Holes in the Ozone Layer.

Ozone in the stratosphere is caused by the photochemical reaction of oxygen in sunlight, and is formed in greater amounts around the tropics where sunlight is strongest.

The least amount of ozone is found around the poles when they are in darkness. In the mid-1980's it was shown that measurements were showing decreasing amounts of ozone above the Antarctic. This depletion of ozone eventually became a large hole by the early 1990's, and this same series of events has recently been observed to be occurring over the arctic. In fact ozone depletion has even occurred at lower latitudes and exposure to sunlight is not recommended in the UK without UV protecting sun cream.

The depletion of ozone comes from chemical emissions, which then react in the stratosphere breaking down ozone. The worst of these chemicals are called halocarbons and include CFC's (chlorofluorocarbons) but nitrous oxide and methane also deplete ozone.

In winter the problem is worst as CFC's build up in clouds at very low temperatures. When the polar area is again hit by sunlight, the sunlight triggers a photochemical reaction. This causes the release of free radical chlorine atoms from the CFC's and they react with ozone:

Cl· + O3 --> ClO· + O2

The ClO· then reacts with other components of the atmosphere. In the stratosphere usually an oxygen atom:

ClO· + O --> Cl· + O2

This equation shows how the chlorine molecule is not used up in the reaction so just one CFC molecule can destroy hundreds of thousands of ozone molecules.

CFC's are rarely used now but were common is products like aerosols during the 70's and 80's. Pesticides have also contained halocarbons. However once the ozone holes were discovered pressure was exerted on countries to find replacement less harmful chemicals. The Montreal Protocol was the first big treaty on the environment and aimed to half CFC use. Since 1987 and its introduction modifications have been made and reductions in CFC's have been significant.

Although this is good there are problems. CFC's have a large lifespan and can stay in the stratosphere for about 70 years; so even after a total end to CFC use for 70 years damage would still continue. The protocol was mostly accepted by MEDC's and lots of LEDC's are still using large amounts of CFC's. Pesticide use was not restricted till 2002 and bromine used in the pesticides is 40 to 100 times more efficient at destroying ozone than chlorine.

Unless some more action is taken the problem will just become worse and countries like Australia near the Antarctic will be more and more at risk. already the depletion of ozone is thought to have caused over an extra quarter of a million cases of skin cancer a year.