# air

Gas composition of
the air in% by volume

As air is called the gas mixture of the Earth's atmosphere . Dry air mainly consists of the two gases nitrogen (around 78.08  % by volume ) and oxygen (around 20.95% by volume). There are also the components argon (0.93 vol .-%), carbon dioxide (0.04 vol .-%) and other gases in traces.

Solid and liquid particles, called aerosols , are also components of air. Water and water vapor are on average 0.4% by volume in the entire earth's atmosphere. These components are listed separately.

In addition, air also contains dust and biological particles (e.g. pollen , fungus and fern spores ). In its natural state it is odorless and tasteless to humans.

As a rule of thumb, the density of air at sea level is around 1.25 kg / m 3 - i.e. only 1/800 that of water - and is significantly influenced by temperature, water vapor content and pressure, which decreases with sea level. In compressed air cylinders with a pressure of typically 5 to 300 bar (0.5 to 30 megapascals), air at 27 ° C has about 5 to 270 times the density of air at 1 bar (roughly atmospheric pressure at sea level).

Rayleigh scattering in the atmosphere leads to blue skies and yellow to red sun

## Light scattering

The gas molecules (nitrogen, oxygen, etc.) occurring in air scatter the incident sunlight to different degrees, depending on its wavelength ( Rayleigh scattering ). The short-wave blue light is most strongly scattered. This process gives the air its typically natural blue color , a partially polarized pattern . If the path of light through the air is longer, the scattering shifts to a reddish hue. This depends on the angle of incidence of sunlight. If the sun is at its zenith (directly above the viewer −90 ° to the ground), the light passes through the earth's atmosphere over a length of 90 km. During a sunrise / sunset, the sun is on the horizon (in the horizontal line of the observer's eyes −0 ° to the ground) and crosses the atmosphere with a length of approx. 12 times (approx. 1075 km). As the light travels through the atmosphere, the blue part is superimposed by the red part of the light, since the blue part is more and more scattered and the red part is less strongly scattered because of its longer wavelength. This atmospheric effect can only be observed with a cloudless horizon in the morning shortly before sunrise and in the evening shortly after sunset. It is known as dawn or sunset and ranges in color from light pink to purple through full red to deep orange.

Apart from the elastic scattering of the photons, inelastic scattering can also be observed in the atmosphere: rotational raman scattering leads to a redistribution in the energies of the incoming radiation within a few 10 cm −1 and thus leads to a "filling up" of the Fraunhofer lines, the so-called ring effect . With a spectral resolution of 0.5 nm, this effect leads to optical thicknesses of typically up to 2% when the scattered sky light from different positions of the sun is compared. This effect must be corrected in various DOAS remote sensing methods for measuring trace gases.

Furthermore, vibrational ram scattering on air molecules can shift the wave number of the incoming photons from 1550 cm −1 (O 2 ) and 2330 cm −1 (N 2 ) and thus place a wavelength-shifted image of the sunlight over the observed sunlight. Its intensity is up to 0.04% of the original intensity.

## composition

Composition of the air
gas formula Volume fraction Mass fraction
Main components of dry air at sea level
nitrogen N 2 78.084% 75.518%
oxygen O 2 20.942% 23.135%
argon Ar 0.934% 1.288%
Subtotal 99.960% 99.941%
Trace gas content (a selection)
carbon dioxide CO 2 0.040%
or 400  ppm
0.061%
or 610 ppm
neon No 18.18 ppm 12.67 ppm
helium Hey 5.24 ppm 0.72 ppm
methane CH 4 1.85 ppm 0.97 ppm
krypton Kr 1.14 ppm 3.30 ppm
hydrogen H ~ 500  ppb 36 ppb
Nitrous oxide N 2 O 328 ppb 480 ppb
Carbon monoxide CO 100-250 ppb 100-250 ppb
xenon Xe 87 ppb 400 ppb
Dichlorodifluoromethane ( CFC-12 ) CCl 2 F 2 520  ppt 2200 ppt
Trichlorofluoromethane ( CFC-11 ) CCl 3 F 234 ppt 1100 ppt
Chlorodifluoromethane ( HCFC-22 ) CHClF 2 253 ppt 480 ppt
Carbon tetrachloride CCl 4 81 ppt 510 ppt
Trichlorotrifluoroethane ( CFC-113 ) C 2 Cl 3 F 3 71 ppt 520 ppt
1,1-dichloro-1-fluoroethane ( HCFC-141b ) CCl 2 F-CH 3 26 ppt 70 ppt
1-chloro-1,1-difluoroethane ( HCFC-142b ) CClF 2 -CH 3 23 ppt 50 ppt
Sulfur hexafluoride SF 6 8 ppt 25 ppt
Bromochlorodifluoromethane CBrClF 2 4 ppt 25 ppt
Bromotrifluoromethane CBrF 3 3.4 ppt 13 ppt
Radiocarbon 14 CO 2 10 −13  %
radon Marg 10 −19  %
Total mass (dry) 100% 5.135 · 10 15  t
water + 0.4% +0.013 · 10 15  t
Total mass (wet) 100.4% 5.148 · 10 15  t

