Air is the essential source for almost all living being. It is around us above us below us in one word it is wrapping the whole planet. The Greek Philosophers once considered it as one of the four basic elements. It is not too long, when early chemists began to learn that air was not an element neither a compound instead a mixture of different gases though the process of understanding these gases individually and also to learn their correlation took several more decades.
Lavoisier was the first to prove that air is composed mainly of two gases with his theory of combustion in 1770. The gas he named Azote is known today as Nitrogen identified by Daniel Rutherford and the reactive one in Lavoisier's experiment was credited to Presley for discovering and naming it as Oxygen. In late 1700 Cavendish confirmed the composition of air as78% of nitrogen and 21% of oxygen with traces of carbon dioxide, argon, water vapor and other elements; he also proved by performing hundred more samples to confirm that the composition of air was the same irrespective of its geographic origin.
Alas it now seems to be a fairy tale, as modern science invented tools to rule over nature and Cavendish's sample became an extinct and air lost its purity making its composition a variable one regarding on the quality of locality and also other geological factors.
However the standard composition of dry air by volume is as below:
Nitrogen: 78.08%, Oxygen 20.95%, Argon 0.93%, Carbon Dioxide 0.38%, Neon 0.0018%,Helium 0.0005%, Krypton 0.0001%, Hydrogen 0.00005%, Xenon 0.0000087%.
There is also trace amount of water vapor or moisture varying primarily with temperature. It would be noteworthy that the average air temperature diminishes at the rate of 6 degree c for each 100.00 m vertical height. Now let's see why and how the composition differs. To understand the diverse nature of air we must take a quick tour through our Earth's atmosphere.
Our Earth's atmosphere is composed of several layers:
First layer is called troposphere. This is our neighbor layer and the air we breathe in flows here. This is the layer where all weather takes place. It is the region of rising and falling packets of air. It is closest to the Earth's surface and of air approximately 10 to 15 kilometers thick that is constantly in motion. The conditions in this layer determine practically all of the Earth's weather patterns. "Tropos," in Greek means "turning" or "mixing." The constant motion in this layer is significant in understanding air quality because it results in the dispersion of pollutants.
The stratosphere and Ozone layer: This layer is just above the troposphere, where air flow is mostly horizontal. The thin ozone layer in the upper stratosphere has the high concentration of ozone. The ozone layer absorbs most of Sun's ultra violet ray. This is the protective Ozone layer and the harmful ozone pollution is found in Troposphere near the ground level. The stratosphere is approximately 40 kilometers thick and is composed mostly of dry stable air. In contrast to the troposphere, pollutants in the stratosphere do not disperse, and tend to remain in the atmosphere for long periods of time.
The mesosphere and ionosphere: Mesosphere extends above the stratosphere and above this is the ionosphere or thermosphere. Ionosphere is very thin but the important layer where the aurora takes place and it absorbs the most energetic photons from the sunlight and reflects the radio waves making long distance radio communication possible.
With the advancement of industrial age human race pumped tons of pollutants to increase the harmful constituents in the air composition hence damaging the environment. Most of the pollution arose from burning of fossil fuel at power plant and motor vehicles. Especially in urban areas there is a severe decrease in air quality arising from the smog or man made ozone thus affecting the growth rate of trees and crops.
Incomplete combustion of fossil fuel increases the level of deadly CO and carbon dioxide which in turn by photochemical reaction produces ozone and traps the heat causing green house effect. Carbon monoxide also readily absorbed in our red blood cell and affects other body cells shrinking our longevity.
The effects of these man made hazards took ages for common habitants to understand the consequences. Now we are alarmed around the globe about global warming and rising of sea levels and current change in weather pattern and scientists are trying to invent echo friendly essentials to reduce the impact we caused and make it breathable for our next generation.
There is also a term called global cooling arising from dust particles of metal ash or aerosols put into the atmosphere from different sources. These particles are believed to increase the reflectivity of atmosphere and hence shielding the solar radiation to reach Earth's surface. Nitrogen dioxide and carbon dioxide come from the burning of the fossil fuels and react with moisture in the atmosphere to cause acid rain. Particulates can also harm our respiratory system causing lung disease and cancer. CFC's generated from refrigerators and aerosols, solvents, jet fuels, air conditioners etc. depleting the ozone layer and letting the UV to sieve through to cause skin cancer, eye complication and other life threatening diseases. Though the western world regulated the use of CFC's, the third world countries are still lacking in a firm ban.
So, we can conclude that the primary components of air are Nitrogen, Oxygen Carbon dioxide, trace amount of water vapor, argon and other noble gases. The variation of the composition arises from the pollutants pumped or caused by civilization making the constituent elements to react and to differ in their proportion. Also the sweep of winds and the exchange of gases, liquids and solids between air masses and water surfaces affect air quality.