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Environment & Ecology


The word smog is derived from smoke and fog. This is the most common example of air pollution that occurs in many cities throughout the world

There are two types of smog:

Classical smog occurs in cool humid climates : It is a mixture of smoke, fog and sulphur dioxide. Chemically it is a reducing mixture and so it is also called as reducing smog.

Photochemical smog occurs in warm, dry and sunny climates :  The main components of the photochemical smog result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles and factories. Photochemical smog has high concentration of oxidising agents and is, therefore, called as oxidising smog. Formation of photochemical smog

When fossil fuels are burnt, a variety of pollutants are emitted into the earth's pours in air. Examples are sulphuric acid mist and troposphere.

Two of the pollutants that are emitted are hydrocarbons (unburned fuels) and nitric oxide (NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which NO is converted into nitrogen dioxide (NO2). This NO2 in turn absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atom

  • Oxygen atoms are very reactive and combine with the O2 in air to produce ozone.
  • The ozone formed in the above reaction reacts rapidly with the NO(g) formed in the reaction to regenerate NO2. NO2 is a brown gas and at sufficiently high levels can contribute to haze.
  • Ozone is a toxic gas and both NO2 and O3 are strong oxidising agents and can react with the unburnt hydrocarbons in the polluted air to produce chemicals such as formaldehyde, acrolein and peroxyacetyl nitrate (PAN).

Effects of photochemical smog

The common components of photochemical smog are ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate (PAN). Photochemical smog causes serious health problems. Both ozone and PAN act as powerful eye irritants Ozone and nitric oxide irritate the nose and throat and their high concentration causes headache, chest pain, dryness of the throat, cough and difficulty in breathing

Photochemical smog leads to cracking of rubber and extensive damage to plant life. It also causes corrosion of metals, stones, building materials, rubber and painted surfaces

Stratospheric Pollution

Formation and Breakdown of Ozone

The upper stratosphere consists of a considerable amount of ozone (O3), which protects us from the harmful ultraviolet (UV) radiations (λ 255 nm) coming from the sun. These radiations cause skin cancer (melanoma) in humans. Therefore, it is important to maintain the ozone shield. Ozone in the stratosphere is a product of UV radiations acting on dioxygen (O2) molecules.

The UV radiations split apart molecular oxygen into free oxygen (O) atoms. These oxygen atoms combine with the molecular oxygen to form ozone. O2 (g) O(g) + O(g) O(g) + O2 (g) O3 (g) Ozone is thermodynamically unstable and decomposes to molecular oxygen.

There have been reports of the depletion of this protective ozone layer because of the presence of certain chemicals in the stratosphere.

The main reason of ozone layer depletion is believed to be the release of chlorofluorocarbon compounds (CFCs), also known as freons. These compounds are nonreactive, non flammable, non toxic organic molecules and therefore used in refrigerators, air conditioners in the production of plastic foam and by the electronic industry for cleaning computer parts etc.

Once CFCs are released in the atmosphere, they mix with the normal atmospheric gases and eventually reach the stratosphere.

In the stratosphere, they get broken down by powerful UV radiations, releasing chlorine free radicals.

The chlorine radicals are continuously regenerated and cause the breakdown of ozone. Thus, CFCs are transporting agents for continuously generating chlorine radicals into the stratosphere and damaging the ozone layer.

Ozone Hole

  • In the 1980s atmospheric scientists working in Antarctica reported the depletion of the ozone layer commonly known as ozone hole over the South Pole.
  • In the summer season, , preventing much ozone depletion, whereas in winter, special types of clouds are formed over Antarctica.
  • These polar stratospheric clouds provide a surface on which chlorine nitrate formed gets hydrolysed to form hypochlorous acid.
  • It also reacts with hydrogen chloride to give molecular chlorine. When sunlight returns to Antarctica in the spring, the sun’s warmth breaks up the clouds and HOCl and Cl2 are photolysed by sunlight.
  • The chlorine radicals thus formed, initiate the chain reaction for ozone depletion.

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