Class 11 Chemistry Environmental Chemistry Air pollution

Air pollution

“The contamination of air with harmful and undesirable substances “

Sources of pollutants:

  • Natural sources
  • Artificial sources


Natural –like volcanic eruption, pollen grains, forest fires, etc.


  • Burning of fossil fuels
  • Combustion of flukes in automobiles
  • Increasing population
  • Industries
  • Agricultural activities
  • Wars

The atmospheric pollution is basically contamination of troposphere and stratosphere.

Troposphere is mainly polluted by:

  • Gaseous pollutants like carbon, Sulphuretc.
  • Particulatepollutant’s like dust,mist,smoke etc.


There are five major pollutants of troposphere are like Carbon dioxide, Carbon monoxide etc.

Carbon monoxide: It is colourless and odourless gas and it is released in air by human activities like:

  • incomplete combustion of fuels,
  • forest fires
  • dissociation of carbon dioxide at high temperature etc or
  • natural sources like conversion of methane to carbon monoxide
  • from decay of chlorophyll etc.

 Though the CO is produced still its level do not rise in atmosphere because of conversion of CO to carbondioxide by the bacteria present in soil so we can say microorganisms act as a sink for carbon monoxide but this sink is not available in urban areas due to less soil.

Effects of carbon monoxide:

The carbon monoxide is a poisonous gas if inhaled can combine with haemoglobin (Hb) to form defective type of complex that can lead to suffocation and also deathTherefore it is a fatal gas.

Carbon dioxide: it is released in atmosphere due to burning; respirationetc.Excess of it is removed by plants as they utilize it for photosynthesis.However, increased in its concentration leads to global warming.

Green House Effect

Global warming

Global warming “is the increase in the concentration of certain gases like carbon dioxide, methane, etc. in air due to which infra-red rays get trapped in it and it leads to increase in overall temperature of earth.”

  • The rays that come from the Sun consist of visible light, infra-red rays and ultra violet rays. Out of these, only infra-red rays and visible light reaches earth’s surface. On reaching earth, the infra-red rays are reflected back and a portion of it is absorbed by earth’s surface. The reflected ones when pass through earth’s atmosphere, get trapped inside the gas molecules. As we know, itleads to heating effect. Therefore, the overall temperature of earth’s surface rises. One of the reasons for global warming is greenhouse effect.
  • In winters, it is advised to keep plants in green houses only. The roof allows infra-red rays to enter but after reflection, they are not allowed to go back. As a result, they get trapped inside the green house and growth of plants is stimulated because optimum temperature is achieved inside the green house.



Oxides of Sulphur:They are produced by burning of Sulphurcontaining fossil fuels. This Sulphur dioxide can cause respiratory problems like asthma, emphysema, irritation to eyes, redness in eyes etc.

Oxides of nitrogen: The nitrogen oxides are formed due to lightning strikes at higher altitudes and also due to burning of fossil fuels.It causes irritant red haze, damage leaves of plant, retard rate of photosynthesis also respiratory disorders.

Both Sulphur dioxide and nitrogen dioxide are cause of acid rain

Acid rain

  • It is the one of the major causes of causing cancer. It is formed when rain water mixes with pollutants present in air. These pollutants are mainly Nitrogen dioxide and Sulphur dioxide gas emitted in exhaust of vehicles.

 When rain falls, these gases combine and the following reaction takes place:

SO2  + H2O             H2SO4

NO2+ H2O            HNO3

As a result, rain water no longer remains only pure water it becomes acidic

Harmful effects:

  1. It causes cancer.
  2. It combines with marble and deteriorates the marble.
  3. It affects ph. of soil and makes it unfit for growing plants.
  4. It affects aquatic life too

Hydrocarbons:They are produced due to combustion of fuels are actually carcinogenic that causes cancer. They also harm plants as they cause ageing in plants, shedding of leaves etc.

