The
atmosphere of the Earth serves as a key factor in sustaining the planetary
ecosystem. The thin layer of gases that envelops the Earth is held in place by
the planet's gravity. Dry air consists of 78% nitrogen, 21% oxygen, 1% argon
and other inert gases, such as carbon dioxide. The remaining gases are often
referred to as trace gases,
among which are the greenhouse gases such as water
vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes
trace amounts of many other chemical compounds. Air also contains a variable
amount of water vapor and suspensions of water droplets and ice crystals seen
as clouds. Many natural substances may be present in tiny amounts in an
unfiltered air sample, including dust, pollen and spores, sea spray, volcanic
ash, and meteoroids. Various industrial pollutants also may be present, such as
chlorine (elementary or in compounds), fluorine compounds, elemental mercury,
and sulphur compounds such as sulphur dioxide [SO2].