may 

choose to use average fires to represent these highly variable (from year to year) and large contributors 

to emissions. Biogenic sources are not generally forecast into the future because they are so dependent 

on meteorology. Section 5 covers the details on projected inventories.

3.8.2 Rate of Progress and Reasonable Further Progress Emissions Reductions

As described in Section 3.4 and the Ozone Implementation Rule, Moderate and above NAAs must have a 

plan for meeting a 15 percent VOC emissions reduction (ROP plan) within 6 years after the NAA 

designation. Serious and above NAAs must also have a plan (RFP plan) to meet a requirement for an 

additional 3 percent per year VOC reduction (annual RFP) for each 3-year period after the initial 6 years 

(after the NAA designation) until the attainment date. In both cases, the VOC reductions can be 

substituted with NOX reductions when specific conditions apply. Serious and above areas may submit 

two separate plans since the annual RFP plan is due 1 year later. Collectively, this guidance calls these 

plans the ROP/RFP plan(s).

In particular for the ROP requirement, an attainment year projected inventory for the NAA can also 

meet the ROP requirement by providing a projected inventory that shows the 15 percent VOC (or NOX, 

as applicable) reductions when compared to the ROP/RFP baseline NAA inventory. For Serious and 

above areas, an additional projected inventory may be needed to meet the ROP requirement. This is 

because the ROP requirement at 6 years can be earlier than the expected attainment date. In the case 

for Serious and above areas, ROP plans cannot use the projected attainment inventory for the NAA, but 

rather should use an inventory projected at the 6-year interval.

To estimate the emissions reductions associated with ROP/RFP plans, air agencies should rely on the 

same techniques as for other projection purposes, which are described in Section 5. When these 

reductions are calculated by creating projected inventories, the on-road portion of the inventories may 

also need to follow transportation conformity guidance

(http:/www.epa.gov/otaq/stateresources/transconf/) so that it can be used to set the motor vehicle 

emissions budgets.

The emissions projections should include emissions growth where it occurs, emissions reductions 

intentionally included to meet the reductions needed for ROP/RFP, as well as emissions reductions 

occurring for other reasons (e.g., as co-benefits from implementation of other NAAQS and from national 

mobile source or stationary source rules). These emissions projections will result not only in estimated 

reductions (that are required to be included in the ROP/RFP plans), but also in projected emissions 

inventories. Thus, the EPA recommends that summaries of these inventories are included in the 

ROP/RFP plan(s) to communicate the emissions reductions by comparing against the ROP/RFP baseline 

NAA inventory. 
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3.8.3 Regional haze emissions reductions

The requirement of 308(f)(6)(v) that states provide for estimates of future projected emissions supports 

the purpose of the SIP to focus on emission reduction measures that are designed to achieve reasonable 

further progress by 2028. As described in the preamble of the proposed regional haze rule, the state’s 

long-term strategy to address regional haze must include enforceable emission limitations.” The EPA 

expects that the estimates of future projected emissions to meet the requirements of 308(f)(6)(v) would 

therefore include the projected impacts of these enforceable emission limitations.

The proposed revision to the Regional Haze Rule does not assume that all sources in a state must be 

subject to an analysis of the four factors laid out in 308(f)(2)(i), but subsection 308(f)(2)(v) requires the 

state to identify the sources considered by the state in developing its long-term strategy and the criteria 

used to select the sources considered. As proposed, once the emission reductions measures for these 

sources are identified pursuant to the four-factor analysis, the emission reductions from those measures 

would be part of the projection of end-of-implementation period emissions (2028, 2038, etc.) needed to 

set the reasonable progress goals. The emissions projection techniques provided in Section 5 below can 

provide the necessary technical guidance to support states in projecting their future emissions.

3.9 Public Hearings

For ozone and PM2.5 implementation, some emissions inventories are considered a part of the SIP under 

the CAA (see sections 172(c)(3), 182(a)(1)(A) and 182(a)(3)(A)), and in this guidance we have called these 

planning inventories. Thus, for ozone and PM2.5 SIPs, detailed emissions inventories data are required to 

be included in the public participation associated with SIPs as specified in CAA section 110(a)(2). For 

regional haze, the detailed emissions inventory data are not expected to be a part of the information 

provided for a public hearing on the regional haze SIP. If emissions inventory information (but not the 

detailed data) is included as part of a SIP submission, the emissions information in the SIP submission 

stills need to be included as part of the information provided as specified in CAA section 110(a)(2). 

In addition, 40 CFR 51.102 (a) further explains that a public hearing requirements means “The State 

must hold a public hearing or provide the public the opportunity to request a public hearing.” Thus, the 

phrase “or provide the opportunity to request” means that it is possible that public hearings on 

inventories will have no one request a public hearing18.

There are policy reasons that support the need for detailed emissions inventories data to be approved 

with the SIP. Some emissions inventories must be used as a basis for ozone RFP plans for areas classified 

as Moderate or above. Additionally, all NAAs seeking redesignation to attainment will rely on an 

emissions inventory for purposes of submitting a maintenance plan under CAA section 175(A). Because 

the emissions inventories and projections play such a fundamental role in determining the reductions 

 

18 A public hearing must be offered for every SIP submittal, but a hearing does not need to actually occur if no one requests a 

hearing. The detailed requirements for public notice and hearings are given in 40 CFR 51.102. See also Attachment B of 

“Regional Consistency for the Administrative Requirements of State Implementation Plan Submittals and the Use of Letter 

Notices,” EPA memorandum dated April 6, 2011, from Janet McCabe, Deputy Assistant Administrator for the Office of Air & 

Radiation to the Regional Administrators, www.epa.gov/airquality/urbanair/sipstatus/docs/FINALSIPGuidelinesSubLtrsPN.pdf.
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necessary to attain and maintain the standard, they need to receive full public scrutiny and be formally 

incorporated into the SIP.

As explained in the final Ozone SIP Requirements Rule, the EPA is no longer advising states to follow our 

existing September 29, 1992, guidance titled, ‘‘Public Hearing Requirements for 1990 Base-Year 

Emissions Inventories for Ozone and Carbon Monoxide Nonattainment Areas” in implementing certain 

SIP adoption and submission procedures for the emissions inventory requirements under CAA sections 

182(a)(1) and 182(a)(3)(A) for purposes of the 2008 ozone NAAQS. In that guidance, the EPA indicated it 

could provide states with a time-limited “de minimus” deferral of the CAA’s state public hearing 

requirement for the emissions inventory SIP revision until the inventory needs had “regulatory 

significance.” As there is nothing in the CAA provisions that provides for waiver or delay of the public 

notification and hearing requirements specified in CAA section 110(a) de minimus or otherwise, we no 

longer believe it is appropriate to advise states to follow the 1992 guidance. We remind states that the 

EPA’s implementing regulations at 40 CFR part 51 (Requirements for Preparation, Adoption, and 

Submittal of Implementation Plans) provide flexibility for states to streamline SIP-related public 

notification and hearing procedures (for example, only holding a public hearing if one is requested, per 

40 CFR 51.102), and we encourage states to take advantage of those provisions in meeting the 

emissions inventory requirements under CAA sections 182(a)(1) and 182(a)(3)(A).

For areas classified as Moderate or above, the state would need to provide the EPA with verification 

that a public hearing was offered for the emissions inventory for any emissions inventory SIP revision at 

the time the state submits the emissions inventory SIP.

In instances where a public hearing is held for the emissions inventory SIP revision, the EPA clarifies here 

what is required by EPA regulations to be published and present for public review. Because emissions 

inventory data can be voluminous, it is common to summarize that data for public hearing purposes. 

The EPA does not interpret 40 CFR 51.102 to require that at a public hearing, held in accordance with 

that provision, every database entry related to the emissions inventory must be summarized, presented, 

and/or made available. Rather, the EPA expects that, in general, emissions inventory summaries would 

be the primary source of information for public review, so long as the detailed emissions inventory data 

are available to the public in accordance with the CAA and EPA regulations. Air agencies must comply 

with 40 CFR 51.102(d)(2), and 40 CFR part 51, Appendix V.

At a minimum, the state should include summaries of the actual emissions. The EPA recommends using

the same tables as those recommended in the EI Guidance for inclusion in the SIP documentation for 

the purpose of the public review (see Section 3.10.3 of this guidance “What to Submit”).

3.10 Submit Inventory Parts of SIP to the EPA

40 CFR part 51 contains the requirements for SIPs, and many general provisions in Part 51 impact the 

requirements for emissions inventory SIPs. Section 3.10.1 summarizes the applicable regulations. In 

addition, the two key concerns for submitting information on inventories are: (1) how to submit, 
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including the format and (2) what to submit. These concerns are addressed in Sections 3.10.2 and 

3.10.3, respectively.

3.10.1 Regulatory Requirements in Addition to Implementation Rules

The general regulatory requirements for SIP submissions are available in several subparts of 40 CFR part 

51 that have an impact on the emissions inventory SIPs. 

 Subpart F – Procedural Requirements

o §51.102 Public hearings (see Section 3.9 above)

o §51.103 Submission of plans, preliminary review of plans: 

Cites Appendix V as the source of the requirements that constitutes an official 

plan submission.

 Subpart G – Control Strategy

o §51.112 Demonstration of adequacy

This section requires a presentation of emissions levels expected to result from 

implementation of each measure of a control strategy (in the modeled 

attainment demonstration).

o §51.114 Emissions data and projections

This section has three subsections:

(a) requires a detailed inventory of emissions from point and area sources that 

are based on measure emissions or, where measured emissions are not 

available, documented emission factors;

(b) requires that the plan contain a summary of emission levels projected to 

result from application of the new control strategy; and 

(c) requires that each plan must identify the sources of the data used in the 

projection of emissions.

o §51.116 Data availability

Subsection (c) requires that each plan must provide for public availability of 

emission data reported by source owners or operators or otherwise obtained by 

a state or local agency. It also requires correlation of emission data with 

applicable emission limitations or other measures.

 Subpart Q – Reports

o §51.322 – Sources subject to emissions reporting

This section points to Subpart A, which is the AERR.

o §51.323 – Reportable emissions data and information

This section also points to Subpart A.

 Appendix V – Criteria for Determining the Completeness of Plan Submissions

o 2.2 Technical Support. This subsection of Appendix V lists various parts of the 

SIP that must be included that can be met as part of the EI SIP, including:

(b) Identification of the location of affected sources included the EPA 

attainment/nonattainment designations of the locations and the status of the 

attainment plan for the affected area(s).
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(c) Quantification of the changes in plan allowable emissions from the affected 

sources; estimates of changes in current actual emissions from affected sources 

or, where appropriate, quantification of changes in actual emissions from 

affected sources through calculations of the differences between certain 

baseline levels and allowable emissions anticipated as a result of the revision.

(e) Modeling information required to support the proposed revision, including 

input data (which includes emissions).

o 3.2 Paper Plan Submissions. This section addresses the EPA’s voluntary 

guidelines for which states are requested to adhere for paper and electronically 

formatted submissions.

3.10.2 How to Submit Inventory Parts of a SIP

The EPA generally encourages air agencies to provide all parts of the SIP related to emission inventories 

using digital media. For the Ozone SIP Requirements Rule, the preamble specifically indicates that 

“states must make available the inventory data to the EPA as electronic files in ... electronic media, such 

as FTP, zip drives, or DVDs.” In most cases, it’s simply best practice to provide this information digitally. 

Because of the challenges that can arise with the use of formats that are supported by one agency and 

not the other, air agencies should provide documentation at least in formats that are readily 

transferrable, such as PDF for documentation files. Other formats can also be considered and agreed 

upon in advance, and such details would be ideal to be included in an IPP.

When paper plans are submitted, some guidelines provided in 40 CFR part 51, Appendix V apply. In 

accordance with section 3.2(a) of Appendix V, the EPA requires that when submitting one paper plan, it

must be accompanied by an electronic duplicate of the entire paper submission, preferably as a word 

searchable document format (PDF).

For the format, the above information is provided as guidance. There are no specific formatting 

requirements added by this guidance for the inventory data, emissions summaries, or documentation 

submitted as part of the SIP. Requirements are included in implementation rules, and the ozone, PM2.5, 

and regional haze implementation rules do not specify required formats for submitting inventory 

information. For ozone and PM2.5, however, the data elements required for submitted emissions 

inventories are to use the elements required by the AERR.

Currently, the AERR data format does not directly support the use of partial counties. Thus, inventories 

with partial counties are not currently able to use the EIS formats and states would need to use some 

identifier in the format to denote partial counties.

The following provides a list of best practices that the EPA encourages air agencies to use when 

submitting the Inventory Parts of a SIP:
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 In accordance with Subpart A, detailed inventory data are required as part of emission inventory 

SIPs. When providing full detailed inventory data (i.e., facility-SCC-pollutant data for point 

sources or county-SCC-pollutant data for nonpoint sources):

o Use the data elements required by the AERR. This is required by the ozone and PM2.5

implementation rules.

o Coordinate with your Regional office about what format is going to be used. If EIS 

format is used, consider submitting this to the EIS, but first contact the EPA NEI program

to make sure any such submission will not conflict with other inventories already 

submitted to the EIS.

o If not using the EIS format, use Excel®, Access® or a rigorous delimited text format. For a 

delimited text format, make sure that the text fields have double quotes and that the 

delimiter chosen (e.g., a tab) does NOT appear in any of the text fields, since this will 

prevent the delimited dataset from being imported easily into other data systems.

 For documents, use a readily available word processor format (e.g., Microsoft Word®) or convert 

the document to PDF format.

 At times, it can be very time consuming to convert tables from a spreadsheet and insert them 

into a word processor, especially if the information changes and has to be updated. The 

guidance below recommends a lot of summary tables. Rather than cut/pasting into a text file, 

states may consider creating a spreadsheet with individual worksheets (tabs) labeled as each 

table included in the document. Then, just reference those tables/tabs from document. This can 

be done with words only (e.g., “see Table 1 in Excel file”) or using a more sophisticated data link 

between the word processor and spreadsheet software. 

3.10.3 What to Submit

With regard to exactly what to submit, the information provided here provides guidance on what would 

be acceptable in most cases. The list of tangible data items and documentation forms the basis of what 

the EPA will review and ultimately approve for a SIP. Table 11 provides the suggested inventory 

elements of an Emissions Inventory SIP and indicates whether they are relevant for ozone, PM2.5, and/or 

Regional Haze SIPs. Refer to the tables in Section 2.3 for more information on which elements are 

required and which are optional.

This section (and Table 11) does not include RFP inventories since those are submitted with a separate 

ROP/RFP plan. Similarly, it does not include information needed for a modeling attainment 

demonstration submittal to the EPA, with the exception of the inclusion of statewide inventory 

summaries for ozone and PM2.5 SIPs for areas that have modeled attainment demonstrations as part of 

their SIP requirements.
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Table 11: Suggested Elements of an Emissions inventory SIP Document

SIP Planning Inventory Documentation Element Ozone PM2.5 Haze 

Background: Includes key elements explained in this guidance, such as 

the nature of the air quality problem (or arrival at attainment 

status for maintenance SIPs), priority source categories and point 

sources, pollutants covered, spatial extent, and other key 

inventory aspects.

Y

 Overall base year emissions summaries: All emissions are for 

relevant pollutants only. Data categories would minimally be 

point, nonpoint, nonroad mobile, and on-road mobile, and fires, 

though more detailed categories are acceptable. Biogenic 

sources and other sources can optionally be included as well, 

and may be required by Regional offices for areas doing 

modeled attainment demonstrations.

See Table 12

 Overall projected attainment year emissions summaries: The 

same as the base year summaries except using the attainment 

projected inventory for the NAA. Required for PM2.5 SIPs. These 

summaries are optional for the ozone Emissions Inventory SIP. 

For ozone and when developed in accordance with 

transportation conformity guidance, the on-road portion also 

can set the motor vehicle emission budgets for VOC and NOX. If 

not provided in the Emission Inventory SIP, the summaries would 

be needed in the Modeled Attainment Demonstration.

See Table 12 n/a

 Overall recent year emissions summaries compared to 

projected emissions: All emissions are for relevant pollutants 

only. Data categories would minimally be point, nonpoint, 

nonroad mobile, on-road mobile, and fires.

See 

Table 

12

Stationary Point Sources:

 Introduction with more detailed documentation of the 

emissions from stationary point sources, including the point 

source emissions threshold, how the point sources were 

identified, how emission estimates were made including 

emission factors, activity data, and control assumptions.

Y

 This section includes additional point source emissions 

summaries. 

See Table 13

Stationary Area (Nonpoint) Sources: 

 Introduction with more detailed documentation of the 

emissions from stationary nonpoint sources, including how the 

nonpoint sources were identified, the model version used, 

which sources were not addressed and why, how partial county 

emissions were estimated, and how emission estimates were 

made including models or tools, emission factors, activity data, 

and control assumptions.

Y

 This section includes additional nonpoint source emissions 

summaries. See Table 14
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SIP Planning Inventory Documentation Element Ozone PM2.5 Haze 

Nonroad Mobile Sources

 Introduction with more detailed documentation of the 

emissions from nonroad mobile sources, including how the 

nonroad mobile sources were identified, which sources were 

not addressed and why, how partial county emissions were 

estimated, and how emissions estimates were made including 

model version, source of input data, input and output data files, 

and any post-processing methods used to develop the 

inventory.

Y

 This section includes additional nonroad source emissions 

summaries. See Table 14

On-road Mobile Sources

 Introduction with more detailed documentation of the 

emissions from on-road mobile sources, the model and version 

used, how partial county emissions were estimated, and how 

emissions estimates were made including sources of input data, 

input and output data files, and any post-processing methods 

used to develop the inventory.

Y

 This section includes additional on-road source emissions 

summaries. See Table 14

Other Sources

 If applicable, a description of other sources in the emissions 

inventory not included and reasons why they are not included. Y

 If applicable, this section includes summaries of other sources of 

emissions. 

See Table 14

Demonstration of Public Hearing: A brief description of the public 

hearing process, a summary of the results, and any other relevant 

materials. The purpose of this section is to provide evidence for the 

public hearing and what the air agency learned through the process.

Y Y N

Table 12 provides the summaries to include in the background section. In the table, “data category” is 

consistent with the definition provided in Table 11 and Section 2.5.1. If both the base year inventory and 

the attainment projected inventory are included in the background, then the same summaries should 

generally be provided for both.

Table 12: Summaries to include in the background 

Emissions Summaries Ozone PM2.5 Haze

Annual baseline, recent year emissions, projected emissions, and emissions 

difference for the state by data category. The baseline emissions may 

from the original SIP may be used.

Y

Annual base year emissions and/or average season day emissions for the 

NAA by data category Y

+

Annual base year emissions and/or average season day emissions for the 

NAA by data category and county (or partial county) λ

Y

+

Ozone season day base year emissions for the NAA by data category Y
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Emissions Summaries Ozone PM2.5 Haze

Ozone season day base year emissions for the NAA by data category and 

county (or partial county) ∆

Y

+ Either annual or season-day emissions need to be reported (but not necessarily both), depending on the temporal basis used 

for the emissions in the SIP. For areas that have nonattainment for both the annual standard and the 24-hour standard and 

are using average-season-day inventory for their 24-hour SIP, then summaries of both temporal resolutions would be 

needed.

‡ For SIPs that include air quality modeling, this section can include both summaries of the planning and the modeling 

inventories. Since modeling domains are usually much larger, multi-state areas, the resolution of the modeling inventory 

summaries depends on the modeling guidance and the preferences of the Regional office. One option is to include countylevel detail for the state(s) included in the NAA, but state-level detail for surrounding states.

λ Partial county emissions can be included rather than, or in addition to, full county, where applicable.

∆ Providing both the county and partial county could provide more flexibility for states when submitting periodic inventories.

Table 13 provides the recommended minimal contents for summaries to include in the point sources 

section. If both the base year inventory and attainment projected inventory are included in the 

background, then the same summaries should generally be provided for both.

Table 13: Summaries to include in the point sources section

Emissions Summaries Ozone PM2.5 Haze

Annual recent year emissions, projected emissions, and emissions 

difference for each point source as a facility total, within the state.

Y

Annual base year emissions and/or PM2.5 average season day base year 

emissions for each point source as a facility total, within the NAA. 

Y

Ozone season day base year emissions for each point source as a facility 

total, within the NAA.

Y

These summaries should include: the county, Agency Facility ID, plant name, physical address, total 

control efficiency, key control devices, NAICS code, and actual and/or projected emissions. Other 

summaries may be included as well, such as by SCC or other fields that are necessary to describe the 

inventory effectively. The Agency Facility ID or some other ID should be included that can easily map 

to the EPA facility records in EIS. This can be more difficult when air agencies change their Agency 

Facility IDs.

Table 14 provides the recommended minimal contents for summaries to include in the nonpoint and 

mobile sources sections. If both the base year inventory and the attainment projected inventory are 

included in the background, then the same summaries should generally be provided for both. 

Table 14: Summaries to include in the nonpoint and mobile sources sections

Emissions Summaries Ozone PM2.5 Haze

Annual recent year emissions, projected emissions, and emissions 

difference by county and SCC (or SCC group‡), within the state. Y

Annual base year emissions and/or average season day emissions by county 

and SCC (or SCC group‡), within the NAA. Y

Ozone season day base year emissions by county and SCC (or SCC group‡), 

within the NAA.

Y
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For county and emissions processes reported that have reductions associated with rules, these 

summaries should include any control efficiencies or rule effectiveness assumed and actual and/or 

projected emissions. Projected attainment year emissions summaries may also be required for areas 

performing modeled attainment demonstrations.

‡ See additional information below on SCC groups.

SCC groups for on-road and nonroad mobile sources can be useful in limiting the size of summaries 

without much loss of information. For example, SCC groups can be formed by vehicle type and fuel, 

dropping the road classifications. Vehicle types can be further grouped into light duty and heavy duty. 

The amount of grouping appropriate depends on what a particular summary and SIP are trying to 

communicate. SCC groups can be suggested as appropriate by Regional offices or air agencies as part of 

the consultation for what will be included in the emissions inventory SIP.
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4 Developing Current Emissions inventories

This section covers the decisions that states will need to make regarding any “current” emissions 

inventory, including a base year NAA or state inventory, RFP baseline inventory, periodic inventory (for 

ozone or regional haze), or maintenance inventory. As a reminder, all inventories described in this 

section are inventories of actual emissions; these inventories do not contain emissions based on 

potential to emit nor allowable emissions. In accordance with 40 CFR 51.5(b), states are urged to use 

applicable, state-of-the art techniques for estimating emissions.

4.1 Inventory Years

A fundamental step in creating a base year inventory is the choice of inventory year. The Ozone, PM2.5

and Regional Haze Implementation Rules include specific language regarding the choice of inventory 

years, and those regulatory requirements supersede any guidance provided in this section. Since the 

purpose of the base year inventory is to represent relatively current emissions conditions, there is a 

general preference to use as recent a year as practical. This means that the year of emissions estimates 

is usually several years earlier than the year in which the inventory work is done, given the time that it 

takes to compile an emissions inventory (for example, an inventory with 2014 emissions may not be 

compiled until 2016).

The primary considerations in choosing a baseline inventory year as covered in this guidance are: (1) the 

implementation rule requirements, (2) transportation conformity requirements, and (3) the availability 

of data. This is in contrast to inventories used for air quality modeling, for which meteorology, and 

availability of ambient data are the primary considerations, with inventory availability secondary. This is 

because the meteorological conditions selected should be associated with ozone formation that is 

representative of either the ambient data relied upon for designation to nonattainment or recent 

ambient data, as appropriate. Choosing a year based solely on inventory availability could result in 

choosing an inappropriate year (considering ambient measurements and the meteorology that led to 

those measured concentrations). As a result, the base year(s) used in the planning inventories can be 

different than the modeling base year and associated modeling inventory.

4.1.1 Ozone baseline year

For ozone, states should become familiar with the specific rule requirements. The 2008 Ozone 

Implementation Rule in 40 CFR 51.1115(a) specifies that the “inventory year shall be selected consistent 

with the baseline year for the RFP plan as required by Section 51.1110(b), which reads as follows:

(b) Baseline emissions inventory for RFP plans. For the RFP plans required under this section, at 

the time of the designation for the 2008 ozone NAAQS the baseline emissions inventory shall be 

the emissions inventory for the most recent calendar year for which a complete triennial 

inventory is required to be submitted to EPA under the provisions of subpart A of this part. States 

may use an alternative baseline emissions inventory provided the state demonstrates why it is 

appropriate to use the alternative baseline year, and provided that the year selected is between 

the years 2008 to 2012. Allstates associated with a multi-state nonattainment area must consult 

and agree on a single alternative baseline year.
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Thus, for the 2008 ozone standard, the 2011 inventory year is the presumed inventory year. Guidance 

on the considerations for using an alternative baseline year is provided below in Section 0.

The consideration of transportation conformity requirements is important because the rule has its own 

language about selecting a base year. The applicable section of the rule is 40 CFR 93.119(e), which reads: 

(e) Baseline year for various NAAQS. The baseline year is defined as follows:

...

(4) The most recent year for which EPA’s Air Emissions Reporting Rule (40 CFR Part 51, Subpart 

A) requires submission of on-road mobile source emissions inventories as of the effective date 

of designations, in areas designated nonattainment for a NAAQS that is promulgated after 

1997.

This requirement impacts the 2008 and 2015 ozone standards. For convenience, OTAQ provides an 

online table that lists the appropriate base year for use for transportation conformity purposes (see

https://www.epa.gov/state-and-local-transportation). Unlike the ozone implementation rule, the 

transportation conformity rule does not include a provision for an alternative baseline year; thus, states 

that wish to select an alternative year are advised to consult with their Regional office as part of their 

SIP development. 

Lastly, the consideration of “availability” is simply what data are (or will be) available to the air agency 

during the time alloted for SIP preparation. Often, the triennial NEI year inventories (as described above) 

are useful starting points for meeting the various “current” inventory requirements associated with SIPs. 

For these triennial NEI years, states are already required to have prepared and provided to the EPA

inventories of criteria pollutants and precursors as well as model inputs for on-road and nonroad mobile 

sources. In some cases, the states have submitted their data to the EPA, but the complete NEI is not yet 

available because there is a 6-12 month lag between when the emissions have been submitted and 

when the inventory is released by the EPA. In this case, the state should make every effort to use the 

most currently available information and incorporate any new information that is made available by the 

NEI release at the earliest practical time.

Other benefits of using an NEI year is that NEI data are available for all states, and information from 

more than one state may be needed for attainment demonstration modeling purposes. Additionally, 

other data needed for modeling (e.g., meteorology and SMOKE inputs) are more likely to be available 

for the triennial NEI years.

4.1.2 PM2.5

For PM2.5, states should become familiar with the specific rule requirements. The PM2.5 SIP 

Requirements Rule at 40 CFR 51.1008(a)(1)(i) states:

(i) The inventory year shall be one of the 3 years for which monitored data were used for the 

designations or another technically appropriate year if justified by the state in the plan 

submission.
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For the 2012 PM2.5 NAAQS, the EPA used ambient data from 2011 through 2013 to designate PM2.5

NAAs; thus any of these three years can be used as the baseline year. The idea behind this 

connection is that the inventory should represent the emissions conditions in the local area 

associated with the nonattainment problem. Given this purpose, situations may arise where the 

most applicable available inventory year is not one of these 3 ambient-data years. 

Two examples of such situations are offered here. In the first, a newer inventory may be advisable. 

