How does SO₂ cause pollution?
Or
What are the principal environmental effects of SO₂?
Or
Describe sources, sinks and polluting effects of SO_2.

The two oxides of sulphur i.e. SO₂ and SO₃ (i.e. SO_x) are the most serious pollutants of the atmosphere. Both are colourless gases with a pungent smell.
(i) Natural sources: Volcanic erruption is the natural source for the release of SO₂ into the atmosphere.
(ii) Man-made sources:
(a) Combustion of sulphur containing coal and fuel oil in the thermal power plants.

(b) A significant amount of SO₂ is formed from the roasting of metallic sulphide ores.

(c) A small amount of CO₂ undergoes photolytic oxidation in air to form SO₃.

Sinks:
(i) Sulphur dioxide in the air can undergo several reactions. It can combine with water droplets to form sulphurous acid.

(ii) Sulphur dioxide also reacts with O₃ in the air by photolytic and catalytic processes to form SO₃ When then combines with water vapour to form droplets of H₂SO₄.

Sulphuric acid comes down in the form of acid rain which then reacts with ammonia emitted from the biological process and limestone or marble. 

Hence marble statues or structures, limestone mineral and sources of ammonia act as sinks for the atmospheric oxide of sulphur.
Harmful effects of SO_x:
(i) SO₂ at a concentration of 5 ppm causes throat and eye irritation.
(ii) SO₂ at a concentration of 1 ppm causes severe discomfort.
(iii) SO₂ has harmful effects on buildings and statues made up of marble and limestone (CaCO₃). The gas released from Mathura refinery is a constant threat to the Taj Mahal at Agra.
(iv) Atmospheric SO₂ is also harmful to plants. It damages vegetables, crops and affects plant growth and nutrient quality of plant products. 
(v) High concentration of SO₂ may cause lung diseases and lung cancer.