The ozone layer
Earth's ozone layer lies in the stratosphere, and extends from about 20 - 40 kilometers above the surface of the Earth. Ozone is a molecule consisting of three bound oxygen atoms, and is often called O3 or trioxygen.
Ultraviolet (UV) light has a shorter wavelength than visible light. The energy of light is inversely proportional to its wavelength, so UV light has higher energy than visible light.
Short term exposure to UV light (10 - 15 minutes) can be beneficial and is sometimes used to treat skin afflictions like eczema, psoriasis and lupus.
We divide the UV portion of the light spectrum into three types, UVa, UVb and UVc. UVa has the longest wavelength, and is sometimes called UV-aging, because it is the cause for long-term damage to skin, like wrinkling, sun spots and premature aging.
UVb light is sometimes referred to as UV-burning rays. It is responsible for sunburns and can also cause skin cancer.
UVc has the shortest wavelength of UV light, and is the most energetic and the most dangerous.
UV light is absorbed by the ozone in the atmosphere. In particular, UVc is absorbed very efficiently by ozone. In effect, the ozone layer protects us from harmful UV rays.
Some common products like aerosol spray, foam insulation, styrofoam and dry cleaning products contain chemicals called chlorofluorocarbons (CFC).
CFC's can rise through the atmosphere via convection and enter the ozone layer, where they interact with ozone molecules.
This simulation illustrates how CFC molecules interact with ozone molecules. UV radiation from the Sun splits off a chlorine atom. The chlorine atom then binds to one of the oxygen atoms in O3 and splits the O3 molecule into O2 and a molecule with one chlorine atom bound to an oxygen atom. That molecule goes on to interact with another O3 molecule, producing O2 and a freed chlorine atom. This chlorine atom can go on to produce the chemical reaction again and again, destroying as many as 100,000 O3 molecules in the process.
Other substances that can produce chemical reactions that similarly destroy O3 are methane (CH4) and nitrous oxide (N2O).
These satellite images show a marked increase in the size of the ozone hole above the south pole. The good news is that ban on CFC emissions in the Montreal Protocol agreement seems to have had a positive effect on the ozone hole. NASA scientists report that it varies in size, but seems to be generally shrinking. It is expected to be smaller than 8 million square miles (the size shown in 2000) by the year 2040.
How does the ozone layer protect Earth? What can we do to affect it? Please watch this recap of Earth's Ozone Layer and Us.