Because of time that it takes for designations to occur and the period allowed for SIP submittal, a 

significant increase in emissions could occur between the designation period and the time SIPs are 

submitted. This has occurred in some areas due to oil and gas development, for example. In this case, it 

may be more appropriate to use an inventory year that is newer than one of the 3 years of data used for 

designations, to represent the higher emissions or different source mix associated with a more recent 

year. Another example is that in an area where mobile sources are a small part of the emissions 

inventory, emissions may not have changed much from a year prior to the 3-year period of ambient 

data. In this case, an available inventory prior to one of the 3 years may sufficiently represent the 

inventory. In both cases, the air agency preparing the SIP should explain these issues as part of its 

justification for not using one of the three years.

The inventory baseline year is the same year used for the RFP baseline year. In addition, states must 

meet quantitative milestone requirements as described in 40 CFR 51.1013(a). Within the 3 years 

available for choosing a base year, a large source of emissions may be reduced. If a state chooses a year 

after those reductions have occurred, then the state would not get the benefit of those reductions in 

meeting their RFP and quantitative milestone requirements. In addition, if those emissions were a key 

part of the nonattainment problem, then the choice of the later year would not follow the guidance that 

the year selected be representative of the conditions leading to nonattainment.

The considerations for availability of emissions for PM2.5 planning inventories are the same as for ozone 

as described in the previous subsection.

4.1.3 Regional Haze

As described in Section 2.3.3, the year of the baseline inventory has previously been set. For the initial 

Regional Haze SIPs, the year used should have fallen between and inclusive of the 2000 to 2004 baseline 

period. Most areas used 2002 as their baseline inventory year in their initial SIPs based on an OAQPS 

memo about this issue (see http://www3.epa.gov/ttn/naaqs/aqmguide/collection/cp2/

20021118_wegman_2002_base_year_emission_sip_planning.pdf).

The proposed Regional Haze Rule revision addresses what states must do for subsequent planning 

periods, and this proposed revision would require that states create and use a recent year inventory as 

part of their SIP analysis. As described in Section 2.3.3 above, the 5-year progress reports and 10-year 

SIP revisions include specific language regarding the inventory year to use for developing those plans, 

which we address here in more detail.

For the progress reports, an inventory is used to compare back to the most recent previous inventory 

from the previous SIP revision. The proposed analysis year for this comparison is the most recent year 
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for which the state has submitted emission inventory information to the EPA in compliance with the 

triennial reporting requirements of the AERR. In addition, the proposed rule revision states the 

following: “With respect to sources that report directly to a centralized emissions data system operated 

by the Administrator, the analysis must extend through the most recent year for which the 

Administrator has provided a State-level summary of such reported data or an internet-based tool by 

which the state may obtain such a summary.” This latter requirement refers (at least) to the direct 

reporting of the CEM data for EGU sources. Thus, the emissions year(s) for the progress report tracking

analysis would include the most recent NEI year that had been reported by the state and any newer EGU 

(or other) data available through direct facility reporting. 

For the 10-year SIP revisions, an inventory is used as a starting point to examine future emissions 

reductions and assess expected progress on visibility improvements. Similar to the progress report 

requirement, the proposed rule revision at 40 CFR 308(2)(f)(iv) would require this starting year to be the 

most recent year for which the state has submitted emission inventory information to the EPA in 

compliance with the triennial reporting requirements of the AERR. This section also allows for a different 

year if “the State adequately justifies the use of another inventory year.” Section 4.1.4 provides more 

information on considerations for choosing an alternative year.

4.1.4 Considerations for choosing an alternative base year 

For both ozone and PM2.5 SIPs and regional haze, the governing implementation rules allow states to 

choose a different year than one of the presumptive years as described in the subsections above. In 

general, states seeking to use newer inventory years will be easily able to justify that choice on the basis 

of improvement in meeting the CAA requirement for a “current” inventory (see CAA 172(c)(3) for PM2.5

and 182(a)(1) for ozone). It is presumed that because of ongoing control programs for mobile and 

stationary sources, which get additional reductions each year, more recent year inventories will reflect 

lower emissions levels.

Another common reason for choosing a different base year includes a desire to have the base year for 

planning inventories consistent with the base year used for modeling inventories. The modeling base 

year is determined in part by meteorology that is conducive to formation of ambient levels of a NAAQS 

pollutant that are above the NAAQS. Other reasons may also exist for choosing a different year, and 

these should be provided as part of SIP documentation. Ideally, any such decisions would be made in the 

planning phase of SIP development, in consultation with the Regional office.

4.2 Pollutants and Pollutant Precursors to Include in Inventories

This section describes the pollutants and precursors that should be included in the planning inventories

for the ozone, PM2.5, and regional haze rules. Pollutants for ROP/RFP inventories are also described in 

Section 3.4.2. Because many sources emit more than one of the precursor pollutants, and because the 

precursor pollutants have the potential to be transported across state boundaries, the EPA encourages 

air agencies to develop inventories for all pollutants, where possible, to support integrated, regionalscale modeling and control strategy development for ozone, PM2.5, and regional haze.
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Section 172(c)(3) of the CAA requires “the relevant pollutant or pollutants” to be included in inventories 

for SIPs. The ozone, PM2.5 and regional haze implementation rules include specific language regarding 

the pollutants and precursors to be included, and those regulatory requirements supersede any 

guidance provided in this section.

For the 8-hour ozone NAAQS, the pollutants to be inventoried are VOC and NOX. The ROP/RFP baseline 

NAA inventory for ozone should also include VOC and NOX. For the PM2.5 NAAQS, the pollutants to be 

inventoried are primary emissions (including both the filterable and condensable portions) of PM10 and 

PM2.5, and emissions of SO2, NH3, VOC, and NOX. The EPA has finalized the requirement that PM10

emissions are to be reported with the PM2.5 inventories because PM10 emissions are often used as the 

basis for calculating PM2.5 emissions. For regional haze, the pollutants to be inventoried include all of the 

pollutants and precursor pollutants identified for ozone and PM2.5, excluding CO.

4.2.1 Condensable PM emissions

As mentioned previously, the AERR identifies reporting requirements for the implementation rules. In 

the past, a source of confusion has been the AERR requirement for reporting primary PM10 and primary 

PM2.5 emissions, which states, “As applicable, also report filterable and condensable components.” The 

AERR also defines condensable PM as:

Material that is vapor phase at stack conditions but which condenses and/or reacts upon 

cooling and dilution in the ambient air to form solid or liquid PM immediately after 

discharge from the stack. Note that all condensable PM, if present from a source, is 

typically in the PM2.5 size fraction and, therefore, all of it is a component of both primary 

PM2.5 and primary PM10.

While the wording in the AERR is clear that the EPA expects reporting of all components of primary PM 

including condensable portion (and as individual components), the rule does not clarify “as applicable” 

by identifying those sources that are expected to have condensable PM. To clarify this issue, Table 15

provides a list of source types for which condensable PM is expected by the AERR.

Table 15: Source types expected to include condensable PM

EIS Sectors (and subsectors)

Subsectors and 

Degree of 

Condensable

Data 

Categories

Commercial Cooking High Nonpoint

Fuel Combustion – Electric Generating Utilities – all fuel types Low, except Gas 

Turbines are 

negligible

Point

Fuel Combustion – Industrial Boilers – all fuel types Point

Nonpoint

Fuel Combustion – Commercial/Institutional – all fuel types

Moderate and 

depends on 

control technology

Point

Nonpoint

Fuel Combustion – Residential – Oil, Natural Gas, Other Significant Nonpoint

Industrial Processes – Cement Manufacturing 

(Kilns) Low Point
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EIS Sectors (and subsectors)

Subsectors and 

Degree of 

Condensable

Data 

Categories

Industrial Processes – Chemical Manufacturing 

(Fuel Fired Process Heaters, Carbon Black Fuel Fired Furnaces 

and Dryers, Charcoal Manufacturing, Fuel Fired Ore Calciners, 

Ammonia Production Fuel Fired Reformers, Fuel Fired 

Incinerators, other fuel fired equipment)

Low – Moderate, 

depends on 

industry

Point

Industrial Processes – Ferrous Metals 

(Silicon Metal Electric Smelting Furnaces, Electric Arc 

Furnaces, Electric Induction Furnaces, Basic Oxygen Furnaces, 

Iron & Steel Pouring/Casting, Foundry Cupolas, fuel fired 

equipment)

Primary metals: 

High, others Low -

Moderate

Point

Industrial Processes – NEC+

(Brick Manufacturing Kilns, Lime Manufacturing Kilns, Glass 

Manufacturing Furnaces, Mineral Wool Blow Chambers, Fuel 

Fired Process Heaters, Coke Manufacturing Ovens, other fuel 

fired equipment)

Coke Ovens: High, 

others Low Point

Industrial Processes – Non-ferrous Metals 

(Aluminum Ore Electro-reduction, Smelting Furnaces, Zinc 

Electrothermal Furnaces, Pouring/Casting, 

Rolling/Drawing/Extruding, Fuel Fired Process Heaters, other 

fuel fired equipment)

Primary metals: 

High, Asphalt 

electrode 

facilities: High, 

Drying: High; 

Others: Low

Point

Industrial Processes – Oil & Gas Production

(Gas Well Dehydrators, Gas & Oil Well Heaters, Fuel Fired 

Compressor Engines, Heater Treaters, Process Heaters, 

Steam Generators)

High Point

Nonpoint

Industrial Processes – Petroleum Refineries 

(Process Heaters, Catalytic Cracking Units, Coke Calciners, 

Flares, Process Gas Incinerators, other fuel combustion)

Low Point

Industrial Processes – Pulp & Paper

(Recovery Furnaces, Lime Kilns, Paper Machine/Pulp Dryer, 

Sawmill Sanding: Cyclone Exhaust, Particleboard Cooler or 

Dryer, Smelt Dissolving Tank, Particleboard Presses)

Moderate – High Point

Waste Disposal

(Landfill Flares, Other Landfill Gas Waste Gas Combustion, 

Incinerators)

Low - Moderate Point

For stationary point and nonpoint sources, the EPA recognizes that emission factors for condensable PM 

are limited, as well as emission factors for many industrial sources of filterable PM2.5. Resource 

constraints for many years have made it infeasible for EPA to conduct sufficient testing programs to 

populate all of the necessary emission factors. Nevertheless, state and local air agencies and their 

facilities are responsible for estimating emissions from sources required by the AERR. 

In response to this issue, the EPA continues to work to help state and local air agencies meet their 

responsibilities. The EPA has devised a mechanism to more readily collect source test data for use in 
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developing improved emission factors. More information on this approach can be found at 

https://www.epa.gov/electronic-reporting-air-emissions/electronic-reporting-tool-ert. The EPA invites 

industry, state, local and tribal agencies to provide source test data they may collect during the 

preparation of PM inventories to support the development of emission factors.

Additionally, the EPA has used available existing emission factors to develop a “PM augmentation tool”

(see https://www.epa.gov/air-emissions-inventories/pm-augmentation). The tool uses ratios of the 

most applicable available emission factors to calculate one PM component from another. In the absence 

of improved PM condensable emission factors or facility-specific source data, inventory developers can 

use this tool to calculate PM condensable emissions from filterable PM2.5 or total PM2.5 emissions.

Condensable PM emissions are not an issue associated with mobile sources. The MOVES model used for 

these sectors produces primary emission estimates for PM, including particle components (e.g., 

elemental carbon and organic carbon) and gaseous hydrocarbons (e.g., VOC, non-methane organic 

gases). These pollutant metrics would include both “filterable” and “condensable” PM. These are 

currently not separable in MOVES, and for SIP inventory purposes, they can be reported as they are 

output from MOVES without additional modification.

4.2.2 Regulatory Definition of VOC

The EPA’s current regulatory definition of VOC (40 CFR 51.100(s)) excludes constituents considered to be 

negligibly reactive in atmospheric photochemistry. These include methane, ethane, methylene chloride, 

1,1,1-trichloroethane (TCA), several Freon compounds, acetone, perchloroethylene, and others. States 

should use the CFR reference above to find the complete list. Over time, additional compounds may be 

exempted from this VOC definition. The exempt compounds are considered negligibly reactive, although 

some can influence the formation of ozone when present in sufficient amounts; for example, methane

impacts on ozone have been observed in areas of high methane emissions associated with oil and gas 

development. The emission factors used to estimate organic emissions generally account for

nonmethane hydrocarbons (NMHCs). 

For SIP purposes, state, local and tribal agencies should report VOC as defined by the EPA (40 CFR

51.100(s)). This is the case regardless of the treatment of methane for modeling inventories, which is 

addressed in more detail below.

4.2.3 Treatment of Methane

The treatment of methane in emissions inventories may be relevant for emissions used in modeled 

attainment demonstrations. Methane is not expected in other planning inventories associated with SIPs. 

This section is included to explain the context for which methane emissions can be important and to 

specifically note its possible relationship to the VOC emissions that are associated with SIP planning 

inventories. In some cases, air agencies interested in considering non-VOC-based estimates of methane 

in their modeling inventories may optionally choose to consider methane as part of non-modeling 

inventories as well.

The air quality models used for modeled attainment demonstrations use methane as one of the 

modeled pollutants. However, these models have typically used a fixed concentration of methane that 
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does not change. Thus, typical applications have not used methane emissions from emissions 

inventories.

Unlike the air quality models, the emissions processing steps to prepare emissions inventories for use in 

air quality models do compute methane emissions. Methane has traditionally been calculated by 

applying a factor to the VOC emissions to calculate total organic gases (TOG). These TOG emissions are 

then split during emissions processing into “model species” that include methane. With the low 

reactivity of methane and the absence of its use in air quality modeling for ozone, the VOC-to-TOG 

factors and the methane part of the speciation factors were not given much scrutiny. Previous versions 

of this guidance encouraged inventory developers to simply use the default VOC-to-TOG factors that 

were associated with the speciation profiles without further analysis, since the methane emissions were 

not used. This approach may still be appropriate for most or all NAAs for their modeled attainment 

demonstration.

In limited situations, air agencies may want to investigate the extent that methane emissions play a role 

in ozone episodes. Hence, additional efforts going beyond the use of the default VOC-to-TOG profiles 

and default methane fractions from the speciation profiles may develop over time. In this case, it may 

be of interest to include methane emissions inputs to air quality models to achieve sufficient model 

performance for a modeled attainment demonstration. Approaches can include incorporating methane 

emissions from other sources into emissions inventories used for modeled air quality. For example, the 

MOVES model estimates methane emissions directly, so there would be no need to use VOC speciation 

to estimate those emissions. Air agencies considering methane as a contributing factor should consult 

with their EPA Regional office to discuss the best sources for such data as this inventory development 

area is undergoing many changes at this time. Furthermore, more specific VOC-to-TOG factors and 

speciation profiles may be useful, particularly for analysis related to areas of very high methane 

emissions. These issues are all associated with modeled attainment demonstrations, and do not affect 

the VOC emissions reported as part of the emissions inventory SIP.

4.3 Identification of Priority Sources

The Ozone Implementation Rule and PM2.5 SIP Requirements Rule refer to the AERR requirements for 

creating the inventories used for SIPs. This includes the AERR requirement that inventories must include 

all sources of emissions. Thus, inventories used for SIPs are expected to be complete, with emissions 

from all sources characterized in some way. In addition, to better understood routine emissions sources, 

emissions from known or quantifiable fugitive sources such as leaking tanks or valves should be included 

in inventories when these emissions can be quantified in both the base and future years and because 

their reductions could help in achieving attainment. These emissions should not be excluded based on 

an invalid assumption that they are “malfunctions.” In addition, startup/shutdown emissions should be 

included in inventories (see Section 4.8.1).

While completeness is required, there is no requirement that all parts of an emissions inventory be 

completed with the same degree of rigor. Because emissions inventory development is time consuming 

and expensive, it is wise for inventory developers first to focus their resources on the sources of 

emissions making up the bulk of the pollutants relevant for the NAAQS in question. To the extent that an 
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inventory is to be used for multiple NAAQS or for multi-pollutant analysis, then the larger sources 

important for all relevant pollutants should be characterized with as much technical rigor as is feasible.

Fortunately, existing inventories can help states guide their initial understanding of the sources of 

pollutants in a particular area. Air agencies can rely on the latest NEI or previously compiled SIP 

inventories to guide further development. The NEI is commonly used as a starting point for state and 

regional planning purposes for NAAQS implementation and regional haze planning. The EIS can help by 

providing emissions summaries for counties or groups of counties and for emissions sectors or SCCs.

For additional information, the Particulate Emissions volume of the EIIP technical report (see

https://www.epa.gov/air-emissions-inventories/volume-9-particulate-emissions) provides thorough 

recommendations for identifying sources of PM emissions. These recommendations can be useful for 

other pollutants as well.

Starting with an existing inventory poses some risks and challenges. Here we try to address the most 

common issues associated with SIP inventories when starting with existing inventories:

1. How do I know which sources in an existing inventory should get further review?

The EPA suggests the following considerations in deciding which sources to be reviewed:

a. The largest contributors to the pollutants of interest (i.e., the pollutants contributing to 

ozone and/or PM2.5 formation) using some coarse groupings, such as the EIS Sectors, 

Tier 2, or Tier 3 groups. 

b. Sources within the NAA (or contributing to it in the case of modeling attainment 

demonstration inventories). Special considerations to sources closest to and upwind of 

violating monitors may deserve special attention.

c. Knowledge about the growth or retraction of a particular sector of the economy. Oil and 

gas production is a recent example of a quickly growing industrial source of emissions.

d. Considering whether the proportion of emissions associated with a particular source is 

likely to have an impact on the outcome of decisions. Emissions methods/changes for 

sectors that are likely to have a large impact on the planning process should get higher 

priority.

e. Assessing whether the approaches used in the preparation of SIP emissions are updated 

with current approaches, including appropriate emission factors. This is particularly 

important for source categories for which there is large uncertainty and those with 

ongoing research.

2. What if the original inventory does not use good approaches?

The EPA recommends starting with inventories that have available and clear documentation. If 

an inventory developer cannot figure out how the emissions were estimated, then it will be 

difficult to build upon those results.

With documentation, inventory developers should be able to evaluate the quality of approaches 

used. As part of the NEI program, the EPA publishes updated methods for many nonpoint 
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emissions categories. The highest quality point source emissions estimates are based on facilityspecific testing, and where not available, current emission factors. Mobile emissions should be 

based on the latest mobile models, using as many location-specific inputs as possible.

If it is determined that a starting inventory does not use the latest methods, it is advisable to 

reassess those emissions with newer methods (or other available inventories) prior to deciding if 

that category requires even further work.

3. If I need to make updates, where are the latest approaches available?

The EPA’s NEI program is a good source of the latest approaches and approaches in 

development. In many cases, multi-jurisdictional organizations also have updated approaches 

for various emissions sectors. The Emissions Inventory Improvement Program volumes still have 

a lot of valuable information on emissions, though some sections are outdated (see

https://www.epa.gov/air-emissions-inventories/emission-inventory-improvement-programeiip). Approaches have historically included AP-42 and WebFire emission factors, though these 

resources are outdated for some sectors. For more information on updated emission factors, 

see Section 4.6.

4. What do I do if the NEI approach has added PM or other emissions that I did not report, and I 

think that they are invalid?

The EPA commonly augments PM emissions provided by states in the NEI because many states 

do not report total PM2.5 and neglect the condensable portion of PM emissions (see also Section 

4.2.1). Consistent with the requirements for complete SIP inventories, the EPA works to create a 

complete and accurate NEI. If emissions have been added erroneously, states should make 

those improvements, but should also let the NEI program staff know that there may be a flaw in 

their methods and provide a better source of information. Just because a source has not 

reported condensable emissions does not mean that they do not exist or should not have been 

characterized. In some cases, states may need to go back to sources to clarify that condensable 

PM emissions must be provided to the state agencies.

Another source for added EPA criteria pollutant and precursor emissions is NH3 for point sources 

from the TRI. Since these are emissions that the facility submits to the EPA through the TRI 

program, they should potentially be considered as a source of information when identifying 

sources of NH3.

Only the facility total emissions are available from the TRI program (with stack and fugitive 

emissions denoted separately). This is not generally enough detail for a SIP inventory. For SIP 

inventories for PM2.5, air agencies would need to follow up to develop more refined information 

about the facility NH3 so that NH3 emissions could be included in the SIP inventories. If the 

source is a “point source” based on the AERR thresholds for other pollutants, then including NH3

emissions as part of the point source emissions is required by the AERR and, therefore, in PM2.5

SIP inventories in accordance with the PM2.5 SIP Requirements Rule. If the source is not a point 
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source based on thresholds of other pollutants, than the AERR NH3 threshold of 100 tpy 

potential to emit would apply in determining if the facility should be reported as a point source. 

Otherwise, the emissions should be included as a nonpoint source in the SIP inventory. If a state 

air agency believes that these emissions are erroneous or unimportant, some discussion of 

these sources should be included in their SIP as to why the NH3 emissions as identified by TRI 

are not being included in a SIP inventory in some form. For incorrect TRI emissions, states 

should consider coordinating with the facility and the EPA’s TRI program to improve the 

emission estimates.

4.4 Spatial Extent

The spatial extent of an inventory refers to counties, states, or other geographic regions covered by the 

emissions data. As described in previous sections, inventories needing to be included in SIPs are either 

for the NAA area only (for ozone and PM2.5) or are for the entire state (for regional haze and ozone or 

PM2.5 when modeled attainment demonstrations are needed). Modeled attainment demonstrations are 

likely to use emissions from a regional multi-state area. While it is not a SIP requirement to submit an 

inventory for other states, the emissions are needed to do the modeling.

A number of complicating issues related to spatial extent arise with inventories. Below, more 

information is provided on partial counties, when point-source locations are needed for emissions that 

have traditionally been nonpoint sources, shapefile-based inventories, and link-based mobile emissions.

4.4.1 Nonattainment Areas with Partial Counties

In some cases, NAAs do not follow county boundaries. Since most inventories and many underlying 

datasets are available at the county level, this poses a challenge for emissions calculations, in particular 

for nonroad and mobile sources.

Air agencies may wish to use full-county emissions in meeting some SIP requirements, despite the 

designation of partial-county NAAs. The EPA has considered the advisability of using whole-county 

emissions to represent partial county emissions and has determined that this is not possible. In 

particular, using whole-county emissions would pose a challenge for using these inventories for 

ROP/RFP analyses, which can only include reductions within the NAA. Thus, areas designated as partial 

counties should estimate those partial county emissions using one of the approaches described here.

For point sources, Geographic Information System (GIS) technology enables straightforward selection of 

point sources into a partial-county NAA by intersecting a dataset of point source latitude/longitude 

coordinates with a shapefile of the NAA boundaries.

For nonpoint and mobile sources, available inventories and methods are often designed to create 

county totals, making partial counties much more difficult. A variety of acceptable approaches exist for 

calculating partial county emissions, though the particular approaches chosen should be agreed to as 

part of developing the plan for the SIP planning inventory as described in Section 2.2.

1. Custom calculations: Air agencies could develop a custom approach to estimate emissions using 

activity data estimated for the partial county area. Since many source categories need to be 
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estimated, this could be a very labor intensive approach, or could be used for just key categories 

where other approaches are insufficient.

2. Shapefile-based inventories: See Section 4.4.3.

3. Use of spatial allocation surrogates: Some sources of data exist at sub-county resolution, 

including road networks, travel demand model results (see also Section 4.4.4), and census data 

that are available at census tracts or blocks. When the emissions activity can be reasonably

assumed to follow an allocation method based on one such surrogate dataset, GIS techniques 

can be followed to apportion a county emissions estimate to a partial county NAA based on the 

patterns of these sub-county data.

4. Use of gridded modeling data: Modeled attainment demonstrations rely on air quality modeling 

that uses gridded emissions data, which could be resolved at a sub-county resolution. Gridding 

resolutions of 12-km, 4-km, or even 1-km have been used. Similar to what was described above 

in the previous approach, emissions processing relies on spatial surrogates to apportion countylevel data to grid cells needed for modeling. Rather than take a custom spatial surrogate 

approach, these gridded data may be sufficient to apportion county-level data to a partialcounty NAA using GIS overlay techniques. Such an approach would sum the parts of the gridded 

data intersecting the partial county area to provide the partial county area inventory. The 

primary difference in this approach compared to the previous approach is the intermediate step 

of using the gridded data. This could be appealing since those gridded data may already be 

available from modeling done for attainment demonstrations.

5. Expert judgment: When there is no appropriate sub-county data surrogate available, when air 

quality modeling is not part of the SIP process, or when a category is relatively unimportant to 

the SIP planning process, other methods may be advisable. In this case, an air agency should 

explain their rationale to the respective EPA Regional office and come to agreement on a 

technically reasonable approach. Such approaches could include using the entire county 

estimated emissions, assuming some fraction based on understanding of county activities, or 

taking an area-proportional (uniform distribution) approach. In this latter approach, the partial 

county emissions are assumed to be the total county emissions times the area of the partial 

county divided by the area of the total county.

Since these partial county emissions calculations could need to be done for a variety of inventories and 

for SIP revisions, considerations should be made for the repeatability of the approach so that it does not 

become overwhelming to the analysis. In collaboration with the EPA Regional office, a balance needs to 

be found among level of effort, technical credibility, and repeatability.

The latest spatial approaches and data are available with the latest emissions modeling platform data on 

the Emissions Modeling Clearinghouse (EMCH). The EIIP Volume III, Chapter 1, Section 4.3 provides 

some additional information on spatial allocation; however, the examples used for spatial allocation 

approaches are much too old for most applications, and more sophisticated spatial surrogate 
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approaches are available than those given as examples. In particular, except in rare circumstances, the 

use of population data as an allocation method is outdated and not recommended. 

4.4.2 Point Source Locations Needed for Modeling of Traditional Nonpoint Sources

In some cases, the details needed in a modeled attainment demonstration push inventory developers to 

use point-source locations for source categories that have traditionally been nonpoint sources. Some 

examples include sources of agricultural ammonia, dry cleaners, and gas stations. Rather than creating a 

much larger point source inventory, one approach is to keep a source as nonpoint, but use an emissions 

processing technique for modeling known as area-to-point allocation.

While this topic is primarily one for consideration in modeling inventories, it is included here because 

the planning inventory often provides a starting point for the modeling inventory (for the NAA counties). 

To the extent that modeling needs can drive additional detail in the planning inventories, this area-topoint feature can allow small sources to be included as nonpoint sources in a planning inventory but still 

allow point source locations to be used for modeling.

In area-to-point allocation, the emissions processor for modeling starts with county-wide estimates and 

a user-provided dataset that allocates county wide emissions to discrete latitude/longitude points 

(within the county). The inventory developer still must obtain the point source locations and proportion 

of countywide emissions, but the point source inventory remains more focused on the largest sources. 

Using an area-to-point allocation approach also avoids having to define stack parameters for inclusion of 

small sources in the point source inventory, which is useful because the stack parameters are not 

necessarily relevant to or available for these source categories.

4.4.3 Shapefile-based Inventories

The EPA collects inventories for the NEI for commercial marine and locomotives using GIS shapefilebased inventories. Rather than including emissions as a county total, the emissions are allocated to 

individual sub-county areas or lines, each with its own GIS polygon shape. This approach was developed 

primarily to support fine scale and risk modeling that are done at a scale finer than most modeled 

attainment demonstrations.

These inventories can be useful for computing partial county emissions for those categories using this 

approach. Otherwise, it is not expected that states would need to start with the shapefile-based 

inventories for SIP planning. In some cases, states may wish to use such inventories in circumstances 

where the CMV and locomotive emissions spatial distribution is critical to the modeled attainment 

demonstration in some way. This may be important in NAAs that contain large ports and/or are near 

major coastal or inland waterways. Even in this case, use of county total inventories with appropriate 

spatial allocation using other techniques will still be able to allocate emissions sufficiently for both 

nonattainment area inventories (if partial counties are needed) and for modeled attainment 

demonstrations. 

