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COURSE MATERIAL

SUBJECT Air Pollution and Control (19A01704a)

UNIT 2

COURSE B.TECH

DEPARTMENT EEE

SEMESTER 41

PREPARED BY Dr. P.Selvaraj

(Faculty Name/s) Professor

Version V-1

PREPARED / REVISED DATE 27-09-2022

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TABLE OF CONTENTS – UNIT 1

S. NO CONTENTS PAGE NO.
1 COURSE OBJECTIVES 3
2 PREREQUISITES 3
3 SYLLABUS 3
4 COURSE OUTCOMES 3
5 CO - PO/PSO MAPPING 3
6 LESSON PLAN 3
7 ACTIVITY BASED LEARNING 4
8 LECTURE NOTES 4
1.1 INTRODUCTION 4
1.2 COMPOSITION AND STRUCTURE OF THE EARTH’S ATMOSPHERE 4
1.3 STRUCTURE OF THE ATMOSPHERE 6
1.4 WIND SYSTEMS 8
1.5 SOLAR RADIATION 12
1.6 LAPSE RATES 14
1.7 ATMOSPHERIC STABILITY CONDITIONS 15
1.8 WIND VELOCITY PROFILE 17
1.9 MAXIMUM MIXING DEPTH 18
(MMD)
1.10 TEMPERATURE INVERSIONS, 19
1.11 WIND ROSE DIAGRAM 21
9 PRACTICE QUIZ 22
10 ASSIGNMENTS 23
11 PART A QUESTIONS & ANSWERS (2 MARKS QUESTIONS) 23
12 PART B QUESTIONS 25
13 SUPPORTIVE ONLINE CERTIFICATION COURSES 25
14 REAL TIME APPLICATIONS 25
15 CONTENTS BEYOND THE SYLLABUS 25
16 PRESCRIBED TEXTBOOKS & REFERENCE BOOKS 25
17 MINI PROJECT SUGGESTION 27

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1. Course Objectives
The objective of this course is to
To identify the sources of air pollution
To know the composition and structure of atmosphere
To know the pollutants dispersion models
To understand the working of air pollution control equipment
To identify the sources of noise pollution and their controlling methods
2. Prerequisites
Students should have knowledge on
Environmental Science
Chemistry

3. Syllabus
UNIT II
Meteorology - composition and structure of the atmosphere, wind circulation, solar
radiation, lapse rates, atmospheric stability conditions, wind velocity profile,
Maximum Mixing Depth (MMD), Temperature Inversions, Wind rose diagram
4. Course outcomes
Understand the composition and structure and structure of atmosphere
To understand the maximum mixing depth and wind rose diagram

5. Co-PO / PSO Mapping

APC PO1 PO2 PO3 PO4 PO5 PO6 PO7 PO8 PO9 P10 PO11 PO12 PSO1 PSO2

CO1 3 3 2 2

CO2 3 3 2 2

CO3 3 3 2 2

CO4 3 3 2 2

CO5 3 3 2 2

6. Lesson Plan

Lecture No. Weeks Topics to be covered References

Meteorology -
1
1
composition and structure of the atmosphere
2 R1

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, wind circulation,
3 R1

solar radiation, R1
4

lapse rates, R2
5
.
atmospheric stability conditions, R1
6
2
wind velocity profile, R4
7

Maximum Mixing Depth R3

8
(MMD),
9 Temperature Inversions, R1
3
10 Wind rose diagram R2

7. Activity Based Learning

What’s up there besides air? Make a DIY pollution catcher

8. Lecture Notes
1.1 INTRODUCTION
Meteorology:

The branch of science that deals with the study of the earth's atmosphere and majorly
focus on the weather processes and forecasting is known as meteorology. Weather
forecasting is made depending on the various variables like temperature, air pressure,
winds, etc. as they vary with time. Storms, lightning, rainfall, etc. are also studied under
meteorology. Predictions made by the meteorological department are important to
citizens, aviation services, farmers, and various other organizations. There are various
models which are used weather prediction:
Mathematical Model: In this model, supercomputers are used to analyze complex data
collected from observations and to find the optimal solution.
Holistic Model: In this model, the output from other models is also taken into consideration,
and then collectively final prediction is made.

1.2 Composition and Structure of the Earth’s Atmosphere

We all know that earth is a unique planet due to the presence of life. The air is one among
the necessary conditions for the existence of life on this planet. The air is a mixture of
several gases and it encompasses the earth from all sides. The air surrounding the earth is
called the atmosphere.

• Atmosphere is the air surrounding the earth.

