Dispersal of Air pollutants and Air Plume behaviuor

Dispersal of the air pollutants in the atmosphere depends upon the prevailing wind, temperature & pressure conditions

With the increase in the height, temperature of the normal air decreases above the earth surface (in troposphere) & the rate at which temperature decreases with increase in height is termed as LAPSE RATE or ENVIRONMENTAL LAPSE RATE (ELR). Its value is 6.5ᵒC/km (average). Lapse rate is not constant and varies from place to place and from time to time. A Rawinsonde’s thermometer measures ELR.

The rate at which temperature of parcel of air pollutant (from automobile exhaust, chimney etc.) decreases with increase in height above the earth’s surface is termed  as ADIABATIC LAPSE RATE (ALR).
[If a material changes its state (P, V or T) without any heat being added to it or withdrawn from it, the change is said to be adiabatic]
It is taken as 6.0ᵒC/km for Wet Process (Wet ALR) and 9.8ᵒC/km for Dry Process (Dry ALR)

WALR is lesser because of the heating produced by the condensation of water vapour.

Depending upon the relative magnitudes of ELR and ALR, following types of atmospheric conditions are defined –

• Unstable Atmosphere
• Stable Atmosphere
• Neutral Atmosphere

Unstable Atmosphere

• The environment become unstable for the pollutants when ELR > ALR.
• This is a favourable condition which leads to rapid vertical mixing of the pollutants.
• The prevailing ELR in such a condition is known as Super Adiabatic Lapse Rate, as it is more than ALR.
• In diagram we can observe that at elevation of 1000 m, ELR is less than ALR which means the enviroment cools more rapidly than air pocket. Thus ELR>ALR.

Stable Enviroment

• The environment is said to be stable with respect to pollutants when it doesn’t lead to their thorough vertical mixing.
• Such condition occurs when ELR < ALR
• It is unfavourable condition and the prevailing ELR  is called as Subadiabatic Lapse Rate as it is less than ALR.

• At elevation of 1000 m, the temperature of environment is more than the temperature of air pocket or parcels. or We can say that ELR< ALR. 

Inversion

• If temperature above the surface of the earth increases with the increase in height, this condition is termed as Inversion.
• The rate at which temperature increase with the increase in height is called negative lapse rate.
• Inversion is generally found when earth cools rapidly in comparison to the air surrounding it (Radiation Inversion) – Occurs in winter nights and becomes normal with the rays of the morning sun.
• Inversion also exists if high pressure region is surrounded by low pressure (Subsidence Inversion).

PLUME BEHAVIOUR

Gases evolved into the atmosphere are termed as Plume and their source of emission is termed as Stack.

Depending upon the type of Environment, following types of plume are observed –

a. Looping Plume (ELR > ALR) – This type of plume is observed in Super- adiabatic Environment and is characterized by rapid downward mixing which requires the higher stacks to avoid these interferences in atmosphere.

b.Neutral Plume ( ELR = ALR ) - This type of plume is found in neutral environment and is characterized by the vertically upward rise of the pollutant into the atmosphere.

c.Coning Plume ( ELR < ALR ) - This type of plume is observed in stable atmosphere generally when wind velocity is more than 32km/h.

d.Fanning Plume - This plume is observed in inversion condition and is characterized by horizontal spread of the pollutant. Higher stacks are needed if Fanning plumes are observed.

e.Lofting Plume – This type of plume is observed if Super- adiabatic environment exist over surface inversion. This plume is characterized with minimum downward mixing as it is restrained by the inversion layer below the stack. This type of plume is generally observed in winter.

f.Fumigating Plume – This type of plume is observed when inversion condition exists over Superadiabatic Environment. It is the worst possible case of plume. Gases are never able to escape as inversion layer exists over the stack.

g.Trapping Plume – This type of plume is observed when superadiabatic environment is present between two inversion layers. In this case, pollutant are trapped in between the two inversion layer. Hence it is termed as Trapping Plume.