4.4.4 Link-based On-road Mobile Emissions and Travel Demand Models

SIP planning and modeling inventories often put great focus on mobile sources through the use of 

detailed emissions by roadway segments (or links) and the use of travel demand models (TDMs). This 
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possibility is simply mentioned here for completeness and is not necessarily recommended, since these 

efforts are often substantial. The approach can also help with partial county emissions estimates. 

4.5 Temporal Basis

Various temporal bases may be needed for meeting different inventory requirements for SIPs. These can 

be different for ozone, PM2.5, and regional haze. The emissions that should be used in planning 

inventories for ozone SIPs are ozone season day emissions. The emissions that should be used in 

planning inventories for PM2.5 SIPs are annual emissions for SIPs addressing the annual or 24-hour PM2.5

NAAQS or seasonal emissions for SIPs addressing the 24-hour PM2.5 NAAQS. At a state’s option, an 

inventory may contain both annual and average season data emissions for SIPs addressing the 24-hour 

PM2.5 NAAQS. The temporal basis for emissions in Regional Haze SIPs is always annual actual emissions.

4.5.1 Temporal Considerations for Ozone SIPs

For ozone SIPs, ozone season day emissions (as defined in Section 2.5.2) are used for the base year 

inventory for the NAA. Since other planning inventories must be consistent with the base year inventory, 

inventories such as the projected attainment year inventory for the NAA and the periodic inventories 

must also include ozone season day emissions. Additionally, for states that include modeled attainment 

demonstrations in their SIP, statewide (or modeling domain-wide) ozone season day emissions and 

statewide annual emissions may also be important for Regional office evaluation and approval of the 

modeled attainment demonstration.

Ozone season day emissions are defined in Section 2.5.2. As explained in the definition, states must 

select the representative months and work week days to include in the calculation of the ozone season 

day emissions. The temporal basis for these emissions should be representative of the conditions 

leading to nonattainment, as recommended by the state and approved by the EPA prior to finalizing the 

emissions approach to be used. The EPA acknowledges that there could be instances where ozone 

values are elevated above the level of the NAAQS only in non-summer months or on weekend days, 

including when winter emissions or weekend emissions dominate the episode. In recognition of this 

situation, the terminology used has changed from “summer day emissions” to “ozone season 

emissions.” The general expectation of high emissions activity on weekdays is the basis for the weekday 

specification in the definition of ozone season day emissions. However, in the event that weekend 

emissions were to be higher than weekday emissions, the state should consider whether weekday 

emissions are sufficiently representative of the nonattainment problem. Since the goal of the definition 

of ozone season day emissions is representativeness of the emissions contributing to the ozone 

nonattainment problem, if including the weekend emissions resulted in a better representation of 

emissions, then states may be able to justify including weekend emissions in coordination with the EPA 

Regional office.

In general, to calculate ozone season day emissions, reporting agencies should estimate emissions for a 

specific period (month, week, selected days, or a day) where emissions (and other factors) resulted in 

ozone values above the level of the NAAQS leading to nonattainment. The preference should be given 

for weekdays as previously described, unless there is a reason to use other days. An ozone season day 
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emissions value should be calculated by summing the emissions on those days and dividing by the 

number of days included in the sum.

For mobile source emissions, mobile source emissions models can produce output for an average 

weekday or weekend day. Consult the EPA’s MOVES Technical Guidance, “MOVES2014 and 2014a 

Technical Guidance: Using MOVES to Prepare Emission Inventories for State Implementation Plans and 

transportation Conformity,” for details (https://www3.epa.gov/otaq/models/moves/). This guidance 

provides numerous recommendations including how to determine temperatures to use in calculating 

emissions for base year and attainment year NAA inventories.

As noted in the preamble for the Ozone Implementation Rule, HEDD periods could have higher 

emissions than during other periods, which could contribute to ozone formation in a particular 

nonattainment area. Additional contributing factors to HEDD-period impacts include location and timing 

of the HEDD-related emissions, emissions amounts from other sources during the HEDD period, and 

meteorology. If an air agency determines that emissions increases (within a NAA) associated with HEDD 

periods significantly contribute to ozone nonattainment, then the air agency should include those higher 

emissions in the planning inventories: the base year inventory for the NAA (which also serves as the 

ROP/RFP baseline), and the attainment projected inventory for the NAA. As described above, in 

consultation with the EPA Regional office, air agencies should select periods to use for defining the 

appropriate days to include in the calculation of ozone season day emissions that are representative of 

the conditions leading to nonattainment. Thus, air agencies that have determined HEDD periods are 

important should select such periods as part of their ozone season day emissions estimation approach. 

As a result, air agencies could estimate HEDD emissions in a way that represents an appropriate portion 

of their NAA emissions inventories. More information on how to estimate HEDD-related emissions is 

provided in Section 4.8.1.

4.5.2 Temporal Considerations for PM2.5 SIPs

The emissions must be reported as annual total emissions, average-season-day emissions, or both, as 

appropriate for the relevant PM2.5 NAAQS. The rationale for the type(s) of emissions provided must be 

included as part of the attainment plan. When seasonal emissions are included, the rationale for the 

seasonal period must also be included as part of the attainment plan. A discussion of the EPA’s rationale 

for including the option of seasonal or annual inventories is provided in Section IV.B.2.d of the PM2.5

Implementation Rule preamble. The regulatory text for this requirement is also provided at 40 CFR 

51.1008(a)(1)(iii). 

In the case of the 24-hour NAAQS, the form of the NAAQS is based upon monitored values on particular 

days with high levels of ambient PM2.5, and in some nonattainment areas, those days may occur only 

during a distinct and definable season of the year. For the PM2.5 NAAQS, states can meet the inventory 

requirement with different combinations of temporal resolutions for the emissions. For the annual 

standard, annual emissions must be submitted. For the 24-hour standard, states must submit either an 

annual or an average-season-day inventory and optionally may submit both. For a nonattainment area 

for both the annual and 24-hour standard, states can meet the inventory requirement with only an 

annual inventory or with both an annual and average-season-day inventory. 
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In contrast with the annual PM2.5 NAAQS, the 24-hour PM2.5 NAAQS is designed to protect against peak 

exposures. Thus, for the 24-hour PM2.5 NAAQS, there are circumstances in which the EPA believes that a

seasonal emissions inventory alone may be sufficient for attainment planning purposes. 40 CFR 

51.1008(a)(1)(iii) allows states to use seasonal inventories for attainment plan development for attaining 

the 24-hour PM2.5 standard in areas that are designated nonattainment for only the 24-hour standard. 

Use of a seasonal emissions inventory will also be appropriate only if the monitored violations of the 

24-hour PM2.5 NAAQS in the area occur during an identifiable season. In the event that it is appropriate 

to rely solely on a seasonal emissions inventory, the state should confer with the EPA concerning the 

exact length of the season and the start and stop dates of the season. The duration and start and stop 

dates of the season will be an important component of the attainment plan and must be approved by 

the EPA along with other elements of the attainment plan for a given nonattainment area. Further, 

seasonal inventories must use average-season-day emissions values for this purpose, as defined by 40 

CFR 51.1000. The nature of some seasonal PM2.5 emissions sources (e.g., residential wood combustion) 

does not allow for only weekday emissions to be included in the inventory; therefore, all days must be 

included. The state would need to explain the rationale for the duration of the season used for the 

inventory as part of the attainment plan submission. To justify the use of a seasonal emissions 

inventory, the state must demonstrate why a seasonal emissions inventory is appropriate for the 

particular PM2.5 nonattainment area in question. 

4.6 Emission Factors for SIP Emissions Inventories

As expressed in 40 CFR 51.114(a), inventories “must be based on measured emissions or, where 

measured emissions are not available, documented emission factors.” Given that the preference is 

measured data, the intent of the regulations is for emissions to be specific to the source of the 

emissions. To ensure that emissions estimates that use factors are high quality, the emission factors for 

SIP development should come from credible sources. The possible sources necessarily differ for 

stationary, on-road mobile, and nonroad sources. The mobile sources with EPA models are the most 

straightforward. For all states but California, on-road mobile emissions should be estimated with the 

latest EPA on-road model, MOVES. For California, the most recent EPA-approved version of EMFAC 

should be used. For stationary sources and remaining nonroad mobile categories, there are several 

approaches that can be acceptable, with the guiding principle of using credible sources of data. Where 

emissions inventory SIPs are to be approved by the EPA, inventory developers should work with the 

appropriate EPA Regional office to make sure the emission factors are acceptable. 

Air agencies should pay close attention to the methods that they are using to calculate emissions to 

ensure updated approaches are used. It would be difficult for an EPA Regional office to approve an 

inventory that used an outdated method for an important part of the inventory if an updated method 

was available at the start of the SIP planning inventory development effort. Since the EPA could have 

multiple approaches posted for the same sector and some of those approaches could be outdated, air 

agencies should not assume that all EPA-posted methods are equivalent and acceptable. For example, 

some emission factors from AP-42 for nonpoint sources are outdated when compared to nonpoint 

methods and factors used for the NEI. The EPA recommends that air agencies refer to the latest NEI 
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development web pages and contact Regional offices to confirm that their planned approaches are up 

to date. The IPP process described above can be helpful in this process.

For most stationary point sources, air agencies have rules in place that require the submission of point 

source data to the state for air planning purposes. Much of these data are then submitted to the EPA as 

part of the NEI and used as the basis for point source inventories in SIPs. These state rules may have 

their own requirements about emission factors and source tests to be used; where possible, these rules 

should follow this guidance.

The EPA recommends selecting emission factors based on the hierarchy of emission factor sources 

shown in Table 16, which is consistent with the requirement in 40 CFR 51.114(a) that emission 

inventories for SIPs use measured data or emissions factors if measured data are not available. The 

higher up on this hierarchy the emission factor chosen, the better the quality of the emissions estimate. 

Based on the emission factor approach used, the states should also set the required “emission 

calculation method code” (see Section 2.5.14) for inclusion in their inventory data.

Table 16: Hierarchy of emission factors for use in point source emissions inventories

Rank Emission Factor Approach

Emission factor 

method code to use

1 Continuous emissions monitoring system 1

2

Facility/process-specific emission factor. Average of

representative stack tests (on one stack) downstream of controls 

(or if no controls).

4

3

Single representative facility/process-specific stack test

downstream of controls (or if no controls).

4

4

Facility/process-specific uncontrolled emission factor, plus control 

efficiency. Average of representative stack tests (on one stack) 

upstream of controls, adjusted based on expected control 

efficiency.

24

5

Single representative facility/process-specific stack test upstream 

of controls, adjusted based on expected control efficiency.

24

6

Site-specific emission factor. Average of representative stack tests 

across multiple stacks or processes, downstream of controls (or if 

no controls).

10

7

Site-specific uncontrolled emission factor, plus control efficiency. 

Average of representative stack tests across multiple stacks or 

processes, upstream of controls, adjusted based on expected 

control efficiency. Only makes sense when all processes 

measured are fed to the same type/combination of control 

devices.

30

8 Material balance 3

9 Manufacturer specification 7

10

Best available other emission factor for uncontrolled or including 

controls, which is one of:

EPA emission factor 8

S/L/T emission factor 9
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Rank Emission Factor Approach

Emission factor 

method code to use

Vendor emission factor 11

Trade group emission factor 12

Other emission factor 13

11

Best available pre-control emission factor, plus control efficiency , 

which is one of:

EPA emission factor, plus control efficiency 28

S/L/T emission factor, plus control efficiency 29

Vendor emission factor, plus control efficiency 31

Trade group emission factor, plus control efficiency 32

Other emission factor, plus control efficiency 33

12 Engineering judgement 2

13

Emission factor with best available speciation profile, one of:

EPA speciation profile 5

S/L/T speciation profile – e.g., to calculate metals emissions 

from filterable PM2.5 or volatile HAP from VOC. 6

4.7 Inventory Data Codes

Building inventories includes the use of various codes, such as SCCs, state/county FIPS, tribal, control 

measure, NAICS, pollutant and emission method codes. A full accounting of all possible codes is 

available as part of the EIS “Reporting Code Tables” available to EIS users using the “Reporting Code 

Tables” link at the left-hand side. The codes are also available to non-EIS users (on a somewhat more 

delayed timetable) at https://www.epa.gov/air-emissions-inventories/emission-inventory-system-eisgateway under the “EIS Code Tables (Including SCCs)” link. There are more than 60 separate lists of 

codes that define all of the required and optional coded data elements based on the AERR. As we have 

noted above, the AERR is relevant as the rule that codifies the correct development of all inventories 

submitted to the EPA.

4.7.1 Codes to Use for Nonattainment Area Inventories

Air agencies developing inventories for SIP planning should use valid EIS codes in all of their inventories. 

Since codes change over time, the EPA has adopted the approach in the EIS to “retire” codes when no 

longer needed and to add codes when needed. Codes are generally retired in coordination with NEI 

development cycles, ideally retiring codes only when plenty of notice has been provided to states in 

advance of an NEI inventory submission period. As such, codes that are retired may have already been 

used as part of a SIP development process, and that is okay. This is the advantage to retiring codes 

rather than removing them. Conversely, the EPA may not be able to retire a code until an appropriate 

point in the triennial NEI cycle. If an air agency wishes to submit a SIP-related inventory to the EIS for the 

NEI or another purpose, it will need to use valid (and not retired) codes for the year being submitted.

4.7.2 How to Request New Codes or Code Retirements

In general, if a new code is needed for compiling an inventory, then an air agency staff member should 

contact the EPA to request a new code. The recommended contact method is to use the call or email 

using the contact information listed for “Facility Inventory and Point Emissions” on the EIS contacts page 
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at https://www.epa.gov/air-emissions-inventories. Requests for codes should include which code 

table(s) are affected, why existing codes are not sufficient, and a recommendation for a code and its 

definition and purpose. While the EPA anticipates that code retirement requests will usually come from 

the EPA rather than from external groups, a code retirement request should be made using the same 

mechanisms.

The EPA most often gets requests for new SCC codes since new emissions processes arise in industries, 

or processes are newly identified as having emissions associated with them. If an air agency inventory 

developer cannot find an appropriate SCC code for an inventory process, a choice needs to be made to 

either use a generic (“miscellaneous”) SCC or to request a new SCC. For cases in which the contribution 

of the emissions process to any one precursor pollutant is minimal, inventory developers may choose to 

report using one of the miscellaneous SCCs. If the process is a significant source of one or more 

pollutants or that process might be a part of a control program, then an SCC request is probably 

warranted. The EPA staff will evaluate the SCC request to make sure that a similar code does not already 

exist, consider codes that have been created but not yet activated for use in submitted inventories19, 

assess the need for the SCC, and either indicate an existing appropriate SCC to use or assign a new

appropriate code.

4.8 Estimation of Base Year Emissions

This section describes the estimation of base year emissions for use in planning inventories for ozone, 

PM2.5 and regional haze implementation. Subsections cover point, nonpoint, on-road mobile, nonroad 

mobile, biogenic sources, and fires, respectively.

4.8.1 Stationary Point 

Stationary point sources can be estimated through continuous measurements and through more 

traditional methods. EIIP Volume II, Chapter 1, Sections 1 through 7 provide still-relevant information on 

compilation and calculation of point source inventories. Readers developing point source inventories 

should also use that report as a resource.

Sources estimated with CEMS

The best estimate of stationary point source emissions is obtained through the use of CEMS. Several 

pollutants, including NOX, SO2, CO2, and mercury (Hg) can be measured with CEMS. Fuel heat input on an 

hourly basis is usually also be collected as part of the EPA’s CEMS programs, and heat content can be 

used to estimate emissions for other pollutants using emission factors (with specific test-based factors 

preferred). More information and guidance on CEMS can be found at and 

http://www.epa.gov/airmarkets/emissions/continuous-factsheet.html. 

The vast majority of CEMS-equipped sources use these measurements as part of reporting programs to 

the EPA. In particular, the Acid Rain Program and NOX SIP call and other trading programs collect data as 

 

19 Some SCCs are set up for use in collecting source test data and developing emission factors. Sometime after that, 

the SCCs can be used for creating and submitting inventories, depending on the availability of emission factors 

and/or the interest for using those SCCs by industry and air agencies.
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part of the EPA’s Clean Air Markets. Through that program, CEMS-based emissions are available through 

the “Data and Maps” resource, including emissions summaries and files ready to be used in the SMOKE 

system used for emissions modeling.

For a few sources, emissions must be reported only for part of the year. In these cases, users of the data 

should be careful not to override the annual emissions estimates for such units with the partial-year 

data included in the CEMS databases. This can be a particular challenge for modeling inventories, where 

the more temporal resolution is useful for the time periods covered, but the other time periods must 

use a different approach to apportion the remaining emissions to the rest of the year.

Other (non-CEMS) sources

EIIP Volume II, Chapter 1, Section 4 provides general emission estimation procedure information. It 

provides a table similar to Table 16, but also focuses on the cost associated with the different 

approaches. It acknowledges that the cost incurred with collecting the more specific source test data 

should be commensurate with the need for a higher data quality. Like the EIIP figure, Table 16 also lists 

the more costly (and better) approaches higher on the list. The EIIP volume provides guidance on each 

of the various methods of computing emissions.

The remainder of EIIP Volume II includes chapters that address various combustion, manufacturing, and 

production activities that are point sources. Information in these chapters can be used to estimate 

ozone, PM2.5, and regional haze precursor emissions where they address the source categories of 

interest. In some cases, the EIIP chapters are somewhat dated and so may not cover newer emissions 

processes. In these cases, inventory developers should adapt the techniques or develop new techniques 

to ensure there estimates are relevant to the sources. The EIIP point source chapters within Volume II 

cover topics as follows (numbers correspond to chapter numbers):

2. Boilers

3. Hot mix asphalt plants

4. Fugitive emissions from equipment leaks

5. Wastewater collection and treatment

6. Semiconductor manufacturing

7. Surface coating operations

8. Paint and ink manufacturing

9. Secondary metal production

10. Oil and gas production

11. Plastic products manufacturing

12. Effects of control device efficiencies and malfunctions

13. Stone mining and quarrying

14. Uncontrolled emission factors for criteria pollutants

15. Packaging and graphic arts

16. Chemical manufacturing
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Each industry- or source-specific document contains a brief description; identification of emission 

points; an overview of methods available for estimating emissions; example calculations for each 

technique presented; a brief discussion on quality assurance (QA) and quality control (QC); and the SCCs 

needed for entry of the data into a database management system. The SCCs included in each volume 

apply to the process emission points, in-process fuel use, storage tank emissions, fugitive emissions, and 

control device fuel (if applicable).

For NH3 sources, a report is also available in the “nonpoint” section of EIIP entitled “Estimating 

Ammonia Emissions from Anthropogenic Nonagricultural Sources - Draft Final Report” (April 2004) and 

covers many point source processes as well as nonpoint ones. Although a lot of research has been done 

on possible new approaches to agricultural NH3 estimation as point sources, these “farm-based” 

approaches have not been completed yet. At this time, the EPA augments agricultural NH3 sources only 

as nonpoint sources, rather than as point sources. Readers should see Section 4.8.2 for more 

information on emissions estimation from nonpoint sources.

A final resource is to use the facility-total NH3 from the TRI. For SIP planning inventory purposes, these 

emissions should be apportioned to individual processes, but at least can be a starting point in the 

absence of other information. TRI provides emissions by stack and fugitive, with all stack emissions 

summed together and all fugitive emissions together. This level of detail would not be sufficient for a 

significant NH3 source and so, while TRI can be a starting point, additional information would need to be 

gathered from the facility to apportion the emissions to the processes at the facility.

Startup/shutdown emissions

The regulatory expectations for SSM emissions are included as part of 40 CFR part 52 in the EPA’s “State 

Implementation Plans: Response to Petition for Rulemaking; Restatement and Update of EPA’s SSM 

Policy Applicable to SIPs; Findings of Substantial Inadequacy; and SIP Calls to Amend Provisions Applying 

Excess Emissions During Periods of Startup, Shutdown, and Malfunction” final rule (SSM rule). A fact 

sheet and list of affected states is available on the website for this action as previously noted at 

https://www3.epa.gov/airquality/urbanair/sipstatus/emissions.html. 

The requirements of the Ozone and PM2.5 Implementation Rules and the Regional Haze Rule require the 

reporting of historical annual-total emissions only (and in some areas “typical” seasonal and/or daily 

emissions for certain pollutants), not day-to-day emissions. As noted in the preamble of the final SSM 

rule:

“Actual emissions during SSM events should be included in these annual emissions. While 

data formats are available from the EPA to allow a state to segregate the total annual 

emissions during SSM events from annual emissions during other types of operation, 

segregation is not a requirement and few states do so” (80 FR 113, page 33950).

The key takeaway message from this guidance is that since startup/shutdown emissions are planned and 

predictable, they should be included in SIP planning inventories. While the final SSM rule generalizes 

that “SSM” emissions should be included, this guidance clarifies that this means startup/shutdown 
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rather than malfunction as further described below. In addition, states should pay particular attention to 

estimating startup/shutdown emissions if these types of emissions are believed by the air agency to 

have a significant contribution to the NAAQS design values.

Since malfunctions are, by nature, unpredictable and given the myriad different types of malfunctions 

that can occur, malfunction emissions would be difficult to estimate and future-year malfunction events 

cannot be readily predicted. Thus, states are not obligated to include malfunction emissions in the base 

inventory for the NAA, ROP/RFP plans, or attainment projected inventory for the NAA. However, to the 

extent that malfunctions become a regular and predictable event, then such emissions should be 

quantified with regular and predictable emissions and included in emission inventories for planning 

purposes. The elimination of high emissions during routinely reoccurring malfunctions could potentially 

help achieve significant emissions reductions needed by a state in attaining the relevant NAAQS.

For sources with CEMs that capture emission rates at all times, the CEM will capture the 

startup/shutdown emissions without taking any of the other steps described below. Without CEMs, 

estimating startup/shutdown emissions becomes more complicated.

Startup/shutdown emissions can be quantified by assessing the emissions during startup or shutdown 

periods using traditional emissions estimation approaches of throughput and emission factors. However, 

the throughput and emission factors must be customized for the activity during these periods. 

Generally, emissions rates during these periods are higher and less well characterized than a typical 

operating emissions rate, and so some degree of expert judgement may be needed in assessing these 

emissions. For example, if control devices are not effective or active during these periods, the emissions 

during these types would not be able to reflect controlled emissions rates. If device emissions rates are 

associated with an operating temperature and the emissions are higher during a warm-up period, 

adjustments to the emissions during the startup period could be made.

In addition to estimating the actual emissions during startup/shutdown periods, another approach to 

estimate startup/shutdown emissions is to adjust control parameters via the emissions calculation 

parameters of rule effectiveness or primary capture efficiency. Using these parameters for 

startup/shutdown adjustments is not their original purpose, but can be a simple way to account for the 

emissions and still have a record of the routine versus startup/shutdown portions of the emissions. For 

example, a lower capture efficiency or rule effectiveness can adjust the estimate of emissions to be 

higher, accounting for extra emissions from startup/shutdown activities. In the absence of 

measurements of throughput and/or emissions rates during startup/shutdown periods, expert judgment 

may need to be used to define the adjustment factors to use. This approach, however, can complicate 

the use of the control parameters during development and documentation of control strategies since 

the control parameters reflect things other than controls – thus, care should be taken when using this 

approach.

Emissions associated with HEDD periods

As described in Section 4.5, when air agencies have determined that emissions associated with HEDD 

periods are important for addressing their nonattainment problem, those air agencies should include 
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HEDD-related emissions in their planning inventories. This subsection first describes what information is 

available to help assess when HEDD periods occur and to help provide activity data that could be used as 

part of estimating the associated emissions. Second, this subsection describes how to consider HEDDrelated emissions in planning inventories (e.g., the base year inventory for the NAA) as compared to 

those in modeling inventories.

Emissions from HEDD periods can include additional emissions from more typical EGU boilers, turbines, 

and other units (collectively called peaking units), and also from combustion-based sources of 

distributed generation such as large back-up generators and other engines. A key challenge in 

developing emissions estimates for HEDD-periods is determining when the peaking units and distributed 

generation units were operating and consequently produced emissions. In the case of peaking units, 

many of these sources have CEMS that record activity and emissions levels, and these data are reported 

to EPA. In the case of distributed generation units, little or no information may be readily available in 

part because the energy produced by the backup generators and other engines is not measured by the 

electric utilities and independent system operators (ISOs), but rather may be experienced by the utilities 

and ISOs as an unmeasured demand reduction.

During HEDD periods, energy customers may use backup generators to participate in demand response 

programs. Though some useful information may be available from the utilities and ISOs running those 

programs, there may also be no information available. The specific rules, reporting, and frequency of 

use will vary by the utility or ISO that is offering the demand response program. Demand response 

program requirements vary and can include: (a) completely excluding the use of backup generators, (b) 

allowing the use of backup generators and requiring some reporting, and (c) not requiring customers to 

report anything about actions taken to provide the demand response. The lack of data on demand 

response backup activity has been noted by others based on past analyses.

20,21 Where emissions from 

demand response programs may be important, air agencies should first collaborate with their state

energy and public utility commission counterparts who may have established mechanisms for 

information collection and sharing. Air agencies may also need to assess the demand response programs 

in their area by working directly with utilities and/or ISOs.

Starting in 2015, the National Emission Standard for Hazardous Air Pollutants (NESHAP) and New Source 

Performance Standards for Reciprocating Internal Combustion Engines (RICE) (40 CFR part 60, Subparts 

IIII and JJJJ, and 40 CFR part 63, Subpart ZZZZ), also called the RICE rules, now require owners/operators 

of emergency engines participating in emergency demand response programs to report the dates and 

times that the engines operate for emergency demand response as well as engine information such as 

horsepower and model year. While the RICE rules do not require the reporting of NOX emissions or 

information about non-emergency programs, they will start to help provide information useful for 

 

20 White paper: “Air Quality, Electricity, and Back-up Stationary Diesel Engines in the Northeast,” Northeast States 

for Coordinated Air Use Management (NESCAUM), August 1, 2012.

21 See also the Mid-Atlantic Distributed Resources Initiative (MADRI) working group meeting information available 

at http://sites.energetics.com/madri/meetings_2012.html.
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understanding the activity of these back-up generators and engines, which may also be helpful for ozone 

planning purposes.

While it may continue to be difficult to find data needed for assessing HEDD-related emissions, air 

agencies may still include these emissions in both their modeling inventories and planning inventories. 

The HEDD-related emissions included by air agencies in planning inventories should have some logical 

relationship with the emissions in modeling inventories. The planning inventories will of course be 

limited spatially to the NAA, while the modeling inventories will cover a larger area. Air agencies should 

extract the appropriate NAAs to include in the planning inventories. Temporally, the modeling 

inventories will include day- and hour-specific emissions for seasonal or episodic periods, which can 

include HEDD periods, while the planning inventories will be an ozone season day emissions value. As 

explained in Section 4.5, the days used to calculate the ozone season day emissions are chosen for the 

particular SIP in consultation with the Regional office. Some examples of possible relationships between 

the modeling inventories (episode-specific) and the planning inventories (ozone season day) are as 

follows:

 Ozone season day emissions could be an average across all days of the episode-specific 

emissions used in the modeled attainment demonstration.

 Ozone season day emissions could be an average of a longer episode or series of episodes 

that overlap with the episodic period used for the modeled attainment demonstration.

