The USEPA requires States to inventory and submit emissions from air pollution sources. These sources include Point Sources (industrial stationary sources),
Nonpoint Sources (small facilities, lawn mowers, wood stoves, etc.), and Mobile Sources (cars, trucks, boats, etc.). Only the Point Source emissions are required
to be reported to the USEPA every year. Nonpoint and Mobile sources are reported every three years. The most recent comprehensive three-year inventory was for the 2020 calendar year.
Inventory data is used to verify whether air programs previously enacted are being effective and to aid in designing air programs needed to ensure air quality is improved or maintained.
Additionally, for new rule development, the USEPA uses these emissions estimates to model the risk to nearby communities; especially for toxic air pollutants.
Area/Nonpoint Sources
Nonpoint sources, also known as area sources, include emissions estimates for sources which
individually are too small in magnitude to report as point sources. These emissions sources are
included in emissions inventories as a county total or tribal total (for participating tribes).
Examples include residential heating, commercial combustion, asphalt paving, wildfires, and
commercial and consumer solvent use.
Mobile Sources
Mobile sources, which includes onroad and nonroad sources, are emissions from vehicles
that use gasoline, diesel, and other fuels. Onroad sources include vehicle emissions operating
on roads, highway ramps, and during idling. Nonroad sources include off-road construction
equipment, lawn and garden equipment, aircraft ground support equipment, locomotives, and
commercial marine vessels.
Oil and Gas Sources
While the major emissions sources associated with oil and gas collection, processing, and
distribution have traditionally been considered point sources (e.g. gas processing plants,
pipeline compressor stations, and refineries), the activities occurring “upstream” of these types
of facilities are considered nonpoint sources. This includes emissions from upstream
exploration activities, the drilling of oil and gas wells, and the equipment used at the wellsite to
extract the product from the well and deliver it to a central collection point or processing facility.
The types of unit processes found at upstream sites include separators, dehydrators, storage
tanks, and compressor engines.