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Point Source Modeling

The factors that affect the transport, dilution, and dispersion of air pollutants can be grouped into:

  • emission or source characteristics

  • the nature of the pollutant material

  • meteorological characteristics

  • the effects of terrain and anthropogenic structures.

Source characteristics

Most industrial pollution is discharged vertically from a stack or duct into the open air. As the contaminated gas stream is emitted, the plume (body of polluted air) expands and Plume means the body of pulluted air, Wind, that is horizontal air movement will bend the plume in the downwind direction. AT some distance from the source, the plume will level off. While the plume is rising, bending, and starting to move with the wind in the downwind direction, the flue gas is being mixed and diluted by the ambient air. As the gas is being diluted by increasing volumes of air, the contaminant will eventually reach the ground.

The initial rise of the plume is due to the upward inertia of the gas stream exiting the stack, and by its buoyancy. The vertical inertia is related to the exit velocity and mass of the gas. The buoyancy is related to the density relative to the surrounding air, primarily determined by temperature. Increasing exit velocity, and increasing exit temperature will increase the plume rise.

The plume rise, together with the physical stack height, is called the effective stack height

For a given set of stack and discharge conditions, the ground level concentration is proportional to the mass flux, i.e., the amount emitted per unit time. Increasing emission rates will therefore lead to a proportional increase in ambient concentrations.

Downwind distance

The greater the distance from the discharge point, the greater the volume of air available for dilution. However, since the plume starts above the ground and needs some time to reach the ground (by bending and spreading), there is no concentration observable in the immediate vicinity of the stack, then we can observe an increase for some distance as the plume approaches the ground. After this, the ground-level concentration will decrease with increasing distance from the emission source.

Wind speed and direction

The wind direction will determine the direction in which the plume will move across local terrain. Wind speed affects the plume rise (fast wind will bend the plume faster), and will increase the rate of dilution.

Thus, the effects of wind speed work in two opposite directions:

  • increasing wind speed will decrease plume rise, thus increase ground level concentrations;

  • increasing wind speed will increase mixing, thus decreasing ground level concentrations.

Depending on the specific conditions, one or the other of these phenomena will prevail. These effects also determine the distance from the source where the maximum concentration will occur.

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