Chemistry of the Stratosphere

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.

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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.

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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).

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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:

[IMAGE][IMAGE][IMAGE]X + O3 XO + O2

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.

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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