Atmosphere of Earth
Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly 78% nitrogen, (normally inert except upon electrolysis by lightning[1] and in certain biochemical processes of nitrogen fixation), 21% oxygen, 0.93% argon, 0.04% carbon dioxide, and trace amounts of other gases, in addition to about 3% water vapor. This mixture of gases is commonly known as air. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night.
There is no definite boundary between the atmosphere and outer space. It slowly becomes thinner and fades away into space. Three quarters of the atmosphere's mass is within 11 km of the planetary surface. In the United States, persons who travel above an altitude of 50.0 miles (80.5 km) are designated as astronauts. An altitude of 120 km (75 mi or 400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. The Karman line, at 100 km (62 miles), is also frequently used as the boundary between atmosphere and outer space.
Temperature and the atmospheric layers
The temperature of the Earth's atmosphere varies with altitude; the mathematical relationship between temperature and altitude varies between seven different atmospheric layers:
- Troposphere: From the Greek word "τρέπω" meaning to turn or mix. The troposphere is the lowest layer of the atmosphere starting at the surface going up to between 7 km (4.4 mi) at the poles and 17 km (10.6 mi) at the equator with some variation due to weather factors. The troposphere has a great deal of vertical mixing due to solar heating at the surface. This heating warms air masses, which then rise to release latent heat as sensible heat that further uplifts the air mass. This process continues until all water vapor is removed. In the troposphere, on average, temperature decreases with height due to expansive cooling.
- Ozonosphere: or ozone layer is the part of the Earth's atmosphere which contains relatively high concentrations of ozone. "Relatively high" means a few parts per million - much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from approximately 15 km to 35 km above Earth's surface, though the thickness varies seasonally and geographically.
- Stratosphere: from that 7–17 km range to about 50 km, temperature increasing with height.
- Mesosphere: from about 50 km to the range of 80 km to 85 km, temperature decreasing with height.
- Thermosphere: from 80–85 km to 640+ km, temperature increasing with height.
- Ionosphere: is the part of the atmosphere that is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. It is located in the thermosphere.
- Exosphere: from 500-1000 km up to 10,000 km, free-moving particles that may migrate into and out of the magnetosphere or the solar wind.
The boundaries between these regions are named the tropopause, stratopause, mesopause, thermopause and exobase.
The average temperature of the atmosphere at the surface of Earth is 14 °C.
Pressure and thickness of the atmosphere
- Barometric Formula: (used for airplane flight) barometric formula
- One mathematical model: NRLMSISE-00
The average atmospheric pressure, at sea level, is about 101.3 kilopascals (about 14.7 pounds per square inch).
Atmospheric pressure is a direct result of the total weight of the air above the point at which the pressure is measured. This means that air pressure varies with location and time, because the amount (and weight) of air above the earth varies with location and time.
Atmospheric pressure decreases with height, dropping by 50% at an altitude of about 5.6 km (equivalently, about 50% of the total atmospheric mass is within the lowest 5.6 km). This pressure drop is approximately exponential, so that each doubling in altitude results in an approximate decrease in pressure by half. However, because of changes in temperature throughout the atmospheric column, as well as the fact that the force of gravity begins to decrease at great altitudes, a single equation does not model atmospheric pressure through all altitudes (it is modeled in 7 exponentially decreasing layers, in the equations given above).
Even at heights of 1000 km and above, the atmosphere is still present (as can be seen for example by the effects of atmospheric drag on satellites).
The equations of pressure by altitude in the above references can be used directly to estimate atmospheric thickness. However, the following published data are given for reference:[2]
- 50% of the atmosphere by mass is below an altitude of 5.6 km.
- 90% of the atmosphere by mass is below an altitude of 16 km. The common cruising altitude of commercial airliners is about 10 km.
- 99.99997% of the atmosphere by mass is below 100 km (almost all of it). The highest X-15 plane flight in 1963 reached an altitude of 354,300 ft or 108 km.
