While Earth's atmosphere is more than 300 miles thick, gravity keeps 80% of its mass within the Troposphere, the layer closest to our planet's surface. About 5 miles thick near the Poles, the Troposphere balloons to 11 miles at the equator, an effect of Earth's rotation; even here, its thickness is minuscule when compared to our planet's radius (almost 4000 miles).
Composed of 78% nitrogen, 21% oxygen and 1% argon, the atmosphere contains a variable amount of water vapor, carbon dioxide, particulates, methane, nitrous oxide and sulfides; these variable components, though generally in very small concentrations, are responsible for our weather and our climate. With the exception of the tallest thunderstorms at mid latitudes (which may poke into the Stratosphere), all of Earth's weather occurs within the Troposphere and it is this "low altitude" band that is primarily subject to pollution; indeed, two thirds of its mass is found below 18,000 feet.
The ozone layer, which absorbs ultraviolet light and other ionizing radiation, is vital to all life on Earth (at least to all species on land and in the upper waters of the sea). This band, about 6 miles thick, lies just above the Troposphere, at its interface with the Stratosphere. Unlike the ozone near Earth's surface (a pollutant resulting from solar radiation acting on industrial and vehicle emissions), this protective layer of ozone results from the ionization of oxygen molecules in the Stratosphere. Holes in the ozone layer, first discovered in the 1980s, have been attributed to pollutants (especially chlorofluorocarbons) in the underlying Troposphere. Once again, human activity may be slowly destroying the natural environment that permitted our own evolution.
