What is the National Emissions Inventory (NEI)?

Background

EPA's Emission Inventory and Analysis Group (EPA/OAR/OAQPS/AQAD/EIAG) prepares a national database of air emissions information with input from numerous State and local air agencies, from tribes, and from industry. This database contains information on stationary and mobile sources that emit criteria air pollutants and their precursors, as well as hazardous air pollutants (HAPs). The database includes estimates of annual emissions, by source, of air pollutants in each area of the country, on an annual basis. The NEI includes emission estimates for all 50 States, the District of Columbia, Puerto Rico, and the Virgin Islands. Emission estimates for individual point or major sources (facilities), as well as county level estimates for area, mobile and other sources, are available currently for years 1990 and 1996 through 2002 for criteria pollutants, and for years 1999 and 2002 for HAPs. A final version of the 2002 NEI was posted in February 2006.

Data from the NEI are used for air dispersion modeling, regional strategy development, regulation setting, air toxics risk assessment, and tracking trends in emissions over time. For emission inventories prior to 1999, criteria pollutant emission estimates were maintained in the National Emission Trends (NET) database and HAP emission estimates were maintained in the National Toxics Inventory (NTI) database. Beginning with 1999, criteria and HAP emissions data were prepared in a more integrated fashion in the National Emission Inventory (NEI), which takes the place of the NET and the NTI.

Criteria air pollutants (http://www.epa.gov/air/urbanair/6poll.html) are those for which EPA has set health-based standards. Four of the six criteria pollutants are included in the NEI database:

Carbon Monoxide (CO)
Nitrogen Oxides (NOx)
Sulfur Dioxide (SO2)
Particulate Matter (PM10 and PM2.5)

The NEI also includes emissions of Volatile Organic Compounds (VOCs), which are ozone precursors, emitted from motor vehicle fuel distribution and chemical manufacturing, as well as other solvent uses. VOCs react with nitrogen oxides in the atmosphere to form ozone. Ammonia (NH3) is an additional pollutant included in the NEI.

The NEI database defines three classes of criteria air pollutant sources:

Point sources - stationary sources of emissions, such as an electric power plant, that can be identified by name and location. A "major" source emits a threshold amount (or more) of at least one criteria pollutant, and must be inventoried and reported. Many states also inventory and report stationary sources that emit amounts below the thresholds for each pollutant.

Area sources - small point sources such as a home or office building, or a diffuse stationary source, such as wildfires or agricultural tilling. These sources do not individually produce sufficient emissions to qualify as point sources. Dry cleaners are one example, i.e., a single dry cleaner within an inventory area typically will not qualify as a point source, but collectively the emissions from all of the dry cleaning facilities in the inventory area may be significant and therefore must be included in the inventory.

Mobile sources - any kind of vehicle or equipment with a gasoline or diesel engine; airplane; or ship.

The main sources of criteria pollutant emissions data for the NEI are:

For electric generating units - EPA's Emission Tracking System / Continuous Emissions Monitoring Data (ETS/CEM) and Department of Energy fuel use data.
For other large stationary sources - state data and older inventories where no state data was submitted.
For on-road mobile sources - the Federal Highway Administration's (FHWA's) estimate of vehicle miles traveled and emission factors from EPA's MOBILE Model.
For non-road mobile sources - EPA's NONROAD Model.
For stationary area sources - state data, EPA-developed estimates for some sources, and older inventories where no state or EPA data was submitted.

State and local environmental agencies supply most of the point source data. EPA's Clean Air Market program supplies emissions data for electric power plants.

Hazardous air pollutants (http://www.epa.gov/ttn/atw/pollsour.html), also known as toxic air pollutants, are those pollutants that are known or suspected to cause serious health problems. Section 112 of the Clean Air Act (CAA) currently identifies a list of 188 pollutants as HAPs. The list of HAPs includes relatively common pollutants such as formaldehyde, chlorine, methanol, and asbestos, as well as numerous less common substances.

The NEI database includes emission estimates for the 188 HAPs from stationary major and area sources and mobile sources, as defined in the Clean Air Act (CAA). (http://www.epa.gov/oar/oaqps/peg_caa/pegcaain.html) The NEI includes three classes of HAP emission sources:

Major sources - stationary (point) sources that emit or have the potential to emit 10 tons per year or more of any listed HAP or 25 tons per year or more of a combination of listed HAPs. The NEI includes data for each individual major source, including the name and location of the facility.

Area and Other sources - stationary sources that emit or have the potential to emit less than 10 tons per year of a single HAP and less than 25 tons per year of all HAPs combined. The Clean Air Act (CAA) defines "area source" as any stationary source of HAPs that does not qualify as a major source.

Examples of area sources include neighborhood dry cleaners and gas stations. Although emissions from individual area sources are often relatively small, collectively their emissions can be of concern. The NEI includes facility data for some area sources and aggregated emission estimates at the county level for the remaining area sources.

Other stationary sources of emissions, such as wildfires and prescribed burning, are addressed through the burning policy agreed to by EPA and the Department of Agriculture. The NEI includes aggregated emission estimates at the county level for these other sources.

Mobile sources - on-road vehicles, non-road 2- and 4-stroke and diesel engines, off-road vehicles, aircraft, locomotives, and commercial marine vessels. The NEI includes aggregated emission estimates at the county level for mobile sources.

The EPA compiles the NEI HAP emission estimates from five primary sources:

State and local HAP inventories.
Existing databases related to EPA's Maximum Achievable Control Technology (MACT) programs to reduce HAP emissions.
Toxic Release Inventory (TRI) data (www.epa.gov/tri/).
Emissions estimated by using mobile source methodology developed by EPA's Office of Transportation and Air Quality (OTAQ).
Stationary non-point source emission estimates generated using emission factors and activity data.

More information about the NEI database and the compilation of criteria pollutant and HAP emissions inventories, and links to the database, are available on the CHIEF NEI webpage http://www.epa.gov/ttn/chief/net/.

What is the National Emissions Inventory (NEI)?

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