Ozone Depletion

High in the earth's stratosphere, chemical processes maintain a balanced concentration of ozone. This protects the earth by absorbing much of the harmful ultraviolet radiation from the sun.

If a gas can stay in the atmosphere long enough to reach the stratosphere, and if the gas carries bromine or chlorine atoms, the ozone balance may be threatened as free bromine and chlorine can accelerate the breakdown of ozone. This is especially true when bromine and chlorine are on the surface of tiny ice crystals and sunlight catalyses the reaction.

The ice crystals may form at the Arctic and Antarctic poles during their winters, when sunlight does not reach them. When sunlight first reaches them in the spring, ozone holes may be created for about a month before the crystals melt and the ozone is regenerated.

An ozone depletion indicator is derived through several properties of a gas. These include its stability to reach the stratosphere and the amount of bromine or chlorine the gas carries. These properties are then compared to CFC-11, a once common refrigerant*. The properties of each gas are then compared to the properties of CFC-11 and converted into CFC-11 equivalents. Then the individual equivalents are added together for the overall ozone depletion indicator score, which represents the total quantity of ozone depleting gases released.

*Although CFC-11 is now banned by the Montreal Protocol in industrialized nations, it is still manufactured in many developing economies.