• The atmosphere is a mixture of different gases. It contains life-giving gases like
Oxygen for humans and animals and carbon dioxide for plants.
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• It envelops the earth all round and is held in place by the gravity of the earth.
• It helps in stopping the ultraviolet rays harmful to the life and maintains the
suitable temperature necessary for life.
• Generally, atmosphere extends up to about 1600 km from the earth’s
surface. However, 99 % of the total mass of the atmosphere is
confined to the height of 32 km from the earth’s surface.

Composition of the atmosphere

• The atmosphere is made up of different gases, water vapor and dust particles.
• The composition of the atmosphere is not static and it changes according to
the time and place. The atmosphere is a mixture of different types of gases.

• Nitrogen and oxygen are the two main gases in the atmosphere and 99
percentage of the atmosphere is made up of these two gases.
• Other gases like argon, carbon dioxide, neon, helium, hydrogen, etc. form the
remaining part of the atmosphere.
• The portion of the gases changes in the higher layers of the atmosphere in
such a way that oxygen will be almost negligible quantity at the heights of 120
km.
• Similarly, carbon dioxide (and water vapour) is found only up to 90 km from
the surface of the earth.

CARBON DIOXIDE:

• Carbon dioxide is meteorologically a very important gas.

• It is transparent to the incoming solar radiation (insolation) but opaque to the
outgoing terrestrial radiation.
• It absorbs a part of terrestrial radiation and reflects back some part of it
towards the earth’s surface.
• Carbon dioxide is largely responsible for the greenhouse effect.
• When the volume of other gases remains constant in the atmosphere, the
volume of the carbon dioxide has been rising in the past few decades mainly
because of the burning of fossil fuels. This rising volume of carbon dioxide is the
main reason for global warming.

OZONE GAS:

• Ozone is another important component of the atmosphere found mainly

between 10 and 50 km above the earth’s surface.
• It acts as a filter and absorbs the ultra-violet rays radiating from the sun and
prevents them from reaching the surface of the earth.
• The amount of ozone gas in the atmosphere is very little and is limited to the
ozone layer found in the stratosphere.

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Water Vapour

• Gases form of water present in the atmosphere is called water vapour.

• It is the source of all kinds of precipitation.
• The amount of water vapour decreases with altitude. It also decreases from
the equator (or from the low latitudes) towards the poles (or towards the high
latitudes).
• Its maximum amount in the atmosphere could be up to 4% which is found in
the warm and wet regions.
• Water vapour reaches in the atmosphere through evaporation and
transpiration. Evaporation takes place in the oceans, seas, rivers, ponds and
lakes while transpiration takes place from the plants, trees and living beings.
• Water vapour absorbs part of the incoming solar radiation (insolation) from the
sun and preserves the earth’s radiated heat. It thus acts like a blanket allowing
the earth neither to become too cold nor too hot.
• Water vapour also contributes to the stability and instability in the air.

Dust Particles

• Dust particles are generally found in the lower layers of the atmosphere.
• These particles are found in the form of sand, smoke-soot, oceanic salt, ash,
pollen, etc.
• Higher concentration of dust particles is found in subtropical and temperate
regions due to dry winds in comparison to equatorial and polar regions.
• These dust particles help in the condensation of water vapour. During the
condensation, water vapour gets condensed in the form of droplets around
these dust particles and thus clouds are formed.

1.3 Structure of the atmosphere

The atmosphere can be divided into five layers according to the diversity of temperature
and density. They are:

1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere (Ionosphere)
5. Exosphere

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Fig 1. Structure of atmosphere

Troposphere

• It is the lowermost layer of the atmosphere.

• The height of this layer is about 18 km on the equator and 8 km on the poles.
• The thickness of the troposphere is greatest at the equator because heat us
transported to great heights by strong convectional currents.
• Troposphere contains dust particles and water vapour.
• This is the most important layer of the atmosphere because all kinds of
weather changes take place only in this layer.
• The air never remains static in this layer. Therefore this layer is called ‘changing
sphere’ or troposphere.
• The environmental temperature decreases with increasing height of the
atmosphere. It decreases at the rate of 1 degree Celsius for every 165 m of
height. This is called Normal Lapse Rate.
• The zone separating troposphere from the stratosphere is known as
tropopause.
• The air temperature at the tropopause is about – 80 degree Celsius over the
equator and about – 45 degree Celsius over the poles. The temperature here
is nearly constant, and hence, it is called tropopause.

Stratosphere

• Stratosphere is found just above the troposphere.

• It extends up to a height of 50 km.
• The temperature remains almost the same in the lower part of this layer up to
the height of 20 km. After this, the temperature increases slowly with the
increase in the height. The temperature increases due to the presence of
ozone gas in the upper part of this layer.

