Different types of plume classified on the basis of ELR and ALR?
The dispersion of emitted gases from the source of their production is known as plume and the source is known as stack.
The diffusion or dispersion of pollutants into the atmosphere is governed by the Environmental Lapse Rate (ELR) as well as Adiabatic Lapse rate (ALR), i.e., atmospheric temperature profile or atmospheric stability.
On the basis of ELR and ALR the nature of plume can be of different types.
Coning Plume :
It is calculated that when horizontal wind velocity exceeds 32km/hour and under the condition of cloud blocking solar radiations at day time and terrestrial radiation at night, neutral plume tends to form cone like structure known as coning plume.
Under sub-adiabatic conditions (ELR < ALR), when there is limited vertical mixing and environment is slightly stable, the plume also attains cone like structure and is coning plume. Such plumes are not, however, suitable for dispersion of pollutants.
Fanning Plume :
Under extreme inversion condition (due to negative lapse rate), fanning plume is obtained. Under condition of inversion, stable environmental condition exists just above the stack and plume does not move upwardly but horizontally.
Looping Plume :
Looping plume is of wavy character and occurs in super adiabatic environment (ELR>ALR), which produces highly unstable atmosphere because of rapid mixing.
In an unstable atmosphere, rapid air movements take place vertically, both upward and downward and the plume becomes a looping plume.
As a result of this, high concentrations of pollutants may occur near the ground. To disperse these pollutants, it is advisable to design high stack where atmosphere is generally super adiabatic.
Neutral Plume :
Neutral plume occurs in neutral atmospheric conditions (ELR=ALR). Such type of plume rises vertically in an upward direction. The upward lifting of the plume will continue till it reaches a height where density and temperature of surrounding air are equal to it.
Lofting Plume :
Under conditions of strong super adiabatic lapse rate just above the stack and negative lapse rate (inversion) just below the opening of stack, lofting plume is obtained.
The upward mixing of plume is very rapid and turbulent, but downward mixing is less because the downward movement is prevented by inversion.
The dispersion of pollutants therefore becomes rapid and pollutants cannot come down to the ground. Such kind of a plume is ideal for dispersion of air pollutants and protection of living beings to a great extent.
Fumigating Plume :
Fumigating plume is just opposite to lofting plume. Under conditions of negative lapse rate (inversion) just above the stack and strong super adiabatic lapse rate below the stack, the type of plume obtained is known as fumigating plume.
Under these set of conditions, the pollutants cannot escape above the stack, rather they come down near the ground due to turbulence and mixing. Fumigating plume is therefore extremely bad for dispersion of pollutants.
Trapping Plume :
When the inversion layer exists above the stack and as well as below the stack, the plume neither goes up nor goes down, rather, it gets confined or trapped between these two inversion layers. Such type of plume is therefore, termed as trapping plume. This plume is not ideal for dispersion of pollutants as it cannot go above a certain height.
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.