KEY DEFINITIONSKEY DEFINITIONS

Ozone: an molecule with 3 oxygen atoms

Troposphere: the lowest layer of the Earth’s atmosphere, extending about 7km above the poles and 20km above the tropics.

Stratosphere: the second layer of the Earth’s atmosphere “containing” the ozone layer. It’s about 10-50km above the Earth’s surface

Dobsons: The unit in which ozone concentration is measured.

What is the ozone layer?What is the ozone layer?

The ozone layer is found between the stratosphere and the troposphere. The ozone layer filters out and converts UV light into heat energy- this makes it a higher temperature than other parts of the upper atmosphere.

Keeping track of the Ozone Keeping track of the Ozone layer…layer…

Dobson set up a worldwide network of ozone monitoring stations which still operate today.

Ozone concentration is measured in units called Dobson's (in his honor).

What’s good and bad about What’s good and bad about the ozone layer?the ozone layer?

Bad: Ozone near the Earth’s surface in the troposphere is an air pollutant with harmful effects on animals and their respiratory systems.

Good: ozone in the stratosphere protects living organisms by preventing harmful UV from reaching the Earth’s surface, by converting it into heat.

UV radiation continually breaks down and creates the ozone layer as shown.

There are three types of UV radiation:

1. UV-a (320-400nm); it causes little damage with only 5% absorbed by the ozone

2. UV-b (280-320nm); causes sunburn, genetic damage and skin cancer over prolonged exposure. 95% absorbed by the ozone

3. UV-c (200-280nm); 100% absorption by the ozone

O₂ + (radiation < 240nm) 2O

High energy UV radiation breaks the oxygen into two oxygen atoms

O₂ + O O₃ + heat

The oxygen atom reacts with the oxygen to form the ozone

The heat is absorbed by the air molecules and raises the temperature of the stratosphere

The ozone is mainly formed on the upper reaches of the stratosphere

O₃ + (radiation < 310nm) O₂ + O

Ozone molecules absorb UV radiation (240-310nm)

This is, chemically, the reverse of the formation of the ozone layer

O₂ + O O₃ + heat

The cycle continues as the oxygen atom immediately reacts with O₂

Chemical energy released when oxygen and the oxygen atom combine is converted into kinetic energy of molecular motion (aka heat)

Overall, penetrating UV radiation is converted into heat without any net loss of the ozone

O₂ + O ↔ O₃

The cycle keeps the ozone in balance

The ozone is broken down and formed at the same steady rate

O₃ + O 2O₂

Luckily this reaction is very slow because the concentration of oxygen atoms is low

However, this balance can be affected by human activity

Ozone DepletionOzone Depletion

Chlorine radicals in the stratosphere mainly come from CFCs, which can only be broken down by the extremely energetic UV radiation found above most of the ozone layer

UV radiation strikes a CFC molecule, producing a chlorine radical Cl•

E.g. CFCl3 Cl• + • CFCl2

The breakdown of ozone takes place in 2 propagation steps

Step 1: Cl• + O3 ClO• + O2

Step 2: ClO• + O Cl• + O2

The propagation steps repeat in a cycle

Overall: O3 + O 2O2

A single CFC molecule can destroy 100,000 ozone molecules

Another radical that destroys ozone is nitrogen oxide (NO) from lightning or aircraft engines

Step 1: • NO + O3 • NO2 + O2

Step 2: • NO2 + O • NO + O2

Overall: O3 + O 2O2

Ozone levels over the northern hemisphere have been dropping by 4% per decade

Around the north and south poles, much larger and seasonal declines have been seen

These are the ozone holes

Regulation Regulation

Montreal Protocol

One of the most successful global envirnomental agreements to be signed.

Nations around the world realised that CFC emissions deplete the O-zone layer and they can damage both human health and the environment

The protocol contains restrictions which limit the use of products with CFC (unless there is no other alternative). At first 30 countries signed to this protocol and now (by 2006) 197 nations have signed the protocol.

The restrictions are:

CFCs - zero production by 2000

Tetra chloromethane (used in solvents) - zero production by 2000

Production of halons (used in fire extinguishers) - zero production by 2000 – if no alternative is found

1,1,1 – trichloroethane (used in solvents) - zero production by 2005

HCFCs and HFCs – To replace CFCs in about 15% of applications

However the problem with HCFCs and HFCs are that they are now thought to also contribute to global warming. Now the protocol wants to call out the use of HCFCs by 2030. There is also the risk that HCFCs are 10 000 times more potent than carbon dioxide. Although there may be enough time to find an alternative to CFCs.

On the other hand there is no restriction to the use of HFCs.

On the mendOn the mend

Satellites confirm that the stratospheric ozone depletion may

be slowing down thanks to the more recent regulations. CFCs have a very long atmospheric

lifespan, so the final recovery of the ozone layer will take a couple

of lifetimes to complete.

The Fridge Legacy The Fridge Legacy

Since fridges help control and prevent germs and disease when CFCs were first introduced they were good for fridges because they were not toxic or flammable.

CFC fridges are still being used worldwide because to begin with CFCs were used as aerosol propellants as well as fridges. If they had been used as aerosol propellants alone CFCs would have been banned straight away. However because they were used in fridges and saved lives that way, their use was continued.

Since 2002 the EU states that they will remove CFCs and HCFCs from fridges before they are scrapped. As it is up to 3 million domestic fridges are deposed each year in the UK and half of them are then replaced with new ones.