Ozone Depletion

The second layer of the atmosphere is the stratosphere. This is the layer where the most ozone (O3) is found, and therefore where the ozone layer is. The ozone layer absorbs UV-B and UV-C rays from the Sun, which protects the organisms on Earth from DNA damage.  Without the ozone layer, there would be much higher rates of cancer, sunburns, and cataracts.

The ozone layer forms here due to UV rays. UV-C breaks O2 into 2O, which reacts with O2, forming O3.

Formerly heavily used as refrigerants, solvents, and spray-can propellants, chlorofluorocarbons (CFCs) were commonly released into the atmosphere.

UV light splits the molecule apart and chlorine atoms are released. The Cl reacts with ozone, breaking it apart.

As the hole in the ozone layer was noticed and its cause identified (a discovery that won a Nobel Prize), nations came together in the '80s and signed the Montreal Protocol, which required the phasing out of CFCs in favor of chemicals that do not deplete the ozone layer. They were primarily replaced by hydrofluorocarbons (HFCs) which do not damage the ozone layer, but are a strong greenhouse gas. As CFCs survive in the atmosphere for long periods of time, it has not been an immediate solution, but things are improving.

Most people have heard about there being a hole in the ozone layer. What a lot of people don't know is that not only are there multiple holes, but the "main ozone hole" is seasonal; it appears over Antarctica in Antarctic Spring (mid-September to October). Polar stratospheric clouds become widespread in the Antarctic winter, and reactions on their crystals form reactive gases of chlorine.

When spring comes, the sunlight degrades these gases and free radicals of chlorine destroy ozone.

As the seasons continue, it becomes too warm for these clouds to form, and the hole closes until the following year. The size and duration of this hole have been shrinking.