text: * Types of air pollutant emission sources &ndash; named for their characteristics
  ** Sources, by shape &ndash; there are four basic shapes which an emission source may have. They are:
  *** Point source – single, identifiable source of air pollutant emissions (for example, the emissions from a combustion furnace flue gas stack). Point sources are also characterized as being either elevated or at ground-level. A point source has no geometric dimensions.
  *** Line source – one-dimensional source of air pollutant emissions (for example, the emissions from the vehicular traffic on a roadway).
  *** Area source – two-dimensional source of diffuse air pollutant emissions (for example, the emissions from a forest fire, a landfill or the evaporated vapors from a large spill of volatile liquid).
  *** Volume source – three-dimensional source of diffuse air pollutant emissions. Essentially, it is an area source with a third (height) dimension (for example, the fugitive gaseous emissions from piping flanges, valves and other equipment at various heights within industrial facilities such as oil refineries and petrochemical plants). Another example would be the emissions from an automobile paint shop with multiple roof vents or multiple open windows.
  ** Sources, by motion
  *** Stationary source &ndash; flue gas stacks are examples of stationary sources
  *** Mobile source &ndash; buses are examples of mobile sources
  ** Sources, by urbanization level &ndash; whether the source is within a city or not is relevant in that urban areas constitute a so-called heat island and the heat rising from an urban area causes the atmosphere above an urban area to be more turbulent than the atmosphere above a rural area
  *** Urban source &ndash; emission is in an urban area
  *** Rural source &ndash; emission is in a rural area
  ** Sources, by elevation
  *** Surface or ground-level source
  *** Near surface source
  *** Elevated source
  ** Sources, by duration
  *** Puff or intermittent source &ndash; short term sources (for example, many accidental emission releases are short term puffs)
  *** Continuous source &ndash; long term source (for example, most flue gas stack emissions are continuous)