Why has there been an ozone hole over Antarctic？

There are natural reasons and anthropogenic reasons for this question.

Natural reasons:

There is a long period of low temperatures ( below 78° C ) over Antarctic in the spring, which causes a vital process. It is the polar air sinks by the cold that forms a strong westward circulation called the "polar vortex". Its major role is to isolate the Antarctic air from the rest of the atmosphere, making the atmosphere inside the vortex a giant reactor. Low temperature conditions can also help ice cloud or liquid aerosols to absorb atmospheric pollutants. Thus the sunlight is able to activate those pollutants especially chlorine and bromine, which increases ozone consumption (the chemical process refers to last post).

There are other three theoretical explanations for the formation mechanism of ozone hole: (1) the changes in polar meridional circulation cause the ozone transported to the Antarctic to decrease and form an ozone hole; (2) the heterogeneous chemical reaction under the polar ice crystal effect causes ozone reduction; (3) the combined effect of the dynamic meteorological factors and photochemical reactions related to solar radiation causes the formation of the ozone hole.

Anthropogenic reasons:

Synthetic chlorine and bromine-containing substances are responsible for the Antarctic ozone hole, most notably CFCs (CFCs) and bromine compounds (Halon). Molecules of CFC and Halon released by humans are heavier than air molecules, but they are chemically inert in the troposphere. Even the most active atmospheric component, the oxidation of free radicals to CFC and Halon, is negligible. Therefore, they are very stable in the troposphere which can not be removed by normal atmospheric chemical reactions. Over the course of a year or two, these compounds are distributed evenly across the troposphere in the world and mainly brought into the stratosphere by the atmospheric circulation over the tropics, which in turn transports them from low latitudes to high latitudes, with layer being evenly mixed. In the stratosphere, intense UV radiation causes dissociation of CFC and Halon molecules, releasing highly reactive atomic states of chlorine and bromine which are also free radicals. Chlorine radicals and bromine radicals are the main substances destroying the ozone layer which is catalyzed, so do Bromine radicals. It has been estimated that one chlorine atom radical will destroy 104-105 ozone molecules while the bromine atom radical released by Halon has 30-60 times as much destruction of ozone as chlorine atoms. In addition, there is a synergistic effect between chlorine atom radicals and bromine radical radicals. In other words, when both of them are present at the same time, the ability to destroy ozone is greater than the simple summation of the two.