a report on environmental pollution control

8/19/2019 A report on environmental pollution control

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BITS PilaniPilani Campus

Lecture 8

Environmental Pollution Control

CHE F411Utkarsh Maheshwari

Chemical Engineering [email protected]


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BITS Pilani, Pilani CampusCHE F411: Environmental Pollution Control 2

Meteorological Aspects

of Air Pollutant

Dispersion

Temperature Lapse

rates and stability

Atmospheric Stability

Inversion

Wind Velocity and

Turbulence

Revision Content


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The tendency of the atmosphere to resist or enhance

vertical motion is termed stability.

It is related to both wind speed and the change of air

temperature with height (lapse rate).

A comparison of the adiabatic lapse rate with the

environmental lapse rate gives an idea of the stability

of the atmosphere.

Atmospheric Stability


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If an air parcel is displaced from its original height it can:

Return to its original height - StableAccelerate upward because it is buoyant - Unstable

Stay at the place to which it was displaced - Neutral


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When theΓ

en v and theΓ

adia are exactly same, a rising parcel of air willhave the same pressure, temperature and density of the

surroundings and would experience no buoyant force.

Such an atmosphere is said to be neutrally stable where a displaced

mass of air neither tends to return to its original position nor tends to

continue its displacement


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When the Γ en v

is less than the Γ adia

, a rising air parcel becomes cooler

and more dense than its surroundings and tends to fall back to itsoriginal position.

Such an atmosphere condition is called stable and the lapse rate issaid to be sub-adiabatic.

Under stable conditions there is very little vertical mixing and pollutants

can only disperse very slowly


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When the Γ en v

is greater than the Γ adia

, the atmosphere is said to be

super-adiabatic. Hence, a rising parcel of air, cooling at the adiabatic

rate, will be warmer and less dense than the surrounding environment.

As a result, it becomes more buoyant and tends to continue its upward

motion.

Since vertical motion is enhanced by buoyancy, such an atmosphere is

called unstable.

In the unstable atmosphere the air from different altitude mixes thoroughly.



Absolute instability

• If the environmental lapse rate is greater than the dry

adiabatic lapse rate

Absolute stability

• If the environmental lapse rate is less than the wet

adiabatic lapse rate

Conditional stability

• If the environmental lapse rate is between the dry

adiabatic lapse rate and wet adiabatic lapse rate



• The extreme case of a stable atmosphere, called an

inversion, occurs when temperature increases with

altitude

• Such a lapse rate is known as negative lapse rate.

• Under these conditions, the atmosphere is very stable

and practically no mixing of pollutants takes place

• Atmospheric inversions influence the dispersion of

pollutants by restricting vertical mixing

• There are several ways by which inversion layers can be

formed

Inversion



• This is usually associated with subtropical anticyclone where the air is warmed by compression as it descends

in a high pressure system and achieves temperature

higher than that of the air underneath.

• If the temperature increase is sufficient, an inversion willresult.

• The subsidence is caused by air flowing down to replace

air which has flowed out of the high pressure region

Subsidence Inversion

Ref:

http://www.earthonlinemedia.com/ebooks/tpe_3e/te

mperature/inversion.html



• This results from the normal diurnal cooling cycle.

• After sunset, the ground cools quickly by radiation heat

transfer , and the lowest layer of air in contact with the

surface loses sensible heat through conduction and

small scale mixing.

• Consequently, a temperature inversion is set up between

the cool low-level air and the warmer air above, in the

first few hundred meters above the surface.

Radiational Inversion

Ref:

http://www.earthonlinemedia.com/ebooks/tpe_

3e/temperature/inversion.html



Valleys and low lying areas are particularly affected by this type ofnocturnal inversions because denser, colder air tends to sink down

beneath the warmer air.

The next day sunlight destroys the inversion as the Earth is warmed

and the air previously stratified by inversion is overturned by

convective currents.

The temperature profile for a combined radiation and subsidence

inversion is illustrated below



• Is formed when warm air moves over a cold surface orcold air.

• The inversion can be ground-based in the former case,or elevated in the latter case.

• An example of an elevated advective inversion occurswhen a hill range forces a warm land breeze to flow athigh levels and a cool sea breeze flows at low levels inthe opposite direction

Advective Inversion

Ref:http://www.globalchange.umich.edu/gctex

t/Vince%20Abreu/Unit%209.3.1_files/imag

e006.jpg



• Occur along windward coasts bordered by cold oceancurrents.

• The bottom layer of a warm maritime air mass originating

over the ocean becomes cooler upon coming in contact

with the cooler water bordering the coast.• This creates cooler air near the surface with warmer air

aloft.

Sea breeze inversion

Ref:

http://www.earthonlinemedia.com/ebooks/tpe_

3e/temperature/inversion.html



Differential solar heating of the Earth’s surface producespressure and temperature gradient.

As a result, the atmosphere is practically in continuous

motion with air movements being always turbulent.

Thermal Turbulence:

The motion of air near the surface of the Earth is retarded

by friction, which varies with surface roughness.

The planetary boundary layer, in which friction is significant,

extends to about 1 km above the Earth’s surface.

Wind Velocity and Turbulence



The wind velocity profile within the layer is not onlyinfluenced by the surface roughness but also by the timeof day.

During the day, solar heating causes thermal turbulence or

eddies and these eddies set up convective currents sothat turbulent mixing is increased.

This results in a more flat velocity profile in the day thanthat at night.

Thermal turbulence also depends on the thermal stability ofthe atmosphere.

It is maximum on a clear sunny day in the afternoon andminimum at night or in the early morning.



Wind Velocity profiles duringday and night



Is produced by shearing stress generated by air movementover the Earth’s surface- the greater the surface

roughness, the greater the turbulence.

For smooth surfaces, the air velocity profile becomes very

steep near the ground.For rougher surfaces such as those in urban areas more

mechanical turbulence is generated and the velocity

profile becomes less steep and reaches deeper into the

atmosphere

Mechanical Turbulence



The mean wind speed variation with altitude in theplanetary boundary layer can be represented by a simpleempirical power law such as,

Where

u is the wind speed at altitude z ,

u1 is the wind speed at altitude z 1 and

α varies between 0.14 and 0.40 depending on the roughness of theground surface as well as on the temperature stability of theatmosphere. The exponent α is observed to increase with increasingstability or with increasing surface roughness.

11 z

z

u

u Adiabatic lapse rate: 7/1



Values along curves represent percentages of gradient wind value

Effect of terrain roughness onthe wind-speed profile:


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BITS PilaniPilani Campus

CHE F411 : Environmental and Pollution Control 21

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