 Ozone season day emissions could be a representative selection of the episodic-specific 

emissions, such as a median emissions value or using a single day emissions value that 

somehow is representative of other days.

These are not the only possible examples. Any approach chosen to determine appropriate HEDD-related 

ozone season day emissions should be developed in consultation with the Regional office and should 

seek to meet the overall goal for the base year inventory. Namely, the NAA inventory should be 

representative of the emissions conditions within the NAA that typically lead to nonattainment 

problems.

4.8.2 Stationary Nonpoint 

Nonpoint sources collectively represent individual sources that have not been inventoried as specific 

point or mobile sources. These individual sources treated collectively as nonpoint sources are typically 

too small, numerous, or difficult to inventory using the methods for the other classes of sources. 

Stationary nonpoint sources can be estimated through numerous different approaches. Though some 

approaches are outdated, the EIIP Volume III, Chapter 1, Sections 1 through 6 provide relevant 

information on compilation and calculation of nonpoint source inventories. Also, the Volume III 

webpage contains numerous supplemental reports for specific source categories. 

For many categories, more updated approaches are included in the EPA tools developed for use by air 

agencies in developing inventories. The latest tools are provided as part of the materials for the latest 

NEI – either as developmental materials for an ongoing NEI effort, or as supporting materials to 

document a completed NEI effort.
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Table 17 provides a list of emissions categories, pollutants, and available approaches documentation 

and/or tools. Older versions of the tools used for the 2011 NEI are still available as part of the 2011 NEI 

documentation at https://www.epa.gov/air-emissions-inventories/2011-national-emissions-inventorynei-documentation under the “Data and documentation” link.

Table 17: List of nonpoint categories, pollutants, and available approaches and tools

EPA-estimated 

emissions source 

description

Possible 

Overlap 

with 

Point

Available Nonpoint Resources (Posted year)

Agricultural Fertilizer 

Application

2014_Fertilizer_Application_v1.0_22apr2016.zip (2016)

Old: ag_fertilizer_application_2011.zip (2013)

Agricultural Fires 2014 Draft Agricultural Burning and Grass/Pasture Estimated Emissions (2016)

Agricultural Tilling 2014_Agricultural_Tilling_v3.1_10mar2016.zip (2016)

Animal Husbandry 2014_Ag_Livestock_v1.0_20may2016.zip (2016)

Old: animal_livestock_emissions_2011.zip (2009)

Architectural Coatings Solvent_Tool_v1_5.zip (2016)

EIIP, Volume III, Chapter 3 (1995)

Asphalt Paving Y

2014_NPt_Asphalt_18nov2015_edit03302016.zip (2016)

EIIP, Volume III, Chapter 17 (2001)

Asphalt Roofing Kettles Y See abstract available with the EIIP Volume III website (2000)

Autobody Refinishing Y Solvent_Tool_v1_5.zip (2016)

Aviation Gasoline 2014_Av_Gas_Stage_1_15nov2015.zip (2015)

2014_Av_Gas_Stage_2_15nov2015.zip (2015)

Backyard Barbecues 2014_Backyard_BBQs_15nov2015.zip (2016)

Commercial Cooking 2014_Commercial_Cooking_v1.2_08mar2016.zip (2016)

Consumer & Commercial 

– All personal care 

products

Solvent_Tool_v1_5.zip (2016)

EIIP Volume III, Chapter 5 (1996)

Consumer & Commercial 

– All household products

Solvent_Tool_v1_5.zip (2016)

EIIP Volume III, Chapter 5 (1996)

Consumer & Commercial 

– All coatings and 

related products

Solvent_Tool_v1_5.zip (2016)

Consumer & Commercial 

– All adhesives and 

sealants

Solvent_Tool_v1_5.zip (2016)

EIIP Volume III, Chapter 5 (1996)

Consumer & Commercial 

– All FIFRA related 

products

Solvent_Tool_v1_5.zip (2016)

Degreasing Y Solvent_Tool_v1_5.zip (2016)

Dental Preparation and 

Use 

epa_mercury_dataset_ 62714(2).zip (2015)

Documentation for the 1999 Base Year Nonpoint area source National 

Emissions inventory for HAPs, page A-30 (2000)

Dry Cleaning Y

Solvent_Tool_v1_5.zip (2016)

EIIP, Volume III, Chapter 4 (1996)

Dust – Paved Roads 2014_Road_Dust_v2.1_09mar2016.zip (2016)

Dust – Unpaved Roads 2014_Road_Dust_v2.1_09mar2016.zip (2016)

Dust – Construction 2014_Construction_Dust_v3.0_18feb2016.zip (2016)
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EPA-estimated 

emissions source 

description

Possible 

Overlap 

with 

Point

Available Nonpoint Resources (Posted year)

Gasoline 

Distribution/Marketing Y

2014_Gasoline_Distribution_v1.0_with_PT_subtraction_01apr2016.zip (2016)

EIIP, Volume III, Chapter 11 (2001)

General Laboratory 

Activities epa_mercury_dataset_ 62714(2).zip (2015)

Graphic Arts Y

Solvent_Tool_v1_5.zip (2016)

EIIP Volume III, Chapter 7 (1996)

Human Cremation Y cremation_2011.zip (2013)

Industrial, 

Commercial/Institutional Fuel CombustionY

ICI Tool v1_4.zip (2015)

Estimating Ammonia Emissions From Anthropogenic Nonagricultural Sources -

Draft Final Report (April 2004)

Industrial Surface 

Coating Y

Solvent_Tool_v1_5.zip (2016)

EIIP, Volume III, Chapter 8 (1997) 

Lamp Breakage (Landfill 

emissions) epa_mercury_dataset_ 62714(2).zip (2015)

Lamp (Fluorescent) 

Recycling epa_mercury_dataset_ 62714(2).zip (2015)

Landfills Y EIIP, Volume III, Chapter 15 (2001)*

Mining and Quarrying 2014_Mining_and_Quarrying_v2.3_09mar2016.zip (2016)

Oil and Gas Production Y

OIL_GAS_TOOL_2014_NEI_EXPLORATION_V1_5.zip (2016)

OIL_GAS_TOOL_2014_NEI_PRODUCTION_V1_5_Access2007.zip (2016)

OIL_GAS_TOOL_2014_NEI_PRODUCTION_V1_5_Access2013.zip (2016)

Open Burning (Leaves, 

Brush, Residential 

Household Waste, Land 

Clearing)

2014_Open_Burning_v1.1_03mar2016.zip (2016)

Other Special Purpose 

Coatings Y Solvent_Tool_v1_5.zip (2016)

Pesticide Application 2014_Agricultural_Pesticides_v2.0_18feb2016.zip (2016)

EIIP Volume III, Chapter 9 (2001)

Portable Gas Cans 

(Residential and 

Commercial)

2014_Portable_Fuel_Containers_25nov2015.zip (2016)

Publicly Owned 

Treatment Works Y 2014_POTW_nonpoint_emissions_23march2016.zip (2016)

Residential Heating -

Fossil Fuels

2014_Residential_Heating_non-Wood_v1.2_09mar2016.zip (2016)

See also abstracts available for coal, fuel oil & kerosene, gas & liquified 

petroleum gas (LPG) available with the EIIP Volume III website (1999).

Residential Heating –

Wood

2014_RWC_v3.0_28apr2016.zip (2016)

EIIP, Volume III, Chapter 2 (2001)

Structure Fires EIIP, Volume III, Chapter 18 (2001)

Traffic Markings Solvent_Tool_v1_5.zip (2016)

EIIP, Volume III, Chapter 14 (1997)

Vehicle Fires See abstract available with the EIIP Volume III website (2000)

Waste Disposal 

(Incineration)

Estimating Ammonia Emissions From Anthropogenic Nonagricultural Sources -

Draft Final Report (April 2004)

* Landfill emissions estimates are available in the NEI as point sources, based on the activity information submitted to the EPA 

as part of the Greenhouse Gas Reporting Rule.
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As indicated in Table 17 by the column “Possible Overlap with Point,” some source types can have 

emissions in both the point source data category and the nonpoint one. Some categories that may have 

both types of sources are denoted, and this list is based on what has been seen in the NEI. The list of 

categories in this situation depend on an individual state’s policies, such as what the emissions size 

threshold is for a point source and whether any categories are treated as point sources despite having 

low individual emissions.

To prevent overestimation of nonpoint emissions for sectors that also have point sources, it is important 

to perform point/nonpoint reconciliation. This reconciliation starts by subtracting the activity associated 

with the point sources from the total activity data (prior to computing the nonpoint emissions). Since 

point sources often have emissions controls, it is usually not sufficient to just subtract point source 

emissions from total activity-based emissions to prevent double counting. Subtracting controlled point 

source emissions from uncontrolled total activity-based emissions would result in too much nonpoint 

emissions. Rather, the point source activity must be determined and subtracted from total activity to get 

the nonpoint activity estimate. Alternatively, uncontrolled point source emissions can be calculated and 

subtracted from the total activity-based emissions estimates to derive a nonpoint emissions estimate.

Part of developing refined nonpoint emissions estimates for a SIP could involve the use of surveys to 

collect data. Inventory preparers often use survey questionnaires to gather point source emissions 

inventory data. The EIIP Volume III, Chapter 24, written in 2000, provides guidance on developing and 

conducting surveys for area source inventories. Since this document predates some of the newer online 

survey techniques, the sections describing the use of the internet are outdated. As part of the 

documentation for inventories, survey design and results should be kept and included in any 

documentation reports or appendices.

4.8.3 On-road Mobile 

Emissions from on-road vehicles are the result of several emission processes, including the combustion

of fuel while vehicles are starting, idling, and moving, evaporation of fuel from the fuel system and 

during refueling, and from brake wear and tire wear.

For all states but California, on-road mobile emissions should be estimated with the latest EPA on-road 

mobile model, MOVES. MOVES estimates emissions for many combinations of vehicle classifications and 

fuel types. Readers should follow the latest MOVES guidance available at 

http://www3.epa.gov/otaq/models/moves/. For California, the most recent EPA-approved version of 

EMFAC should be used.

The available guidance covers using MOVES for inventory development in SIPs in states other than 

California. The guidance focuses on determining what the appropriate inputs are and how MOVES 

should be run to develop emissions estimates for on-road vehicles. It describes when default MOVES

inputs are appropriate for SIPs and regional conformity analysis and what factors to consider for 

providing locality-specific MOVES inputs.
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MOVES estimates emissions for multiple source use types (the terminology that MOVES uses to describe 

vehicle type) and fuel types. MOVES also estimates emissions for different road types that represent 

rural and urban unrestricted and restricted (accessed only by an on or off ramp) access roads, as well as 

off-network emissions such as vehicle starts, evaporative emissions while parked, and hoteling 

emissions when long-haul combination trucks are parked for long periods with the engine running under 

load.

One of the key elements for use of MOVES is creating a Run Specification (RunSpec) file, which defines 

the place and time period of the analysis as well as the vehicle types, road types, fuel types, and the 

emission-producing processes and pollutants that will be included in the MOVES run. The MOVES 

Technical Guidance describes using the MOVES graphical user interface GUI to set up a MOVES run for a 

particular scale (e.g., county), time period, set of pollutants, and the vehicle types, road types, and fuel 

types applicable to a particular area and time.

Another key element for use of MOVES is entering local meteorology, fleet, activity, fuel, and control 

measure information using the County Data Manager. The MOVES Technical Guidance discusses when

local data should be used in place of model default data, as well as potential sources of local data. 

For some input parameters, there is overlap with the inputs needed for other models, such as the 

nonroad mobile models (e.g., meteorology and fuels). Efforts should be made to use the same source of 

data across multiple categories when the inputs are shared. Not doing so calls into question the validity 

of one or both approaches. Air agencies should explain the use of shared inputs, or reasons for not using 

them, as part of the documentation provided with their inventories.

4.8.4 Nonroad Mobile Equipment

Nonroad mobile equipment emissions result from the use of fuel in a diverse collection of vehicles and 

equipment, including:

 recreational vehicles, such as all-terrain vehicles and off-road motorcycles;

 logging equipment, such as chain saws;

 agricultural equipment, such as tractors;

 construction equipment, such as graders and back hoes;

 industrial equipment, such as forklifts and sweepers;

 residential and commercial lawn and garden equipment, such as leaf and snow blowers;

 recreational marine vessels, such as power boats.

Starting with MOVES 2014, MOVES incorporates and expands upon the capabilities of the older 

NONROAD model. States are required to use MOVES for any nonroad source emissions estimates for SIP 

development that is started since the release of MOVES 2014.

MOVES nonroad capabilities include predicting emissions for all of these nonroad equipment categories

listed above. Emissions from CMVs, locomotives, and aircraft are estimated in other ways and described 

in Section 4.8.5. The MOVES nonroad capabilities include more than 80 basic and 260 specific types of 
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nonroad equipment and further stratifies equipment types by horsepower rating. Fuel types include 

gasoline, diesel, compressed natural gas (CNG), and Liquified Petroleum Gas (LPG).

The nonroad capabilities of the MOVES model estimates emissions for all criteria pollutant and 

precursors from both exhaust and non-exhaust processes (diurnal, refueling spillage, vapor 

displacement, hot soak, running loss, take permeation, hose permeation, and crankcase emissions). 

NONROAD supports many SIP-related inventory development needs, including support for partial 

counties and seasonal emissions.

Air agencies have the option of replacing default model inputs with more representative data. Common 

input adjustments include equipment population, geographic allocations, and local growth rates. If 

agencies make changes to default model values, the agency should submit the input files to the EPA as 

well as a description of why the defaults were changed. Additional guidance on improving nonroad 

estimates for lawn and garden equipment can be found in EIIP, Volume IV, Chapter 3.

For some input parameters, there is overlap with the inputs needed for other models, such as the onroad mobile models (e.g., meteorology and fuels). Efforts should be made to use the same source of 

data across multiple categories when the inputs are shared. Not doing so calls into question the validity 

of one or both approaches. Air agencies should explain the use of shared inputs, or reasons for not using 

them, as part of the documentation provided with their inventories.

4.8.5 Other Nonroad Mobile 

The MOVES model does not provide air agencies with a tool for estimating emissions for CMVs, aircraft, 

and locomotives. Thus, separate methods are used for each of these emissions sectors.

For purposes of reporting data to the EPA, aircraft emissions are often reported as point sources, with 

this being a requirement for reporting data to the EIS for use in the NEI. Likewise, the CMV and 

locomotive data are included in the nonpoint data category in the EIS. While these sources are still 

mobile sources from the traditional standpoint of emissions categories, for data formatting and 

reporting purposes in the EIS, they are treated along with these non-mobile data categories.

4.8.5.1 Aircraft

The aircraft sector includes all aircraft types used for public, private, and military purposes. This includes 

four types of aircraft: (1) commercial, (2) air taxis (AT), (3) general aviation (GA), and (4) military. A 

critical detail about the aircraft is whether each aircraft is turbine- or piston-driven, which allows the 

emissions estimation model to assign the fuel used, jet fuel or aviation gas, respectively. The fraction of 

turbine- and piston-driven aircraft is either collected or assumed for all aircraft types. NEI data are 

available for this sector for use as a starting point in SIPs, though the NEI includes military aircraft only 

where state or local air agencies have submitted military aircraft emissions data.

Commercial aircraft include those used for transporting passengers, freight, or both. Commercial aircraft 

tend to be larger aircraft powered with jet engines. ATs carry passengers, freight, or both, but usually 

are smaller aircraft and operate on a more limited basis than the commercial aircraft. GA includes most 
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other aircraft used for recreational flying and personal transportation. Finally, military aircraft are 

associated with military purposes, and they sometimes have activity at non-military airports.

The national AT and GA fleet includes both jet- and piston-powered aircraft. Most of the AT and GA fleet 

are made up of larger piston-powered aircraft, though smaller business jets can also be found in these 

categories. Military aircraft cover a wide range of aircraft types such as training aircraft, fighter jets,

helicopters, and jet-powered and piston-powered planes of varying sizes.

Emissions at airports also include other sources that are a part of the nonroad equipment emissions 

described in Section 4.8.4. These emissions come from aircraft auxiliary power units (APUs) and aircraft 

ground support equipment (GSE) typically found at airports, such as aircraft refueling vehicles, baggage 

handling vehicles, and equipment, aircraft towing vehicles, and passenger buses. In the NEI point source 

inventory, these APUs and GSE are located at the airport facilities as point sources along with the 

aircraft exhaust emissions (though not as part of the aircraft sector). Emissions from APUs and from GSE 

can be calculated using the EPA’s NONROAD model and by the Federal Aviation Administration (FAA) 

Emissions and Dispersion Modeling System (EDMS). If the air agency chooses to calculate these 

emissions with the EDMS, care should be taken to remove exclude those emissions from emissions 

associated with the NONROAD model.

The EPA collaborates with the FAA to develop and maintain methods for estimating aircraft emissions 

using EDMS, which also estimates emissions from aircraft in addition to the APUs and GSE as noted 

above. The emissions developed in this way are associated with aircrafts’ landing and takeoff (LTO) 

cycle. The EPA encourages state and local air agencies to collect aircraft LTO activity data that can be 

used in improving estimates using the EPA EDMS-based methods. More details on the EPA methods are 

available in the documentation and methods associated with the latest NEI year found at

https://www.epa.gov/air-emissions-inventories/national-emissions-inventory. 

The aircraft sector includes numerous SCCs as shown in Table 18, and all but two of these are associated 

with the NEI point source inventory. Historically, airport emissions have been calculated as a nonpoint 

source, but with the latest approaches having all airport locations readily available, such a simplification 

is no longer needed. The result of this change from nonpoint to point sources is that most of the SCCs 

are shown as 10-digit SCCs (usually used just for nonpoint sources) but are now used as point SCCs. 

Table 18: Aircraft SCCs and Data Categories in EPA Estimates

SCC Data Category SCC Description EPA estimates

2275001000 Point Mobile Sources; Aircraft; Military Aircraft; Total X

2275020000 Point Mobile Sources; Aircraft; Commercial Aircraft; 

Total: All Types X

2275050011 Point Mobile Sources; Aircraft; General Aviation; Piston X

2275050012 Point Mobile Sources; Aircraft; General Aviation; Turbine X

2275060011 Point Mobile Sources; Aircraft; Air Taxi; Piston X

2275060012 Point Mobile Sources; Aircraft; Air Taxi; Turbine X

2260008005 Point Mobile Sources; Off-highway Vehicle Gasoline 2-

Stroke; Aircraft Ground Support Equipment X
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SCC Data Category SCC Description EPA estimates

2265008005 Point Mobile Sources; Off-highway Vehicle Gasoline 4-

Stroke; Aircraft Ground Support Equipment X

2267008005 Point Mobile Sources; LPG; Aircraft Ground Support 

Equipment X

2268008005 Point Mobile Sources; CNG; Aircraft Ground Support 

Equipment X

2270008005 Point Mobile Sources; Off-highway Vehicle Diesel; 

Aircraft Ground Support Equipment X

2275070000 Point Mobile Sources; Aircraft; Aircraft Auxiliary Power 

Total X

2275085000 Nonpoint Mobile Sources; Aircraft; Unpaved Airstrips; Total

2275087000 Nonpoint Mobile Sources; Aircraft; In-flight (non-LandingTakeoff cycle) X

In June 2012, the EPA promulgated standards for NOX emissions from commercial aircraft engines. 

Information about this rule including fact sheets, test procedures, and emissions documentation is 

available on the EPA website for this purpose (https://www3.epa.gov/otaq/aviation.htm). In that rule, 

the EPA adopted Tier 6 and Tier 8 standards for NOX on new aircraft engines, with compliance dates in 

2013 and 2014. Prior rules published in 2005 and 1997 also reduce aircraft NOX emissions with engine 

standards (documented on the same website). When estimating emissions from aircraft, it is best 

practice to consider the impacts of these rules for improving the accuracy of emissions inventories. This 

is why the EPA recommends using the EPA EDMS-based approach with locality-specific data.

State or local air agencies may determine that the EPA default method is not appropriate for SIP 

purposes, though the EPA recommends considering the approach but using more locality-specific

activity information. For areas that have provided the EPA locality-specific airport activity data, the NEI 

estimates may very well provide emissions needed to meet SIP requirements. The EPA method is more 

robust than older methods and includes national control programs as noted below. As shown in the 

table above, the EPA’s methods do not estimate emissions from unpaved airstrips (SCC 2275085000). 

Furthermore, the “in-flight” SCC (2275087000) is covered by the EPA’s method for only lead (Pb) 

emissions and is related to general aviation aircraft that still use leaded fuel. Air agencies may want to 

consider unpaved airstrips and/or in-flight emissions for other pollutants as part of their inventories for 

improved model performance or other reasons, though doing so may not be necessary depending on 

the significance of those emissions in a particular area. Any locality-specific data or custom emissions 

associated with aircraft sources are expected to be included with supporting documentation for the 

emissions inventory used for a SIP.

4.8.5.2 Commercial Marine

The CMV emissions category includes boats and ships used either directly or indirectly in the conduct of 

commerce or military activity. The majority of vessels in this category are powered by diesel engines 

that are either fueled with distillate or residual fuel oil blends. For the purpose of this inventory, we

assume that Category 3 (C3) vessels primarily use residual blends while Category 1 and 2 (C1 and C2) 

vessels typically use distillate fuels. Another consideration for emissions estimation of CMV sources is 
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auxiliary engines. All three types of vessels may have auxiliary engines that are used for onboard power 

generation or other needs.

The C3 inventory includes vessels which use C3 engines for propulsion. C3 engines are defined as having 

displacement above 30 liters per cylinder. C3 inventories typically include emissions from both 

propulsion and auxiliary engines used on these vessels, as well as those on gas and steam turbine 

vessels. Geographically, the inventories include port and interport emissions that occur within the area 

that extends 200 nautical miles (nm) from the official U.S. shoreline, which is roughly equivalent to the 

border of the U.S. Exclusive Economic Zone. For national inventories created by the EPA, only some of 

these emissions are allocated to states based on official state boundaries that typically extend 3 nm 

offshore. The EPA recommends that state water boundaries also be used when determining which 

emissions should be included with a particular state or county.

The C1 and C2 vessels tend to be smaller ships that operate closer to shore, and along inland and 

intercoastal waterways, with engines greater than or equal to 37kW. Naval vessels are not included in 

C1 and C2 vessel inventories, though Coast Guard vessels are included. While these C1/C2 vessels are 

most often associated with near-land applications and thus included as within state waters, sometimes 

these vessels are used outside of state waters, particularly in the Gulf of Mexico. In these cases, as with 

C3 vessels, the EPA recommends that state water boundaries be used when calculating emissions 

associated with a particular state or county.

The CMV source category does not include recreational marine vessels, which are generally less than 

100 feet in length, most being less than 30 feet and powered by either inboard or outboard engines. The

calculation of these emissions is included in the NONROAD model, and these emissions are, therefore,

considered “nonroad equipment.”

The EPA develops and maintains methods for estimating CMVs, and these methods are used as 

“fallback” methods for use in the NEI. They are also available as a fallback for states without other data 

for this sector. At this time, the EPA’s methods are top-down national methods, which apportion 

nationally estimated emissions values to the states and counties. In the latest such approaches (used for 

the 2008 and 2011 NEI), this apportionment includes allocation to GIS shapefiles for ports and underway 

emissions. These shapefiles are available with the latest NEI documentation and include separate files 

for ports and underway. Key differences associated with various EPA methods from the 2002 NEI 

through the 2011 NEI have focused on allocation of emissions to the shapefiles, which has been done in 

collaboration with state and local air agencies.

The current EPA methods also use emissions types. Each of the CMV SCCs requires an appropriate 

emissions type (M=maneuvering, H=hoteling, C=cruise, Z=reduced speed zone), because emission 

factors vary by emission type. For the NEI, each SCC and emissions type combination was allocated to a 

shapefile identifier in the nonpoint inventory. The allowed combinations of SCCs and emission types are 

shown in Table 19. The default values are those assumed when the actual emission type may be 

unknown; for example, emissions that occur in shipping lanes are assumed to be ‘cruising’ and cannot 

be ‘hoteling,’ which only occurs at ports.
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Table 19: Commercial marine SCCs and emission types in EPA emissions

SCC SCC Description Allowed Default

2280002100 Marine Vessels, Commercial Diesel Port M M

2280002200 Marine Vessels, Commercial Diesel Underway C C

2280003100 Marine Vessels, Commercial Residual Port H H

2280003100 Marine Vessels, Commercial Residual Port M H

2280003200 Marine Vessels, Commercial Residual Underway C C

2280003200 Marine Vessels, Commercial Residual Underway Z C

State or local agencies may determine that the EPA default method is not appropriate for SIP purposes 

or may need further refinement by air agencies in developing SIPs. For example, the general calculation 

method may be a reasonable approach, but the activity and spatial allocation achieved from a top-down 

approach could be improved by using the same methods with locally resolved input data to calculate 

emissions for specific areas.

When estimating CMV emissions, reporting agencies should consider national regulations that decrease 

emissions from this sector, such as increasingly stringent emissions requirements for new marine diesel 

engines and sulfur requirements for marine diesel fuel and adjust their estimates accordingly. Emissions 

reductions resulting from requirements for new marine engines are generally not immediately reflected 

in inventories as newer engines are gradually incorporated into the vessel fleet and as older 

engines/vessels are retired. Emissions reductions resulting from the use of lower sulfur fuels are 

generally seen more quickly, though estimating impacts can be complicated by vessels coming from 

other parts of the world with different fuel standards. Table 20 provides a list of more recent relevant 

regulations and associated references for the CMV category. Older Tier 1 and 2 regulations on marine 

diesel engines, dating back to 1999, are also listed on the websites shown in the table.

Table 20: Commercial marine vessel recent regulations and impacted pollutants

Regulation Pollutants 

impacted

Reference

Great Lakes Steamship Repower Incentive Program 

(January, 2012) SO2, PM2.5

www3.epa.gov/otaq/

oceanvessels.htm Tier 2 and Tier 3 Emissions Standards for Category 3 

Marine Diesel (April, 2010). Includes both engine 

standards and fuel sulfur limits.

SO2, NOX, PM2.5

Tier 3 and Tier 4 Emission Standards for Category 1 and 

2 Marine Diesel Engines (June, 2008).

PM2.5, NOX

www3.epa.gov/otaq/

marine.htm#regs Nonroad Diesel Tier 4 Rule (May 2004). Decreased 

allowable levels of sulfur in marine diesel fuel by 99%, 

taking effect in 2007.

SO2

For states needing to refine their emissions estimates beyond the values produced by the EPA’s default 

methods, Table 21 provides a list of the latest resources for states in better understanding how to 

calculate emissions for the CMV sources. The newer references are included higher in the table. The

EPA’s recent method updates have occurred in conjunction with rule development efforts, and so some 
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of these documents are associated with inventories developed for the rules listed above. Even newer 

methods may become available over time and can be considered for use in SIP planning inventory 

development. Much of this information is also available on the EPA nonroad mobile website under 

“Other Nonroad Reference Material,” available at http://www3.epa.gov/otaq/nonrdmdl.htm. Any 

locality-specific data or custom emissions associated with CMV emissions are expected to be included 

with supporting documentation for the emissions inventory used for a SIP.

Table 21: Current EPA methods for commercial marine vessel emissions.

