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Region 1: EPA New England

Glossary of Terms

Atmospheric Transformation (Secondary Formation):
The process by which hazardous air pollutants (HAP) are transformed in the air into other chemicals. When a HAP is transformed, the original HAP no longer exists, but it is replaced by one or more chemicals. Compared to the original HAP, the newer reaction products may have more, less, or the same toxicity. Transformations and removal processes affect both the fate of the HAP and its atmospheric persistence. Persistence is important because human exposure to HAPS is influenced by the length of time the HAP remains in the atmosphere. Note that in NATA, the terms atmospheric transformation and secondary formation are used interchangeably.

Background Concentrations:
In this context, EPA uses background concentrations to mean the contributions to outdoor air toxics concentrations resulting from natural sources, persistence in the environment of past years' emissions, unidentified sources, and long-range transport from distant sources. Background concentrations could be levels of pollutants that would be found in 2002 even if there had been no recent manmade emissions. To accurately estimate outdoor concentrations, it is necessary to account for the background concentrations by adding them to the modeled concentrations. For more detailed information, see Background Concentrations in NATA.

Mobile Source:
A mobile sourceis any source which is not stationary such as a car, truck, train or plane. A mobile source includes on-road sources such as cars trucks and off-road sources such as tractors and lawnmowers.

National Air Toxics Assessment (NATA):
EPA's ongoing comprehensive evaluation of air toxics in the U.S. These activities include expansion of air toxics monitoring, improving and periodically updating emission inventories, improving national- and local-scale modeling, continued research on health effects and exposures to both ambient and indoor air, and improvement of assessment tools.

National Emissions Inventory:
EPA's compilation of quantitative information concerning the mass of air toxics emitted into the atmosphere (through smokestacks, tailpipes, vents, etc.)

Non-point Emission Source:
Non-point sources include area sources that are inventoried collectively because their specific locations are not known. For example, EPA estimated emissions from dry cleaners by using a national estimate of emissions and allocating those emissions to the county level by using dry cleaner employment data. Non-point sources also include wildfires and prescribed burning whose emissions are estimated at the county level.

Nonroad Mobile Sources:
Mobile sources not found on roads and highways (e.g., airplanes, trains, lawn mowers, construction vehicles, farm machinery).

Point Emission Source:
A point source is a stationary of air toxics for which a specific location of air toxics is known. Point sources in the national emissions inventory (NEI) include major and area sources of air toxics. Major sources of air toxics are defined in Clean Air Act as stationary sources that:

  • Have the potential to emit 10 tons per year of one or more air toxics; or
  • Have the potential to emit 25 tons per year of one or more air toxics.

Onroad Mobile Sources:
Vehicles found on roads and highways (e.g., cars, trucks, buses).

For complete glossary of terms go to NATA.

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