Air

This article is about Air. For other uses of the term Air, please see Air (disambiguation).

(CC) Photo: Pascal Lando
Landscape with clear air and and clouds

Air is a colorless, odorless and tasteless mixture of gases consisting mostly of nitrogen (N₂) and oxygen (O₂). It is the part of Earth's atmosphere that humans and all other animals breathe in order to obtain the oxygen needed to sustain life.

The Earth's atmosphere not only contains the air we breathe, it also holds clouds of moisture (water vapor) that become the water we drink. Furthermore, it protects us from meteors and harmful solar radiation and warms the Earth's surface by heat retention. In effect, the atmosphere is an envelope that protects all life on Earth.

The air may contain pollutants that originate from a variety of sources such as our industries and our vehicles, and can directly or indirectly affect our health and the natural environment. These effects may be experienced near the sources of air pollution and some air pollutants may be transported long distances by the wind, even across political boundaries.

Composition of the atmospheric air

Composition of Dry Air Gas Concentration Name Symbol Volume % ppmv Nitrogen N2 78.084 780,840 Oxygen O2 20.947 209,470 Argon Ar 0.934 9,340 Carbon dioxide CO2 0.033 330 Neon Ne 0.001820 18.20 Helium He 0.000520 5.20 Methane CH4 0.000200 2.00 Krypton Kr 0.000110 1.10 Sulfur dioxide SO2 0.000100 1.00 Hydrogen H2 0.000050 0.50 Nitrous oxide N2O 0.000050 0.50 Xenon Xe 0.000009 0.09 Ozone O3 0.000007 0.07 Nitrogen dioxide NO2 0.000002 0.02 Notes: -- ppmv = parts per million parts by volume -- Water vapor varies up to a maximum of 4 volume percent. -- The total volume percent of the listed gases does not equal      exactly 100 percent because of rounding of the numbers.

The adjacent table lists the concentration of 14 gases present in filtered dry air. Two of the gases, nitrogen and oxygen make up 99.03 percent of the clean, dry air. The other listed gases total to 0.97 percent.

Note the amounts of greenhouse gases that are present: water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Additional gases (not listed in the table) are also present in very minute amounts.

The atmospheric air is rarely, if ever, dry. Water vapor is nearly always present up to about 4% of the total volume. In the deserts regions, when dry winds are blowing, the water vapor content will be near zero. This climbs to near 3% on extremely hot/humid days. The upper limit of 4% is for tropical climates.

Unfiltered air contains minute amounts of various types of particulate matter derived from sources such as from dust, pollen and spores, sea spray, volcanoes, meteoroids and industrial activities.

The nitrogen cycle

The nitrogen in the air is also essential for life on Earth. It is incorporated into amino acids and proteins, and is part of the nucleic acids, such as DNA and RNA. In plants, nitrogen is used in chlorophyll which is essential for photosynthesis and further growth. However, the free nitrogen in the atmosphere is mostly unusable by plants. Since nitrogen is inert, it requires considerable energy to remove nitrogen from the air.

The process by which nitrogen in the air is extracted to be useful for living things is called nitrogen fixation, and it can happen by several ways, by nature, by living things, and by human activity. One way nitrogen is fixed is during atmospheric lightning strikes; these have enormous energy to break down inert nitrogen molecules, allowing them to combine with oxygen to make nitrogen oxides; these, in turn, can dissolve in rain, forming "nitrates" that are carried by gravity to the earth, but most of the recycling of nitrogen does not occur by lightning. A second way, which accounts for much nitrogen fixation, is when symbiotic bacteria which have the nitrogenase enzyme combine gaseous nitrogen with hydrogen to produce ammonia; this, in turn, is further converted by the bacteria to make organic compounds needed by the bacteria for internal processes. Some nitrogen fixing bacteria, such as Rhizobium, live in the root nodules of legumes and work with the plant as a form of symbiosis and produce ammonia in exchange for carbohydrates. A third way to fix nitrogen is by means of an industrial process which uses great pressure at high temperatures, such as over 600°C, and uses a catalyst; this process combines atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) to form ammonia (NH3) which can be used directly as fertilizer, but for economic reasons, mostly it's processed further to yield urea and ammonium nitrate (NH4NO3). This industrial process enables humans to grow more food than would otherwise have been the case, but there is concern that this activity is disturbing the natural cycling of nitrogen.

By many methods, natural and man-made, nitrogen circulates through the atmosphere, down to the soil of the Earth's crust and into lakes and streams and reservoirs and oceans of the Earth. The nitrogen cycle is an example of a biogeochemical cycle similar to the water cycle. There is speculation that these different cycles are related to each other, and may influence each other in ways yet unknown; scientists continue to explore how these cycles affect each other.

Why the daytime sky is usually blue

Light is electromagnetic radiation that travels in waves of vibrating electric and magnetic fields and is a small part of a larger range of vibrating electromagnetic fields called the electromagnetic spectrum. Visible light is electromagnetic radiation visible to the human eye and is only a small part of the electromagnetic spectrum.

Light from the sun looks white, but is actually a combination of many colors. The colors blend continuously into one another. At one end of the spectrum are the red colors which have the highest wavelength and therefore the lowest frequency. At the other end of the spectrum are the blues and violets with the lowest wavelengths and therefore the highest frequency.

As light from the sun travels through Earth's atmosphere, it bumps into a bit of dust or a water droplet or a gas molecule. Dust particles and water droplets are very much larger than the wavelength of visible light. When light bumps into them, it gets reflected and bounced off in a different direction but the reflected light still appears white because it still contains all of the same colors it had before it was reflected.

However, gas molecules are much smaller than the wavelength of visible light. When light bumps into a gas molecule, it behaves differently than it does when it bumps into a dust particle or a water droplet in that some of the colors in the light are actually absorbed by the molecule. Subsequently, the molecule radiates or releases the light in a different direction. The colors in the radiated light are the same colors that were absorbed. The higher frequency colors (blues and violets) are absorbed more often than the lower frequency colors and this phenomena is called Rayleigh scattering. It was named after Lord Rayleigh, an English physicist who first described it in the 1870's.

Thus, the lower frequency (higher wavelength) red, orange and yellow colors in the light from the sun mostly pass right through the atmosphere and are unaffected by the air. The higher frequency (lower wavelength) green, blue and violet colors in the light from the sun are absorbed by the gas molecules in the air and are then scattered all over the sky. That is why we see the daytime sky as being blue colored.