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• Weather related incidents do not take place in this layer. The air blows
horizontally here. Therefore this layer is considered ideal for flying of aircraft.
• The upper limit of the stratosphere is known as stratopause.
• One important feature of stratosphere is that it contains a layer of ozone gas.
• The relative thickness of the ozone layer is measured in Dobson Units.
• It is mainly found in the lower portion of the stratosphere, from approximately
20 to 30 km above the earth’s surface.
• It contains a high concentration of ozone (O3) in relation to other parts of the
atmosphere.
• It is the region of the stratosphere that absorbs most of the sun’s ultra-violet
radiations.

Mesosphere

• It is the third layer of the atmosphere spreading over the stratosphere.

• It extends up to a height of 80 km.
• In this layer, the temperature starts decreasing with increasing altitude and
reaches up to – 100 degree Celsius at the height of 80 km.
• Meteors or falling stars occur in this layer.
• The upper limit of the mesosphere is known as mesopause.

Thermosphere

• This layer is located between 80 and 400 km above the mesopause.

• It contains electrically charged particles known as ions, and hence, it is known
as the ionosphere.
• Radio waves transmitted from the earth are reflected back to the earth by this
layer and due to this, radio broadcasting has become possible.
• The temperature here starts increasing with heights.

Exosphere

• The exosphere is the uppermost layer of the atmosphere.

• Gases are very sparse in this sphere due to the lack of gravitational force.
Therefore, the density of air is very less here.

1.4 Wind Systems

• It is the horizontal movement of air from high pressure zones to low pressure areas to
maintain atmospheric equilibrium. Due to the Coriolis force, winds do not flow in a
straight path. The direction of the wind is identified by an instrument called a wind
vane. Anemometer is an instrument that measures the speed of the wind.
• Types of winds – Winds are classified into three:
o Primary winds,
o Secondary winds and
o Tertiary winds.

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Primary Winds

• Primary winds are also called planetary winds, permanent winds (blow constantly
throughout the year), global winds, invariable winds and prevailing winds.
• There are three types of primary winds – Trade Winds, Westerlies and Easterlies.
• Trade winds
o These winds flow between 30°N and 30°S latitudes from sub-tropical high-
pressure belts towards equatorial low pressure belts (in Hadley cell).
o These trade winds flow towards the equator from the north-east in the
northern hemisphere and from the south-east in the southern hemisphere.
o North-east and south-east trade winds get warm and pick up moisture on
their way to the equator. Near the equator, they rise and cause heavy rains.
• Westerlies
o These winds flow between 30°N and 60°N in the northern hemisphere and
between 30°S and 60°S in the southern hemisphere (in Ferrell cells). These
winds blow from subtropical high pressure towards subpolar low-pressure
belts.
o Westerlies blow from south-west to north-east in the northern hemisphere and
north-west to south-east in the southern hemisphere.
o Westerlies are stronger in the southern hemisphere because there are no
large landmasses to interrupt them.
• Easterlies
o These winds blow from polar high-pressure belts towards subpolar low
pressure belts between 90° and 60° latitudes in both the hemispheres (in Polar
cells).
o These polar easterlies blow from north-east to south-west in the northern
hemisphere and from south-east to north-west in the southern hemisphere.
Secondary Winds
Also called seasonal winds, periodic winds, variable winds and regional winds. Seasonal
winds change their direction in different seasons. Monsoons are seasonal winds that are
characterised by seasonal reversal of wind direction.
Land and Sea Breezes
During the day, the land heats up faster than water and the air over the land warms and
expands leading to the formation of a low-pressure zone. At the same time, the air over
the ocean is comparatively cool because of water’s slower rate of heating and forms a
high-pressure area. Thus, the pressure gradient from sea to land is created and the wind
blows from sea to the land as the sea breeze. In the night, the reversal of condition takes
place. The land loses heat faster than the sea and is cooler than the sea. The pressure
gradient is developed from the land to the sea and this results in land breeze.

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Fig 2. Breezes

Mountain and Valley Winds

• In mountainous regions, during the day the slopes get heated up and air moves up
the slope. To fill the resulting gap, the air from the valley blows up and this wind is
known as the valley breeze or Anabatic wind or upslope wind.
• During the night, the slope gets cooled, and the dense air descends downhill into
the valley. This wind is known as mountain wind or Katabatic wind or downslope
wind.
• On the leeward side of the mountain ranges, warm winds may occur. While
crossing the mountain ranges, the moisture in these winds condenses and
precipitate. The resulting dry winds descend the leeward side of the slope and get
warmed up by the adiabatic process. This warm wind may melt the snow in a short
time.