Therefore, most of the atmosphere (99.9997%) is below 100 km, although in the rarefied region above this there are auroras and other atmospheric effects.
Composition
| ppmv: parts per million by volume | |
| Gas | Volume |
|---|---|
| Nitrogen (N2) | 780,840 ppmv (78.084%) |
| Oxygen (O2) | 209,460 ppmv (20.946%) |
| Argon (Ar) | 9,340 ppmv (0.9340%) |
| Carbon dioxide (CO2) | 383 ppmv (0.0383%) |
| Neon (Ne) | 18.18 ppmv |
| Helium (He) | 5.24 ppmv |
| Methane (CH4) | 1.745 ppmv |
| Krypton (Kr) | 1.14 ppmv |
| Hydrogen (H2) | 0.55 ppmv |
| Not included in above dry atmosphere: | |
| Water vapor (H2O) | typically 1% to 4%(highly variable) |
Minor components of air not listed above include:
| Gas | Volume |
|---|---|
| nitrous oxide | 0.5 ppmv |
| xenon | 0.09 ppmv |
| ozone | 0.0 to 0.07 ppmv |
| nitrogen dioxide | 0.02 ppmv |
| iodine | 0.01 ppmv |
| carbon monoxide | trace |
| ammonia | trace |
The mean molar mass of air is 28.97 g/mol. Note that the composition figures above are by volume-fraction (V%), which for ideal gases is equal to mole-fraction (that is, fraction of total molecules). By contrast, figures for mass-fraction abundances of gases, particularly for gases with significantly different molecular (molar) mass from that of air, will differ accordingly in their molar(mole) fraction abundance figures. For example, helium is 5.2 ppm by volume-fraction and mole-fraction, but only about (4/29)* 5.2 ppm = 0.72 ppm by mass-fraction.
Mass percentage calculations
 |
In general, assuming that the gases act like ideal gases as discussed (so that each molecule occupies the same average volume in the gas as any other molecule), then the total mass m of each particular gas in the atmosphere is its volume-percentage (mole-fraction percentage) V%, multiplied by its particular molar mass (M), multiplied by the total moles of gas in the atmosphere T, or m = V%· M· T. We may sum these m's for all gases to get a total atmospheric mass M = Σ(V%·M·T) = T·Σ(V%·M). Any given element's mass-fraction (m/M) in the total atmosphere, is given by the ratio of the element's mass to the total mass: m/M = (V%·M) / Σ(V%·M). Note that the total mole figure T cancels in this ratio, as do the % ratios. Thus, results for m/M are direct mass-fraction, and must be re-converted to a % mass fraction by multiplication by 100, or else mass fraction by parts per million (ppm) by multiplication by 1 million. The figure given above for the mean molar mass of air (28.97 g/mole) is the convenient summation of Σ(V%·M), so that mass fractions for individual gases may be calculated simply by multiplying the gas volume fraction in the atmosphere by its particular molecular weight, and dividing by this number, as was done for the helium example above (if volume fraction % is input, then mass fraction % is output).
When we do this using volume-percentages of the gases given above, we get that, by mass-fraction% (m%), the mass% composition of the atmosphere is 78.084 V% x [28.014/28.97] = about 75.51 m% nitrogen, etc. Accepted mass fraction % figures (since the gases do NOT act perfectly as ideal gases, as was assumed for this calculation) are:[citation needed]
- 75.523% nitrogen
- 23.133% oxygen
- 1.288% argon
- 0.035% carbon dioxide
- 0.001267% neon
- 0.00029% methane
- 0.00033% krypton
- 0.000724% helium
- 0.0000038 % hydrogen
Atom-fraction abundances, if needed, can be calculated but must be corrected for numbers of atoms in the relevant molecule, so helium makes up 5.2 ppm of the molecules in air, but (since most air molecules are diatomic) helium only constitutes about half as many (2.6 ppm) of the total atoms in air.