Inventory approach resource CMV Approaches For... Reference

Current Methodologies in 

Preparing Mobile Source 

Port-Related Emissions 

Inventories (April, 2009)

Ocean going (generally C3) 

vessels, harbor craft, cargo 

handling,

https://archive.epa.gov/sectors/web

/pdf/ports-emission-inv-april09.pdf

Draft Regulatory Impact 

Analysis: Control of 

Emissions of Air Pollution 

from Category 3 Marine 

Diesel Engines (June, 2009)

Bottom-up port C3 emissions for 

89 deep water and 28 Great Lake 

ports in the U.S. and interport 

emissions from the Waterway 

Network Ship Traffic, Energy and 

Environmental Model (STEEM).

https://www3.epa.gov/otaq/regs/

nonroad/marine/ci/420d09002.pdf. 

See Chapter 3.

Category 2 Vessel Census, 

and Spatial Allocation 

Assessment and Category 1 

and Category 2 in-Port/AtSea Splits (EPA-HQ-OAR2003-0190-0423)

C1 and C2 vessels

https://www.regulations.gov/docum

ent?D=EPA-HQ-OAR-2003-0190-

0423

Final Regulatory Impact 

Analysis: Control of 

Emissions of Air Pollution 

from Locomotive Engines 

and Marine Compression 

Ignition Engines Less than 30 

Liters per Cylinder (May 

2008)

C1 and C2 propulsion, C1 marine 

auxiliary, <37kW commercial 

propulsion, and <37kW marine 

auxiliary.

http://nepis.epa.gov/Exe/ZyPURL.cgi

?Dockey=P10024CN.txt. Chapter 1 

provides more in-depth description 

of engine types, while Chapter 3 

provides emissions calculation 

approach.

Emissions Inventory

Improvement Program

Evaporative VOC emissions from 

marine transport of petroleum 

liquids, including the loading and 

unloading of the petroleum 

liquids.‡

EIIP, Volume III, Chapter 12 “Marine 

Vessel Loading, Ballasting, and 

Transit.”

‡ These emissions are often included in as part of the nonpoint inventory rather than associated with the CMV 

sector.

Older approaches are also available. In 1999, the EPA sponsored two studies, “Commercial Marine 

Activity for Great Lake and Inland River Ports in the United States,” and “Commercial Marine Activity for 

Deep Sea Ports in the United States.” Both of these reports are available at 

https://www3.epa.gov/otaq/oceanvessels.htm. These studies provide activity profiles for approximately 

150 ocean, lake, and river ports, and present a method for an inventory preparer to allocate time-in-
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mode activity data from one of four typical ports to another port that has similar characteristics. Activity 

profiles for a typical port include: 1) number of vessels in each category; 2) vessel characterization, 

including propulsion size (horsepower), capacity tonnage, and engine age; and 3) number of hours at 

each time-in-mode associated with cruising, reduced speed, maneuvering, and hoteling. These methods 

provide alternative approaches for state and local air agencies in preparing SIP inventories, but care 

should be taken to incorporate the impact of national rules incorporated into the newer approaches as 

documented in Table 21. 

4.8.5.3 Locomotives

The locomotive emissions category includes diesel-electric engines, which use 2- or 4-stroke diesel 

engines and an alternator or a generator to produce the electricity required to power its traction 

motors. The EPA default approach does not include locomotives powered by electricity or steam. It is 

believed that the number of wood or coal driven steam locomotives in the U.S. is currently very small; 

therefore, these types of locomotives are not generally included. In specific instances, if such sources 

are relevant to a NAA, then they may need to be included. The locomotive sources are further divided 

into categories: Class I line haul; Class II/III line haul; Passenger; Commuter; and Yard.

In the NEI, rail yard emissions are treated as point sources. Whether rail yards should be treated as a 

point source for SIP planning purposes depends on the importance of these emissions to the NAA and 

the analyses that need to be done for any modelled attainment demonstration. Emissions at these 

sources can include both stationary sources (e.g., switching engines) and mobile sources (locomotives). 

Because of the mobile source component, a rail yard doesn’t necessary meet the technical definition of 

a major source. Nevertheless, for NAAs where rail yards are a significant source, the EPA recommends 

that states consider treating these as point sources so that the best spatial allocation can be used for 

modeled attainment demonstration.

Several definitions come into play in determining the locomotive category, which impacts the SCC that 

should be used.

 Class I railroads are those that operate in many different states over thousands of miles of track. 

They are defined by the U.S. Surface Transportation Board as having annual carrier operating 

revenues of $250 million or more. As of 2014, there were 8 such railroads in the U.S.: Amtrak 

and seven freight railroads, designated Class I based on 2011 measurements released in 2013

("Class I Railroad Statistics". Association of American Railroads. April 17, 2013.)

 Class II/III railroads are also known as short line and regional railroads, and they range in size 

from very small (a few carloads of freight per month) to multi-state railroads just under Class I 

size. Class II railroads are defined as having revenues greater than $20.5 million but less than 

$277.7 million for a least 3 consecutive years. Switching and terminal railroads are excluded 

from Class II status. A Class III railroad is defined as a rail company with annual operating 

revenue of less than $20 million.

 Line haul refers to higher power usage of locomotive engines during extended mileage. It is also 

associated with a test cycle used for estimating emission factors intended to represent 

emissions during line-haul activity.
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The EPA develops and maintains methods for estimating locomotive emissions, and these methods are 

used in the NEI in the absence of more updated data from air agencies. They are also available as a 

fallback for states without other data for this sector. The EPA’s 2011 national rail estimates were 

developed by applying growth factors to the 2008 NEI values based on railroad freight traffic data from 

the 2008 and 2011 “R-1” reports submitted by all Class I rail lines to the Surface Transportation Board 

and employment statistics from the American Short Lines and Regional Railroad Association for Class II 

and III. In the latest such approaches (used for the 2008 and 2011 NEI), this apportionment includes 

allocation to GIS shapefiles for line-haul and rail yard emissions. The EPA project report Development of 

2011 Railroad Component for National Emissions Inventory (September 2012) provides the EPA’s latest 

estimation approach and references the 2008 approach as well. The report for the 2008 NEI was another 

project report, called Documentation for Locomotive Component of the National Emissions Inventory 

methodology (May 2011) and available in the zip file posted with the 2008 NEI documentation. Ongoing 

work for the 2014 NEI and future inventories will further improve upon past methods.

The 2008 and 2011 EPA default method was developed with a consortium of the EPA, state, and 

industry, led by the Eastern Regional Technical Advisory Committee (ERTAC). The ERTAC group provided 

data and reviewed methods as is described in the 2008 NEI Technical Support Document and associated 

documentation. Table 22 lists the locomotive SCCs used in the EPA default methodology. Other SCCs 

exist as valid SCCs, but are not used in the EPA method, so states should consider this when comparing 

the EPA estimates to locality-specific estimates and when merging emissions inventories that include 

both state/local components and the EPA estimates. 

Table 22: Locomotive SCCs

SCC Description EPA Estimated?

Data 

Category

2285002006 Mobile Sources Railroad Equipment Diesel Line Haul 

Locomotives: Class I Operations

Yes – in shape 

files Nonpoint

2285002007 Mobile Sources Railroad Equipment Diesel Line Haul 

Locomotives: Class II / III Operations

Yes-in shape 

files Nonpoint

2285002008 Mobile Sources Railroad Equipment Diesel Line Haul 

Locomotives: Passenger Trains (Amtrak) no Nonpoint

2285002009 Mobile Sources Railroad Equipment Diesel Line Haul 

Locomotives: Commuter Lines no Nonpoint

2285002010 Railroad Equipment Diesel Yard Locomotives no Nonpoint

28500201 Internal Combustion Engines Railroad Equipment 

Diesel Yard

Yes – as point 

sources

Point

State or local agencies may determine that the EPA default method is not appropriate for SIP purposes 

or may need further refinement by air agencies in developing SIPs. A number of ways to improve the 

EPA estimates were noted in the original 2011 report (see 

ftp://ftp.epa.gov/EmisInventory/2011/doc/2008nei_locomotive_report.pdf) that documented the 

approach used for the 2008 NEI. For the Class I line-haul emissions, the report identifies the most 

important considerations such as the rail link-level freight tonnage. Other variables were not addressed 
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in the methodology, including track grade and track speed. While the method calculated railroadspecific fleet- averaged emission factors from industry fleet data, it is noted that emissions from 

individual engines are highly variable even for the regulated engine categories depending on variables 

such as the specific locomotive model, operation cycle, and conditions of operation. Furthermore, the 

EPA approach does not address the seasonality of the emissions, while activity levels of different 

engines do vary seasonally and could impact emissions for seasonal inventories. Any locality-specific

data or custom emissions associated with locomotive sources are expected to be included with 

supporting documentation for the emissions inventory used for a SIP.

For the rail yard emissions, we have already noted above the consideration of rail yards as point sources. 

In addition, the 2011 report mentioned in the last paragraph notes additional considerations for 

improvements. For example, using the tonnage hauled as an indicator of the amount of switching 

activity is an assumption that could reduce uncertainty. The variability associated with actual switching 

duty-cycles and in the number of switchers operating at some rail yards seasonally was not considered 

in the method.

Locomotive idling emissions is another issue that is not clearly considered in the default EPA 

methodology and could have an impact on emissions estimates. During normal operations, locomotives 

have numerous reasons why they may remain idling including the switching operations noted above. 

The EPA has included regulations on idle emissions to help address this source. More details on idling 

emissions from locomotives is available in “Control of Emissions from Idling Locomotives” (EPA, March 

2008).

The EPA website on locomotive regulations and locomotive emissions 

(https://www3.epa.gov/otaq/locomotives.htm) provides a resource for understanding the regulations 

that apply to locomotives and how those may impact emissions. Only one recent regulation impacts 

emissions from locomotives, while several older regulations also apply. In June 2008, the EPA published 

the Control of Emissions of Air Pollution for Locomotive Engines and Marine Compression Ignition 

Engines Less than 30 Liters Per Cylinder. This rule includes standards for existing and remanufactured 

locomotives, Tier 3 and Tier 4 standards for newly built locomotives, and idle reduction requirements for 

new and remanufactured locomotives. The rule reduces emissions of PM2.5 and NOX. The emissions 

estimation approach used for this rule is available in Chapter 3 of the Regulatory Impact Assessment for 

the rule, and this approach differs from the EPA method used for the NEI. Associated with the rule, the 

EPA developed a fact sheet on locomotive emission factors (April, 2009) that can be used in estimating 

emissions from locomotives.

4.8.6 Biogenic and Geogenic Sources

Biogenic sources are a subset of natural emissions sources that may contribute significantly to an 

emissions inventory. Vegetation (i.e., forests and agriculture) is the predominant biogenic source of VOC 

and is typically the only source that is included in a biogenic VOC emissions inventory. Microbial activity 

in the soil contributes to natural biogenic NOX and CO emissions. 
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The EPA makes available biogenic emissions estimates as part of the NEI. Starting with the 2008 NEI, the 

biogenic vegetation and soil emissions have been included in the official NEI releases. In some cases, 

these EPA-generated emissions may be sufficient for use in SIP inventories other than modeling 

inventories.

Biogenic emissions from vegetation and soils are computed using a model which utilizes spatial 

information on vegetation and land use and environmental conditions of temperature and solar 

radiation.

22 The model inputs are typically horizontally allocated (gridded) data, and the outputs are 

gridded biogenic emissions that can then be speciated and utilized as input to photochemical grid 

models. Several models exist, as described in the following subsections.

Modeled biogenic vegetation emissions are significant for understanding ozone formation, particularly 

in the Eastern U.S., and the dependence on meteorology makes using case-specific meteorology very 

important. In addition, the impact and model characterization of biogenic terpene and sequiterpene on 

secondary aerosol formation for PM modeling is thought to be important. Emissions data developers 

should consider the latest available information on this issue where biogenic sources are key sources in 

their modeling region.

Biogenic Emissions Inventory System (BEIS)

The EPA has developed the Biogenic Emissions Inventory System, version 3.6 (BEIS3.6) for use in 

estimating biogenic emissions. This model can be run both as part of a CMAQ model run and 

alternatively as a module of the SMOKE system. The CMAQ approach is described by the CMAQ wiki at 

http://www.airqualitymodeling.org/cmaqwiki/index.php?title=

CMAQv5.1_Biogenic_Emissions_(BEIS)_Update. The latest documentation on the SMOKE approach to 

run BEIS is available at http://www.cmascenter.org/smoke/. 

The key inputs to BEIS include: 

 Temperature data at 2 meters, which can be obtained from modeled meteorological data, such 

as data from the Weather Research and Forecasting (WRF) Model. Such data are also needed for 

input to air quality models used for modeled attainment demonstrations sometimes needed as 

part of SIPs. The BEIS3.14 documentation has more information on the meteorological inputs.

 Land-use data from the Biogenic Emissions Land Use Database, version 4.1 (BELD4). BELD4 data 

provides data on the 230 vegetation classes at 1-km resolution over most of North America. 

These data are available at https://www.epa.gov/air-emissions-modeling/biogenic-emissionsources. These land use data can be created with the Multimedia Integrated Modeling System

(MIMS) Spatial Allocator (https://www.cmascenter.org/sa-tools/). Air agencies can alternatively 

rely on the EPA’s 12-km gridded data for modeling domains that overlap with the EPA national 

 

22 In most cases, ozone, PM, and regional haze modeling includes the emissions from vegetation and soils, but not other

potentially relevant sources such as volcanic emissions, lightning, and sea salt.
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domain, provided with the EPA’s latest modeling platform on the Emissions Modeling 

Clearinghouse.

Other Biogenic Models

Other biogenic models such as Model of Emissions of Gases and Aerosols from Nature (MEGAN)

(http://wiki.seas.harvard.edu/geoschem/index.php/MEGAN_v2.1_plus_Guenther_2012_biogenic_emissions) are acceptable as well, 

provided that the users have demonstrated their suitability for the modeling application through model 

performance evaluation (see Modeling Guidance for more information). A version of MEGAN has also 

been incorporated into an air quality model called the WRF model coupled with Chemistry (WRF-CHEM) 

(http://www.pnl.gov/atmospheric/research/wrf-chem/). California has developed a GIS-based biogenic 

model called BEIGIS (http://www.arb.ca.gov/ei/biogenicei.htm). The EPA previously made a comparison 

between BEIS and MEGAN (available at 

http://www.cmascenter.org/conference/2008/slides/pouliot_tale_two_cmas08.ppt).

BEIS, MEGAN, and other models continue to evolve and so agencies using these models should evaluate

the suitability of one model over the other as part of a modeled attainment demonstration. Once a 

choice has been made for modeling purposes, emissions should also be summed across the NAA for 

inclusion in the base year inventory for the NAA and/or the attainment projected inventory for the NAA.

Geogenic and Other Natural Sources

Geogenic emissions are primarily the result of oil or natural gas seeps and soil wind erosion. In addition, 

lightning may also be a significant contributor to natural NOX emissions in an inventory area. Volcanoes 

and fumaroles (i.e., vapor or gas vents in a volcanic region) can be additional sources of geogenic 

emissions.

As a source of ozone precursor emissions, geogenic sources are usually less significant than biogenic

sources. However, geogenic wind erosion may contribute substantially to PM emissions in an area. In 

2012, the EPA released CMAQ v5.0 that includes a module that dynamically estimates hourly natural 

emissions of fine and coarse dust particles due to wind action over arid and agricultural land. This allows 

the contribution of this source to be included in a modeled attainment demonstration without the need 

to prepare an emissions inventory first. The windblown dust approach is based on the work of J.S. Fu et 

al. (2016). For the emissions inventory SIP, air agencies using this feature should evaluate the results of 

the CMAQ-based approach to determine if it provides a reasonable representation of dust emissions in 

their modeling domain. Often times, CMAQ overestimates ambient crustal matter concentrations 

without the addition of windblown dust and so adding emissions may not be advisable. 

If windblown dust emissions are included in SIP planning, such emissions should be included in the base 

year and attainment projected inventories to meet the requirements for a complete emissions 

inventory. In the case of using CMAQ, this would require that the CMAQ-based emissions be summed 

for the NAA. Since CMAQ produces gridded emissions estimates, air agencies would need to sum 

emissions from the relevant grid cells or portions of cells associated with the NAA and/or other 
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geopolitical regions as appropriate. One relevant detail about the AERR regarding windblown dust is that 

they are specifically exempted from the reporting requirements in 40 CFR 51.20(d)

State, local and tribal agencies should also prepare an inventory of all other significant geogenic sources 

in the inventory area. Methods for estimating VOC emissions from oil and gas seeps, as well as NOX

emissions from lightning, are described in the EIIP document, Volume V, Biogenic Sources Preferred 

Methods available at https://www.epa.gov/air-emissions-inventories/volume-5-biogenic-sources. For oil 

and gas seeps, the preferred method is to develop a local emission factor based on the study of oil or 

gas seeps in the inventory area.

Lightning produces NO, which is oxidized to NO2 in the presence of ozone or in a photochemically 

reactive atmosphere. This NOX is formed in the mid-upper troposphere and can impact the distribution 

of reactive nitrogen as well as ozone. NO emissions from lightning can be estimated by collecting activity 

data on the cloud-to-ground lightning flashes, assuming a frequency of intra-cloud flashes based on the 

value for cloud-to-ground lightning flashes, and applying appropriate emission factors (in molecules NO 

per flash) to these activity levels. Because lightning is not a direct source of NO2, accounting for this 

source category is more important for air quality modeling purposes than for SIP planning inventory 

purposes. Given the altitude of these emissions, it is not usually important to include these emissions in 

the base year inventory for the NAA or in the attainment projected inventory for the NAA. 

CMAQ v5.0 includes a parameterized approach to estimate NOX emissions and its vertical distribution 

that is generated from lightning. Over the Continental U.S., the approach can use lightning flash totals 

from the National Lightning Detection Network (NLDN). For cases where such data are not available, an 

alternate algorithm can be used that estimates the number of lightning flashes based on the modeled 

convective precipitation. The peer reviewed literature contains both a description of the approach (Allen 

et al., 2011) and a model evaluation (Appel et al., 2010). More information is available from the CMAQ 

v5.0 wiki (see http://www.airqualitymodeling.org/cmaqwiki/index.php?title=CMAQv5.0_Lightning_NO). 

4.8.7 Wildland Fires 

Because the CAA specifies that planning inventories (e.g., the base year inventory for the NAA) must 

include all sources of emissions, the inventories should, therefore, include fires. Thus, wildfires and 

prescribed fires, including wildland fire use, should be included in the NAA inventories when they 

overlap with NAA boundaries. Generally, areas will find that the estimates available in the NEI will 

suffice for providing a reasonable estimate of these emissions, with the purpose of providing a general 

idea of the level of contribution to the overall emissions within the NAA.

Wildland fires (WLFs) are generally defined as any non-structure fire that occurs in the wild land (an area 

in which human activity and development are essentially non-existent, except for roads, railroads, 
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power lines, and similar transportation facilities). The National Wildfire Coordinating Group23 has 

identified the following two types of WLFs.

24

 Prescribed fire is any fire intentionally ignited by management actions in accordance with 

applicable laws, policies, and regulations to meet specific land or resource management 

objectives.

 Wildfire is any fire started by an unplanned ignition caused by lightning; volcanoes; other acts of 

nature; unauthorized activity; or accidental, human-caused actions, or a prescribed fire that has 

developed into a wildfire.

Wildland fires are a unique source of emissions with respect to NAAQS attainment planning. For ozone, 

PM2.5, and regional haze planning purposes, these fires are major sources of primary PM2.5 and VOC and 

a moderate source of NOX. They can consequently impact the ambient PM2.5 and ozone; however, they 

have a number of unique properties:

 Depending on the fuel source and acreage burned, both prescribed and wildfires can emit 

significant emissions, but emissions from prescribed fires are generally lower due to the burning 

characteristics and the relative amounts of flaming and smoldering emissions. In addition, 

prescribed fires can be used to prevent future wildfires, which are generally larger, hotter, more 

dangerous, and more polluting.

 Wildfires are naturally occurring events for which the emissions are generally not reasonable to 

prevent or control. As such, air agencies may request that the EPA exclude from regulatory 

determinations ambient data influenced by the wildfire if the request meets the criteria, 

schedule, and procedural requirements for an exceptional event set forth in 40 CFR 50.14.25

 Fire emissions can rise very high in the atmosphere on plumes, and the pollutants can be 

transported beyond state boundaries. Fires occurring outside NAAs can have impacts within 

NAAs on days that may not be determined to be exceptional events.

 Since wildfires and prescribed fires generally occur in forests and rangelands, they are less likely 

to be within the boundaries of NAAs, which are usually urbanized areas.

 Fires can also reduce ozone formation because their smoke can block sunlight that otherwise 

would result in more ozone.

SmartFire emissions

The EPA has adopted for use in the NEI a satellite-based approach to estimate fire emissions at specific 

locations and as daily emissions. The SmartFire approach (see http://www.airfire.org/smartfire/) was 

developed by researchers with support from the U.S. Forest Service (USFS). The EPA and USFS have 

 

23 The National Wildfire Coordinating Group was formed under a cooperative agreement between the 

Departments of Agriculture and the Interior and consists of representatives from each of the Federal land 

management agencies and the National Association of State Foresters.

24 Definitions are derived from the Glossary of Wildland Fire Terminology, National Wildfire Coordinating Group, 

PMS 205, July 2012. Available at http://www.nwcg.gov/glossary-of-wildland-fire-terminology.

25 “Treatment of Data Influenced by Exceptional Events; Final Rule” (72 FR 13560, March 22, 2007). EPA is 

updating the 2007 exceptional events rule and expects to release the revised final rule before October 1, 2016. 
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worked together to implement approaches for collecting activity data from states to feed SmartFire for 

use in calculating emissions. The SmartFire approach uses the following general equation to estimate 

wildfires and prescribed fires. Accurate estimates of fire emissions rely on accurate estimates of the 

terms in the equation below. 

Emissions = Area burned * Fuel Load Available * Fuel Consumed (Burn Efficiency) * Emission Factors 

The SmartFire2 (SF2) approach has been used most recently for the 2011 and 2014 NEIs.

SF2 estimates the “area burned” term in the above equation, in conjunction with the BlueSky 

framework model that estimates the last three terms in the above equation. The “fuel load available” 

term is estimated using the Fuel Characteristic Classification System (FCCS) maps in the BlueSky model. 

The “fuel consumed” term is estimated from BlueSky using the CONSUME3 model, which predicts the 

fraction of fuel that burns based on many parameters including fuel moisture. Finally, the “emission 

factors” term is estimated in BlueSky using the Fire Emissions Prediction Simulator which relies on EFs 

from the literature apportioned by flaming and smoldering combustion. Since SF2 was recently 

developed, direct references to its development in conjunction with updated BlueSky methods are not 

yet available; however, the following reference can be used in general for past applications of these 

process models in the SF/BlueSky process: http://getbluesky.org/smartfire/. Reid (2013) provides more 

exacting details on the specific procedures used in developing the NEI prescribed and wildfires. As new 

NEI years are developed and resources are available, additional improvements to SF2 are expected.

Some states have determine that SF2 is not sufficient for their needs because the satellite-based 

methods can miss detection of prescribed burning that occurs under thick tree canopies. As a general 

matter, the EPA encourages all users of SmartFire to work with their forestry agencies to obtain and use 

local-specific information about area burned, fuel types, fuel load, burn efficiency, and emission factors.

Exceptional Events and Inventories

When developing design values for a particular NAA, certain ambient monitoring data may be excluded 

if an exceptional events submittal is approved by the EPA (see 40 CFR §50.1, 40 CFR §50.14, and 40 CFR 

§51.930). It has been suggested during the course of developing this guidance that emissions associated 

with exceptional events should be removed from the inventories. The purpose of this recommendation 

was the improvement of model performance evaluations and projected attainment modeling.

In spite of potential impacts on modeled concentrations, in most cases, the EPA does not recommend 

removing fire emissions from inventories for the following reasons. First, it is not necessary to remove 

fires from use in the model performance evaluation because the EPA does not expect ambient data to 

be removed from the model performance evaluation (www3.epa.gov/ttn/scram/guidance_sip.htm). 

Second, the EPA recommends holding the emissions of fires constant between the base and future years 

for the purpose of future year inventories (such as are required for PM2.5 NAAQS) and for calculating a 

modeled relative response as part of a modeled attainment demonstration. This approach ensures that 

the modeled fires have a minimal influence on estimated future year concentrations that are calculated 

using the relative change in modeled concentrations. Third, even though in some cases there may be a 
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direct causal link between fire emissions and monitored air quality data, resulting in exceptional events, 

removing fire emissions directly associated with an exceptional event from an inventory requires

knowledge of exactly which fire(s) at specific locations on specific days and hours contributed to the 

exceptional event. This may be especially difficult when multiple fires over multiple days lead to 

identified exceptional events. Therefore, from an inventory perspective, it may not be straightforward 

to remove emissions specifically associated with any particular exceptional event. Determining which 

emissions to remove from the inventory could be a significant effort with limited benefit.

The EPA therefore encourages states to use a representative mix of prescribed fire and wildfire in their 

inventories. In the past, some plans under previous PM2.5 NAAQS have estimated the actual fire 

emissions and temporal and spatial patterns from a given year and used this estimate as the assumed 

future baseline for planning, while others have used average emissions over multiple years. Other 

approaches may be appropriate as well. Moreover, regardless of the approach used, the EPA still 

encourages states to submit actual wildfire and prescribed fire activity data that are critical to 

developing emissions estimates to the NEI as suggested in the AERR. Additionally, the EPA encourages 

states to consult with their EPA Regional office about the fire approach to be used. While other 

approaches may be more suitable in particular situations, the recommendations provided here provide 

a starting point for discussions about the most appropriate approach for a particular area.

4.9 Quality Assurance

As part of the 8-hour ozone NAAQS, PM2.5 NAAQS, and regional haze rules, state, local and tribal

agencies are strongly encouraged to perform QA checks and procedures on their inventories. State, local 

and tribal agencies can develop and select the QA procedures they will perform, and should include the 

details of their QA program (including specific procedures) in their IPPs.

The purpose of QA is to ensure the development of a complete, accurate, and consistent emissions 

inventory. A well-developed and well-implemented QA program fosters confidence in the inventory and 

in any resulting regulatory and/or control program.

When air agencies are using data from the NEI, additional QA is warranted considering the purpose for 

SIPs is not the purpose for which the NEI was developed. Since a great deal of QA is done on the data 

used for the NEI, most of the QA work has been done and so starting with the NEI can often save time. 

The overall QA program provides routine and consistent checks and documentation points in the 

inventory development process to verify data integrity, correctness, and completeness; identifies and 

reduces errors and omissions; maximizes consistency within the inventory preparation and 

documentation process; and facilitates internal and external inventory review processes. Quality 

assurance activities include technical reviews, accuracy checks, and the use of approved, standardized 

procedures for emission calculations, and should be included in inventory development planning, data 

collection and analysis, emission calculations, and reporting.
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4.9.1 QA Reference Materials and Planning

The EPA has developed several guidance documents designed to assist state, local and tribal agencies in 

designing and implementing their QA programs. The EIIP Volume VI, addresses QA issues, including the 

following:

 Chapter 1: Introduction - The Value of QA/QC

 Chapter 2: Planning and Documentation

 Chapter 3: General QA/QC Methods

 Chapter 4: Evaluating the Uncertainty of Emission Estimates

 Chapter 5: Model QA Plan

 Appendix A: Example Audit Report

 Appendix B: Technical Systems Audit Checklist Example

 Appendix C: Example 1 of Data Quality Audit Checklist

 Appendix D: Example 2 of Data Quality Audit Checklist

 Appendix E: Performance Evaluation Checklist Example

 Appendix F: EIIP Recommended Approach to Using the Data Attribute Rating System (DARS)

On October 20, 2008, the EPA issued the latest version of the EPA Quality Program Policy (CIO 2106.0). 

