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Stratospheric Ozone

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STRATOSPHERIC OZONE: What it is, what it does and why it's needed

What is ozone?

An unstable moelcule consisting of three oxygen atoms.

Is ozone good or bad for us?

That depends where the ozone is located. In the troposhere at ground level, it damages lung tissue and plants. It is also a greenhouse gas which contributes to global warming. Higher up, in the stratosphere, it shields the earth from harmful UV-B radiation.

How is ozone formed?

In the stratosphere it is formed and destroyed by ultraviolet light. When high energy ultraviolet rays strike molecules of ordinary oxygen (O2), they split the molecule into two single oxygen atoms, known as atomic oxygen. A freed oxygen atom then can bump into an oxygen molecule (O2), and form a molecule of ozone (O3).

   When an ozone molecule (O3) absorbs even low energy ultraviolet radiation, it splits into an ordinary oxygen molecule (O2) and a free oxygen atom (O). The free oxygen atom then may join up with an oxygen molecule to make another ozone molecule, or it may steal an oxygen atom from an ozone molecule to make two ordinary oxygen molecules.  These processes of formation and destruction are known as the Chapman reactions.

So, destruction of ozone is a natural process?

Yes, ozone is made and destroyed all the time, and as long as there is balance, the ozone layer is not in danger.

Does ozone react with anything other than ultraviolet light?

Yes. Ozone reacts very easily, donating its "extra" oxygen molecule to nitrogen, hydrogen, and chlorine found in natural compounds. These elements always have existed in the stratosphere, released from the soil, water vapor, and oceans.

   Ozone levels change periodically as part of natural cycles such as the changing seasons, sun cycles and winds. Volcanic eruptions inject materials into the stratosphere that can destroy ozone.

When people talk about destruction of the ozone layer, what do they mean?

It means that more ozone is destroyed than created. Human activity is responsible for this by emitting ozone-destroying agents, eg chlorofluorcarbons, into the atmosphere. Ultraviolet radiation breaks apart CFCs, freeing chlorine which reacts react with ozone molecules, taking one oxygen atom to form chlorine monoxide and leaving an ordinary oxygen molecule.

   When a chlorine monoxide molecule encounters a free atom of oxygen, the oxygen atom breaks up the chlorine monoxide, stealing the oxygen atom and releasing the chlorine atom back into the stratosphere to destroy more ozone. This reaction happens over and over again, allowing a single atom of chlorine to destroy many thousands of molecules of ozone.

   Ozone destruction is accelerated on the surface of polar stratospheric clouds, where active chlorine destroys ozone.  

What has been done to limit CFCs and other ozone-dstroying agents?

Faced with strong evidence that CFCs caused serious ozone damage, politicians from around the world signed a treaty known as the Montreal Protocol in 1987. This set strict limits on the production and use of CFCs. By 1990, the growing amount of scientific evidence against CFCs prompted diplomats to strengthen the requirements of the Montreal Protocol. The revised treaty called for a complete phase out of CFCs by 2000.

Has, and will, banning CFCs make a difference to the damage already done?

Chlorine atoms do not remain in the stratosphere forever. When a free chlorine atom reacts with gases such as methane (CH4), it is bound up into a molecule of hydrogen chloride (HCL), which can be carried from the stratosphere into the troposphere, where it can be washed away by rain. If we stop putting CFCs and other ozone-destroying chemicals into the stratosphere, the ozone layer may in time repair itself.

(With thanks to NASA for ozone chemistry facts)

Research into stratospheric ozone is carried out by NASA and the European Space Agency (ESA). In this photograph, a balloon carrying instruments for such research is being launched from the ESA base in Kiruna, Sweden