Heterosphere
Below the turbopause at an altitude of about 100 km, the Earth's atmosphere has a more-or-less uniform composition (apart from water vapor) as described above; this constitutes the homosphere.[4] 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.[5]
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 due to 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 5,000 trillion metric tons. According to the National Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480×1018 kg with an annual range due to water vapor of 1.2 or 1.5×1015 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 1.27×1016 kg and the dry air mass as 5.1352 ±0.0003×1018 kg."
Evolution of the Earth's atmosphere
The history of the Earth's atmosphere prior to one billion years ago is poorly understood, but the following presents a plausible sequence of events. This remains an active area of research.
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 nitrogen, carbon dioxide, and very recent simulations run at the University of Waterloo and University of Colorado in 2005 suggest that it may have had up to 40% hydrogen[6]. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide and methane, kept the Earth from freezing. In fact temperatures were probably very high, over 70 degrees C (158 degrees F), until some 2.7 billion years ago.
One of the earliest types of bacteria were 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 convert carbon dioxide into oxygen, playing a major role in oxygenating the atmosphere.
Photosynthesising plants would later evolve and convert more carbon dioxide into oxygen. 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 modern day level of nitrogen are due mostly to 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, resulting in mass extinctions 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". 200 - 250 million years ago, up to 35 percent of the atmosphere was oxygen (bubbles of ancient atmosphere were found in an 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, as well as by animals, bacteria, and even land plants at night), while CO2 tends to be produced by respiration and decomposition and oxidation of organic matter. Oxygen would vanish within a few million years due to 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.
Air pollution
Air pollution is a chemical, physical (e.g. particulate matter), or biological agent that modifies the natural characteristics of the atmosphere. Stratospheric ozone depletion due to air pollution has long been recognized as a threat to human health as well as to the earth's ecosystems.
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 are actually mobile sources, principally the automobile. Gases such as carbon dioxide, which are thought to contribute to global warming, have recently been viewed by some as pollutants.
References
- ^ Silberberg, Martin S. Chemistry, the Molecular Nature of Matter and Change. McGraw-Hill Publishing, 2006.
- ^ Lutgens, Frederick K. and Edward J. Tarbuck (1995) The Atmosphere, Prentice Hall, 6th ed., pp14-17, ISBN 0-13-350612-6
- ^ Source for figures: NASA. carbon dioxide (updated 2007.01). Methane updated (to 1998) by IPCC TAR table 6.1 [1]. The NASA total was 17 ppmv over 100%, and CO2 was increased here by 15 ppmv. To normalize, N2 should be reduced by about 25 ppmv and O2 by about 7 ppmv.
- ^ http://amsglossary.allenpress.com/glossary/search?id=homosphere1
- ^ Vercheval, J. "The thermosphere: a part of the heterosphere". (offline, see Internet Archive copy)
- ^ http://newsrelease.uwaterloo.ca/news.php?id=4348
See also
- Aerial perspective
- Air glow
- Atmosphere (for information on atmospheres in general)
- Atmospheric chemistry
- Atmospheric dispersion modeling
- Atmospheric electricity
- Atmospheric models
- Aviation
- Biosphere
- Compressed air
- Global warming
- Greenhouse effect
- Historical temperature record
- Hydrosphere
- Intergovernmental Panel on Climate Change (IPCC)
- Lithosphere
- Meteorology
- US Standard Atmosphere
External links
- NASA atmosphere models
- NASA's Earth Fact Sheet
- American Geophysical Union: Atmospheric Sciences
- Layers of the Atmosphere
- The AMS Glossary of Meteorology
- Paul Crutzen Interview Freeview video of Paul Crutzen Nobel Laureate for his work on decomposition of ozone talking to Harry Kroto Nobel Laureate by the Vega Science Trust.
- Slides describing the Earth's modern atmosphere