The proportions of the atmospheric gases are not natural constants . In the development of the earth's atmosphere , which has been going on for billions of years , the composition changed constantly and fundamentally several times. The main components have been largely stable for 350 million years. The current mixture for dry air is shown in the table on the right, whereby a distinction is made between main components and trace gases. The stated concentrations represent global mean values for the free troposphere . The chemically stable components are uniform throughout the homosphere apart from sources , ie up to an altitude of about 100 km. There are significant gradients for reactive trace substances .

Separation into the components

Liquefied at low temperature, liquid air can be broken down into its constituent parts by fractional distillation , usually with the help of the Linde process .

### Main ingredients

#### nitrogen

The main constituent of air is chemically inert . It is organically bound by the natural (biotic and abiotic) nitrogen fixation and thus usable for living beings . Technically, the nitrogen in the air is used to manufacture fertilizers via the Haber-Bosch process . The opposite chemical process - denitrification runs faster, so that the nitrogen cycle hardly changes the nitrogen content in the atmosphere.

Cosmic radiation produces small amounts of radioactive carbon ( 14 C) from the nitrogen in the air , which is used for archaeological dating with the radiocarbon method.

#### oxygen

The molecular oxygen in the air was mainly formed from water by photosynthesis , the amount produced in the course of the earth's history being around twenty times the amount present in the atmosphere today. It gives the atmosphere its oxidizing character and is the most important oxidizing agent required for biological respiration and chemical combustion processes .

The oxygen in the air is necessary for all aerobic organisms to live. Through breathing, they feed oxygen to their metabolism for combustion ( catabolism ). Plants use the carbon dioxide contained in the air for photosynthesis and split off the oxygen in the process. For almost all plants this is the only source of carbon for vital processes and body substance ( anabolism ). In this organic process, almost all of the oxygen in the air is regenerated. The oxygen cycle enables the maintenance and distribution of a permanent supply of resources for aerobes and photosynthetically active plants.

The current global atmospheric oxygen content remains remarkably constant at a level of 20.946 ± 0.006 vol.%, With a slight decrease of 0.0004 vol.% / Year (4 ppmv / a), which is counter-correlated with carbon dioxide from fossil fuels and biomass combustion.

#### argon

As a noble gas, argon is extremely inert and relatively common with a content of almost 1%. It is inexpensive and is used as an inert gas, for example in metal welding and for filling incandescent lamps. There, and as a filling of multi-pane insulating glass, the somewhat lower thermal conductivity compared to air is used. ( In special cases, expensive, rare krypton is used as an even better heat insulating gas.)

Argon is created slowly by radioactive decay of potassium -40, is stable and denser than air and therefore remains in the atmosphere.

#### Steam

The ambient air is not "dry", but contains the gas water vapor , one speaks of air humidity . The water vapor content fluctuates between a tenth percent by volume at the poles and three percent by volume in the tropics , with an average of 1.3% near the ground. Since the water vapor content reduces the density of the air (62.5% of the density of "dry" air), more humid air is pushed upwards, where condensation then occurs in cooler layers , i.e. the water vapor content in the gas mixture drops. The water vapor content is very low above the condensation layers, so that, averaged over the entire atmosphere, there is only 0.4% by volume of water vapor in the air.

### Trace gases

Larger fluctuations over a few years and decades can also be recorded for trace gases. Their low concentrations can be influenced by comparatively low emissions. Volcanic eruptions also often have a short-term impact.

#### carbon dioxide

In terms of its share, carbon dioxide is a trace gas, but as the fifth most common atmospheric gas - taking into account water vapor. Because of its importance for the climate and living beings, it is often counted among the main components of the air.

