Atmosphere

The Earth's atmosphere serves as a major factor in sustaining the planetary ecosystem. The thin layer of gases surrounding 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, [12] among which are greenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide and ozone. Filtered air includes small amounts of many other chemical compounds. The 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 minute quantities in samples of unfiltered air, including dust, pollen and spores, sea air, volcanic ash and meteoroid. Various industrial pollutants may also be present, such as chlorine (elemental or in compounds), fluorine compounds, mercury in elemental form, and sulfur compounds such as sulfur dioxide [SO2] manual de auditoria ambiental.

The ozone layer in the Earth's atmosphere plays an important role in reducing the amount of ultraviolet (UV) radiation that reaches the surface. As DNA is easily damaged by UV light, this serves as protection for life on the surface. The atmosphere also retains heat at night, thus reducing extremes of temperature during the day.

Atmospheric layers
Main article: Terrestrial atmosphere
Main layers
The Earth's atmosphere can be divided into five main layers. These layers are determined primarily by increasing or reducing the temperature according to height. From the highest to the lowest, these layers are:

Exosphere
Thermosphere
Mesosphere
Stratosphere
Troposphere
Other layers
Ozonosphere
Ionosphere
Homosphere and heterosphere
Atmospheric boundary layer