This policy supersedes the previous CIO 2105.0 (formerly EPA Order 5360.1 A2). This policy established 

among other things, the requirement for QAPPs for all EPA-supported programs that generate 

environmental data. As described by the “EPA Requirements for QA Project Plans” guidance:

The purpose of a QA Project Plan is to document planning results for environment data operations and to 

provide a project-specific blueprint for obtaining the type and quality of environmental data needed for a 

specific decision or use.

The orders describing the QA requirements are available at the EPA’s website for Agency-wide Quality 

System Documents (https://www.epa.gov/quality/agency-wide-quality-system-documents). That 

website also includes specifications for non-EPA organizations: “EPA Requirements for Quality 

Management Plans” (EPA/240/B-01/001; March 2001) and “EPA Requirements for QA Project Plans”

(EPA/240/B-01/003; March 2001). Additional documentation is also available on the same website 

under the subheading of “General Guidance” that air agencies may find helpful in developing their own 

quality assurance plans for their inventory development efforts.

States should also consult with their EPA Regional office to determine how to meet the quality 

assurance planning requirements for the SIP planning inventory development. 

4.9.2 Types of Problems in Emissions Inventories

Emissions inventories are compiled from a large number of disparate data sources, which can lead to 

many types of data problems. These problems can be divided into value errors and structure errors. 

Value errors are errors in the values of emissions inventory data fields, such as the emissions amounts, 

incorrect codes, missing values that are required, or incorrect data such as facility names, stack heights, 

or latitude/longitude locations. Correct quality assurance can identify these errors and allow data 

developers to correct them before the data are used for their intended purpose.
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Structure errors refers to those data problems that cause databases to not work, such as missing records 

in relational database tables (e.g., “widows” and “orphans”), duplicate records (i.e., multiple records 

that represent the same emissions, causing double counting), or incorrect formats (e.g., the wrong 

number of fields, missing fields that are required, mismatched data types, such as integer versus text). 

Data with no structure errors are described as having good “data integrity.” Generally, these types of 

problems should be automatically prevented by using robust data systems to manage the data in ways 

that match the expected downstream data use, but this is not always the case. For example, data 

systems managing emissions inventory data should be developed with the AERR requirements in mind, 

since these are the governing requirements for SIP-related inventories.

4.9.3 Checks Available through the EPA’s Emissions Inventory System

While many inventories used for SIPs are not also submitted to the EPA through its EIS (the system the 

EPA uses to build the NEI), the EIS does have a large number of documented QA checks that the system 

performs as it is loading data submitted by air agencies. This automated QA provides a large number of 

data integrity checks that air agencies can consider including in their own data systems or checking as 

part of their inventory QA. To access a list of these checks, EIS users must use the EIS Gateway and use 

the “QA Checks” link under the “Reference Data” heading on the left of the Gateway interface. The 

checks are not otherwise published on the EPA’s website and can be requested by non-EIS users by

using the “Contact Us” link provided at https://www.epa.gov/chief. The list has more than 600 data 

checks that are performed automatically by the EIS. While many of these checks are only relevant for 

getting data into the EIS, they promote standardization of inventories and proper formatting in 

accordance with the AERR, which (as noted previously) provides the EPA’s standards for inventory 

preparation including for SIP inventories.

Since the timing of development of SIP inventories is rarely, if ever, coincident with the timing required 

for submitting emissions for the NEI, it is generally not feasible for air agencies to submit SIP inventories 

to EIS. However, the same types of checks can and should be done on SIP inventories to insure 

appropriate inventory quality.

The EIS checks include both critical errors and warnings. Critical errors prevent data getting into the EPA 

data system until they are corrected, whereas warnings allow the data to go through, but may need to 

be resolved to ensure the data transmitted to the EPA are correct. Examples of critical errors include:

 Checks that data codes for required fields use valid values (for the given inventory year; see also 

Section 4.7)

 Fields required by the AERR and for data integrity are present

 Certain fields are within a critical range check (e.g., a year is between 1900 and 2050, stack 

height is between 1 and 1300 meters)

 Certain fields have appropriate lengths (e.g., ZIP codes are between 5 and 10 characters)

 Percentages are provided as percentages (1 to 100) and not as a fraction

 Data element types (e.g., integer, text, numerical) of required data fields are provided 

consistent with their required type.

 MOVES county database submission contains the correct files
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 NMIM county database submission contains the correct files

 Conflict with pollutants reported for grouped pollutants versus individual members of the 

group.

 If stack diameter information is reported, the release point exit gas flow rate or velocity should 

be reported. Both may also be reported.

Examples of warnings include:

 Checks that data codes for optional fields use valid values (for the given inventory year)

 Data value is longer than the allowable width (some text fields will be truncated if longer than 

what the EIS allows).

 Emissions value is outside of a presumed valid range

 Exit Gas Stack temperature is outside an appropriate range

 When submitting monthly data, all months are not submitted together

 File names of mobile data files use incorrect file extensions

 Data element types (e.g., integer, text, numerical) of optional data fields are inconsistent with 

their required type.

4.9.4 Inventory Comparisons

Inventory comparisons can be used in multiple ways to effectively identify problems in new inventories. 

This is most useful when a high-quality prior inventory is already available. In states with a long history 

of developing SIPs, past inventories will be readily available for such comparisons. In states with new or 

emerging attainment problems, the EPA suggests comparing SIP inventories to the NEI. Even when the 

comparisons reveal problems not with the new inventory but with the one being compared, this can 

increase confidence in the inventory of interest.

Emissions are one of the obvious things to compare, and these comparisons are usually done on a 

pollutant-by-pollutant basis for pollutants important to a given analysis. As noted in the section on 

ranking techniques (Section 4.9.7), ranked emissions (e.g., facility, unit, county, SCC group) can be 

compared to the same ranked emissions from other inventories. Such comparisons will often identify 

differences worth further exploration when one large source appears on one list and not the other. 

In addition to ranking, all records across an inventory (or inventory summary) can be compared and 

percent differences can be computed. This is usually done by computing “new emissions – old 

emissions” divided by “old emissions.” Large or unexpected percent differences can be investigated 

further to make sure that the differences have some reasonable explanation, such as newer emissions 

being controlled, a downturn in activity for one inventory, process changes, fuel changes, or a partial or 

complete facility closure. In some cases, methods changes may be the reason for such differences, 

including updated emission factors or test data.

For nonpoint emissions, graphical inventory comparisons can be useful. This can be done by not only 

using calculated percent differences or ranking, but also using GIS techniques to map county level 

emissions and their differences. It can be very useful to map the old emissions, new emissions, and the 
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computed difference or percent difference side-by-side. This can reveal areas of large emissions that 

have significantly changed from one inventory to the next, raising questions for further exploration. 

When comparing to an EPA inventory that used EPA methods for a particular sector, this could reveal 

that the SIP planning inventory is a better inventory with updated methods, or it could reveal that 

outdated methods are being used. In any case, making the comparison will help inventory developers 

understand the potential improvements that could be made or that have been made, which will help in 

understanding reviewers’ expected reactions to the data.

For multi-state inventories needed for SIP development, state-to-state comparisons may be useful in 

addition to county maps. This is especially true if emissions for one state were developed differently 

from emissions for another state. The EPA encourages states to work together to promote consistent 

approaches across a multi-state NAA, but a final QA check of the consistency can be done using the 

comparison techniques described here. 

4.9.5 Checking Data Codes

As described in Section 4.7, air agencies should check the inventory data codes they are using against 

valid codes. Valid codes are defined by the EPA within the EIS. Air agencies that start with the NEI may 

significantly minimize the burden of this recommendation.

4.9.6 Point Versus Nonpoint and Reconciliation

Several quality assurance issues arise related to point versus nonpoint sources and inventories: 

(1) correct identification of point sources, (2) prevention of double counting across point and nonpoint, 

and (3) prevention of missing nonpoint emissions.

Minimum thresholds are specified in the AERR for which sources with facility total potential to emit

emissions above those thresholds must be treated as a point source in SIP planning inventories. This is 

more fully explained in Section 2.5.7, with lower thresholds relevant for ozone and PM NAAs. Air

agencies should implement a QA check (or otherwise have a data collection process) that ensures all 

facilities with potential emissions (potential controlled emissions, if applicable) that are greater than 

these thresholds are included in their inventory as point sources. See also Section 2.5.2 for more 

information on calculating potential emissions.

When a particular type of source includes contributions from both point and nonpoint sources, double 

counting can occur. Quality assurance steps should be taken to ensure that double counted emissions 

have been prevented. A good example of this is smaller boilers. Both point source facilities (based on 

the potential to emit thresholds) and smaller facilities are likely to have emissions from non-industrial 

boilers. For instance, some large institutional facilities such as hospitals may be included as a point 

source with their associated boiler emissions, whereas smaller institutional facilities are not included as 

point sources and would thus be included as a nonpoint source under an “institutional/commercial” 

boiler SCC. 

The EPA provides calculation approaches for many nonpoint emissions that allow for subtracting out the 

activity of the point source parts prior to estimating nonpoint emissions, including that for the 

institutional/commercial boiler example. This is called “point/nonpoint reconciliation.” Whether 
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nonpoint emissions are estimated by the EPA or by air agencies, an assessment of quality should be 

done to ensure that sectors associated with both point and nonpoint sources do not double count 

emissions. See also Section 4.8.2 for additional information on preventing double counting. A list of 

source sectors with potential point/nonpoint overlap is provided in Table 17 above and in the latest NEI 

Technical Support Document, available at https://www.epa.gov/air-emissions-inventories/nationalemissions-inventory.

A third type of QA should be done for categories that have both point and nonpoint contributions. It 

involves an assessment of potentially missing nonpoint emissions. Because the requirement for SIP 

inventories is to include emissions from all sources, it is not sufficient to exclude nonpoint emissions 

when part of a sector is covered by point sources, unless it can be shown that the nonpoint portion is 

negligible. At a minimum, air agencies should consider the tools available from the EPA to estimate 

nonpoint sectors (see Section 4.8.2) and evaluate whether or not their point source inventory covers all 

of the activity associated with the nonpoint categories. For inventory years not covered by EPA tools, 

agencies should compare point total activity as collected from point sources versus the same type of 

activity data used in the EPA tools. For example, U.S. DOE fuel consumption data can be used for 

industrial, commercial, and institutional boilers. The source of the activity data used for the latest EPA 

nonpoint methods is included in the descriptions of those methods in the nonpoint tools described in 

Section 4.8.2 and with the latest NEI documentation.

4.9.7 Ranking Techniques

Ranking emissions records can be a very useful tool to identifying value errors in emissions values or 

other numeric inventory data fields, or at least in finding records that need further investigation. 

Ranking involves first defining a set of sources and a level of aggregation of emissions and then ordering 

those emissions values from highest to lowest or lowest to highest. This analysis must be done one 

pollutant at a time. An analyst then considers whether those sources at the top of the list make sense.

Point sources, facility level emissions ranking

For point sources, ranking can be done at the facility total, unit total, or process level (i.e., at the SCC 

level), and each level has its place for reviewing inventories. A facility level ranking can often be followed 

by subsequent investigation to understand the reasons (at the unit and/or process level) for very high or 

very low values. An analyst using a ranking procedure would assess the facilities with the largest 

emissions and seek to understand whether they make sense. For example, for combustion products 

such as NOX or SO2, one would expect to see large EGUs and other industrial sources. Ranking of 

emissions values is usually done for one pollutant at a time, reviewing most carefully those pollutants 

that are most critical to the particular purpose of the inventory. For example, an inventory supporting 

ozone analysis would want to review NOX and VOC most carefully.

It can be useful to compare the list of top facilities for a given pollutant from one inventory to a previous 

or alternative inventory for a given state or NAA. This can raise questions regarding large changes from 

one inventory to another, which could be explained (e.g., a decrease due to controls) or could identify a 

problem (e.g., a missing value or data entry problem). In addition, facility-level ranking can use 
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additional grouping such as by industry code (i.e., NAICS), to identify unexpectedly high or low emissions 

for facilities within a particular industry.

Point sources, sub-facility emissions ranking

Unit- and process-level summaries can additionally be ranked for more specific analyses. For example, 

all boilers could be ranked and particularly high or low emissions values investigated. Similarly, all 

natural gas processes at particular unit types (e.g., boilers) or by industry (NAICS) could be ranked. These 

rankings can show real variations, but they can also find incorrect values that stand out when ordered 

and reviewed in this way. There are many variations that could produce useful results, and which ones 

are useful for a particular inventory depends on many factors and the purpose of the inventory.

Nonpoint and mobile source ranking

Nonpoint and mobile emissions can also be ranked as technique to help with QA. Since these inventories 

are primarily compiled at the county/SCC resolution, the ranking can be done only at combinations of 

county, SCC, and for specific pollutants. Total county nonpoint emissions could be ranked to look for 

overly high or low values, but the same effect can be accomplished in a visual way with mapping 

approaches (see Section 4.9.9). Specific SCCs can also be selected and the emissions by county ranked, 

or individual counties selected and SCCs ranked and compared to other counties.

Since nonpoint emissions are based on various types of activity data that also has a county or other 

spatial resolution (such as census tract), these activity data can be used to “normalize” the nonpoint 

emissions prior to ranking. For example, emissions coming from households (e.g., residential heating, 

residential wood combustion, consumer solvents) can be divided by population or housing data and 

ranked for identify possibly strange results. Since some of the activity data used for these categories 

starts with state or regional resolution and is subdivided to apportion to the county resolution, such 

comparisons can reveal illogical outcomes of such approaches. For example, while it makes sense that 

residential wood combustion occurs more per household in rural areas, it wouldn’t make sense for a 

very low population county to have a disproportionate amount of emissions and urban areas have none.

4.9.8 Outlier Analyses

One result of the ranking approaches described above will be to identify outliers. However, such ranking 

is a relatively unsophisticated way of identifying such outliers. A more robust approach can be used for 

point sources where states have the underlying activity data reported by facilities on a per-process 

basis. Dividing emissions by activity values would produce a set of implied emission factors, or rates. The 

rates for a similar set of processes (based on SCC) and control devices in use might be expected to be 

within an order of magnitude of each other. Emission rate values that are too far outside the expected 

relationship could be further investigated to better understand the source of any unexpected values, 

which might include an error in the units of measure or the presence of controls or an incorrect SCC, as 

well as data entry errors for emission values or other related fields.
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4.9.9 Using Maps to QA Nonpoint Emissions

Nonpoint and mobile source emissions data are associated with states and counties and, therefore, are 

ideal candidates for mapping with GIS software. By mapping emissions values for individual SCCs or 

groups of SCCs, data analysts can identify unexpected patterns in the data that could signal an error in 

creating the data. Though it is technically more challenging to use GIS software, this approach can often 

be more valuable than ranking in identifying data problems.

Like the ranking techniques, GIS maps can also use emissions data normalized by some other underlying 

or related spatial data, such as population, land use, urban/rural designations, or housing. This type of 

information is increasingly available online from the U.S. Census Bureau and other sources. Data 

analysts can use the other data to identify unexpected relationships that possibly indicate bad 

assumptions made in the underlying methods.

4.9.10 Expected Pollutants

Air agencies can use their expertise with emissions sources to identify which pollutants are expected 

from particular processes. In addition, for the 2014 NEI cycle, the EPA has provided a list of expected 

pollutants for each SCC. While this list is not comprehensive for any pollutant that can be associated 

with a process, it provides a bare minimum of essential pollutants associated with SCCs. For example, 

NOX is always expected as a by-product of combustion processes. In addition to expected pollutants, 

analysts can also identify pollutants that are not expected from a particular process, such as SO2 from an 

evaporative process. 

Once identified, such lists can be used to cross-check inventories to identify either problems with 

reported emissions, pollutant codes used, or process codes used. For example, if a combustion process 

does not have NOX emissions, it could be that the wrong pollutant code was used in reporting the NOX

and its showing up as something else. Or, the process itself may be incorrectly using a combustion 

process code.

Expected pollutants can also help with making sure the correct PM components are provided for PM 

inventories, in particular that condensable PM emissions are included in the total PM emissions and that 

both filterable and condensable PM emissions are reported. This issue was described in Section 4.2.1

along with a list of the types of sources expected to have such emissions.

Some emissions data analysts may not be familiar with various sources enough to know which pollutants 

should be expected. For point sources, agency staff can rely on the sum total of all reported data across 

multiple sources for a particular process to get a good idea of what should be expected. For example, if 

all but one industrial boiler burning natural gas reported VOC, then it would be worth further 

investigation as to why VOC was not reported for that one. Since the AERR specifies that all criteria 

pollutants must be reported if any one pollutant exceeds the potential to emit threshold, one should 

expect that similar processes from multiple facilities would have many, if not all, the same pollutants.

There are other ways to identify expected pollutants. These include looking at past inventories created 

by the same air agency or looking at the NEI. Another way is to see what emission factors are available

for the process in Webfire. While these are not foolproof ways of finding expected pollutants, they can 
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be very helpful. Comparing a draft inventory with such a list can also identify unexpected pollutants to 

consider. Used with ranking techniques, knowing what pollutants are expected can also help when you 

find zero or null values for a facility, unit, or process.

4.9.11 Facility and Release Point Locations

Facility and release point (e.g., stack) locations are key elements of inventories that are used primarily in 

preparation of inventories for air quality modeling and also in permit modeling. Facility locations are 

also used in mapping point sources for various public communications purposes or other reasons 

needing less precision. While facility resolution is acceptable for many modeling applications, including 

12-km gridded modeling, the need for release point locations increases for finer resolution modeling. 

Thus, for 4-km or 1-km modeling, release point location information should be considered.

With regard to the more detailed release point coordinates, the EPA recognizes the need for air agencies 

to leverage SIP inventory development efforts for possible use for other inventory needs. Thus, air 

agencies should be particularly attuned to release point geographic coordinates for sources that could 

be involved in permitting or risk analysis. For regulatory dispersion modeling, release point location is 

critical (rather than only facility location).

The EPA’s EIS and other inventory data systems support two types of locations: facility and release 

point. There are four key errors that can occur, and each can be identified with their own type of check, 

as listed below.

 Facility is not consistent with the state/county FIPS code

This error can be found using GIS, SAS® or other calculations to compare the latitude/longitude 

location with the county boundaries using some tolerance. Because county boundaries have 

varying degrees of accuracy, a tolerance should be used that allows for locations near, but not 

always 100% within, county boundaries.

 Facility location is not consistent with the physical location of the facility

Since some facilities can be on very large tracts of land, facility locations developed using 

geocoded addresses or facility entrances are often not accurate with regard to the actual 

location of the facility. For emissions inventory purposes, the EPA encourages that facility 

locations be positioned on top of a part of the facility that emits one or more pollutants, and the 

facility centroid is often sufficient for doing so. Using freely available mapping software such as 

Google Maps, latitude/longitude locations can be plotted on satellite images to ensure that 

values are on top of an emitting source and not on the road, at the corporate headquarters, or 

in some other invalid location.

 Release point location is not consistent with facility location

This check is only useful once the facility location is accurate. Such an error can be found using 

GIS, SAS® or other calculations to determine the distance between the facility location and the 

release point locations. Alternatively, a subtraction of the absolute values of the latitudes and 

longitudes can be used to determine the difference in degrees. If that linear distance or the 

distance in degrees is greater than some preset criteria, then the locations can be investigated 

further. For some types of facilities such as refineries or major chemical complexes, the facilities 
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may be very spread out and therefore may have legitimately large distances between release 

points.

 Release point location is not consistent with the physical location of the release points

For many applications, including some SIP modeling applications, all release points using the 

same (usually facility) latitude/longitude is sufficient. However, with the availability of 

inexpensive GPS technology, it is more realistic for facilities to collect and submit individual 

release point location information. Furthermore, for some types of analysis, particular fine scale 

modeling for permitting and risk analysis, correct release point locations can make the 

difference between a good analysis and a useless one. It is very difficult to check the accuracy of 

release point locations, but using mapping software overlaid on satellite images can provide 

some sense of whether the release points are meaningful or not. Coordinates reported to oneten-thousandth of a degree (four digits after the decimal point) give a precision of 

approximately 30 feet. Thus, a simple check to see if such values are useful is to examine the 

number of decimal places included in a given database.

Since a facility’s location is not likely to change, the EPA has implemented a procedure in the EIS that 

allows latitude/longitudes to be locked, meaning that they cannot be changed accidently during a data 

exchange with a state. We encourage air agencies to implement a similar approach in their data systems 

so that the efforts of QA done on one inventory can be inherited by other inventories created by that 

agency. Locking mechanisms prevent the data from being changed inadvertently through a new data 

collection that relies on data from an unverified data source. Thus, a comparison of the locations used in 

the SIP inventory to any locked and verified site coordinates in EIS would also highlight potential errors.

4.9.12 Release Point Parameters

Like release point locations, release point parameters (also called stack parameters) are key elements of 

inventories that are used primarily in preparation of inventories for air quality modeling. The release 

point parameters impact the modeling of emissions from facilities because they specify details of the 

way the emissions are released (e.g., they specify how high off the ground the pollutants are emitted 

and the exit velocity and temperature of those emissions). One basic and important distinction for 

release points is whether the release point is a stack or a fugitive emissions, and different types of 

parameters are specified for stacks than for fugitive emissions sources.

Correct release point parameters for stacks are important for modeling done for ozone and PM2.5

modeled attainment demonstrations as well as modeling done for Regional Haze SIPs, whereas fugitive 

release parameters are not as critical for that purpose. This is because stack emissions parameters 

impact the modeling of plume rise and therefore determine how high the emissions rise in the modeled 

layers of the air quality model. Fugitive emissions are emitted in the modeled surface layer with no 

additional characterization. This is in contrast to more detailed plant-level modeling (i.e., done for 

permit modeling or risk modeling purposes), where both stack and fugitive parameters are critical. 

Because the focus of this document is on SIP purposes, we focus on stack parameters here.

Stack parameters included in the EIS and historically are stack height, stack diameter (at the exit point of 

the emissions), exit gas velocity, exit gas flow rate, and exit gas temperature. Additionally, a release 
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point type is available in the EIS to specify a code for whether the “stack” has a particular orientation of 

vertical, horizontal, goose neck, vertical with rain cap, or a downward facing vent. Valid release point 

codes are available in the EIS reporting code tables in the EIS Gateway.

Quality assurance for stack parameters can be done using several of the QA methods already described. 

Range checks can be applied; these can be the default EIS range checks (see Section 4.9.3) or range 

checks developed by an air agency. A more robust range check could be developed for specific types of 

processes associated with a release point or for specific NAICS codes. For example, one would expect 

higher stacks for EGUs than for lumber mills. Additionally, ranking techniques can be applied to stack 

parameters (see Section 4.9.7), including options for grouping ranked stack parameters by facility, unit, 

process and/or specific types of facilities, units, or processes. This is somewhat complicated by the fact 

that multiple units or processes can feed a given release point, and when this happens, the predominant 

unit or process could be used for grouping purposes. Finally, more sophisticated outlier analysis could 

additionally be used, by finding correlations between stack parameters such as height and diameter and 

investigating those combinations that statistically fall outside an expected relationship or ratio.

4.9.13 Biogenic QA/QC

Biogenic emissions are somewhat more challenging to QA than emissions for other source categories 

because they may be generated differently (i.e., as part of a gridded, hourly modeling exercise rather 

than more basic inventory development techniques). The following list provides some basic checks that 

can be done as part of the generation of biogenic emissions:

1. Check the biogenic emissions tool logs for warnings/errors.

2. Check the logs to confirm that the expected inputs were used.

3. Compare state or county total emissions to another year or case’s biogenic emission totals.

4. Ensure that summer emissions are greater than winter emissions. This can be done by summing 

the emissions across the modeled summer days (e.g., May-October) and for the other models 

and then comparing the totals on a state and/or county basis. As a quick (but less rigorous) 

check, example summer hourly or daily emissions can be visually compared to example winter 

hourly or daily emissions using software that displays the gridded emissions.

5. Visually inspect sample days of the gridded, hourly emissions using software that displays the 

gridded emissions. The VOC emissions should generally be higher in southern latitudes because 

of warmer temperatures. This difference should be more pronounced in winter in cases where 

the southern areas have more actively growing vegetation. The NO emission should generally be 

higher in agricultural areas with significant fertilizer application.

4.9.14 Timing of QA and Follow-up

As air agencies, as well as the EPA Regional offices, work quickly to meet relevant inventory and SIP 

deadlines, data problems are exacerbated when they are discovered too late in the process. A key part 

of making sure that a QA plan is effective is deciding (as part of the planning) when a given QA check is 
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to be done and ensuring (as part of implementing that plan) that it is actually done. To do this 

effectively, QA steps should be timed in a QA plan at the earliest possible time that the step can be 

performed so that errors are caught as early in the process as possible. Furthermore, assurances must 

be put in place that someone is confirming that the checks have been done along the way, so that errors 

are caught. At the end of the data development process, some accounting for the history of the QA 

should be available along with the data, so that others can confirm that the checks have been done and 

can be reassured. Where possible, evidence of completion of each QA step should be referenced, such 

as a comparison report.
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5 Developing Projected Emissions Inventories

5.1 Projected Emissions Overview

Emissions estimates for future years26 are called “emissions projections” or “projected emissions.” These 

projections include emissions growth (due to increased or decreased activities) and emissions controls 

due to regulations, settlement agreements (or consent decrees), fuel-switches, or other programs that 

reduce emissions in specific ways in the future. Emissions controls can be either on-the-books or 

considered as part of a control strategy to reduce emissions as part of SIP planning. For point sources, 

emissions projections can also include facility and/or unit-level shutdowns due to settlement

agreements (or consent decrees) and/or ownership decisions. 

The primary goal in making projections is to obtain a reasonable and technically credible estimate of 

future-year emissions that accounts for key variables. The EPA encourages the air agencies to 

incorporate in their analyses the variables that have historically been shown to drive their economy and 

emissions, as well as the changes in growth patterns and regulations that are expected to take place 

between the time of their base year and projected attainment year. For details on which future year(s) 

should be modeled for attainment demonstrations, readers should refer to the Modeling Guidance 

(https://www3.epa.gov/ttn/scram/guidance_sip.htm).

5.1.1 Readily Available Projections Information

The EPA’s latest projection approaches and data are available as a part of the EPA’s “emissions modeling 

platform” development. These platforms are released and updated periodically in accordance with the 

EPA regulatory and analysis activity on the Emissions Modeling Clearinghouse at

https://www.epa.gov/air-emissions-modeling. While each platform comes with a single or limited set of 

future years included, the data and thoroughly documented approaches can help air agencies to 

develop and improve their own emissions projections.

Additionally, as part of SIP planning, various situations lead to requirements for states to implement 

Reasonably Available Control Technology (RACT) and Best Available Control Technology (BACT) as part 

of their attainment planning. The emissions reductions expected based on these programs would need 

to be include in emissions projections used for modeled attainment demonstrations and other projected 

emissions such as the projected attainment inventory for the nonattainment area (see also Section 

3.8.1).

5.1.2 Background Materials

For point sources, Volume II “Point Sources” of the EIIP series includes Chapter 12 “How to Incorporate 

the Effects of Air Pollution Control Device Efficiencies and Malfunctions into Emission Inventory 

Estimates” was published in 2000 and still provides the authoritative information related to point source 

projections. In particular, Appendix A “Clearing Up the Rule Effectiveness Confusion” and Appendix B 

 

26 Technically, projected emissions could be made from a base year to a year that has already occurred when data 

are not yet available for a very recent year. However, for the purposes of the remainder of this guidance, we will 

assume projected emissions refers to future year projected emissions.
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“Emission Inventories and Proper Uses of Rule Effectiveness” provide a thorough discussion of rule 

effectiveness that is relevant for current emissions projections.

