Geochemistry defines the earth's living space as a unit consisting of the air and water envelope as well as the upper part of the solid earth crust. Within this range, a closed circuit of the elements which form this air envelope takes place.
The subject of atmospheric air is governed by the elements contained in air, i.e. nitrogen, oxygen, carbon dioxide, and the rare gases argon, neon, helium, krypton and xenon. Nitrogen forms part of the more widespread elements. By far the largest quantities are found in the air envelope, containing about 78.1% by volume. Smaller quantities of nitrogen occur in the gases emitted by some springs and occlusions in rock. Oxygen is the most widespread element of our living space and its weight share of the earth's upper crust is estimated as being 49.5% in this context. About 90% of the available space of petrified rocks is occupied by oxygen. However, as oxygen is restricted to the outer crust of our earth, its weight share of the entire globe is reduced to 29%. The atmosphere contains on average 1 x 1015 (metric) tonnes of oxygen i.e. a weight share of 23.1%.
Carbon dioxide is contained in the air envelope of the earth with a weight of about 23 x 1012 tonnes. About 10 x 1011 tonnes of carbon dioxide per year emerge from the oceans into the atmosphere while the same quantity returns from air into the water. Assimilation by green plants annually extracts 6 x 1010 tonnes of carbon dioxide from the air but nearly the same quantity is released by the breathing of humans, animals, micro organisms and decay. Since the beginning of the 19th century, the carbon dioxide content has increased, this being caused mainly by the increasing use of fossil fuels and the clearing of forests. Assuming a continuing population growth of 2% per annum and counting on a growth in consumption of 1% per year, doubling the carbon dioxide content of the atmosphere in the next 50 years can be expected. Hydrogen makes up an estimated share of the upper earth crust including water and air envelope of about 0.88 weight percentage.
At the height of 2000 - 20000 km the earth is surrounded by a layer of dilute hydrogen. In comparison, the sun consists of about 84% by weight of hydrogen, the fuel for the generation of solar energy. Argon is by far the most widespread of the rare gases. Air contains 0.93% by volume on average and a human being breathes in and out 20 litres per day. Spring water, too, contains dissolved argon, particularly if the spring emerges from greater depths such as in the case of geysers.
Helium occurs most frequently in natural gases in concentrations of up to 7.5% by volume. The major occurrences are in the USA and the former USSR. Percentage occurrence in the upper earth crust including the oceans and the atmosphere is estimated at 4.2 x 10-7% and it is thus more rare than gold, silver or platinum.
Neon also belongs to the rare elements with an estimated share of 5 x 10-7%. In outer space, however, neon forms the third most frequent element after hydrogen and helium.
Krypton is contained in air to an amount of 0.000108%. Content in the upper earth crust including the air and water envelope is estimated to be only 1.9 x 10-8%, thus making it one of the most rare elements.
The share of xenon in the upper earth crust including air and water envelope is estimated to be about 2.4 x 10-9% and the volume-wise share in air is 0.0000086% = 86 parts per billion (ppb), thus extremely low.
Nitrogen, oxygen, carbon dioxide, hydrogen, in addition to the rare gases argon, neon, helium, krypton and xenon form the gas mixture which makes up our air in the lower regions of the atmosphere. Air is ever present for us all. It surrounds all life at every step from the beginning to the end. Let us take a deep breath of air for new spectacular or vital aims. There is much to this medium: Air.
