Geology Reference
In-Depth Information
has an important property of absorbing ultraviolet (UV) radiation from
the sun, which would otherwise be harmful to life on earth. The
stratopause at approximately 50 km altitude marks the boundary
between the stratosphere and the mesosphere, which extends upwards
to the mesopause at approximately 90 km altitude. The mesosphere is a
region of large temperature extremes and strong turbulent motion in the
atmosphere over large spatial scales.
1
Above the mesopause is a region characterised by a rapid rise in
temperature, known as the thermosphere.
2
In the thermosphere, the
atmospheric gases, N
2
and O
2
, are dissociated to a significant extent into
atoms so the mean molecular mass of the atmospheric species falls. The
pressure is low and thermal energies are significantly departed from
the Boltzmann equilibrium. Above 160 km gravitational separation of
the constituents becomes significant and atomic hydrogen atoms, the
lightest neutral species, moves to the top of the atmosphere. The other
characteristic of the atmosphere from mesosphere upwards is that above
60 km, ionisation is important. This region is called the ionosphere. It is
subdivided into three regimes, the D, E and F region, characterised by
the types of dominant photo ionisation.
3
With respect to atmospheric chemistry, though there is a great deal of
interesting chemistry taking place higher up in the atmosphere,
1-3
we
shall focus in the main on the chemistry of the troposphere and strato-
sphere.
2.2 SOURCES OF TRACE GASES IN THE ATMOSPHERE
As previously described, the troposphere is the lowest region of the
atmosphere extending from the earth's surface to the tropopause at 10-18
km. About 90% of the total atmospheric mass resides in the troposphere
and the greater part of the trace gas burden is found there. The tropo-
sphere is well mixed and its bulk composition is 78%N
2
,21%O
2
,1%Ar
and 0.036% CO
2
with varying amounts of water vapour depending on
temperature and altitude. The majority of the trace species found in the
atmosphere are emitted into the troposphere from the surface and are
subject to a complex series of chemical and physical transformations.
Trace species emitted directly into the atmosphere are termed to have
primary sources, e.g. trace gases such as SO
2
, NO and CO. Those trace
species formed as a product of chemical and/or physical transformation
of primary pollutants in the atmosphere, e.g. ozone, are referred to as
having secondary sources or being secondary species.
Emissions into the atmosphere are often broken down into broad
categories of anthropogenic or ''man-made sources'' and biogenic or
