Air Pollution: Dispersion of pollutants

DISPERSION OF POLLUTANTS

 Dispersion characteristics of stack plumes

Dispersion is the process of spreading out pollution emission over a large area and thus reducing their concentration. Wind speed and environmental lapse rates directly influence the dispersion pattern. Five classifications of plume behaviour, which may occur under some commonly encountered metrological conditions.

Coning

A coning plume, shown in Figure 1 (a), occurs under essentially neutral stability, when environmental lapse rate is equal to adiabatic lapse rate, and moderate to strong winds occur. The plume enlarges in the shape of a cone. A major part of pollution may be carried fairly far downwind before reaching ground.

Looping

Under super-adiabatic condition, both upward and downward movement of the plume is possible. Large eddies of a strong wind cause a looping pattern, Figure 1

(b). Although the large eddies tend to disperse pollutants over a wide region, high ground level concentrations may occur close to the stack.

Fanning

A fanning plume occurs in the presence of a negative lapse rate when vertical dispersion is restricted, Figure 1 (c). The pollutants disperse at the stack height, horizontally in the from of a fanning plume.

Fumigation

As shown in Figure 1 (d), when the emission from the stack is under an inversion layer, the movement of the pollutants in the upward direction is restricted. The pollutants move downwards. The resulting fumigation can lead to a high ground level concentration downwind of the stack.