Chemistry of the Stratosphere
Essay Preview: Chemistry of the Stratosphere
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here are several techniques used to investigate the chemistry of the
stratosphere. The first of these is monitoring which involves
analysing the air using spectroscopy. Given that ozone absorbs in the
infra-red and ultra-violet regions of the spectrum, the concentration
on ozone in a sample can be calculated form the strength of its
absorption (figure1). This has to be carried out at different times
and in different conditions to ensure any decrease is not due to
natural fluctuations.
Figure 1 – Ozone Distribution in the Atmosphere
Once the molecules present in the stratosphere are identified,
laboratory measurements can be carried out to investigate the
reactivity of the molecules concerned and how radiation affects them.
Special techniques such as flash photolysis have to be used to work
out how fast the reactions are occurring. This technique allowed
scientists to work out that the reactions breaking down and making
ozone are generally occurring at the same rate and consequently there
is a steady concentration of ozone. The study of meteorology involves
learning about the movements of air currents which circle around the
lines of latitude and help gas to mix within a layer. Meteorology
allows scientists to obtain a better idea of how the reactions occur
in the stratosphere as opposed to the very different conditions under
which they take place in a laboratory. Information from these
different sources is fed into a powerful computer that produces a
�model’ of what scientists think happens in the stratosphere. The more
data that becomes available the closer to reality the model becomes.
These overall simulations can be used to make predictions about future
variations in the atmosphere.
In 1972, James Lovelock, who was interested in the global spread of
gasses in the troposphere, developed a method for detecting CFCs in
the troposphere. He detected small concentrations of CFC 11 in rural
areas, far away from potential sources. He recognised that such a
stable gas would accumulate and move in the atmosphere. Sherwood
Rowland and Mario Molina found that when CFCs reached the troposphere
they are broken down by the absorption of UV light.
Years after these discoveries, a group of scientists flew to the ozone
hole and measured concentrations of ClO radicals and ozone. They found
that the correlation of ozone fell considerably as that of ClO
radicals rose therefore proving that Cl radicals must be involved in
ozone depletion (figure 2).
Figure 2 – Measurement of ozone and chlorine monoxide from a flight
over the Antarctic
Ozone is destroyed in a catalytic cycle, by reacting with radicals
present in the atmosphere. If X is a radical the two reactions
involved can be written as:
XO + O X + O2
Overall reaction: O + O3 O2 + O2
The radical X could be a HO, NO or Cl radicals. The equation shows
that ozone is being lost and the X radical involved in the reaction is
acting as a catalyst.
The formation of the ozone hole over Antarctica is a consequence of
the special atmospheric conditions which occur there. In particular,
the low stratospheric temperatures, the isolated wind patterns and the
disappearance of the sun in winter, for six months. A vortex of winds
develops around the pole and isolates the polar stratosphere. This
causes the temperature to drop, meaning clouds begin to form in the
stratosphere. These clouds are called polar stratospheric clouds
(PSCs), and they are made of a mixture of water ice crystals, crystals
of water ice mixed with nitric acid, as well as droplets of liquid
water mixed with nitric acid and sulphuric acid. These clouds are a
catalyst for the chemical reaction between ClONO2 and HCl.
Figure 3
When the sun returns in the southern spring the Cl2 molecules are
split by the UV radiation into chlorine radicals, which catalyse the
destruction on ozone.
CFC’s first emerged as refrigerants after other chemicals were found
to be less affective. Afters the first commercially successful
refrigerator was used

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Special Techniques And Disappearance Of The Sun. (April 18, 2021). Retrieved from