Previous Year Question

Smog is a condition of atmospheric pollution involving Smoke, Particulate Matter, gaseous pollutants, and, under certain circumstances, Fog. It forms mainly due to pollutants emitted from vehicles, industries, and fossil fuel combustion. Oxides of Nitrogen (NOₓ) and Oxides of Sulfur (SOₓ) are important air pollutants associated with smog formation. Smog is broadly classified into Classical or Sulfurous Smog and Photochemical Smog.

Classical Smog is also known as London Smog or Reducing Smog. It is primarily associated with Sulfur Dioxide (SO₂), smoke, and particulate matter released from burning sulfur-containing fuels such as coal. Its effects can become more severe under cold, humid conditions with limited air circulation. The Great Smog of London in December 1952 is a historical example, following which important legislative measures were introduced in Britain to control air pollution.

Photochemical Smog forms mainly when Nitrogen Oxides (NOₓ) and Volatile Organic Compounds (VOCs) react in the presence of sunlight. These reactions produce Ground-level Ozone (O₃), Peroxyacetyl Nitrate (PAN), and other secondary pollutants. This type of smog commonly develops in warm, sunny urban environments. Nitrogen Oxides are primarily emitted from vehicles, thermal power plants, and industrial combustion processes.

Oxygen (O₂) and Nitrogen (N₂) are normal atmospheric constituents, and their mere presence does not produce smog. Ozone is a major component of Photochemical Smog, but it is formed primarily through reactions involving NOₓ and VOCs. Scientifically, the simultaneous presence of both groups of oxides is not essential for every type of smog. SO₂ and smoke play major roles in Sulfurous Smog, whereas NOₓ and VOCs are important in Photochemical Smog.