Chlorofluorocarbons:They are released into the atmosphere from industries. They are actually manmade industrial chemicals used in air conditioners, refrigeratorsetc. and their vapours do not remain in lower layers of atmosphere they move up and cause thinning of ozone.




Ozone: It is a molecule consisting of three oxygen atoms i.e. O₃. It is present in stratosphere.

Ozone is formed when u-v rays in upper atmosphere split oxygen atoms that are: -

When oxygen molecule splits into atoms then each atom combine with undissociated molecule of oxygen to form ozone.


Ozone protect us from ultra violet radiations as it doesn’t allow ultra violet rays to enter earth’s atmosphere that if enter into earth may harm us in many ways such as:

  • They may raise the temperature of earth
  • It can cause global warming
  • It may cause skin disorders, eye disorders and many more.
  • The thinning of ozone layer by air pollutants is called ozone depletion.

For this some synthetic chemicals are responsible like ChloroFloro Carbons that are used in refrigerators, air conditioners, etc.

Ozone hole

  • Due to continuous use of certain harmful chemicals like CFC’s etc. they have caused thinning of ozone layer to an extent that a hole is observed in ozone layer in the region of Antarctica.
  • This hole is not a hole actually we can say it is the extreme thinning in some parts due to extreme reactions taking place with certain harmful chemicals that are not stable in upper atmosphere and react with ozone causing its depletion

 Montreal protocol:

  •  It is agreement made by industrialized nations to freeze the production of CFC’s and make use of some alternative source.
  • It was done in 1987 UNEP (United Nations Environment Program)

Now, instead of using CFC’s; HFC’S and PFC’s are used that is somehow helping in saving the ozone layer.

Particulate pollutants

  • They are minute solid or liquid particles present in air.These are released by vehicle exhaust, dust and ashes of hydrocarbons.
  • They may be viable like bacteria etc. and non-viable like ash etc.


Non-viable particulates are classified according to nature and size of particles. They are as follows:-


Smoke: Itconsists of solid and liquid particles formed during combustion of fuels etc.

Dust: It is composed of fine solid particles produced during crushing, grinding of solid particles, like saw dust from wood, fly ash from factories etc.

Mist:They are produced by particles of spray liquids and condensation of vapors in air, like insecticides etc.

Fumes:They are obtained by condensation of vapors during sublimation, distillation, boilingetc., like metallic and non-metallic oxides

  • The particulate matter harm but degree of harm depends upon the size of particles particulate pollutants bigger than 5 microns can block nasal passage whereas particle of size micron can even enter lungs.

Smog and Photochemical smog

  • Smog is formed by combination of smoke and fog.
  • A thick fog-like layer in the atmosphere, especially during winters it is known as smog as it is made up of smoke and fog.
  • Smoke may contain oxides of nitrogen which combine with other air pollutants and fog to form smog. The smog causes breathing difficulties such as asthma, cough and wheezing in children


Photochemical smog

  • It is caused due to hydrocarbons and nitric oxide.When their concentration rises beyond limit a chain reaction occur from their interaction with sunlight in which NO is converted into nitrogen dioxide.
  • This nitrogen dioxide in turn absorbs energy and breaks into nitric oxide and free oxygen atoms.
  • These oxygen atoms combine with oxygen gas molecules to produce ozone .This ozone is a toxic gas it combines with un-burnt hydrocarbons in air to produce chemicals like formaldehyde, acrolein and PAN.

Harmful effects of photochemical smog

  • It causes serious earth problems
  • Ozone and PAN are eye irritant
  • Ozone and nitric oxide irritate nose, throat, causes dryness in throat, cough and difficulty in breathing.The smog leads to cracking of rubber and extensive damage to plant life.

Photochemical smog can be controlled as:-

  • Plantation of certain plants like pinesetc can metabolise nitrogen dioxide and Catalytic convertors which prevent the release of nitrogen oxide and hydrocarbons should be used in vehicles

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