How is `SO_2` an air pollutant?

Explanation

`SO_2` acts as air pullutant due to :
(i). `SO_2` is strongly irritating to the respiratory tract.
(ii). At a concentration 5 ppm, causes throat and eye irritation, resulting in cough tears and redness in eyes. It causes breathlessness and affects larynx i.e., voice box.
(iii). At 0.03 ppm concentration causes chlorosis in plants. When the plant is exposed for long time to `SO_2` it slows down the formation of chlorophyll resulting in injury to the leaves including loss of green colour. This is called chlorosis.
(iv). `SO_2` dissolves in rain water and produces acid rain which damages building materials especially marble `CaCO_3`.
`SO_3+H_2OtoH_2SO_3`
`CaCO_3+H_2SO_3toCaSO_3+H_2O+CO_2`