In addition, the 1999 EIIP report entitled “Procedures for Preparing Emissions Projections” 

(https://www.epa.gov/air-emissions-inventories/volume-10-emissions-projections-final-report) 

introduces the basic concepts behind emissions projections issues and approaches. The concepts of 

control efficiency, capture efficiency, rule effectiveness, and rule penetration are introduced in Section 

2.5.18. The EIIP document provides information on how to use these factors to estimate projected 

emissions. However, some key features of this document need to be updated, including the reference to 

the Economic Growth and Analysis System, which is no longer supported by the EPA. Other outdated 

concepts include the assumption that economic growth can be a good indicator of emissions growth. 

While some correlation can exist, it cannot be assumed but rather must be demonstrated for a 

particular industry or facility. The correlation has not been well observed historically in the NEI data 

because of various issues including technology improvements and fuel-switching that often reduce 

emissions without regulation.

In this section, we will address an overall approach for tackling emissions projection issues that 

incorporates use of this document and points out where other approaches or new data are available to 

amend the information included in that report.

Some general aspects of the EIIP projections document that are now out of date are as follows.

 The latest SCC codes are available as described in Section 2.5.14.

 The AP-42 emission factor website is now https://www.epa.gov/air-emissions-factors-andquantification/ap-42-compilation-air-emission-factors.

 The reference in the Table 2.1-1 web link is no longer applicable. Instead, the following 

information can be used to help provide similar information:

o Energy consumption by industry report, 2002: 

http://www.eia.doe.gov/oiaf/analysispaper/industry/consumption.html

o Annual Energy outlook report, early release 2014: 

http://www.eia.gov/forecasts/aeo/er/index.cfm

 The references in Table 2.1-2 are also out of date and should be updated as follows:

o U.S. EPA Emissions trends reports. These reports can be used to provide the historical 

trends of emissions sectors based on the NEI: https://www.epa.gov/air-trends

o Integrated Planning Model: See reference provided above in Section 1.2

o Multiple Projections System - is no longer in use

o California Emission Forecasting System:

http://www.arb.ca.gov/ei/drei/maintain/database.htm

The EPA recommends that developers of projected emissions take the steps in the bulleted list below. 

Each of these steps corresponds to a subsequent subsection.
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 Identify sectors of the inventory that require projections and sectors for which projections are 

not advisable, and prioritize these sectors based on their expected impact on the modeling 

region (Section 5.2).

 Collect the available data and models that can be used to project emissions for each of the 

sectors (Section 5.3).

 For key sectors, determine what information will impact the projection results the most, and 

ensure that the data values reflect conditions or expectations of the inventory region (Section 

5.4.

 Quality assurance (Section 5.5)

5.2 Identify and Prioritize Key Sectors and Regions for Projections

Emissions data developers should evaluate their inventories for those sectors and regions that are most 

critical to their modeling results. The purpose of identifying these key sectors is to direct more resources 

and efforts to estimating future-year emissions for these sectors in the subsequent steps.

Sectors can be subsets of the larger groups of stationary area, on-road mobile, nonroad mobile, and 

point sources. For ozone modeling, it will be most important to ensure correct projections for sectors 

with higher NOX emissions. In larger urban areas that contain VOC-limited regions, sectors with higher 

VOC emissions will also be important. Mobile sources are typically large contributors to both VOC and 

NOX; therefore, they are usually a priority in estimation of future-year emissions, particularly for urban 

areas. For PM and regional haze modeling, the relative importance of primary PM2.5 emissions and its 

precursors will depend on the area of the country. Primary PM2.5 emissions may be important in urban 

areas or areas heavily influenced by local sources such as residential wood combustion or industrial 

facilities. In the East, SO2 emissions may be of primary importance. For PM2.5 planning, NOX, VOC, and 

NH3 will vary in importance depending on the particular situation, which needs to be assessed as part of 

the SIP planning process.

Emissions data developers should consider the proximity of the emissions and the expectation of longrange transport when reviewing projection assumptions. Those emissions that have an impact on the 

nonattainment status (whether transported from outside areas or locally generated) should be given 

highest priority in evaluating the projection assumptions. For example, less priority can be given to 

emissions from distant states and/or for pollutants that do not transport from those states to the NAA.

In some cases, there are sectors that are very difficult or highly uncertain to project into the future. 

Wildfire emissions is a common example of such a sector, since models are not readily available that 

estimate the magnitude and location of future-year emissions. In these cases, it is reasonable to create 

an “average year” inventory and temporal distribution of the emissions. Other approaches to futureyear fire emissions include holding the emissions constant at the same levels as included in the base 

year.

5.3 Collect Available Data and Models for Each Sector Needing Projections

For each sector needing projections (and in particular for the priority sectors), air agency staff should 

collect and review growth and control information. The sources of such information depend upon the 
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sector of interest. Some data resources and approaches are available and provided here, and new data 

and approaches will become available subsequent to the publication of this document. The information 

provided here is a snapshot of information that will grow and adapt over time, and emissions modelers 

should seek out such new information as part of this step.

Emissions modelers can rely on available default data for sectors and regions with lower priority for 

consideration (as described in the previous section). For sectors and regions that are important to 

modeling the NAA, emissions modelers should spend additional effort obtaining improved information 

from other modeling applications (such as modeling done by RPOs or other states) or developing the 

improved information themselves.

As mentioned previously, the EPA provides control assumptions with its modeling data used for 

promulgating air quality rules and impact assessments. The EPA approaches typically include the 

impacts of federal rules that have been finalized. Impacts of proposed rules are rarely included, as the 

changes in emissions impacts can be very large between the proposed and final rules. These data are 

posted at the Emissions Modeling Clearinghouse. Sometimes, more current information is available than 

what is available from the EPA, for example, when reconsideration amendments are developed and 

finalized after the EPA has publicly released its projected inventories. In the subsequent subsections, we 

focus on sources of growth information.

5.3.1 Electric Generating Units

This section conveys the general principles and key criteria that inform development and review of EGU

emission projections in support of CAA-related state regulatory actions (such as SIPs) subject to EPA 

approval. The section includes technical information related to EGU emission inventories, key 

modeling/projection input assumptions, appropriate data sources, and general methodological criteria 

to be used by states and EPA Regional offices.

This guidance does not recommend or restrict the use of any specific tool to generate credible 

projections of EGU emissions. It explains a process for documenting credible data and input assumptions 

with accompanying justifications; describes how to demonstrate a reasonable relationship between 

inputs and outputs for a given application; and identifies relevant resources that should be taken into 

consideration.

Emission projections for EGUs do not tend to follow a simple growth path from historical emission data. 

The composition and behavior of the generating fleet, and resulting power sector emission patterns 

across facilities, states, and regions, vary substantially over time based on changing economic conditions 

as well as changes in fuel markets and regulatory requirements. Therefore, evaluating the credibility of 

power sector emission projections necessitates evaluating the reasonableness of several key projection 

elements or assumptions, while keeping in mind that appropriateness of the method also depends on 

the span of future year(s) being projected (the “modeling horizon”). These projection elements include:

1. Electricity demand
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2. The composition of the future generating fleet

If the future year(s) provides enough lead time, consideration of economic entry (new facility 

construction) and exit (existing facility retirement) that would change the composition of the 

fleet.

3. Generation pattern across facilities that is consistent with economic factors, power system 

operability, reliability, and environmental regulatory constraints

4. Potential emissions at each generator, taking into account pollution content of available fuels as 

well as the ability of control equipment or processes to capture that pollution

State, tribal and local agencies have the choice to independently develop power sector emission 

projections or to use emissions projections from other entities (federal or regional bodies, including 

RTOs and ISOs). If a state, tribal or local agency chooses to develop independent projections (or to 

derive such projections from other available forecasts), data sources should be referenced and clear 

justification should be stated for the projection methodology, including a clear demonstration of the 

relationship between demand, cost, generation capacity and emissions. Documentation supporting any 

set of power sector emission projections should include:

1. Inventory of all sources of generation accounted for in the projections, including but not limited 

to all individual emitting EGUs

2. Information regarding electricity demand for the future year

a. The demand forecast for EGU projections should be referenced and documented 

(potential sources include AEO, NERC, regional transmission organizations, independent 

system operators, state energy offices or public utility commissions, and regional 

councils that develop energy planning).

3. A detailed profile of future capacity, including:

a. Projected changes to the operable fleet, such as retired capacity, new capacity, changes 

to capacity type (e.g., refueling), and pollution control retrofits

b. Physical characteristics of the operable fleet, including capacity, fuel type, heat rate, and 

pollution controls (i.e., list of emissions controls and their associated removal 

efficiencies for pollutants of concern) 

4. A detailed profile of projected power system operation, including generation, fuel use, and 

emissions

5. A detailed list of all of the constraints (with the corresponding future compliance dates) that 

apply to reduce emissions at one or more sources, such as: 

a. Federal, state and local rules

b. NSR settlements

c. Consent decrees

d. Rate or permit requirements
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e. All other on-the-books regulatory requirements

6. Representations of the following, or if not directly factored in, reasoning and methodology 

explaining how these elements are accounted for indirectly:

a. Fuel supplies and/or prices

b. Generator availability constraints

c. Minimum generation requirements (such as qualifying renewable energy (RE)

generation to meet RPS targets)

d. Cost and performance characteristics of existing and potential new generation and 

pollution control technology options

e. Transmission, reserve margin, and other system operating constraints

7. Unit-level generation, fuel type and use, pollution control configuration, and emissions

8. Any other information used in and/or relevant to the projection

Electric generating unit projections for different purposes (such as attainment demonstrations, 

maintenance plans, and reasonable further progress plans), as well as varying factors such as pollutants, 

geographic areas, and/or how far the projection period is into the future, may prompt adaptation of 

different approaches. There are two main types of methodologies states can consider when projecting 

EGU emissions for SIPs: optimization methods and non-optimization methods. 

1. Optimization methods are those that use computational algorithmic models that seek the 

“least-cost” optimal dispatch based on economic assumptions while obeying physical 

constraints of generators within the selected universe of EGUs. Examples of typical constraints 

within an optimization method include, but are not limited to, ramp rates, outage schedules, 

transmission limits, system reserve margin limits, and emission limitations. Optimization 

methods are generally used for longer-term projections at the national or regional scale. In 

some cases, optimization approaches may under-represent non-economic factors that affect 

unit operations.

2. Non-optimization methods are those that follow a prescribed set of rules and relationships to 

simulate the behavior of EGUs based on electricity demand trends, historical electricity 

production trends and data from other forecasting models. Generally, these frameworks employ 

statistical analysis or algorithms to estimate how EGUs will generate in a future year based on 

certain expected conditions. To the extent that these methods are based largely on past EGU 

behavior, their projections can be relatively insensitive to potential changes over time in 

economic and regulatory factors that affect EGU behavior. These non-optimization methods are 

generally used for shorter-term projections at the regional, state, or local scale.

Whether a state uses an optimization or non-optimization EGU projection method, states should 

provide information to EPA that documents the methodology and how the chosen projection method 

adequately addresses the analytic drivers described above, in the context of the associated regulatory 

action and with the timeframe of the projections taken into account. For example, for a given area, 
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there may be a reasonable justification to use recent historical data to represent fleet behavior in the 

near future (e.g., 1-5 years). However, projecting for years farther into the future (e.g., 10-20 years) may 

necessitate an optimization approach in order to allow for a better representation of the economic and 

regulatory drivers that affect fleet behavior and associated emissions. 

A state may also consider whether limited modifications to other EGU projections (such as those 

provided by EPA, EIA, or ISO/RTOs) may reasonably address the analytic drivers described above. Such 

an approach should include documentation of where and how the modified set of EGU projections 

differs from the unmodified (original) set of projections, and why it is appropriate for the given 

regulatory action or future scenario to make such modifications without conducting a more thorough or 

complete analysis. 

5.3.1.1 Existing Material for More Information

The resources listed in this section can provide additional information that may be helpful for EGU 

emissions projections.

Guidance for 1-Hour SO2 Nonattainment Area SIP Submissions. This document describes non-binding 

recommendations on a wide range of issues that are likely to arise as state, local and tribal governments 

develop nonattainment SIPs for the 1-hour SO2 NAAQS. This document is available at 

https://www.epa.gov/so2-pollution/guidance-1-hour-sulfur-dioxide-so2-nonattainment-area-stateimplementation-plans-sip. 

Roadmap for Incorporating Energy Efficiency and Renewable Energy in SIPs/TIPs. This document 

provides a roadmap to assist state, tribal and local agencies with accounting for and incorporating 

energy efficiency (EE) and RE policies and programs in SIPs and TIPs. The roadmap accomplishes this task 

by clarifying guidance the EPA issued in 2004 on incorporating EE/RE policies and programs into SIPs, as 

well as related guidance EPA issued in 2004 and in 2005. This document is available at 

https://www.epa.gov/energy-efficiency-and-renewable-energy-sips-and-tips/energyefficiencyrenewable-energy-roadmap. 

Resource for States on estimating EE/RE relative to EIA’s Annual Energy Outlook for SIPs. This 

document helps states identify their on-the-books energy efficiency and renewable energy policies and 

estimate the incremental electricity savings of these policies; develop a methodology for projecting a 

jurisdiction’s energy demand both with and without the incremental electricity savings; and estimate 

the change in power sector emissions attributable to the incremental electricity savings. For more 

information visit: https://www.epa.gov/sites/production/files/2015-08/documents/

including_ee_and_re_policies_in_ed_projections_03302015_final_508.pdf.

Assessing the Multiple Benefits of Clean Energy Programs: A Resource for States. This document helps 

state energy, environmental, and economic policy makers identify and quantify the energy, 

environmental (specifically greenhouse gas, air, and health), and economic benefits of clean energy. This 

document is available at https://www.epa.gov/statelocalclimate/assessing-multiple-benefits-cleanenergy-resource-states.
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EPA’s AVERT (AVoided Emissions geneRation Tool). This tool can be used to estimate the EGU emission 

impacts of EE/RE. For more information visit: https://www.epa.gov/avert.

5.3.1.2 Examples of Existing EGU Projection Tools

There are specific approaches available that can be used in ways that meet the general principles and 

key criteria explained in Section 5.3.1. The EPA uses the IPM to model future electricity demand and 

emissions trading programs that now dominate the prediction of future-year emissions from EGUs. The 

IPM meets the criteria provided in the EPA resource document listed above. According to the IPM user’s 

document:

“IPM is a multi-regional, dynamic, deterministic linear programming model of the U.S. 

electric power sector. It provides forecasts of least cost capacity expansion, electricity 

dispatch, and emission control strategies while meeting energy demand and environmental, 

transmission, dispatch, and reliability constraints. IPM can be used to evaluate the cost and 

emissions impacts of proposed policies to limit emissions of SO2, NOX, carbon dioxide (CO2), 

mercury (Hg), and HCl from the electric power sector.”

More information on IPM and IPM-based emissions are available at the Power Sector Modeling EPA 

website (https://www.epa.gov/airmarkets/power-sector-modeling). 

Other EGU projection models and tools exist and could be used for estimation of future-year emissions

and, in these cases, the states are advised to make sure that the tools meet the criteria laid out in this 

document because EPA will use these criteria to assess any EGU projection information submitted as 

part of SIPs. Mention of specific tools in this guidance should not be taken as an endorsement or 

guarantee that such tools will meet the criteria that the EPA will use to evaluate EGU emissions 

projections. Alternative tools include the ERTAC Tool, the National Energy Modeling System (NEMS), the 

Electric Power Market Model (EPMM) and the Market Allocation (MARKAL) model. There is also a 

Northeast-specific version of MARKAL called NE-MARKAL. The subsections below provide additional 

information and reference links for some of these alternative approaches.

The ERTAC Tool

The ERTAC tool is an open-source tool developed by the ERTAC group of state and regional associations 

that benefits from an in-depth collaboration among a large multi-state region. More information on this 

tool is available on the ERTAC EGU website (http://www.marama.org/2013-ertac-egu-forecasting-tooldocumentation), including sample files and documentation.

The ERTAC tool is designed to estimate future EGU activity and emissions using user-supplied, unitspecific information, CAMD hourly emissions data, and AEO and NERC growth forecasts by fuel and 

region. The ERTAC tool outputs hourly future-year activity, SO2 emissions, and NOX emissions by keying 

off the historical CAMD hourly data from the SIP base year. For the future-year planning inventories, the 

ERTAC results can be aggregated to match the annual or season-day resolution needed for the 

regulation being supported (see Section 4.5). For modeled attainment demonstrations (not covered by 
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this guidance), the ERTAC hourly future-year data outputs may then be used in conjunction with an 

emissions preprocessor and an air quality model to evaluate the effect of HEDD periods as well as other 

EGU impacts that might be of shorter duration than the modeled period.

Other Approaches

The NEMS model (available at: https://www.eia.gov/forecasts/aeo/info_nems_archive.cfm) was 

developed by EIA and uses Annual Energy Outlook (AEO) projections of energy supply, demand and 

prices, and it couples with regulatory/SIP and trading assumptions and known future control 

installations to create future year NOX, SO2 and heat input estimates. The EPMM is a private sector EGU 

model, but some discussion can be found in these reports: http://www.ladco.org/reports/rpo/

emissions/eug_growth_methodologies_methods_final_report_pechan.pdf. The MARKAL model is a 

data-driven energy system optimization model geared more towards basic, preliminary analyses of 

energy policies at EGUs but does not provide plant-specific detail like IPM. MARKAL is relevant for other 

sectors and more information is included in Section 5.3.6.

Preventing Double Counting of Emissions between EGUs and Other Categories

To prevent double-counting of emissions sources between EGUs and other categories, emissions 

developers should be careful to match the sources of emissions from external models such as IPM with 

the base year emissions sources. This helps to ensure that the EGU part of the point source inventory is 

separated from the non-EGU part based on the facilities included in the future-year EGU model. In the 

case of IPM, the facilities are defined using NEEDS. The NEEDS records should be compared to the base 

year point inventory using the ORIS Plant ID field from both. In some cases (e.g., co-generation facilities), 

only some of the units at the facilities are included in NEEDS; therefore, the separation of EGUs from 

non-EGUs should be done at the unit level, not by facility. One resource for this information is also in the 

NEI, which contains a field that indicates whether the units are NEEDS units or not.

5.3.2 Non-EGU Stationary Sources

Non-EGU stationary sources include both the point and nonpoint sources, but the methods for 

projecting them to the future can rely on many of the same sources of information.

Since 2006 (EPA, 2006a), the EPA has been assuming that emissions growth does not track with 

economic growth for many stationary sources (both point and nonpoint). This “no-growth” assumption 

is based on an examination of historical emissions and economic data. Emissions (as of 2005) had 

declined for several years and those reductions could not be directly attributed to specific control

programs despite increasing economic-based growth factors for many metrics over the same time 

period. While the EPA continues to work toward improving the projection approach in its own work, we 

are still using this no-growth assumption for many emissions sectors. Of course, where air agencies have 

better data about the expected growth in emissions or the possibility of sources reaching their permit 

limits, that information should be included in developing more refined emissions projections.

The EPA creates and uses data that quantify assumed base year growth (projections), controls, 

settlement agreements (or consent decrees), and other unit and facility-specific information on non-
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EGU point and stationary area sources. These are included in the most recent emissions platform 

Technical Support Document (TSD) available at the Emissions Modeling Clearinghouse. The TSDs include

the methodologies and background information for projecting sources such as Portland cement, 

refineries, oil and gas, industrial/commercial/institutional boilers, residential wood combustion, 

agricultural sources, fuel sulfur rules, corn and cellulosic ethanol and biodiesel plants, and several other 

categories. The data sources include national sources with regional and state-specific information as 

well as projection information provided by regional modeling centers and states and local air agencies.

In addition, the EPA encourages air agencies to consider the latest settlement agreements and consent 

decrees that are updated periodically by the EPA’s Office of Enforcement and Compliance Assurance 

(OECA) at: https://www.epa.gov/enforcement/air-enforcement.

The additional resources described in Section 5.3.6 are also used by the EPA to develop projections used 

in its modeling platforms. These same resources are available to state, local, and tribal agencies to 

develop their own projection methods for non-EGU stationary sources.

Finally, for nonpoint sources, Section 3 of the EIIP projections document provides additional details 

about projecting nonpoint-source. However, several of the references or statements in that document 

are now out of date, as follows.

 The web sites for SCCs, AP-42 emission factors, and more information about area-source 

inventories are out of date. These should be updated to the same sites as were listed in the 

point sources section above. Furthermore, this guidance includes more updated information 

than the EIIP report referenced.

 The ASEM model mentioned in Section 3.2 of the EIIP projection document was not completed 

and is not available.

 The references in Table 3.1-2 are out of date and should be updated as follows:

o U.S. EPA Emissions trends reports. These reports can be used to provide the historical 

trends of emissions sectors based on the National Emissions inventory: 

https://www.epa.gov/air-trends/

o Multiple Projections System is no longer in use

o California Emission Forecasting System: this system is no longer available. Interested 

parties should contact California Air Resources Board for more information on their 

latest emissions projection approaches.

The tools and data sources described in Section 5.3.6 below should also be considered for use for 

projection of point and nonpoint stationary source emissions.

5.3.3 On-Road Mobile Sources

As described in Section 4.8.3, for all states but California, on-road mobile emissions should be estimated 

with the latest EPA on-road mobile model, MOVES, following the EPA’s latest guidance available at 

https://www3.epa.gov/otaq/models/moves/. For California, the latest EPA-approved version of EMFAC 

should be used.
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Emissions preparation with MOVES for projected emissions is much like preparation for a base year. 

MOVES includes assumptions about emissions reductions associated with national emissions programs 

including the effects of national vehicle and fuel standards. Some MOVES inputs, such as meteorology, 

should be the same as for the base year inventory. Others, such as VMT and vehicle population should 

be grown/reduced to reflect expected changes in population and activity. MOVES default estimates for 

future VMT and vehicle population are based on national growth rates and should not be used for SIP 

inventory purposes because growth rates vary substantially from county to county.

Because of the change to the using the MOVES model and the newer guidance provided on the OTAQ 

websites, the EIIP projections document is no longer relevant for on-road mobile sources.

5.3.4 Nonroad Mobile Equipment

Section 5 of the EIIP projections document provides details about projecting nonroad mobile sources. 

While some information is still valid, several of the references or statements in that document are now 

out of date, as follows.

 The name “Office of Mobile Sources” has changed to “OTAQ” and its primary website is

https://www3.epa.gov/otaq/.

 The EPA’s NONROAD model has been incorporated into the MOVES model and should no longer 

be used for new SIP development.

 According to a California Air Resources Board website, category-specific inventory models and 

methods have replaced most uses of California’s OFFROAD model. The information on 

California’s nonroad emissions is available at http://www.arb.ca.gov/msei/categories.htm. A link 

to the OFFROAD2007 model is also available from this page to be used for categories not listed 

on that page.

MOVES includes assumptions about emissions reductions associated with nonroad source national 

emissions programs. MOVES also includes assumptions about national growth in nonroad activity and 

equipment population. Local estimates of future activity and equipment populations should be 

substituted for the national estimates when possible.

5.3.5 Other Nonroad Mobile Sources

Aircraft, locomotives, and CMV sources of emissions do not have a specific model designed for 

projections, and so additional effort is needed for these three inventory components.

For aircraft emissions, FAA data can be used to estimate growth for aircraft emissions. The FAA provides 

summary historical and forecast statistics on passenger demand and aviation activity at U.S. airports. 

The summary level forecasts are based on individual airport projections. The Terminal Area Forecast 

(TAF) includes forecasts for active airports in the National Plan of Integrated Airport Systems (NPIAS). 

The TAF system can be accessed at https://aspm.faa.gov/main/taf.asp. The TAF system allows users to 

create their own forecast scenarios. The TAF database, which contains a query data application, allows 

the public to access and print historical and forecast (up to 2040) aviation activity data by individual 

airport, state or FAA region. Air agencies can use this information to adjust their base year emissions 

based on projected airport activity.
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For locomotives and marine vessel emissions, the best source of data on the impact of national rules 

from OTAQ is the websites provided for each of the rules and their associated Federal Register notices. 

As described for the base year emissions in Section 4.8.5, the locomotive marine rule information is 

included at the website https://www3.epa.gov/otaq/nonrdmdl.htm in the subsection “Other Nonroad 

Reference Material.” For CMV sources, Table 20 (from the base year emissions section) provides a good 

list of individual rules and references. Since each rule is different, there is no single guidance that can be 

made about how these should be incorporated. However, the EPA incorporates these data into their 

modeling platforms, and the most recent projection methodologies are documented in the TSDs

included with the modeling platforms on the Emissions Modeling Clearinghouse at 

https/www.epa.gov/air-emissions-modeling .

Additionally for C3 vessels, emissions data are available from an Emissions Control Area-International 

Maritime Organization (ECA-IMO) project. These emissions are available at a fine-grid resolution and are 

subject to various engine and fuel standards in the future. Like base year emissions, air agencies should 

consider the state versus federal waters when deciding what parts of the ECA-IMO data to use (see 

Section 4.8.5.2). Growth/control factors have been developed for this C3 CMV inventory, and the EPA is 

using them for future year C3 CMV inventory development. The ECA-IMO rule for C3 CMV can be found 

at https://www3.epa.gov/otaq/oceanvessels.htm. New approaches are likely to develop over time, so 

air agency staff should consult the web pages listed here for the latest available information. The latest 

EPA modeling platform projection methodology for C3 marine emissions is also available in the latest 

emissions modeling platform TSD at the Emissions Modeling Clearinghouse mentioned previously.

5.3.6 Other Projection Resources

In addition to sector-specific resources, there are other sources of information that projection 

developers can use for more than a single sector as information to help determine the most applicable 

growth rates for the modeling region. These include the Bureau of Labor Statistics “Employment 

Outlook,” Census Bureau data, Trade Organizations and individual facilities, and the Chemical Economics 

Handbook. The following paragraphs provide a brief description and additional references for each of 

the sources of data just listed. 

Sources for EPA nonpoint methods

As described in Section 4.8.2, the EPA develops methods for numerous nonpoint methods. Each of these 

methods depends on some surrogate activity. Many of these activity sources come from USDA, DOE, 

U.S. Census Bureau, and Department of Labor databases available online. Appendix B provides a list of 

nonpoint sectors and associated activity reports and websites. These data do not include future-year 

projections, but may include newer information than were used for estimating the base year 

information. As such, these data can help with understanding the possible changes in activity (and 

perhaps emissions) from the date of the underlying base year activity data to the present day. It should 

be noted that emissions for 1 year (e.g., 2011) may be estimated using activity data from a prior year 

(e.g., 2009) because of timing needed for preparing the data does not match with the timing of the 

release of the underlying data. When emissions are estimated using data earlier than the emissions base 
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year, it further strengthens the benefit of newer activity data. Furthermore, these datasets often include 

historical trends, which could be of use in understanding what may happen in the future.

Since the base year emissions for nonpoint methods are calculated based on direct activity indicators 

such as population, fuel consumption, employment, and VMT, projections of these direct activity 

indicators may also be used to estimate future emissions. Population is related to sectors such as 

consumer products, architectural coatings, portable fuel containers, and commercial cooking. VMT also 

plays a major role in calculation of paved road emissions. Growth projections for population and fuel 

consumption are readily available from the U.S. Census and the DOE’s Annual Energy Outlook (see 

below). VMT projections are provided periodically as part of the EPA emissions modeling platforms and 

may be available in some states from other sources. Employment projections may also be available in 

some states and could be used for industrial categories where the state believes that employment is an 

appropriate surrogate for facility activity and therefore emissions growth or retraction.