The main biological meaning of carbon dioxide (colloquially often referred to as carbon dioxide) lies in its role as a source of carbon for photosynthesis. The atmospheric carbon dioxide concentration has a strong effect on plant growth. Due to the light-dependent metabolic cycle of plants, i.e. the interrelationship between respiration and photosynthesis, the ground-level CO 2 concentrations fluctuate during the day. With sufficient plant cover, there is a nocturnal maximum and, accordingly, a minimum during the day. The same effect is present in the course of the year, as the extra-tropical vegetation has distinct vegetation periods . In the northern hemisphere, there is a maximum in March to April and a minimum in October or November. The heating season also contributes to this through increased consumption of fossil fuels.

Overall, the carbon dioxide content has increased by over 40% since the beginning of industrialization . In connection with the anthropogenic greenhouse effect, this is one of the causes of global warming , for which a reference value in the geological climate of less than 100 years applies. In 2013, the CO 2 concentration at the Mauna Loa measuring station exceeded 400 ppm for the first time.

#### Noble gases

While argon is one of the main constituents of the air with around 1% (see above), the other noble gases neon , helium and krypton with volume fractions of> 1 ppm each belong to the trace gases (see table). Xenon is even rarer (volume fraction <0.1 ppm). Radon is the rarest noble gas in the air (mean volume fraction 1:10 21 ), but - depending on the isotope - it can be well determined via its radioactivity.

Helium is released with every radioactive alpha decay. Helium is much lighter than air and escapes into space. The second lightest noble gas, neon, also evaporates there, so that only traces of these two exist in the atmosphere.

Radon escapes from some rocks as a link in radioactive decay series , which can accumulate in cellars and continue to decay with a radiating effect.

#### ozone

For the stratosphere, ozone values ​​are often given in Dobson units rather than fractions . Since the values ​​also depend on the altitude ( ozone layer , ground-level ozone ) as well as weather conditions, temperature, pollution and time of day, and ozone both forms and decays quickly, this value is very variable. Due to the high reactivity of ozone, it plays a central role in chemical reactions of various kinds in the atmosphere. One example is the ODEs ( ozone depletion events ), during which sharp drops in the ozone concentration from normally 20–40 ppb to <5 ppb can regularly be observed during the polar spring. These phenomena are caused, for example, by the release of halogens through natural processes or by mixing air masses. Typical ozone concentrations in temperate latitudes and populated areas are 30–60 ppb in the northern hemisphere and tend to be around 10 ppb less in the southern hemisphere due to the role ozone plays in nitrogen oxide chemistry .

#### Carbon monoxide

Carbon monoxide (colloquially often referred to as carbon monoxide) is an invisible, flammable, poisonous gas that is produced when substances containing carbon are not completely burned . It blocks the transport of oxygen in the blood ( carbon monoxide intoxication ) and can lead to death even in small doses. It also damages the photosynthesis of plants . It forms z. B. in tobacco smoking and in the internal combustion engine . Car exhaust fumes and the immense flight operations without exhaust aftertreatment by a vehicle catalytic converter can contain up to 4% CO, the standard value for tobacco smoke . Vegetation fires are the main source of carbon monoxide with around 60% of emissions worldwide.

## Physical quantities of air

Temperature dependence
Temperature
[° C]
Sound
speed
[m / s]
Sound
characteristic impedance
[N · s / m 3 ]
Air density
[kg / m 3 ]
−10 325.4 436.6 1.341
0−5 328.5 432.5 1.317
0−0 331.5 428.5 1.293
0+5 334.5 424.6 1,270
+10 337.5 420.8 1.247
+15 340.5 417.1 1.225
+20 343.4 413.5 1.204
+25 346.3 410.0 1.184
+30 349.2 406.6 1.164

### Average molecular weight

The mean molar mass is the sum of the products of the molar masses and molar proportions of the components, mainly oxygen, nitrogen and argon. For dry air the exact value is 28.949 g / mol . If the air still contains moisture, the mean molar mass is lower because the molar mass of water vapor is only approx. 18 g / mol.

### Airtightness

Under normal conditions the air density is equal to 1.293 kg / m 3 .

