The ozone hole
The ozone hole is more a thinning of the ozone layer than a real hole. It occurs every spring above Antarctica since the end of the 1970s. The chemistry of the ozone hole formation at the poles is somewhat different than at higher latitudes. The different key ingredients are the polar vortex, polar stratospheric clouds, chlorine and UV radiation. We explain this shortly here.
During the long dark Antarctic winter, stratospheric winds move in a circular pattern over the polar region, isolating the air above the Antarctic land mass: the polar vortex. In the polar vortex, the stratospheric temperatures can become extremely low (around -80°C) and polar stratospheric clouds (PSCs) can be formed at altitudes between 15 and 25 km. Heterogeneous chemistry then occurs on or in condensed particles (such as liquid water droplets or solid ice particles of the PSCs) that are in contact with gaseous molecules containing clorine, and the species Cl2 and HOCl are formed. These formed chlorine species are short lived and are quickly photolyzed by sunlight (even in visible wavelengths) when the sun returns to the Antarctic in the early spring. The freed Cl and ClO species act as catalysts in chemical reaction cycles (involving the already mentioned ozone photolysis) destroying ozone in the stratosphere.
The size of the ozone hole has increased steadily from the late seventies to the beginning of the nineties, and since then stabilized to an area of about 20-25 million square kilometers, with a maximum value reached of 27 million square kilometers in 2006. The thickness of the ozone layer in the ozone hole can be as low as 100 DU while it should be between 300 and 400 DU at that time. The minimum value reached was 92 DU in 1994. During the most recent years, values around 120 DU are measured. At present, the ozone hole is monitored continuously. The figure below shows the ozone hole in black.