Children in Punta Arenas, Chile, the world’s most southern city, look forward to spring as much as anyone who lives through a frigid, dark winter. But some years, the children are instructed not to go outside because the ozone hole has moved north and the UV radiation is too high.
Ozone Makes Life on Earth Possible
Ozone is a molecule composed of three oxygen atoms, (O3). Ozone in the upper atmosphere absorbs high-energy ultraviolet (UV) radiation coming from the Sun. This protects living things on Earth’s surface from the Sun’s most harmful rays. Without ozone for protection, only the simplest life forms would be able to live on Earth. The highest concentration of ozone is in the ozone layer in the lower stratosphere.
Ozone Keeps Earth’s Temperature Moderate
Along with the oceans, the atmosphere keeps Earth’s temperatures within an acceptable range. Without an atmosphere, Earth’s temperatures 162 would be frigid at night and scorching during the day. Greenhouse gases trap heat in the atmosphere. Important greenhouse gases include carbon dioxide, methane, water vapor, and ozone.
At this point you might be asking yourself, “Is ozone bad or is ozone good?” There is no simple answer to that question: It depends on where the ozone is located (see Figure below).
- In the troposphere, ozone is a pollutant.
- In the ozone layer in the stratosphere, ozone screens out high-energy ultraviolet radiation and makes Earth habitable.
How Ozone is Destroyed
Human-made chemicals are breaking ozone molecules in the ozone layer. Chlorofluorocarbons (CFCs) are the most common, but there are 163 others, including halons, methyl bromide, carbon tetrachloride, and methyl chloroform. CFCs were once widely used because they are cheap, nontoxic, nonflammable, and non-reactive. They were used as spray-can propellants, refrigerants, and in many other products. Once they are released into the air, CFCs float up to the stratosphere.
Air currents move them toward the poles. In the winter, they freeze onto nitric acid molecules in polar stratospheric clouds (PSC) (see Figure below). In the spring, the sun’s warmth starts the air moving, and ultraviolet light breaks the CFCs apart. The chlorine atom floats away and attaches to one of the oxygen atoms on an ozone molecule. The chlorine pulls the oxygen atom away, leaving behind an O2 molecule, which provides no UV protection. The chlorine then releases the oxygen atom and moves on to destroy another ozone molecule. One CFC molecule can destroy as many as 100,000 ozone molecules
The Ozone Hole
Ozone destruction creates the ozone hole where the layer is dangerously thin (see Figure below). As air circulates over Antarctica in the spring, the ozone hole expands northward over the southern continents, including Australia, New Zealand, southern South America, and southern Africa.
UV levels may rise as much as 20% beneath the ozone hole. The hole was first measured in 1981 when it was 2 million square km (900,000 square miles). The 2006 hole was the largest ever observed at 28 million square km (11.4 million square miles). The size of the ozone hole each year depends on many factors, including whether conditions are right for the formation of PSCs.