The ozone hole, discovered over Antarctica (Farman et al., 1985), is a phenomenon of severe ozone depletion at the altitude between 15-20 km. It occurs in late winter / early spring in presence of polar stratospheric clouds (PSCs), sunlight, strong westerly winds and when the air is isolated from other stratospheric regions.
Although located in the stratosphere, it is strongly connected to the troposphere:
The tropospheric emissions of chlorine and bromine containing gases, most notably chlorofluorocarbons (CFCs), led to the formation of the ozone hole. They were intensively used, during the previous century, as refrigerants, propellants (aerosol applications) and solvents. Inert and volatile, the long lifetime of these molecules allow them to be transported to the stratosphere. Then, the UV sunlight initiates a photochemically conversion in halogen gases.
Ecosystems and life in the troposphere are threatened as a lower ozone amount in the upper layers leads to a higher UV radiation at the surface. This also initiates tropospheric photo-oxidation and generation of tropospheric O3
In September 1987, the Montreal protocol was signed by 24 countries and the European Economic Community. Its implementation has allowed to regulate and limit the production and consumption of the CFCs and related substances. In recent years (2017), there is evidence that the ozone layer is recovering: i.e. ozone hole has started closing. However, it will take another 20–30 years to fully recover.