Meteorology and Air Pollution

Plume types are important because they help us understand under what conditions there will be higher concentrations of contaminants at ground level.

• Looping Plume

  • High degree of convective turbulence

  • Superadiabatic lapse rate —> strong instabilities

  • Associated with clear daytime conditions accompanied by strong solar heating and light winds

  • High probability of high concentrations sporadically at ground level close to stack

  • Occurs in unstable atmospheric conditions

• Coning Plume

  • Stable with small-scale turbulence

  • Associated with overcast moderate to strong winds

  • Roughly 10° cone

  • Pollutants travel fairly long distances before reaching ground level in significant amounts

  • Occurs in neutral atmospheric conditions

• Fanning Plume

  • Occurs under large negative lapse rate

  • Strong inversion at a considerable distance above the stack

  • Extremely stable atmosphere

  • Little turbulence

  • If plume density is similar to air, travels downwind at approximately same elevation

• Lofting Plume

  • Favorable in the sense that fewer impacts at ground level.

  • Pollutants go up into environment

  • They are created when atmospheric conditions are unstable above the plume and stable below

• Fumigation

  • Most dangerous plume: contaminants are all coming down to ground level

  • They are created when atmospheric conditions are stable above the plume and unstable below

  • This happens most often after the daylight sun has warmed the atmosphere, which turns a night time fanning plume into fumigation for about a half an hour.

The wind velocity profile is another important characteristic of the atmosphere. The wind’s velocity depends on height above ground, stability of the atmosphere, and roughness of the Earth’s surface. An empirical power law is typically used to describe the dependence of wind speed on height above ground and atmospheric stability.

Another aspect of atmospheric stability is the maximum mixing depth (MMD). MMD describes the maximum height at which the convective mixing layer would be limited. Such information provides how high the convective mixing layer may rise for a given day. Maximum MMDs are expected during the summer and minimum MMDs are expected during winter.

A wind rose is another technique used to describe the atmosphere at a particular location. A wind rose provides the wind speed, wind direction, and the frequency of the wind’s speed and direction.