Ozone (03) is a molecule made up of three oxygen atoms. The oxygen we breathe (02) is similar but has only two oxygen atoms. High up in a region of the upper atmosphere known as the stratosphere, light rays can break down breathable oxygen into two individual oxygen atoms. Single oxygen atoms are quite reactive, and ozone is formed when one of this lone oxygen’s bump into and combines with 02.
But ozone is not very stable either. If a high-energy light ray, in particular ultraviolet-B (UV-B), strikes an ozone molecule, it too will break down, back to the lone oxygen and 02. Other molecules naturally found in the stratosphere, such as nitrous oxide, can also react with 03 and break it down.
Over time, as the atmosphere formed, the reactions that create and break down ozone came into a dynamic equilibrium. The result was a small residual amount of ozone concentrated in a band between 15 and 35 km high in the stratosphere.
This is the band popularly referred to as the ozone layer. But the equilibrium is delicate, and ozone is rare even in the ozone layer. For every ten million molecules of air, two million are breathable 02, and only three are yet this small amount of ozone is enough to prevent most UV-B radiation from reaching the surface of the earth.
Humans have damaged the ozone layer by adding molecules containing chlorine or bromine that lead to ozone destruction. The largest group among these is chlorofluorocarbons (CFCs). At ground level, these molecules are stable and have many uses in industrial and domestic applications. However, when they are released into the atmosphere, they drift up to the stratosphere, pushed by winds and atmospheric mixing.
At that high altitude, energetic light rays (UV-C radiation) can break down such molecules in a reaction that liberates an atom of chlorine (CI). This chlorine atom can react with ozone and break it down to chlorine oxide and 02. Chlorine oxide will break down as well, releasing the CI to go on destroying ozone. In fact, one CI can destroy up to 10,000 ozone molecules.
As a result, ozone in the stratosphere has been reduced to such an extent that a hole opens up above Antarctica each spring that has, in each of the past four years, measured 8.2 million square miles-larger than the United States and Canada combined! The problem is not limited to Antarctica, however. Stratospheric ozone is being reduced over much of the globe and research shows that this allows more dangerous UV-B to reach the surface of the earth.