Smog
Smog
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 climate. 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 climate. 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 oxidizing agents and is, therefore, called as oxidizing smog.
Formation of photochemical smog
When fossil
fuels are burnt, a variety of pollutants are emitted into the earth’s
troposphere. Two of the pollutants that are emitted are hydrocarbons (unburnt
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.
NO2
(g) (hv) → NO (g) + O (g) (i)
Oxygen atoms
are very reactive and combine with the O2 in air to produce ozone.
O (g) + O2
(g) → O3 (g) (ii)
The ozone
formed in the above reaction (ii) reacts rapidly with the NO (g) formed in the
reaction (i) to regenerate NO2. NO2 is a brow gas and at
sufficiently high levels can contribute to haze.
NO (g) + O3
(g) → NO2 (g) + O2
(g) (iii)
Ozone is a
toxic gas and both NO2 and O3 are strong oxidizing agent
and can react with the unburnt hydrocarbons in the polluted air to produce
chemicals such as formaldehyde, acrolein and peroxyacetyl nitrate (PAN).
Effect 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, and 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.
How can photochemical smog be controlled?
Many
techniques are used to control or reduce the formation of photochemical smog.
If we control the primary precursors of photochemical smog, such as NO2
and hydrocarbons, the secondary precursors such as ozone and PAN, the
photochemical smog will automatically be reduced. Usually catalytic converters
are used in the automobiles, which prevent the release of nitrogen oxide and
hydrocarbons to the atmosphere. Certain plants e.g., pinus,
Juniparus, Quercus, Pyrus and Vitis can metabolize nitrogen oxide and
therefore, their plantation could help in this matter.
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