Oxides of nitrogen:
These include nitrogen monoxide (NO) and nitrogen dioxide (NO2). NO is a colourless, odourless and inert gas but like CO it can also combine with haemoglobin to reduce oxygen carrying capacity of blood.
NO2 is reddish brown in colour having pungent smell and is suffocating in nature.
Sources:
(i) Fuel combustion in automobiles and industries.
(ii) Lightening.
(iii) Forest fires
(iv) Bacterial decomposition of organic matter
(v) Natural ionizing radiations.
Sinks: the NO and NO2 undergo various photochemical and chemical reactions in the atmosphere leading to formation of HNO3 within 3 to 4 days. The end product HNO3 precipitates as nitrate salts in either rainfall or on dust particles.O3 plays very important role in the formation of HNO3.
O3 + NO2 NO3 + O2
NO3 + NO2 N2O5
N2O5 + H2O 2HNO3
Health effects:
(i) At high conc. It causes acute bronchitis and prolonged exposure to low concentration of it causes bronchitis in children.
(ii) It causes eye irritation and even lung congestions.
(iii) In sunlight, nitrogen oxides and hydrocarbons form smog which limits visibility of roads, causes eye irritation, difficulty in breathing.
Control measures:
(i) Modifying the engine design: use of two stage combustion process to reduce excess of air in engine.
(ii) Use of two stage catalytic converter for exhaust emissions.
(iii) By using scrubber containing H2SO4 or Ca (OH)2, Mg(OH)2 solution for flue gases.