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8/19/2019 A report on environmental pollution control
1/21
BITS PilaniPilani Campus
Lecture 8
Environmental Pollution Control
CHE F411Utkarsh Maheshwari
Chemical Engineering [email protected]
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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.
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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
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• 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
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• 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
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• 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
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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
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• 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
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• 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
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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
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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.
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Wind Velocity profiles duringday and night
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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
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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
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Values along curves represent percentages of gradient wind value
Effect of terrain roughness onthe wind-speed profile:
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