# Earth's atmosphere

Earth's atmosphere

ppmv

The composition figures above are by volume-fraction (V%), which for ideal gases is equal to mole-fraction (that is, the fraction of total molecules). Although the atmosphere is not an ideal gas, nonetheless the atmosphere behaves enough like an ideal gas that the volume-fraction is the same as the mole-fraction for the precision given.

By contrast, "mass-fraction" abundances of gases will differ from the volume values. The mean molar mass of air is 28.97 g/mol, while the molar mass of helium is 4.00, and krypton is 83.80. Thus helium is 5.2 ppm by "volume-fraction", but 0.72 ppm by "mass-fraction" ( [4/29] × 5.2 = 0.72), and krypton is 1.1 ppm by "volume-fraction", but 3.2 ppm by "mass-fraction" ( [84/29] × 1.1 = 3.2).

Heterosphere

Below the turbopause at an altitude of about 100 km (not far from the mesopause), the Earth's atmosphere has a more-or-less uniform composition (apart from water vapor) as described above; this constitutes the homosphere. [ [http://amsglossary.allenpress.com/glossary/search?id=homosphere1 "homosphere"—AMS Glossary] ] However, above about 100 km, the Earth's atmosphere begins to have a composition which varies with altitude. This is essentially because, in the absence of mixing, the density of a gas falls off exponentially with increasing altitude but at a rate which depends on the molar mass. Thus higher mass constituents, such as oxygen and nitrogen, fall off more quickly than lighter constituents such as helium, molecular hydrogen, and atomic hydrogen. Thus there is a layer, called the heterosphere, in which the earth's atmosphere has varying composition. As the altitude increases, the atmosphere is dominated successively by helium, molecular hydrogen, and atomic hydrogen. The precise altitude of the heterosphere and the layers it contains varies significantly with temperature.

In pre-history, the Sun's radiation caused a loss of the hydrogen, helium and other hydrogen-containing gases from early Earth, and Earth was devoid of an atmosphere. The first atmosphere was formed by outgassing of gases trapped in the interior of the early Earth, which still goes on today in volcanoes. [cite web| url=http://www.oma.be/BIRA-IASB/Public/Research/Thermo/Thermotxt.en.html| title=The thermosphere: a part of the heterosphere| first=J| last=Vercheval (offline, see [http://web.archive.org/web//http://www.oma.be/BIRA-IASB/Public/Research/Thermo/Thermotxt.en.html Internet Archive copy] )]

Density and mass

The density of air at sea level is about 1.2 kg/m³(1.2 g/L). Natural variations of the barometric pressure occur at any one altitude as a consequence of weather. This variation is relatively small for inhabited altitudes but much more pronounced in the outer atmosphere and space because of variable solar radiation.

The atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula. More sophisticated models are used by meteorologists and space agencies to predict weather and orbital decay of satellites.

The average mass of the atmosphere is about 5 quadrillion metric tons or 1/1,200,000 the mass of Earth. According to the National Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480e|18 kg with an annual range due to water vapor of 1.2 or 1.5e|15 kg depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27e|16 kg and the dry air mass as 5.1352 ±0.0003e|18 kg."

Opacity

The atmosphere has "windows" of low opacity, allowing the transmission of electromagnetic radiation. The optical window runs from around 300 nanometers (ultraviolet-C) at the short end up into the range the eye can use, the visible spectrum at roughly 400-700 nm, and continues up through the visual infrared to around 1100 nm, which is thermal infrared. There are also infrared and radio windows that transmit some infrared and radio waves. The radio window runs from about one centimeter to about eleven-meter waves.

Evolution of Earth's Atmosphere

The history of the Earth's atmosphere prior to one billion years ago is poorly understood and an active area of scientific research. The following discussion presents a plausible scenario.

The modern atmosphere is sometimes referred to as Earth's "third atmosphere", in order to distinguish the current chemical composition from two notably different previous compositions. The original atmosphere was primarily helium and hydrogen. Heat from the still-molten crust, and the sun, plus a probably enhanced solar wind, dissipated this atmosphere.

About 4.4 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes which released steam, carbon dioxide, and ammonia. This led to the early "second atmosphere", which was primarily carbon dioxide and water vapor, with some nitrogen but virtually no oxygen. This second atmosphere had approximately 100 times as much gas as the current atmosphere, but as it cooled much of the carbon dioxide was dissolved in the seas and precipitated out as carbonates. The later "second atmosphere" contained largely nitrogen and carbon dioxide. However, simulations run at the University of Waterloo and dn|University of Colorado in 2005 suggest that it may have had up to 40% hydrogen. [cite news |title=Early Earth atmosphere favorable to life: study |publisher=University of Waterloo |date=2005-04-07 |url=http://newsrelease.uwaterloo.ca/news.php?id=4348 |accessdate=2007-07-30 ] It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide and methane, kept the Earth from freezing.