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Fig 3. Mountain and valley Winds

Tertiary Winds
Tertiary winds are formed due to pressure gradients which may develop on a local scale
due to differences in the heating and cooling of the earth’s surface. Local winds are
tertiary winds that blow only during a particular period of the day or year in a small area.
Such winds blow locally and are confined to the lowest levels of the troposphere.
North American local winds –

• Chinook – (snow eater) These are warm dry westerly off the Rocky Mountains.
• Blizzard – These are cold winds that blow in Canada, the USA, Siberia, etc.
• Norte – These are strong cold winds that blow along the Gulf of Mexico.
• Santa Ana – These are warm, dry and strong winds that blow out of the Great Basin
through the upper Mojave desert to California.
South American local winds –

• Pampero – These are cold winds and blow in Argentina and Uruguay.
• Zonda – These are warm and dry winds, and blow on the eastern slope of the
Andes in Argentina and Uruguay.
African winds –

• Sirocco – Also called blood rain as it brings reddish sand along with it from the
Sahara desert. It is warm, dry and dusty. Blows in a northerly direction from the
Sahara desert and crossing over the Mediterranean Sea, reach southern Europe.
• Khamsin – Dry, hot and sandy wind blows from North Africa to the eastern
Mediterranean.
• Harmattan – Also called doctor wind as it makes the weather pleasant. It is a dry
northerly wind across central Africa.
• Berg – A hot dry wind blows from the Kalahari high to the coastal low pressure area.
European winds –

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• Fohn/Foehn – It is a dry, strong and warm wind that blows along the northern slope
of the Alps and Switzerland. The wind helps in melting snow and aids in the ripening
of snow. It is a katabatic wind.
• Mistral – It is a cold northerly wind that blows from central France and the Alps to
the Mediterranean.
• Levante – It is a moist and rainy wind that blows near the Mediterranean sea and
southern France and Spain.
• Bora – It is cold, dry and gusty wind that blows north-easterly from Eastern Europe to
northeastern Italy.
Asian winds –

• Karaburan – (Black storm). It is a dusty fast blowing wind that blows in central Asia.
• Buran – In summer, it is hot and dry. During winters, it is an extremely cold wind that
blows across eastern Asia.
• Simoom – It is a strong, dry desert wind that blows in the Arabian desert.
• Loo – It is a hot and dry wind that blows over the plains of India and Pakistan.
• Yoma – It is a warm and dry wind that blows in Japan.
Australian winds

• Brickfielder – It is a hot and dry wind that blows in southern Australia.

Fig 4. Wind circulation

1.5 Solar Radiation

Solar Radiation – The earth receives almost all of its energy from the sun and it radiates the
energy back to space. As a result, the earth neither warms up nor does it get cooled over
a period of time. The energy received by the earth is termed as insolation- incoming solar
radiation.

• Aphelion and Perihelion – During the revolution of the earth around the sun, it is
farthest from the sun on 4th July (152 million km) and this position is called aphelion.
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o The earth is nearest to the sun on 3rd Jan (147 million km) and this position is
called perihelion.
o Therefore, the solar energy received by the earth on 3rd Jan is slightly more
than the amount received on 4th July.
o However, this variation in solar output does not greatly affect the daily
weather changes on the surface of the earth, because this variation is
masked by other factors like the distribution of land and sea, and the
atmospheric circulation.
Variability of insolation at the surface of the earth
The amount and intensity of solar radiation received by the earth (insolation) vary during
a day, in a season and in a year. The following are the factors that cause these variations:

1. The rotation of the earth on its axis.

2. The angle of inclination of the rays of the sun
3. The length of the day.
4. The transparency of the atmosphere, and
5. The configuration of the land in terms of its aspect.

(The insolation depends more on the first three factors)

The tilted position of the earth’s axis is known as the inclination of the earth’s axis. The
earth’s rotation axis makes an angle of about 66.5° with the plane of its orbit around the
sun and this greatly influences the amount of insolation received at different places.
The amount of insolation also depends on the angle of inclination of the sun’s rays. The
higher the latitude the less is the angle they make with the surface of the earth which
results in slant sun rays. The slant rays cover more area than the vertical rays. When more
area is covered, the energy gets distributed and the net energy received per area
decreases. Also, the slant rays have to pass through a greater depth of the atmosphere
which results in more absorption, diffusion and scattering.
Before striking the earth’s surface, the incoming solar radiation passes through the
atmosphere. The atmosphere is largely transparent to shortwave solar radiation. Water
vapours, ozone and other gases present in the atmosphere absorb most of the near-
infrared radiations. Small suspended particles in the troposphere scatter the visible
spectrum both to space and towards the surface of the earth. The blue colour of the sky
and the red colour of the rising and setting sun are the results of the scattering of light
within the atmosphere.
Duration of the day varies from place to place and season to season. It decides the
amount of insolation received on the earth’s surface.
The amount of solar radiation received at the surface of the earth is more in the tropics
(about 320 watts/m²) and least in the poles (70 watts/m²). The subtropical deserts receive
maximum insolation as the atmosphere is more transparent (least cloudiness). At the same
latitude, the insolation is more over the continents than over the oceans.
Terrestrial Radiation, Heating and Cooling of the Atmosphere

