The air in our atmosphere is composed of molecules of different gases. The most common gases are nitrogen (78%), oxygen (about 21%), and argon (almost 1%). Other molecules are present in the atmosphere as well, but in very small quantities.
While the composition doesn’t change much as you travel up through the lower layers of the atmosphere, what does change is the number of molecules. As you travel higher, the air molecules become less plentiful.
Although dominantly the same composition, there is a very important chemical difference within the stratosphere. For it is within this layer that the highest concentrations of ozone molecules reside. In the stratosphere, ozone molecules -- three oxygen atoms bonded together-- prevent some of the Sun’s most intense rays from reaching the Earth’s surface. Currently, NCAR scientists and researchers worldwide are monitoring this layer; so thin at the South Pole we call it a “hole” where the molecules are being destroyed.
Above the mesosphere, the composition changes. While still dominated by nitrogen and oxygen, gases in the thermosphere are highly ionized and the bonds between oxygen atoms are broken. In the exosphere, the outer layer of Earth’s atmosphere, air molecules can easily escape the Earth’s gravity and float into space.