Environmental Engineering Reference
In-Depth Information
coming from the Sun. In general terms, of the total energy received by the Earth every
year, only 69% is retained by the atmosphere-Earth system, while the remaining 31% is
reflected back into space (27% reflected by the atmosphere and 4% reflected by the Earth's
surface). The 69% retained is distributed inhomogeneously within the system, with the
atmosphere retaining 24% and the Earth's surface retaining the remaining 45%.
Despite there being a global balance between the total energy received and the total
energy absorbed, this balance does not occur in every place. The amount of energy
absorbed by the Earth's surface depends, among other factors, on the nature of the surface
(e.g., soil compared with sea), the altitude of the site considered, and the degree of cloudi-
ness. The heterogeneous distribution of energy, as a result of the different albedo values
across the surface of the Earth, is the cause of the atmospheric movements of air masses.
This is the reason for considering air movements as energy redistribution mechanisms
that tend toward homogenization.
The atmosphere is also a
physical filter
that protects living beings from high-energy
radiation (x-rays and UV light), a
mechanical filter
that traps and burns (to some extent)
the meteorites that impact the Earth, and a
greenhouse
that protects the Earth's surface
from the cold of the outer space (maintaining a mean temperature gradient of about
300 degrees).
Finally, the atmosphere is a
physicochemical reactor,
where a huge amount of circulating
water produces constant physical changes, and a lot of chemical compounds combine, dis-
sociate, and recombine with sunlight, constituting a mixture of substances, which, depend-
ing on their elevation above ground, play a different role in some of the phenomena that
attract our attention today, for example, pollution (excess) by tropospheric ozone, the hole
(deficiency) in the stratospheric ozone layer, acid rain, etc.
The chemical composition of the atmosphere has gradually changed during the last few
million years. Nowadays, 99.95% of the total atmospheric volume is composed of three
species: oxygen, nitrogen, and argon. The remaining atmospheric components (excluding
water vapor) are present in concentrations of the order of parts per million (in volume);
these include carbon dioxide, neon, helium, methane, hydrogen, carbon monoxide, ozone,
etc. Most of these gases remain in fairly constant proportions up to altitudes of around
80 km. However, there are gases whose concentrations vary substantially not only with
height but also over time (for some cases, even on a scale of hours). In this last group of
gases (the so-called “variable gases”) are water vapor, carbon dioxide, stratospheric and
tropospheric ozone, aerosols,* and other VOCs (which include pesticides).
These variable gases play an essential role in meteorological processes because they
drive the energy balance of the Earth system. As a matter of fact, the origin of “Climate
Change” lies mainly in the abrupt change in the mean concentrations of these gases in the
atmosphere by the direct or indirect action of man.
From a thermodynamic point of view, the atmosphere is essentially stable
†
(the rate of
global change can be measured in centuries), but locally it is unstable because of its strati-
fied nature and because of the combination of cyclic isolation, the Coriolis force, the con-
tent of water and its reiterated phase transition, volcanic eruptions, etc. This combination
of global organization and local chaos is what makes a physicochemical description of the
atmosphere so difficult.
*
Although aerosols are not gases, they have been included here because their variability plays a crucial role in
the physicochemical properties of the atmosphere and the global radiative balance.
†
The terms stable atmosphere and unstable atmosphere relate to whether vertical exchanges of matter, momen-
tum, and energy are favored or inhibited.
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