(i) Troposphere is the lowest layer of atmosphere where life exists.
The layer gets polluted due to the presence of particulate matter. Such particulate matter may either be:
(a) Solid matter : e.g., dust, mist, fumes, smoke, smog, etc.
Smoke particulates consist of solid or mixture of solid and liquid particulates formed during combustion of organic matter.
Dust is composed of fine solid particles (over 1 mm in diameter), produced during crushing, grinding and attribution of solid materials.
Mists are produced by particles of spray liquids and by condensation of vapours in air. Fumes are generally obtained by the condensation of vapours during sublimation, distillation, boiling and several other chemical reactions. The word smog is derived from smoke and fog. There are two types of smog : Classical smog occurs in cool humid climate. It is a mixture of smoke, fog and sulphur dioxide. Photochemical smog occurs in warm, dry and sunny climate. The main components of the photochemical smog result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles and factories.
(b) Gaseous matter : e.g., oxides of sulphur and carbon and hydrocarbons, etc.
Oxides of sulphur are produced when sulphur containing fossil fuel is burnt. Even a low concentration of sulphur dioxide causes respiratory diseases e.g. asthma, bronchitis, emphysema in human beings. While most of these pollutants are produced by human activities such as mining, burning of fossil fuels, smoke from industries, etc., they may also be produced by natural activities like volcanic eruptions which throw up large quantities of particulate matter or landslides which create an envelope of dust.
Hydrocarbons are composed of hydrogen and carbon only and are formed by incomplete combustion of fuel used in automobiles. Hydrocarbons are carcinogens, i.e., they cause cancer. They harm plants by causing ageing, breakdown of tissues and shedding of leaves, flowers and twigs.
Carbon monoxide (CO) is one of the most serious air pollutants. It is produced as a result of incomplete combustion of carbon.
Carbon dioxide (CO2) is released into the atmosphere by respiration and burning of fossil fuels for energy. The increased amount of CO2 in the air is mainly responsible for global warming, thereby, leading to tropospheric pollution.