A major stationary source is a source that emits more than a certain amount of a pollutant as defined by the U.S. Environmental Protection Agency (EPA). The amount of pollutants allowed for certain new sources is defined by the EPA's New Source Performance Standards (NSPS).

A stationary source in air quality parlance is any fixed emitter of air pollutants, such as fossil fuel burning power plants, petroleum refineries, petrochemical plants, food processing plants and other heavy industrial sources.

Environmental Protection Agency | Air dispersion modeling