Previous Year Question

Sulfur Dioxide (SO₂) is a Primary Air Pollutant because it is mainly emitted directly into the atmosphere from pollution sources. Primary Pollutants are substances released directly into the atmosphere through industrial activities, vehicles, fuel combustion, or natural processes. In contrast, Secondary Pollutants are formed through chemical or Photochemical Reactions involving primary pollutants and atmospheric constituents.

The principal source of Sulfur Dioxide is the combustion of sulfur-containing fossil fuels such as coal and petroleum. It is also emitted from Thermal Power Plants, Metal Smelting, industrial processes, and volcanic activities. Atmospheric oxidation of SO₂ can produce Sulfate Aerosols and Sulfuric Acid (H₂SO₄), which are examples of secondary pollutants. Sulfur Dioxide contributes to respiratory irritation, breathing difficulties, and the formation of Acid Rain.

Peroxyacetyl Nitrate (PAN) is a secondary photochemical pollutant with the chemical formula CH₃C(O)OONO₂. It forms through reactions involving Nitrogen Oxides (NOₓ) and Volatile Organic Compounds (VOCs) in the presence of sunlight. PAN is an important component of Photochemical Smog and can cause eye irritation and damage to plant leaves. Smog is a mixture of smoke, fine particles, and gaseous pollutants. Photochemical Smog, in particular, contains Ozone, PAN, and other secondary pollutants formed mainly through reactions involving primary pollutants emitted from vehicles and industrial activities.

Ozone (O₃) is considered a secondary air pollutant when it forms near the Earth's surface in the Troposphere. It is produced mainly through reactions involving NOₓ and VOCs in the presence of sunlight. It can affect the respiratory system and damage vegetation and agricultural crops.