The blanket of the air that supports the earth is known as the atmosphere. The atmosphere extends than a thousand of kilometers above the earth's surface. It is held in place by the earth's force of gravity. 
Composition of the atmosphere 
Air which makes up the atmosphere is a mixture of several gases . Nitrogen makes up to 78 percent and oxygen about 21 percent of the total volume of the atmosphere.  Carbon dioxide makes up about 0.03 percent and the remaining 0.97 percent includes other constituent. Some dust particles are also present in the atmosphere. Although the gases are present in the atmosphere are used up in certain processes they are also released back in the atmosphere in other processes. Thus the composition of the lower part of the atmospher remains fairly constant. 
             Atmospheric nitrogen is converted to nitrogen compounds by lightning and by lightning and by certain types of organisms such as nitrogen-fixing bacteria present in the soil or in the roots of the plant. Plant absorb nitrogen compounds and converts them to protien. Animals take in protein by eating plants or other animal's. Nitrogen return to the atmosphere when identifying bacteria act on the animal waste and the dead bodies of the plants and animals. 
              Oxygen is used up in process like the respiration of organisms the burning of fuel and the decay of organic matter.  It is replaced by the oxygen released by the green plant during the photosynthesis. Similarly the carbon dioxide used up during photosynthesis is replaced by the carbon dioxide released by the respiration of the organisms the burning of fuel and decay of organic matter.  Thus nature has of maintaining the balance of gases in the atmosphere. However human activities are disturbing this balance by polluting the air. 
Air pollution 
Any undesirable changes in the composition in the composition of air is known as air pollution. This may happen if the process regulating the composition of the air are distributed or if harmful substances are added to the air .Substances that causes pollutants .
                  In cities  and industrial areas ,automobiles and factories are the chief polluting agents. They emit large amount of gases such as carbon dioxide,carbon monoxide and sulphur dioxide. They also emit solid pollutants such as unburnt carbon particles from fuels. Some of the emitted substance such as compounds of poisonous metals like lead are harmful for us . 
       Natural gases may also cause pollution . Volcanoes for example release huge amounts of dust ash and poisonous gases in the atmosphere. 
Layers of the earth 
On the basis of composition temperature and other properties  the atmosphere may be divided into five layers.  Starting from the bottom layer  are called troposphere,stratosphere,mesosphere,thermosphere and exosphere. However the boundaries between these layers are neither fixed layers nor well defined. The layers overlap and vary in thickness .