Free Radicals and Their Adventures in the Stratosphere

When I say free radical, it may conjure up images of hippies and free spirits doing what they want without a care in the world:

peace symbol
Peace symbol (Photo credit: Wikipedia)

But in reality, free radicals are far more dangerous, not only to ourselves but to our planet.

Here on Earth, we are protected from up to 99% of the harmful UV rays that come from the sun by the ozone layer. Ozone (which is in the form of O3 instead of O2) is dangerous to breathe in when in the lower atmosphere and on the surface of the planet, such as when it is found in smog, but when in the stratosphere it forms a protective, warming layer. Without the ozone layer, we would be exposed to larger amounts of ultraviolet radiation which could result in more cases of sun related diseases such as skin cancer.

Free radicals are defined as being an atom or a group of atoms that contain an unpaired electron. Because of the unpaired electron, the atom is highly unstable and will therefore react with other molecules in order to make itself stable. Sadly for the ozone layer, it can become a target of free radicals such as chlorine (Cl•), which are formed from molecules known as chlorofluorocarbons.

Chlorofluorocarbons have been in use since the 1950’s in normal household products such as aerosols, air conditioning units and refrigerators. As the name suggests, they are composed of a carbon atom in the centre, with chlorine and fluorine atoms surrounding it. They were seen as an ideal component due to their low flammability and the fact that they are non-toxic to humans, they are, however, not water soluble and so could stay in the atmosphere for long periods of time. Eventually, ultraviolet radiation reacts with a CFC, breaking off a chlorine free radical. These chlorine free radicals (Cl•) are then free to wreak havoc on ozone molecules, hence why they are sometimes referred to as ozone depleters:

1) Cl•(g) + O3(g) –> ClO(g) + O2(g)

Chlorine Free Radical + Ozone –> Chlorine Oxide and Oxygen

2) O(g) + ClO(g) –> O2(g) + Cl•(g)

Oxygen Atom + Chlorine Oxide –> Oxygen and Chlorine Free Radical

As you can see from the two equations above, chlorine free radicals don’t disappear when they first react with ozone and are able to break apart thousands of ozone molecules. This ozone depletion is most apparent over Antarctica due to the cold temperatures allowing a faster rate of chlorine free radical production from CFC’s:

English: Ozone Hole
Antarctica Ozone Depletion
Source: Wikipedia

But what is the future for CFC’s and the ozone?

The Montreal Protocol on Substances That Deplete the Ozone Layer, which was first introduced in 1987, is one of the most successful international environmental agreements that has ever been introduced by the UN, with around 196 countries signing up to it. It’s concerned with the reduction of CFC’s in the atmosphere by phasing out their use in manufactured products, which is good news for the ozone layer, as eventually it will repair itself. CFCs are replaced with hydrofluorocarbons (HFCs) which aren’t reactive with ozone molecules as they lack chlorine, which again is good news. The downside however, is that HFCs are very potent greenhouse gases, which as we know is a growing problem, there is however a call for The Montreal Protocol to be expanded to include HFCs as less harmful alternatives can be investigated.


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