• Terrestrial Radiation – The solar radiation received by the earth is in short wave forms
and it heats up its surface. The earth acts as a radiating body and radiates energy
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in the form of long waves to the atmosphere. This process is called terrestrial
radiation and these long wave radiations heat up the atmosphere from below. The
atmosphere in turn radiates and transmits heat to space. This maintains the
constant temperature at the earth’s surface, as the amount of heat received from
the sun is transmitted to space.
• Heating and cooling of the atmosphere (conduction, convection and advection):
o The terrestrial radiation heats up the lower atmosphere which is directly in
contact with the surface of the earth. This process is called conduction in
which there is a flow of energy from the warmer to the cooler body and the
transfer continues till both the bodies attain the same temperature.
o As the lower layer of the atmosphere heats up, it rises vertically in the form of
currents and transmits the heat of the atmosphere. This vertical heating of the
atmosphere is called convection and is restricted only to the troposphere.
o The transfer of heat through the horizontal movement of air is called
advection. During summer in India, the local winds called loo is the outcome
of the advection process. Advection is relatively more important than
convection. In middle latitude, most of the diurnal (day and night) changes
are the result of advection alone.
o

Fig 5. Solar radiation

1.6 Lapse rate

The lapse rate is the rate at which an atmospheric variable,

normally temperature in Earth's atmosphere, falls with altitude.[1][2] Lapse rate arises from
the word lapse, in the sense of a gradual fall. In dry air, the adiabatic lapse rate is 9.8
°C/km (5.4 °F per 1,000 ft).

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It corresponds to the vertical component of the spatial gradient of temperature. Although
this concept is most often applied to the Earth's troposphere, it can be extended to any
gravitationally supported parcel of gas.
A formal definition from the Glossary of Meteorology is:
The decrease of an atmospheric variable with height, the variable being
temperature unless otherwise specified.
Typically, the lapse rate is the negative of the rate of temperature change with
altitude change:

where (sometimes ) is the lapse rate given in units of temperature

divided by units of altitude, T is temperature, and z is altitude

1.7 Atmospheric Stability

When you think of the word “stable,” you typically think of an object that is unlikely
to change or something that is balanced. The opposite is true with something that is
“unstable”. An unstable object is likely to fall or change position with time. The
same is true with clouds. When you see a fluffy cumulus cloud, you might notice
them changing shape from one minute to the next. Such clouds are in a constant
state of change, and thus represent the atmosphere in an unstable state.

Instability in the atmosphere is a concept that is intimately connected with thunderstorms,

cumulus development, and vertical motion. In order to visualize the concept of stability,
you might imagine a boulder sitting at the bottom of a canyon surrounded by steep hills,
as depicted in the figure below by the blue circle. If you were strong enough to push the
boulder from its initial position partway up one of the hills, it would roll back to the bottom
once you let go. Despite exerting a force on the boulder and causing an initial
displacement, it would return to its initial position, and the net displacement would be
zero. To visualize the concept of instability, imagine the same boulder at the top of a hill
(red circle below). If you were able to push the boulder just a little bit in any direction, it
would begin to roll downward and accelerate away from its initial position. However, if
the same boulder were to be placed on flat ground (green circle below) and you were to
push it, it would change position, but remain in its new position. This is an example
of neutral stability.

Each of these concepts can be applied to motions of air parcels in the atmosphere. The
topic of stability in atmospheric science is important because the formation of clouds is
closely related to stability or instability in the atmosphere. In this chapter we will connect
these concepts to the buoyancy of air parcels and learn to use thermodynamic diagrams
to visualize movement.

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Fig 6 Wind circulation

Adiabatic Processes

When discussing stability in atmospheric sciences, we typically think about air parcels, or
imaginary blobs of air that can expand and contract freely, but do not mix with the air
around them or break apart. The key piece of information is that movement of air parcels
in the atmosphere can be estimated as an adiabatic process. Adiabatic processes do
not exchange heat and they are reversible.

Imagine you have a parcel of air at the Earth’s surface. The air parcel has the same
temperature and pressure as the surrounding air, which we will call the environment. If you
were to lift the air parcel, it would find itself in a place where the surrounding
environmental air pressure is lower, because we know that pressure decreases with
height. Because the environmental air pressure outside the parcel is lower than the
pressure inside the parcel, the air molecules inside the parcel will effectively push outward
on the walls of the parcel and expand adiabatically. The air molecules inside the parcel
must use some of their own energy in order to expand the air parcel’s walls, so the
temperature inside the parcel decreases as the internal energy decreases. To summarize,

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rising air parcels expand and cool adiabatically without exchanging heat with the
environment.

