| The atmospheric air is a gaseous mixing with nitrogen
(about 78%) and oxygen (about 21%) as main elements. The
rest is composed of carbon dioxide, rare gases (like argon
and krypton) and organic compounds (like methane and propane).
The troposphere also contains water in the form of vapor,
with an amount going from 0 to 4% of the air volume. In
addition, a large number of other gases like ozone, carbon
monoxide, sulphur or nitrogen compounds need to be added.
Furthermore, the air can contain several types of aerosols.
The driving forces behind the physico-chemical processes
in the atmosphere
are the solar radiations. (picture
1) These have the power to “break”
chemical molecules (photodissociation).
The troposphere and the stratosphere are two atmospheric
zones in which the ozone problem arises (video
1 & 2) We remember that the troposphere contains
about 10% of the atmospheric ozone and that in this
area the impact of ozone is harmful. It has toxic consequences
for humans and vegetation.
The largest part of ozone is to be found in the stratosphere
(about 90%). Opposite to what happens in the troposphere,
the stratospheric ozone (the ozone layer)
(picture 2) plays a good role by absorption of
an important part of the ultraviolet
solar radiations (picture 3),
which are very dangerous for living creatures.
We remember as well that when the complete atmosphere
would be reduced to pressure and temperature conditions
of the ground level, it would have a reduced thickness
of 8 kilometers, while the ozone layer would only be 3
to 5 mm thick.In many ways the mesosphere is the transitional
stage between the “classic” atmosphere and
the spatial environment, in which the satellites move
(picture 4), these are two
completely different environments. The physical and chemical
processes slowly run from one regime into another, what
generates complex interactions. The composition of
the thermosphere is very variable in time and space
due to the extremely rarefied air. From a height of
100 km, the mixing is no longer sufficient to maintain
the air homogeneous as it is observed lower. Molecular
diffusion becomes the dominant phenomenon. Moreover,
because of the very low air pressure, the temperature
changes very fast between day, night and in function
of the solar activity.
While the pressure of the atmosphere constantly decreases
with the height, the atoms behave freely in the exosphere
and can follow different trajectories, some of them
escaping out the Earth’s atmosphere.
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