system
Given the following post, respond as if you're commenting on Reddit:
human
Why does the Ozone deplete over antarctica and not other places? Has it depleted over other places?
gpt
Ozone in the ozone layer has gone down globally, but a particular set of circumstances create the famous "ozone hole" over Antarctica every spring.

First, let me explain a little ozone chemistry. The major problem is CFCs, aka chlorofluorocarbons. These are extremely stable molecules, so they survive in the atmosphere a long time. Long enough to be transported up to the stratosphere where UV light will knock of a Cl atom. The Cl atom then reacts with ozone like so:

* Cl + O3 --> ClO + O2
* ClO + O3 --> Cl + O2 + O2

The net reaction destroys 2 ozone molecules and preserves the Cl atom, so this cycle can repeat until the Cl atom reacts to form a reservoir species that prevents it from destroying more ozone. Two such reactions are:

* Cl + CH4 --> HCl + CH3
* ClO + NO2 --> ClONO2

Now, all these reactions are happening all over the world. What is special about Antarctica is how cold it gets. Because it is at a pole, it gets cold. But because there is a continent over the pole, which prevents heat transport by ocean currents, Antarctica gets even colder than the North Pole. And when it gets so ridiculously cold, a unique phenomenon occurs: the formation of clouds in the stratosphere. These clouds are made up of frozen water, sulfuric acid, and nitric acid. But the important thing is that they provide surfaces. Chemical reactions that normally take a really long time in the gas phase can occur much more quickly when they have surfaces to react on. This allows for this reaction to happen:

* ClONO2 + HCl --> HNO3 + Cl2

Reactions like this happen all through the polar winter. When the sun eventually comes up in spring. the Cl2 is released, UV light breaks it into 2 Cl atoms, the concentration of Cl skyrockets, and then the concentration of ozone plummets, leading to the ozone hole.