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Stratospheric ozone forms when ultraviolet light splits O₂ and the freed oxygen atoms join other O₂ molecules. Chlorine and bromine atoms released from human-made compounds such as chlorofluorocarbons (CFCs) destroy ozone in catalytic cycles, so one atom can remove many ozone molecules before it is locked away. Over Antarctica each spring, reactions on polar stratospheric clouds speed this destruction and produce the "ozone hole." The 1987 Montreal Protocol phased out most ozone-depleting substances, and the ozone layer is now slowly recovering.
Write out the chlorine catalytic cycle and add the two steps to show that chlorine is not used up. Then plot a year of Antarctic ozone column data in Dobson units and link the September–October minimum to the return of sunlight after the polar night. Close with the Montreal Protocol as a case of science informing policy, and compare its timeline with observed chlorine levels in the stratosphere.
You know that the stratosphere warms with height because ozone absorbs ultraviolet light. This topic asks how that ozone is made, how human chemicals destroy it, and why the damage is worst over Antarctica.
Ozone forms in a two-step process first described by Sydney Chapman in 1930. Short-wave UV splits an O₂ molecule into two oxygen atoms. Each atom then joins another O₂ to make O₃. Ozone also absorbs UV and breaks apart, so its amount reflects a balance between making and destroying it. About 90 percent of Earth's ozone sits in the stratosphere.
In 1974, Mario Molina and F. Sherwood Rowland showed that CFCs could upset this balance. CFCs were used in refrigerators, spray cans, and foams because they are stable and non-toxic. That stability lets them survive for decades and drift into the stratosphere. There, UV breaks them and frees chlorine atoms. A chlorine atom attacks ozone (Cl + O₃ → ClO + O₂), and the ClO then meets a free oxygen atom (ClO + O → Cl + O₂). The chlorine comes back unchanged, so a single atom can destroy many thousands of ozone molecules. Bromine from halons works the same way. Molina, Rowland, and Paul Crutzen shared the 1995 Nobel Prize in Chemistry for this work.
The surprise came in 1985, when Joseph Farman and colleagues at the British Antarctic Survey reported a deep springtime ozone loss over Antarctica. The cause is local. In the polar night, a strong vortex isolates very cold air, and polar stratospheric clouds form. On their surfaces, chlorine stored in harmless forms is converted to Cl₂. When the Sun returns in September, light splits the Cl₂, and ozone is destroyed quickly. The "hole" is the area where total ozone falls below 220 Dobson units.
Less ozone means more UV-B reaches the ground, which raises the risk of skin cancer and cataracts and harms some plankton and crops.
The 1987 Montreal Protocol phased out most ozone-depleting gases. Every UN member state has ratified it. Because the gases last so long, recovery takes decades. Current assessments expect Antarctic ozone to return to 1980 levels around 2066. The protocol is often cited as a model for other global environmental agreements.