Plume Dispersion Patterns

Plume Dispersion Patterns

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.

It is however, to be noted that plume rise depends not only on the stability of atmosphere, but also on the buoyancy and momentum of exhaust gases. Momentum depends on mass and velocity of the gases leaving the stack and buoyancy on the molecular weight of the exhaust gases and its temperature compared to the ambient air.

Six classifications of plume behaviour, which may occur under some commonly encountered metrological conditions, are Looping plume Lofting plume Coning plume Fumigation plume Fanning plume Trapping

Looping plume: Under super-adiabatic condition, both upward and downward movement of the plume is possible. Large eddies of a strong wind cause a looping pattern. Although the large eddies tend to disperse pollutants over a wide region, high ground level concentrations may occur close to the stack. 2. Lofting plume: When the stack is sufficiently high and the emission is above an inversion layer, mixing in the upward direction is uninhibited, but downward motion is restricted. Such lofting plumes do not result in any significant concentration at ground level. However, the pollutants are carried hundreds of kilometres away from the source.

3. Coning plume: A coning plume 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. 4. Fumigation plume: 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

When strong or moderate solar insolation occurs around the noon of summer, this stability is seen. The wind velocity will be < 3m/s. Temperature falls at more than 1.9ᵒc per 100m rise of altitude.

It is highly desirable condition as the pollutants are made to drift away from the stack, emitting them and dispersed and diluted over a longer height of infinite atmosphere.

However its duration is much short lived, spreading for about 1 to 1 1/2 hour on either side of noon in summer.

Stability B: It occurs during the clear, sunny day time when strong or moderate solar insolation with the velocity of the blowing wind at 1 to 5m/s. Its temperature gradient lies between -1.9 to -1.7ᵒC/100m rise.

Stability B is likely to occur for most part of the day i.e. before and after stability A for double its duration on sunny day.

Stability C: It occurs during the day time when moderate insolation and a wind velocity between 3 to 7 m/s and slight solar insolation but wind velocity is between 2 and 5m/s. Temperature gradient lies between -1.7ᵒC to -1.5ᵒC/100m