text: Air pollution emission plume &ndash; flow of pollutant in the form of vapor or smoke released into the air. Plumes are of considerable importance in the atmospheric dispersion modelling of air pollution. There are three primary types of air pollution emission plumes:
  * Buoyant plumes – Plumes which are lighter than air because they are at a higher temperature and lower density than the ambient air which surrounds them, or because they are at about the same temperature as the ambient air but have a lower molecular weight and hence lower density than the ambient air. For example, the emissions from the flue gas stacks of industrial furnaces are buoyant because they are considerably warmer and less dense than the ambient air. As another example, an emission plume of methane gas at ambient air temperatures is buoyant because methane has a lower molecular weight than the ambient air.
  * Dense gas plumes – Plumes which are heavier than air because they have a higher density than the surrounding ambient air. A plume may have a higher density than air because it has a higher molecular weight than air (for example, a plume of carbon dioxide). A plume may also have a higher density than air if the plume is at a much lower temperature than the air. For example, a plume of evaporated gaseous methane from an accidental release of liquefied natural gas (LNG) may be as cold as .
  * Passive or neutral plumes – Plumes which are neither lighter or heavier than air.