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Managing Air Quality - Emissions Inventories

An emissions inventory is a database that lists, by source, the amount of air pollutants discharged into the atmosphere during a year or other time period.

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How Does an Emissions Inventory Contribute to the Air Quality Management Process?

Governments use emission inventories to help determine significant sources of air pollutants and to target regulatory actions. Emissions inventories are an essential input to mathematical models that estimate air quality. The effect on air quality of potential regulatory actions can be predicted by applying estimated emissions reductions to emissions inventory data in air quality models.

Emission trends over time can be established with periodic updates of the emissions inventory. Inventories also can be used to raise public awareness regarding sources of pollution.

An emissions inventory includes estimates of the emissions from various pollution sources in a geographical area. It should include all pollutants associated with the air quality problems in the area. For example, an emissions inventory to support the management of ground-level ozone should include sources of nitrogen oxides (NOx) and of volatile organic compounds (VOC).

How are Emissions Inventories Developed?

Methods to determine emissions include, but are not limited to: continuous monitoring of emissions from a source; short-term emission measurements that are extrapolated to a longer time period, and use of emissions factors. The choice of method depends on the availability of data, time, staff and funding.

An emissions factor is a representative value that attempts to relate the quantity of a pollutant emitted with an activity level associated with the emission of that pollutant. An example format of an emissions factor is kilograms of particulate matter emitted per megagram of coal burned.

Emissions factors in the published literature typically are averages of available data of acceptable quality, and are assumed to represent long-term averages for similar facilities. However, variations among facilities, such as the raw materials used, operational conditions and emission controls, can significantly affect the emissions. Whenever possible, the development of local, source-specific emission factors is highly desirable.

EPA’s Clearinghouse for Inventories & Emission Factors contains information on emissions inventories, emissions factors and tools used for emissions inventories to support air quality management. EPA uses models to estimate emissions related to mobile sources.

Where Can I Find Information About Emissions Inventories in the United States?

The National Emissions Inventory (NEI) is a comprehensive and detailed estimate of air emissions of both criteria and hazardous air pollutants from all air emissions sources. EPA prepares the NEI every three years. It is based primarily upon information provided by state, local, and tribal air agencies for sources in their jurisdictions, and supplemented by data developed by EPA.

 Emissions trends and summaries, charts and maps of emissions from the NEI are available.

The Toxics Release Inventory (TRI) program tracks the management of certain toxic chemicals that may pose a threat to human health and the environment. Learn more about the TRILearn how the TRI Program works with other countries related to Pollutant Release and Transfer Registers (PRTRs)

EPA also develops and maintains an inventory of U.S. greenhouse gas emissions and sinks.