Now imagine that you move the same air parcel back to Earth’s surface. The air parcel is
moving into an environment with higher air pressure. The higher environmental pressure
will push inward on the parcel walls, causing them to compress, and raise the inside
temperature.

The process is adiabatic, so again, no heat is exchanged with the environment. However,
temperature changes in the air parcel can still occur, but it is not due to mixing, it is due to
changes in the internal energy of the air parcel.

1.8 Wind Velocity Profile

Wind speed, or wind flow velocity, is a fundamental atmospheric quantity.

Wind speed is caused by air moving from high pressure to low pressure, usually due to changes in
temperature. Note that wind direction is usually almost parallel to isobars (and not perpendicular
as one might expect), due to the rotation of the earth.

Velocity profile is just a graph between velocity of the fluid (along X axis) and distance from
the stationary solid surface (along Y axis).

Velocity profile just means that there are different velocities at different layers in the fluid as
you go along a direction perpendicular to the object. This is why there is a differential velocity
with velocity being zero at the interface of the solid and increasing to being on par with the
free stream velocity at a distance significantly far away from the solid surface.

The term velocity profile just refers to the variation in velocity in a direction which is
perpendicular to the direction of fluid flow. If the fluid flows parallel to the solid surface, then it
results in shear force. Now why the fluid surface at the interface with the solid stays’ stationary
is because of viscosity. It is the measure of the resistance to flow of neighbouring layers
relative to each other (to be taken in a tangential sense).

Let us assume that there are 5 layers in the fluid, it is the outer most layer on which shear force
is applied. This layer suffers the maximum velocity on account of being in proximity with the
force. And the velocity decreases as you go from 5th layer to 4th and so on. The ultimate
layer which touches the fluid will be stationary or suffers the least velocity of all the layers

If you fix the fluid between two walls, with the top wall moving and the bottom wall stationary,
then you will see that the layer of fluid in contact with the moving wall will move with the
same velocity as the moving wall. Similarly, the fluid layer in contact with the stationary wall
will remain stationary. This is the no slip condition.

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Fig 7. Wind velocity Profile

1.9 maximum mixing depth (MMD)

The depth of the convective mixing layer in which vertical movement of pollutants is
possible, is called the maximum mixing depth (MMD)

The amount of air available to dilute pollutants depends on both wind

speed and the extent to which emissions can rise into atmosphere-
the maximum mixing depth.

Imagine a parcel of air at ground level being warmed (by convection). It

will tend to rise but how high?

Remember that as it moves up it will cool at about 1 degree per hundred

metres.

Whenever its temperature is greater than surrounding air it will continue to

rise. But if the temperature becomes less than ambient then the parcel of
air will fall. A temperature profile of the atmosphere allows us to determine
the maximum height of ascent.
And thus we can see that the maximum mixing depth can be
obtained provided we have a temperature profile for the atmosphere.

Fig 8 . maximum mixing depth

A temperature inversion is a layer in the atmosphere in which air temperature

increases with height. An inversion is present in the lower part of a cap. The cap is a
layer of relatively warm air aloft (above the inversion).
Meaning

▪ Under normal conditions, temperature usually decreases with increase in

altitude in the troposphere at a rate of 1 degree for every 165 metres. This is
called normal lapse rate.

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o But on some occasions, the situations get reversed and temperature
starts increasing with height rather than decreasing. This is
called temperature inversion.
▪ 1.10 Temperature inversion:
It is a reversal of the normal behavior of temperature in the troposphere. Under
this meteorological phenomenon a layer of warm air lies over the cold air layer.

o It is caused in stac atmospheric conditions while sometimes, it occurs

due to horizontal or vertical movement of air.
o Temperature inversion is usually of short duration but quite common
nonetheless.

Fig 9 Temperature inversions

Favourable Conditions for Temperature Inversion

▪ Long winter nights: Loss of heat by terrestrial radiation from the ground surface
during night may exceed the amount of incoming solar radiation.
▪ Cloudless and clear sky: Loss of heat through terrestrial radiation proceeds
more rapidly without any obstruction.
▪ Dry air near the ground surface: It limits the absorption of the radiated heat
from the Earth’s surface.
▪ Slow movement of air: It results in no transfer or mixing of heat in the lower
layers of the atmosphere.
▪ Snow covered ground surface: It results in maximum loss of heat through
reflection of incoming solar radiation.
Types of Temperature Inversion

▪ Temperature inversion occurs in several conditions ranging from ground

surface to great heights. Thus there are several kinds of temperature inversions.