MARKAL

The MARKAL model is a public sector, data-driven, energy-system optimization model. A user inputs the 

structure of the energy system to be modeled and data to characterize each of the technologies and 

resources included in the model run. MARKAL calculates the least cost set of technologies over time to 

satisfy specific demands, using linear and mixed-integer linear programming techniques. Outputs of the 

model include the technological mix at intervals in the future, energy demand, and estimates of criteria 

and GHG emissions, among others. The DOE uses MARKAL for its System for the Analysis of Global 

Energy Markets (SAGE) model. SAGE is used to produce EIA’s annual International Energy Outlook.

The EPA has applied MARKAL for technology and emissions evaluations, which required the 

development of the EPA MARKAL technology database (EPANMD). This technology-rich database 

represents the major sectors in the U.S. energy system, including the commercial, industrial, residential, 

transportation, and electricity generation sectors. The DOE’s AEO and the NEMS were used to construct 

the energy supply, demand, and technology characterizations. Data for technologies not represented in 

the AEO and NEMS were derived from other widely recognized authoritative sources (e.g., Electric 

Power Research Institute’s Technical Assessment Guide and DOE’s Office of Transportation Technology’s 

Quality Metrics report). More information on the EPA’s use of MARKAL and the EPANMD can be found 

on the website for “EPAUS9R - An Energy Systems Database for use with the Market Allocation 

(MARKAL) Model” and EPA report number NRMRL-RTP-375 (EPA, 2006b).

Control Strategy Tool (CoST)

The purpose of CoST is to estimate the emission reductions and engineering costs associated with 

control strategies applied to point, nonpoint, and mobile sources of air pollutant emissions to support 

the analyses of air pollution policies and regulations. Control strategy results can be exported to CSV 

files or viewed in a graphical table that supports sorting, filtering, and plotting. The results can also be 

merged with the original inventory to create controlled emissions inventories that can be exported to 
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files that can be input to the emissions model SMOKE. CoST allows for user-supplied control measures 

data, provides tracking of analyses and outputs, and supports built-in QA. 

The Control Measure dataset focuses on criteria pollutants and their precursors, and it includes primary 

control efficiencies, co-pollutant control efficiencies, and control costs. The data also includes an 

extensive set of mobile source control measures. More information on the Control Measure dataset and 

CoST is available at https://www3.epa.gov/ttn/ecas/cost.htm. 

Annual Energy Outlook

The EIA’s AEO forecast data was a primary input for the final version of EGAS. However, with EGAS no 

longer being supported by the EPA, the latest versions of the AEO are the best available source of 

socioeconomic forecast data for many industrial sectors. The latest AEO data are available at: 

http://www.eia.gov/forecasts/aeo/er/index.cfm.

Census Bureau

The U.S. Census Bureau provides data on total employees, total payroll, number of establishments, and 

value of shipments by NAICS code (2 digit - 6 digit) for specific years. This database is ideal for examining 

changes in the number of establishments between the two latest available years and also for historical 

changes. Therefore, the actual census data are useful primarily to evaluate emissions changes from the 

base year to the present (or most recently available year) and comparing to the data included in the

AEO. The census data are available at http://www.census.gov/2010census/data/.

In addition, the Census Bureau projects future population, which is available at 

http://www.census.gov/population/projections/. This information can be used to help project nonpoint 

emissions categories that include population as an activity input for the base year emissions. 

Trade Organizations and Individual Facilities

Trade organizations can also be a helpful resource because they often have projections for future-year 

growth of their industry and may be aware of pending facility closures or new facility construction. This 

resource is most relevant for large industrial sources that usually are included in point source 

inventories. In addition, emissions modelers should consider contacting large sources of emissions in 

their modeling region for their expectations of emissions growth or reduction. In many cases, large 

industries may be willing to provide such information when presented with what will be used if no 

additional information can be provided. Additional information may be found in permit applications for 

future development or controls at individual facilities. Except in the case of permit applications, 

emissions projections developers should be aware of possible conflicts of interest for this source of 

information. For example, industry groups or facilities may have an incentive to under-represent futureyear emissions in hopes of avoiding additional requirements for reducing emissions. However, many 

industrial representations have been valuable stakeholders who are interested in providing accurate 

information.
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Chemical Economics Handbook

The Chemical Economics Handbook, produced by SRI Consulting (a part of IHS, Inc.

27), is a series of 

reports on prices, production and consumption of hundreds of chemical industry products and 

commodities. Past and current information on chemical products and commodities is available, and 

projections of future prices, production and consumption are often available. Reports on specific 

industries are also available. Reports at an industry level can often be used to verify the efficacy of 

future industry modeling results. Each report is updated every 3 years. Projections, some up to 5 years 

from the current day, are often prepared using proprietary methods. Reports are available by 

subscription, and can be obtained as hard copy, CD, or at 

http://www.sriconsulting.com/CEH/Public/index.html.

5.4 Evaluate and Refine Data for Projections in Inventory Region

For key sectors, emissions projections developers should determine what information will impact the 

projection results the most, and ensure that the data values reflect conditions or expectations of the 

modeling region. The key information is identified based on a combination of base-year emissions data 

and growth and control rates. An iterative process is helpful to complete this step, as follows:

1. Estimate future-year emissions by combining the base-year emissions with the projection 

information (as described in the EIIP document referenced in Section 5.1).

2. Review future-year emissions data to identify large sectors of key pollutants (e.g. NOX, SO2, 

PM2.5, NH3 and VOC). Emissions summaries by state/SCC or state/NAICS for point sources can be 

helpful for this step to consolidate the data to review.

3. For the largest future-year emissions sectors, review the growth and control assumptions, 

compare among all available data sources, and/or collect additional information to refine the 

assumptions. A representative growth rate should be identified from the available data sources 

and all information known about the sources and sectors. Stakeholder review of the data can be 

helpful during this step; for example, an industrial facility with large projected emissions may be 

able to review the data and provide additional information (e.g., expected capacities, fuels or 

controls) for a more informed future-year emissions estimate.

4. Additionally, data developers should also compare the future-year emissions to base-year 

emissions, and if economic-based growth is used, to identify overall growth rates for industrial 

sectors and scrutinize excessively high growth rates, especially when associated with significant 

emissions.

5. Using the new information, repeat step 1 and continue these steps until a satisfactory futureyear projection across the entire inventory has been completed.

 

27 A number of other industry reports appears to be available from the IHS home page from the “Industries” tab. 

Some of these websites appear to have databases available for a fee, but they have yet not been assessed by EPA 

for applicability to emissions projections.
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Emissions models (e.g., CoST, SMOKE, MOVES, IPM, etc.) provide the capability to create future-year 

inventories using base year inventories and projection information. Emissions modelers will need to 

convert the projected emissions data into specific formats for input to these models. Prior to starting 

this step, data developers should determine which emissions model will be used to perform the 

calculations and make sure that the type of information needed by the model is being collected.

5.5 QA for Projected Inventories

As mentioned in the previous subsection, final review of the inventories is an iterative process. During 

each iteration, adaptations of the methods described in Section 0 can be used for QA of projected 

emissions inventories. The following list gives examples of some of the ways of the methods for base 

year inventories can be adapted for projected emissions inventories.

 The same checks used for finding structure errors for base year emissions should be used for 

ensuring the projected inventories use the correct structures. The EIS checks are less relevant 

since projected emissions are more likely to be in a modeling format (since they are used for 

modeling) than are base year inventories. Even so, data developers should make sure that data 

structure or formatting problems do not lead to incorrect emissions data being used in planning 

inventories or modeling.

 Like base year inventories, data codes should be compared to lists of valid codes. It is just as 

important to have future-year SCCs be valid as it is for base year SCCs.

 Ranking techniques can be used to identify facilities, units, and processes that have been grown 

to unrealistic values. Ranking sources by emissions values can identify any that rise to the top of 

the list of largest sources without a good reason why. Ranking techniques can also be used for 

projected nonpoint emissions data.

 Inventory comparisons can be made between projected and base year inventories to help make 

sure:

o Location information is consistent between base and future year (needed for modeling 

only).

o Release point parameters are consistent between base and future year. Where controls 

have been added, one may expect that the release point parameters could change, for 

example, with higher stacks or lower exit gas temperatures (needed primarily for 

modeling).

o Emissions increases and decreases at the facility, unit, and process level are consistent 

with what is expected.

o Emissions increases and decreases at the county and county/SCC levels are consistent 

with what is expected. If certain sectors are being held constant (i.e., no better 

data/approaches are available), then data developers can confirm that these sectors did 

not change emissions.
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6 References

Allen, D. J., Pickering, K. E., Pinder, R. W., Henderson, B. H., Appel, K. W., and Prados, A.: Impact of 

lightning-NO on Eastern United States photochemistry during the summer of 2006 as 

determined using the CMAQ model, Atmos. Chem. Phys. Discuss., 11, 17699-17757, 

doi:10.5194/acpd-11-17699-2011, 2011.

Appel, K. W., Foley, K. M., Bash, J. O., Pinder, R. W., Dennis, R. L., Allen, D. J., and Pickering, K.: A multiresolution assessment of the Community Multiscale Air Quality (CMAQ) Model v4.7 wet 

deposition estimates for 2002–2006, Geosci. Model Dev. Discuss., 3, 2315-2360, 

doi:10.5194/gmdd-3-2315-2010, 2010.

EPA, 2006a. Regulatory Impact Analyses, 2006 National Ambient Air Quality Standards for Particle 

Pollution. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, 

October, 2006. Docket # EPA-HQ-OAR-2001-0017, # EPAHQ-OAR-2006-0834. Appendix D: 

http://www.epa.gov/ttn/ecas/regdata/RIAs/Appendix%20D--Inventory.pdf.

EPA, 2006b. Shay, C. L., S. Yeh, J. Decarolis, D. H. Loughlin, C. L. Gage, And E. Wright. EPA U.S. National 

MARKAL Database: Database Documentation. U.S. Environmental Protection Agency, 

Washington, DC, EPA/600/R-06/057, 2006.

Fu, J.S., Dong, X., Huang, K., Tong, D., and Zhuang, G.: Model development of dust emission and 

heterogeneous chemistry within the Community Multiscale Air Quality modeling system and 

its application over East Asia, Atmospheric Chemistry and Physics, 16, 8157-8180, 2016.

Reid, S.B., Technical Memorandum, Sonoma Technology, Inc., Preparation of Version 2 of the Wildland

Fire Emissions Inventory for 2011, April 26, 2013. 
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Appendix A

Emissions Inventory System - Quick Reference

How to Create County and Facility NOx and VOC Summaries for Select Counties

Create a summary of NOX and VOC emissions BY SECTOR and COUNTY for a group of counties

1. Select "Request Reports" from the left hand sidebar on the EIS Gateway.

2. Under the Emissions Summaries heading, select “By Geography.”

3. Select Report Type "County by Sector."

4. Click the Data Set or Selection bar. This brings you to a “filter” that allows you to select an 

inventory on which to run a report. 

To find a version of the NEI, type “NEI V” in the search box under the Data Set or Selection. 

Then, click the appropriate row for the year and version of the inventory for your needs.

5. Click on the Pollutants bar to access the pollutant filter. Use the search box provided and type in 

“VOC” and select that resulting row for VOC (click on it with the mouse). Repeat this for NOX. 

Typing both filters at the same time will not work.

6. Because we are looking at counties, go directly to the County filter. DO NOT SELECT A STATE 

FIRST from the State filter (this will select all counties in selected states, even if you then select 

specific counties in the county filter). Use the search box in the County filter to find the counties 

of interest either by name or State/County FIPS code. Select the counties of interest by clicking 

on those rows. If no counties are selected, all counties will be returned.

7. Lastly, select your Sectors using the Sectors search box. If no sectors are selected, the report will 

be created with all sectors. The EIS Sectors are explained in more detail in Section of the 2011 

NEI technical documentation.

8. Click the "Request Download" button at the bottom of the screen. These reports are run by EIS

every 15 minutes and are available under the Report Download section on the left hand sidebar 

of the Gateway. 

Note: Using the “Run Report” button will give you a faster response and display the summary on 

the screen, but may not include all records for your report. A warning is given at the top of the 

screen if the “Run Report” result is incomplete.

Create a summary of NOX and VOC emissions BY FACILITY for a group of counties 

1. Select "Request Reports" from the left hand sidebar on the EIS Gateway.

2. Under the Emissions Summaries heading, select “By Facility.”

For access to the Emissions Inventory System (EIS), refer to 

https://www.epa.gov/air-emissions-inventories/emission-inventory-system-eis-gateway .

See also Online EIS training “Reports – May 2012” at 

https://www.epa.gov/air-emissions-inventories/emission-inventory-training
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3. Select Report Type “Facility”. For more detailed (sub-facility) data, you may select “Emissions 

Unit” or “Emissions Process.”

4. Follow Step 4 in the list above.

5. Follow Step 5 in the list above.

6. Follow Step 6 in the list above.

7. Lastly, you may refine your search even more using the optional filters of Facility Types, NAICS, 

Regulations, or Source Classifications. If you do not use these filters, all facilities for the selected 

counties that include NOX and VOC emissions will be returned in the report.

8. Follow Step 8 in the list above.
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Appendix B

EPA Nonpoint Methods and Associated Activity Data and 

Websites
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Category Data Type

Last 

Release 

Date

Data Lag 

(years) Report Source Link

Agriculture Production - Livestock Livestock 

Population

May 2014 

(2012 data) 2 Census of Agriculture USDA https://www.agcensus.usda.gov/

Agriculture Production - Livestock Livestock 

Population

Continuous 

Updates USDA NASS Quick Stats USDA https://www.nass.usda.gov/Quick_Stats/

Asphalt Paving - Cutback Cutback 

Asphalt Use

2008 Asphalt Usage Survey for the 

United States and Canada Asphalt Institute http://www.asphaltinstitute.org/

Asphalt Paving - Cutback VMT 2014 2 Highway Statistics 2014 FHWA http://www.fhwa.dot.gov/policyinformation/quickfinddata/qftravel.cfm

Asphalt Paving - Emulsified Emulsified 

Asphalt Use

2008 Asphalt Usage Survey for the 

United States and Canada Asphalt Institute http://www.asphaltinstitute.org/

Asphalt Paving - Emulsified VMT 2014 2 Highway Statistics 2014 FHWA http://www.fhwa.dot.gov/policyinformation/quickfinddata/qftravel.cfm

Aviation Gasoline Distribution -

Stage 1

AvGas 

Consumption

Sept 2015 

(2014 data) 1.5 Petroleum Supply Annual EIA http://www.eia.gov/petroleum/supply/annual/volume1/

Aviation Gasoline Distribution -

Stage 2

AvGas 

Consumption

Sept 2015 

(2014 data) 1.5 Petroleum Supply Annual EIA http://www.eia.gov/petroleum/supply/annual/volume1/

Commercial Cooking Population June 2016 

(2015 data) 1.5 Population Estimates Program GCT-T1: Population Estimates

U.S. Census 

Bureau http://www.census.gov/popest/

Construction Dust -

NonResidential Construction

Value of 

Construction April 2016 0

Annual Value of Construction Put in 

Place

U.S. Census 

Bureau http://www.census.gov/construction/c30/c30index.html

Construction Dust -

NonResidential Construction

Business 

Patterns 2014 2

County Business Patterns (e.g., no. 

establishments, employees)

U.S. Census 

Bureau http://www.census.gov/econ/cbp/index.html

Construction Dust -

NonResidential Construction

Construction 

Costs June 2016 1 mo. Producer Price Index U.S. Dept. of Labor http://www.bls.gov/ppi/#tables

Construction Dust - Residential 

Construction Housing Units (2014 data) 2

Annual Housing Units Authorized by 

Building Permits CO2010A

U.S. Census 

Bureau https://www.census.gov/popest/data/housing/totals/2014/

Construction Dust - Residential 

Construction Housing Starts June 2016 2 mo. New Privately Owned Housing Units Started for previous quarter

U.S. Census 

Bureau http://www.census.gov/construction/nrc/index.html

Construction Dust - Residential 

Construction

Ratio for 5 or 

more units

May 2016

(2015 data) 5 mo

Table 2au. New Privately Owned 

Housing Units Authorized 

Unadjusted Units for Regions, 

Divisions, and States 2009

U.S. Census 

Bureau http://www.census.gov/construction/bps/stateannual.html

Construction Dust - Residential 

Construction

Types of 

Foundation (2015 data) Type of Foundation in New OneFamily Houses Completed

U.S. Census 

Bureau

http://www.census.gov/construction/chars/completed.html 

(see row for “Foundation”).

Construction Dust - Road 

Construction VMT 2014 2 Highway Statistics 2014 FHWA http://www.fhwa.dot.gov/policyinformation/quickfinddata/qftravel.cfm
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Category Data Type

Last 

Release 

Date

Data Lag 

(years) Report Source Link

Construction Dust - Road 

Construction Housing Starts June 2016 2 mo. New Privately Owned Housing Units Started for previous quarter

U.S. Census 

Bureau http://www.census.gov/construction/nrc/index.html

Fertilizer Application Commercial 

Fertilizer Usage 2012 Commercial Fertilizers

Association of 

American Plant 

Food Control 

Officials

http://www.aapfco.org/publications.html#comm (must order)

Fuel Combustion - Commercial Fuel Sales 2014 2 Fuel Oil and Kerosene Sales EIA http://www.eia.gov/dnav/pet/pet_cons_821use_dcu_nus_a.htm

Fuel Combustion - Commercial Business 

Patterns 2014 2

County Business Patterns (e.g., no. 

establishments, employees)

U.S. Census 

Bureau http://www.census.gov/econ/cbp/index.html

Fuel Combustion - Commercial Gov't Payrolls 2016

(2012 data) 4 Census of Governments U.S. Census Bureau https://www.census.gov/govs/cog/

Fuel Combustion - Commercial Fuel 

Consumption 2014 2 State Energy Data System (SEDS) EIA http://www.eia.gov/state/seds/

Fuel Combustion - Industrial Fuel Sales 2014 2 Fuel Oil and Kerosene Sales EIA http://www.eia.gov/dnav/pet/pet_cons_821use_dcu_nus_a.htm

Fuel Combustion - Industrial Business 

Patterns 2014 2

County Business Patterns (e.g., no. 

establishments, employees)

U.S. Census 

Bureau http://www.census.gov/econ/cbp/index.html

Fuel Combustion - Industrial Energy 

Consumption

March 2012 

(2010 data) 3.5 Energy consumption as a fuel, by industry and region EIA http://www.eia.gov/consumption/manufacturing/

Fuel Combustion - Industrial Fuel 

Consumption 2014 2 State Energy Data System (SEDS) EIA http://www.eia.gov/state/seds/

Gasoline Distribution - PFCs

Gasoline Distribution - Stage 1 Business 

Patterns 2014 2

County Business Patterns (e.g., no. 

establishments, employees)

U.S. Census 

Bureau http://www.census.gov/econ/cbp/index.html

Gasoline Distribution - Stage 1 Fuel 

Consumption

Sept 2015 

(2014 data) 1.5 Petroleum Supply Annual EIA http://www.eia.gov/petroleum/supply/annual/volume1/

Gasoline Distribution - Stage 2

Industrial Fuel Combustion Business 

Patterns 2014 2

County Business Patterns (e.g., no. 

establishments, employees)

U.S. Census 

Bureau http://www.census.gov/econ/cbp/index.html

Industrial Fuel Combustion Fuel Sales 2011 2 Fuel Oil and Kerosene Sales EIA http://www.eia.gov/dnav/pet/pet_cons_821use_dcu_nus_a.htm

Industrial Fuel Combustion Energy 

Consumption June 2009 3.5 2006 Manufacturing Energy Consumption Survey EIA http://www.eia.gov/emeu/mecs/mecs2006/2006tables.html

Industrial Fuel Combustion Fuel 

Consumption 2014 2 State Energy Data System (SEDS) EIA http://www.eia.gov/state/seds/

Open Burning - Household Waste Solid Waste 

Generation 2011 2

Municipal Solid Waste in the United 

States: 2009 Facts and Figures EPA http://www.epa.gov/epawaste/nonhaz/municipal/msw99.htm
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Category Data Type

Last 

Release 

Date

Data Lag 

(years) Report Source Link

Open Burning - Household Waste Population June 2016 

(2015 data) 1.5 Population Estimates Program GCT-T1: Population Estimates

U.S. Census 

Bureau http://www.census.gov/popest/

Open Burning - Land Clearing Business 

Patterns 2014 2

County Business Patterns (e.g., no. 

establishments, employees)

U.S. Census 

Bureau http://www.census.gov/econ/cbp/index.html

Open Burning - Land Clearing Housing Units (2014 data) 2

Annual Housing Units Authorized by 

Building Permits CO2010A

U.S. Census 

Bureau https://www.census.gov/popest/data/housing/totals/2014/

Open Burning - Land Clearing Value of 

Construction 2016 0

Annual Value of Construction Put in 

Place

U.S. Census 

Bureau https://www.census.gov/construction/c30/c30index.html

Open Burning - Land Clearing Housing Starts June 2016 2 mo. New Privately Owned Housing Units 

Started for previous quarter

U.S. Census 

Bureau http://www.census.gov/construction/nrc/index.html

Open Burning - Land Clearing Ratio for 5 or 

more units

May 2016

(2015 data) 5 mo

Table 2au. New Privately Owned 

Housing Units Authorized 

Unadjusted Units for Regions, 

Divisions, and States 2009

U.S. Census 

Bureau http://www.census.gov/construction/bps/stateannual.html

Open Burning - Leaves and Brush Solid Waste 

Generation 2013 3

Municipal Solid Waste in the United 

States: 2013 Facts and Figures EPA https://www.epa.gov/smm/advancing-sustainable-materialsmanagement-facts-and-figures-report

Open Burning - Leaves and Brush Population June 2016 

(2015 data) 1.5 Population Estimates Program GCT-T1: Population Estimates

U.S. Census 

Bureau http://www.census.gov/popest/

Paved Road Dust VMT 2014 2 Highway Statistics 2014 FHWA http://www.fhwa.dot.gov/policyinformation/quickfinddata/qftravel.cfm

Publicly Owned Treatment Works Nationwide 

Flow rate

January 

2016 4

Clean Watersheds Needs Survey 

2012 Data and Reports, Detail 

Report

EPA https://www.epa.gov/cwns/clean-watersheds-needs-survey-cwns2012-report-and-data#access

Publicly Owned Treatment Works Population June 2016 

(2015 data) 1.5 Population Estimates Program GCT-T1: Population Estimates

U.S. Census 

Bureau http://www.census.gov/popest/

Residential Coal Coal 

Distribution

Dec 2016 

(2014 data) 2 Annual Coal Distribution EIA http://www.eia.gov/cneaf/coal/page/coaldistrib/a_distributions.html

Residential Coal Fuel 

Consumption 2014 2 State Energy Data System (SEDS) EIA http://www.eia.gov/state/seds/

Residential Coal House Heating 

Fuel Type 2014 2

House Heating Fuel (online report 

from “Advanced Search”)

U.S. Census 

Bureau

http://factfinder.census.gov/faces/nav/jsf/pages/searchresults.xhtml?

refresh=t#none (Enter: “heating fuel” in search field)

Residential Distillate Fuel Fuel 

Consumption 2014 2 State Energy Data System (SEDS) EIA http://www.eia.gov/state/seds/

Residential Distillate Fuel House Heating 

Fuel Type 2014 2

House Heating Fuel (online report 

from “Advanced Search”)

U.S. Census 

Bureau

http://factfinder.census.gov/faces/nav/jsf/pages/searchresults.xhtml?

refresh=t#none (Enter: “heating fuel” in search field)
Emissions Inventory Guidance for Ozone & PM NAAQS Implementation and Regional Haze Regulations

9/29/2016 DRAFT

B-4

Category Data Type

Last 

Release 

Date

Data Lag 

(years) Report Source Link

Residential Kerosene Fuel 

Consumption 2014 2 State Energy Data System (SEDS) EIA http://www.eia.gov/state/seds/

Residential Kerosene House Heating 

Fuel Type 2014 2

House Heating Fuel (online report 

from “Advanced Search”)

U.S. Census 

Bureau

http://factfinder.census.gov/faces/nav/jsf/pages/searchresults.xhtml?

refresh=t#none (Enter: “heating fuel” in search field)

Residential LPG Fuel 

Consumption 2014 2 State Energy Data System (SEDS) EIA http://www.eia.gov/state/seds/

Residential LPG House Heating 

Fuel Type 2014 2

House Heating Fuel (online report 

from “Advanced Search”)

U.S. Census 

Bureau

http://factfinder.census.gov/faces/nav/jsf/pages/searchresults.xhtml?

refresh=t#none (Enter: “heating fuel” in search field)

Residential Natural Gas Fuel 

Consumption 2014 2 State Energy Data System (SEDS) EIA http://www.eia.gov/state/seds/

Residential Natural Gas House Heating 

Fuel Type 2014 2

House Heating Fuel (online report 

from “Advanced Search”)

U.S. Census 

Bureau

http://factfinder.census.gov/faces/nav/jsf/pages/searchresults.xhtml?

refresh=t#none (Enter: “heating fuel” in search field)

Residential Wood Timber Output 2012 4 Timber Products Output Reports USDA http://srsfia2.fs.fed.us/php/tpo_2009/tpo_rpa_int1.php

Residential Wood House Heating 

Fuel Type 2014 2

House Heating Fuel (online report 

from “Advanced Search”)

U.S. Census 

Bureau

http://factfinder.census.gov/faces/nav/jsf/pages/searchresults.xhtml?

refresh=t#none (Enter: “heating fuel” in search field)

Residential Wood

Heating 

Equipment 

Type

2013 3

American Housing Survey for the 

United States: 2013 Summary 

Tables, Table C-03-AO: Heating, Air 

Conditioning, and Appliances – All 

Occupied Units

U.S. Census 

Bureau http://www.census.gov/programs-surveys/ahs/data.html

Residential Wood Metropolitan 

Characteristics 2013 3

American Housing Survey 

Metropolitan Data

U.S. Census

Bureau

http://www.census.gov/programs-surveys/ahs/data/2013/ahs-2013-

summary-tables/metropolitan-summary-tables---ahs-2013.html

Solvent Usage - Other Population June 2016 

(2015 data) 1.5 Population Estimates Program GCT-T1: Population Estimates

U.S. Census 

Bureau http://www.census.gov/popest/

Solvent Usage - Other Business 

Patterns 2014 2

County Business Patterns (e.g., no. 

establishments, employees)

U.S. Census 

Bureau http://www.census.gov/econ/cbp/index.html

Solvent Usage - Surface Coatings Population June 2016 

(2015 data) 1.5 Population Estimates Program GCT-T1: Population Estimates

U.S. Census 

Bureau http://www.census.gov/popest/

Solvent Usage - Surface Coatings Business 

Patterns 2014 2

County Business Patterns (e.g., no. 

establishments, employees)

U.S. Census 

Bureau http://www.census.gov/econ/cbp/index.html

Unpaved Road Dust VMT 2014 2 Highway Statistics 2014 FHWA http://www.fhwa.dot.gov/policyinformation/quickfinddata/qftravel.cfm
Emissions Inventory Guidance for Ozone & PM NAAQS Implementation and Regional Haze Regulations

9/29/2016 DRAFT