Structure of the atmosphere
The atmosphere is normally composed of 79 percent nitrogen, 20 percent oxygen and one percent as a mixture of carbon dioxide, water vapour and trace amounts of several other gases such as neon, helium, methane, krypton, hydrogen and xenon. The general structure of the atmosphere has several important features that have relevance to environmental problems. The atmosphere is divided into several layers. The innermost layer the troposphere extends 17 kilometers above sea level at the equator and about 8 kilometers over the poles. It contains about 75 percent of the mass of the earth’s air. The fragility of this layer is obvious from the fact that if the earth were an apple this particular layer would be no thicker than an apple’s skin.
Temperature declines with altitude in the troposphere. At the top of the troposphere temperatures abruptly begin to rise. This boundary where this temperature reversal occurs is called the tropopause.
The tropopause marks the end of the troposphere and the beginning of the stratosphere, the second layer of the atmosphere. The stratosphere extends from 17 to 48 kilometers above the earth’s surface. While the composition of the stratosphere is similar to that of the troposphere it has two major differences. The volume of water vapour here is about 1000 times less while the volume of ozone is about 1000 times greater. The presence of ozone in the stratosphere prevents about 99 percent of the sun’s harmful ultraviolet radiation from reach ing the earth’s surface thus protecting humans from cancer and damage to the immune system. This layer does not have clouds and hence airplanes fly in this layer as it creates less turbulence. Temperature rises with altitude in the stratosphere until there is another reversal. This point is called the stratopause and it marks the end of the stratosphere and the beginning ofthe atmosphere’s next layer, the mesosphere.
In the mesosphere the temperature decreases with altitude falling up to –110o C at the top. Above this is a layer where ionization of the gases is a major phenomenon, thus increasing the temperature. This layer is called the thermosphere. Only the lower troposphere is routinely involved in our weather and hence air pollution. The other layers are not significant in determining the level of air pollution.