A fuel that can replace ordinary gasoline. Alternative fuels may have desirable energy efficiency and pollution reduction features. Alternative fuels include compressed natural gas (CNG), alcohols, liquefied petroleum gas (LPG), and electricity.

Outdoor air.

For a region to be "in attainment," hourly measurements of air pollution in that region must not exceed those designated to be safe by the Federal Clean Air Act.

For a region to be "in compliance," hourly measurements of air pollution in that region must not exceed those designated to be safe by the Federal Clean Air Act.

Release of pollutants into the air from a source.

A measurement of air pollution that exceeds the level designated to be safe by the Federal Clean Air Act in any given hour or on any given day.

A moving object that emits pollutants. Mobile sources are divided into two categories: on-road sources include cars, trucks, sport utility vehicles, motorcycles, and buses; off-road sources include airplanes, trains, motorboats and lawn mowers.

A criteria air pollutant. Nitrogen oxides are produced from burning fuels, including gasoline and coal. Nitrogen oxides react with volatile organic compounds (VOCs) to form ozone, the main component of urban smog. Nitrogen oxides are also a precursor pollutant that contributes to the formation of acid rain.

A region is said to be in "nonattainment" when hourly measurements of air pollution in that region exceed those designated to be safe by the Federal Clean Air Act.

A region is said to be in "non-compliance" when hourly measurements of air pollution in that region exceed those designated to be safe by the Federal Clean Air Act.