9.1 Stratospheric Ozone Depletion

Ozone (O₃) is a molecule of three oxygen atoms. Most of Earth's ozone sits in the stratosphere, the atmospheric layer above the one you live in, concentrated roughly 15 to 35 km above the surface. This ozone layer absorbs nearly all of the most dangerous ultraviolet radiation (UV-C) and most UV-B, the type that burns skin and damages DNA. Life on land was only possible once this shield formed.

In a healthy stratosphere, ozone is constantly made and broken down in a natural balance: UV light splits O₂ into single oxygen atoms, which join other O₂ molecules to form O₃; UV light also splits O₃ back into O₂ and O. Each time ozone absorbs UV in this cycle, it keeps that radiation from reaching the ground.

Chlorofluorocarbons (CFCs) are compounds of chlorine, fluorine and carbon. They were widely used from the 1930s on as coolants in refrigerators and air conditioners, propellants in spray cans, and in making foam. They are stable and non-toxic at ground level, so they last long enough to drift up into the stratosphere over several years.

Up there, strong UV light breaks a chlorine atom off the CFC molecule. The free chlorine then destroys ozone in a cycle:

• Cl + O₃ → ClO + O₂ (chlorine steals an oxygen atom from ozone)
• ClO + O → Cl + O₂ (the chlorine is released again)
• Because the chlorine comes out unchanged, it acts as a catalyst and repeats the cycle. A single chlorine atom can destroy on the order of 100,000 ozone molecules before it is removed.
• Bromine from halons (used in fire extinguishers) and methyl bromide (a pesticide) destroys ozone the same way, even more effectively per atom.

Ozone thinning is most severe over Antarctica, where a deep seasonal drop called the ozone hole was discovered in 1985. During the dark Antarctic winter, the stratosphere gets cold enough (below about −78 °C) for polar stratospheric clouds of ice crystals to form. Chemical reactions on the surfaces of these ice crystals convert chlorine into forms that are released as active chlorine when sunlight returns in early spring (September and October). The chlorine then destroys ozone rapidly. So the natural cold and ice provide the conditions, and the human-made chlorine provides the destroyer. As the stratosphere warms and the clouds melt, the hole shrinks each year by early summer.

• Higher rates of skin cancer, including melanoma, and sunburn.
• Cataracts, a clouding of the eye's lens that can cause blindness.
• A weakened immune system.
• Damage to phytoplankton at the base of ocean food webs, and lower yields for some crops.