Earth Sciences / 5114. The evolution, composition, and structure of the atmosphere
When the Earth first formed, it may have had a thin atmosphere of light gases, mainly hydrogen and
helium, derived from the stellar nebula. If so, this atmosphere was lost when the Sun began to
radiate, supplying the gas molecules with the energy they needed to escape the planet’s gravitational
attraction. Volcanism then released the gases which formed a new atmosphere. No one knows the
precise composition of that atmosphere, but probably it was rich in carbon dioxide and contained
only traces of nitrogen and free oxygen. Our present atmosphere has evolved to its present state,
partly (some would say mainly) as a result of biological processes.
These processes continue to maintain it. Nitrogen, for example, is chemically somewhat inert but, in
the presence of oxygen and with a sufficient application of energy, it will oxidize to nitrate (NO
3
).
This reacts with water to form nitric acid (HNO
3
) and is washed to the ground. Lightning supplies
enough energy for the oxidation, and in the world as a whole it is estimated that there are about 1800
thunderstorms, with lightning, happening at any one time, delivering some 100 million tonnes of
fixed nitrogen to the surface every year. At this rate it would not take long to strip the atmosphere of
its nitrogen, were it not for the activities of denitrifying bacteria, which utilize nitrogen compounds
in the soil and release gaseous nitrogen as a metabolic by-product. Were the air to be seriously
depleted of nitrogen the proportion (partial pressure) of oxygen would increase and were it to increase
to more than about 25 per cent exposed carbon compounds would burn even more readily than they
do. This would reduce the amount of oxygen, replacing it with carbon dioxide. As it is, carbon
dioxide is removed from the air by green plants as well as by dissolving in rain water.
To a considerable extent, therefore, our present atmosphere has been constructed and is maintained
by living organisms. It is an essential component of the environment. It is also the route by which
many organisms are disseminated and most nutrients cycled.
Air is a mixture of gases. They are not combined, so air itself is not a chemical compound, although
some of its minor constituent gases are.
As the many practical uses for compressed air (in car and bicycle tyres, for example) testify, air is
highly compressible. The total weight of the atmosphere (with a mass of about 5×10^15 tonnes)
compresses its lower layers, so the average density of the air decreases from about 1.2 kg m-3 at sea
level to about 0.7 kg m-3 at a height of 5 km. Half of the total mass of the atmosphere lies below 5 km.
The pressure exerted by the atmosphere is usually expressed in millibars (mb) in weather forecasts
or in pascals (Pa) by physicists (1 mb=100 N m-2=100 Pa ). The average sealevel pressure is 1013.2
mb (or 101320 Pa).
One consequence of its compressibility is that for all practical purposes the atmosphere is quite
shallow. Although it has no clearly defined upper boundary, merging imperceptibly with the solar
atmosphere at an altitude of about 80000 km (ALLABY, 1992, p. 50), pressure and therefore density
decrease logarithmically with height. At 30 km, air density is at only 0.02 per cent of its sea-level
value. The part of the atmosphere in which weather occurs is confined to the lowermost 8–16 km.
None of the familiar meteorological phenomena occur above this height, but they are influenced by
events in the lower reaches of the overlying layer, up to a height of about 30 km.
The air is warmed convectively from below, by contact with the surface. As it warms it expands
and as it expands it cools. Temperature and density determine the amount of water vapour air can
hold (its humidity), and the combination of the humidity, density, and temperature imposes a
layered structure on the atmosphere. Figure 2.15 illustrates this structure, relating it to height,
temperature, and pressure.