(a) How do oxides of nitrogen enter the atmosphere?

(b) What are their harmful effects?

(a) Oxides of Nitrogen enter the atmosphere in the following ways :

• On the burning of fuels in furnaces, temperature increases. At high temperature, nitrogen and oxygen present in the air combine to form oxides of nitrogen.
• When fuel burns in internal combustion engine, oxides of nitrogen are produced and they enter the atmosphere as exhaust gases from automobile engines.
• Nitric oxide is formed by the reaction between atmospheric nitrogen and oxygen in the presence of electric discharge, which happens during thunderstorms, when there is lightening.
• Nitric oxide further reacts with atmospheric oxygen and ozone to form nitrogen dioxide.

b) Harmful effects of oxides of nitrogen are :

• Nitrogen dioxide causes irritation in the mucous membrane.
• Large concentrations of nitrogen dioxide may cause serious lung diseases.
• Nitrogen dioxide causes injuries to vegetation by damaging plant leaves.
• In sunlight, nitrogen dioxide oxidises hydrocarbons to form photochemical smog. It causes eye irritation, asthma attacks, nasal and throat infections.