### Air pressure

The weight of the air column creates a static pressure. According to the barometric altitude formula, this pressure depends on the altitude above sea ​​level . In addition, the air pressure depends on the weather. Wind and general changes in the weather cause fluctuations in air pressure. A barometer for measuring the air pressure is therefore part of the basic equipment of weather stations . Over one square meter of floor space, the air mass is around 10,000 kg, corresponding to the air pressure.

### Air temperature

The air temperature is the temperature of the air close to the ground, which is neither influenced by solar radiation nor by soil heat or heat conduction . The exact definition in science and technology is different. In meteorology , the air temperature is measured at a height of two meters, which is often done by weather houses painted white in the open.

### humidity

The humidity is the proportion of water vapor in the air. It is specified using various humidity measures such as vapor pressure and dew point as well as relative, absolute and specific humidity.

### Other values

Under normal conditions , the speed of sound in air is 331.5 m / s.

The refractive index of air under normal conditions for visible light is approximately 1,00029. The value depends on the pressure, temperature and composition of the air, but above all on the humidity. Because is roughly proportional to the air pressure, the refractive index can be determined with a Michelson interferometer , one arm of which extends through an area of ​​variable air pressure. The refractive index is determined from the resulting optical path length difference with a known pressure difference. ${\ displaystyle n}$${\ displaystyle n-1}$

Specific heat capacity under normal conditions:

${\ displaystyle c_ {p} \, = \, 1 {,} 005 \; \ mathrm {kJ} / (\ mathrm {kg} \ cdot {\ mathrm {K}})}$( isobaric change of state )
${\ displaystyle c_ {v} \, = \, 0 {,} 718 \; \ mathrm {kJ} / (\ mathrm {kg} \ cdot {\ mathrm {K}})}$( isochoric change of state )

The thermal conductivity of air is under normal conditions . ${\ displaystyle \ lambda}$${\ displaystyle 0 {,} 0261 \; {\ mathrm {W}} / ({\ mathrm {m}} \ cdot {\ mathrm {K}})}$

## Air pollution and air pollution control

The air pollution is related to the air aspect of pollution . According to the Federal Immission Control Act , air pollution is a change in the natural composition of the air, in particular through smoke , soot , dust , aerosols , vapors or odorous substances . Increased ozone values are important for smog and sulfur dioxide concentrations for acid rain , but also concentrations of nitrogen oxides and volatile organic compounds , which in turn have a major influence on the chemistry of the air.

In most industrialized countries, local air pollution has fallen sharply in recent decades due to legal requirements for air pollution control . At the same time, the emission of greenhouse gases such as carbon dioxide has continued to increase. Local and regional air pollution is still a significant problem for third world countries and emerging economies like China .

The effects of trace gases are diverse and, to a large extent, also influence one another. For example, ozone not only plays the role of a pollutant and greenhouse gas due to its role in hydroxyl radical chemistry in the air layers close to the ground, it is also essential for the self-cleaning mechanisms of the atmosphere as a whole.

A particularly large amount of microplastic was found in the air in London . Also pesticides are detectable in the air and are blown over long distances.

## Cultural meaning

The Greek natural philosophers considered air to be one of the four basic elements that make up all being. The octahedron was assigned to the element air as one of the five platonic solids . The central central belt asteroid (369) Aëria is named after air.

## literature

Wikiquote: Air  - Quotes
Wiktionary: Luft  - explanations of meanings, word origins, synonyms, translations

## Individual evidence

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15. ^ The Keeling Curve A daily record of atmospheric carbon dioxide
16. Stöcker 2007, p. 714.
17. ^ John H. Seinfeld, Spyros N. Pandis: Atmospheric chemistry and physics: from air pollution to climate change. John Wiley & Sons, 2012.
18. ^ Damian Carrington: Revealed: microplastic pollution is raining down on city dwellers. In: theguardian.com . December 27, 2019, accessed December 28, 2019 .
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20. ^ Lutz D. Schmadel : Dictionary of Minor Planet Names . Fifth Revised and Enlarged Edition. Ed .: Lutz D. Schmadel. 5th edition. Springer Verlag , Berlin , Heidelberg 2003, ISBN 978-3-540-29925-7 , pp.  186 (English, 992 pp., Link.springer.com [ONLINE; accessed October 24, 2019] Original title: Dictionary of Minor Planet Names . First edition: Springer Verlag, Berlin, Heidelberg 1992): “Discovered 1893 July 4 by A. Borrelly at Marseilles. "