One of the earliest types of bacteria was the cyanobacteria. Fossil evidence indicates that bacteria shaped like these existed approximately 3.3 billion years ago and were the first oxygen-producing evolving phototropic organisms. They were responsible for the initial conversion of the earth's atmosphere from an anoxic state to an oxic state (that is, from a state without oxygen to a state with oxygen) during the period 2.7 to 2.2 billion years ago. Being the first to carry out oxygenic photosynthesis, they were able to produce oxygen while sequestering carbon dioxide in organic molecules, playing a major role in oxygenating the atmosphere.

Photosynthesising plants later evolved and continued releasing oxygen and sequestering carbon dioxide. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to release nitrogen; in addition, bacteria would also convert ammonia into nitrogen. But most of the nitrogen currently present in the atmosphere results from sunlight-powered photolysis of ammonia released steadily over the aeons from volcanoes.

As more plants appeared, the levels of oxygen increased significantly, while carbon dioxide levels dropped. At first the oxygen combined with various elements (such as iron), but eventually oxygen accumulated in the atmosphere, contributing to Cambrian explosion and further evolution. With the appearance of an ozone layer (ozone is an allotrope of oxygen) lifeforms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere". Between 200 and 250 million years ago, up to 35% of the atmosphere was oxygen (as found in bubbles of ancient atmosphere preserved in amber).

This modern atmosphere has a composition which is enforced by oceanic blue-green algae as well as geological processes. O2 does not remain naturally free in an atmosphere but tends to be consumed (by inorganic chemical reactions, and by animals, bacteria, and even land plants at night), and CO2 tends to be produced by respiration and decomposition and oxidation of organic matter. Oxygen would vanish within a few million years by chemical reactions, and CO2 dissolves easily in water and would be gone in millennia if not replaced. Both are maintained by biological productivity and geological forces seemingly working hand-in-hand to maintain reasonably steady levels over millions of years.

Currently, anthropogenic greenhouse gases are increasing in the atmosphere and this is a causative factor in global warming.cite web | url= http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_SPM.pdf | format=PDF | title=Summary for Policymakers | work=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change | date=2007-02-05 | publisher=Intergovernmental Panel on Climate Change]

Air pollution

Air pollution is the human introduction into the atmosphere of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or damages the environment. [Starting from [http://www.merriam-webster.com/dictionary/pollution] Pollution - Definition from the Merriam-Webster Online Dictionary] Stratospheric ozone depletion is believed to be caused by air pollution (chiefly from chlorofluorocarbons)

Worldwide air pollution is responsible for large numbers of deaths and cases of respiratory disease. Enforced air quality standards, like the Clean Air Act in the United States, have reduced the presence of some pollutants. While major stationary sources are often identified with air pollution, the greatest source of emissions is actually mobile sources, principally the automobileFact|date=August 2008. Gases such as carbon dioxide, methane, and fluorocarbons contribute to global warming, and these gases, or excess amounts of some emitted from fossil fuel burning, have recently been identified by the United States and many other countries as pollutants.Fact|date=February 2008

ee also

* Aerial perspective
* Air glow
* Airshed
* Atmosphere (for information on atmospheres in general)
* Atmospheric chemistry
* Atmospheric dispersion modeling
* Atmospheric electricity
* Atmospheric models
* Atmospheric Radiation Measurement (ARM) (in the U.S.)
* Atmospheric stratification
* Aviation
* Biosphere
* Compressed air
* Global dimming
* Historical temperature record
* Hydrosphere
* Intergovernmental Panel on Climate Change
* Lithosphere
* US Standard Atmosphere

References

* [http://modelweb.gsfc.nasa.gov/spdf_models_home.html#atmo NASA atmosphere models]
* [http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html NASA's Earth Fact Sheet]
* [http://atmospheres.agu.org/ American Geophysical Union: Atmospheric Sciences]
* [http://www.stuffintheair.com Stuff in the Air] Find out what the atmosphere contains.
* [http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm Layers of the Atmosphere]
* [http://www.scribd.com/doc/22854/Air-Atmosphere-and-Airplanes/ Answers to several questions of curious kids related to Air and Atmosphere]
* [http://amsglossary.allenpress.com/glossary The AMS Glossary of Meteorology]
* [http://www.vega.org.uk/video/programme/111 Paul Crutzen Interview] Free video of Paul Crutzen Nobel Laureate for his work on decomposition of ozone talking to Harry Kroto Nobel Laureate by the Vega Science Trust.
* [http://des.memphis.edu/lurbano/Geog1010/Fall05/chapter_03/chapter_03.html Slides describing the Earth's modern atmosphere]

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