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▪ The following are classified on the basis of relative heights from the earth’s
surface at which it occurs and the type of air circulation:
▪ Non-Advocational

o Radiation Inversion (Surface Temperature Inversion)

• Surface temperature inversion develops when air is

cooled by contact with a colder surface until it becomes
cooler than the overlying atmosphere; this occurs most
often on clear nights, when the ground cools off rapidly
by radiation. If the temperature of surface air drops
below its dew point, fog may result.
• It is very common in the higher latitudes. In lower and
middle latitudes, it occurs during cold nights and gets
destroyed during day time.

Fig 10 Temperature inversions Profile

o Subsidence Inversion (Upper Surface Temperature Inversion)

• When a widespread layer of air descends, it is compressed and

heated by the resulting increase in atmospheric pressure, and as a
result the lapse rate of temperature is reduced.
• The air at higher altitudes becomes warmer than at lower altitudes,
producing a temperature inversion. This type of temperature
inversion is called subsidence inversion.
• It is very common over the northern continents in winter (dry
atmosphere) and over the subtropical oceans; these regions
generally have subsiding air because they are located under large
high-pressure centers.
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• It is also called upper surface temperature inversion because it takes
place in the upper parts of the atmosphere.
Effect

▪ Temperature inversion determines the precipitation, forms of clouds, and also

causes frost due to condensation of warm air due to its cooling.

o Dust particles hanging in the air: Due to inversion of temperature, air

pollutants such as dust particles and smoke do not disperse on the
surface.
o Stops the movement of air: It causes the stability of the atmosphere
that stops the downward and upward movement of air.
o Less rainfall: Convection clouds can not move high upwards so
there is less rainfall and no showers. So, it causes a problem for
agricultural productivity.
o Lower visibility: Fog is formed due to the situation of warm air above
and cold air below, and hence visibility is reduced which causes
disturbance in transportation.
o Thunderstorms and tornadoes: Intense thunderstorms and tornadoes
are also associated with inversion of temperature because of the
intense energy that is released after an inversion blocks an area’s
normal convention patterns.
o Diurnal variations in temperature tend to be very small.
▪ 1.9 wind rose diagram

It summarizes information about the wind at a particular location over a specified

time period. A wind rose was also, before the use of magnetic compasses, a guide
on mariners’ charts to show the directions of the eight principal winds. The modern
wind rose used by meteorologists gives the percentage of the time the wind blows
from each direction during the observation period; it sometimes shows the strengths
of these winds, and the percentage of the time calm air or light winds are
observed. This wind rose usually has eight radiating lines, whose lengths are
proportional to wind frequency, and shows wind strength by the thickness of the
lines or by feathers attached to them. The frequency of calm or nearly calm air is
given as a number in the centre.

The earliest-known wind roses appeared on navigation charts used in the 13th
century by Italian and Spanish sailors. The eight points were marked with the initials
of the principal winds; sometimes the east point had a cross, and the north point
had a fleur-de-lis. When the magnetic compass began to be used in navigation,
the wind rose was combined with it and used as a compass card.
▪

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Fig 11 Wind Rose diagram

9. Practice Quiz

1. Which of the following is one of the secondary air pollutants among the following?
a. Ozone
b. CO
c. CO2
d. PANs
2. In case of pollution due to high Ozone levels one must take which of the following
precautions?
i) Drink lots of water and fluid
ii) Expose oneself less to sunlight
a. Only i
b. Only ii
c. Both i and ii
d. None of the above
3. Which of the following can be considered causes of air pollution?
i) Climate change
ii) Greenhouse gas emission
iii) Heavy vehicles movement
a. Only i and ii
b. i, ii and ii
c. Only i and iii
d. Only ii and iii
4. What is particulate matter causing air pollution called?
a. Smog
b. Soot
c. Foam
d. None of the above
5. Which of the following causes minamata disease?
a. Lead
b. Mercury
c. Magnesium
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d. Methyl chloride
6. Which of the following is considered to be the major sources of CFC?
i) Refrigerants
ii) Aircraft Halon
iii) Aerosol Sprays
a. i and ii
b. i and iii
c. ii and iii
d. ii and iii
7. What is true about AQI?
i) It informs about the color and odour in the air
ii) It can inform about the ozone levels in any area along with particulate matter
a. Only i
b. Only ii
c. Both i and ii
d. None of the above
8. Photochemical smog mainly is
a. H2O2
b. Methyl Chlorate
c. Ozone
d. Peroxyacetyl nitrate
9. Which of the following is an example of greenhouse gas?
a. Methane
b. Carbon dioxide
c. Oxygen
d. Both a and b
10. Which of the following is the major contributor of air pollution in iIndia as per the
Indian Government?
a. Dust and Construction
b. Agricultural burning
c. Transport
d. Industries

10.Assignments

S.No Question BL CO
1 Explain about Maximum Mixing Depth (MMD), 2 1
2 Discuss the stability conditions, wind velocity profile, 2 1
3 Explain the Temperature Inversions 2 1

11. Part A- Question & Answers

S.No Question & Answers BL CO

1 Define Meteorology
The branch of science that deals with the study of the earth's
1 1
atmosphere and majorly focus on the weather processes and
forecasting is known as meteorology.
2 What do you mean by Temperature inversion? 1 1
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It is a reversal of the normal behavior of temperature in the
troposphere. Under this meteorological phenomenon a layer of
warm air lies over the cold air layer
3 Define wind rose map diagram
It summarizes information about the wind at a particular
location over a specified time period. A wind rose was also, 1 1
before the use of magnetic compasses, a guide on mariners'
charts to show the directions of the eight principal winds.
4 What are the three types of solar radiation?
The three relevant bands, or ranges, along the solar radiation
spectrum are ultraviolet, visible (PAR), and infrared. Of the light 1 1
that reaches Earth's surface, infrared radiation makes up 49.4%
of while visible light provides 42.3% 9.
5 How the wind velocity is determined?
Which device is used to measure wind speed?
The instruments used to measure wind are known as
1 1
anemometers and can record wind speed, direction and the
strength of gusts. The normal unit of wind speed is the knot
(nautical mile per hour = 0.51 m sec-1 = 1.15 mph).
6 Define The Lapse Rate
It is the rate at which temperature changes with height in the
Atmosphere. Lapse rate nomenclature is inversely related to the
1 1
change itself: if the lapse rate is positive, the temperature
decreases with height; conversely if negative, the temperature
increases with height.
7 Write the composition of atmosphere
The atmosphere is comprised of layers based on temperature.
These layers are the troposphere, stratosphere, mesosphere 2 1
and thermosphere. A further region at about 500 km above the
Earth's surface is called the exosphere.
8 What are the components of atmosphere?
Earth's atmosphere is composed of about 78 percent nitrogen,
21 percent oxygen, 0.9 percent argon, and 0.1 percent other
2 1
gases. Trace amounts of carbon dioxide, methane, water
vapor, and neon are some of the other gases that make up the
remaining 0.1 percent.
9 What is wind circulation?
The global circulation can be described as the world-wide
system of winds by which the necessary transport of heat from
tropical to polar latitudes is accomplished. In each hemisphere 2 1
there are three cells (Hadley cell, Ferrell cell and Polar cell) in
which air circulates through the entire depth of the
troposphere.
10 Define Solar radiation,
It is called the solar resource or just sunlight, is a general term for
the electromagnetic radiation emitted by the sun. Solar
2 1
radiation can be captured and turned into useful forms of
energy, such as heat and electricity, using a variety of
technologies.
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12. Part B- Questions

S.No Question BL CO
1 Explain the composition and structure of the atmosphere 1 1

2 Write a short notes on wind circulation 2 1

3 Explain the various types of solar radiation, 2 1
4 Explain in detail about Maximum Mixing Depth (MMD) 3 1
5 Draw and Explain the Wind rose diagram 3 1

13. Supportive Online Certification Courses

Air Pollution and Control ,12 week course conducted by IIT Roorkee

14. Real Time Applications

S.No Application CO
1 Measuring Vehicle emissions 1
2 Measuring Pollutant - Fuel oils and natural gas to heat homes, 1
3 Measuring Pollutant - By-products of manufacturing and power 1
generation

15. Contents Beyond the Syllabus

Air pollution emission standards, National and international policies, acts, rules, and
regulations.
16. Prescribed Text Books & Reference Books
REFERENCES:
1. WarkK ., Warner C.F., and Davis W.T., “Air Pollution - Its Origin and Control”, Harper &
Row Publishers, New York.
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2. Lee C.C., and Lin S.D., “Handbook of Environmental Engineering Calculations”,
McGraw Hill, New York. 3. Perkins H.C., “Air Pollution”, McGraw Hill.
4. Crawford M., “Air Pollution Control Theory”, TATA McGraw Hill.
5. Stern A.C., “Air Pollution”, Vol I, II, III.
6. Seinfeld N.J.,, “Air Pollution”, McGraw Hill.
7. Stern A.C. Vol. V, “Air Quality Management”. 8. M N Rao and HVN Rao, Air Pollution”
Tata McGraw Hill publicatio
17. Mini Project Suggestion
Pollution vacuum cleaners: Sucking up the air's contaminants

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