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I thought there was a hole in the ozone layer and thus too little ozone

The hole in the ozone layer is the annual re-occurring decrease of the thickness of the ozone layer above the South Pole in October-November (the Antarctic spring). As already postulated in the answer on the first question, an ozone layer exists in the stratosphere, at great height. This 'good' ozone protects us against the harmful solar UV radiation. Without this ozone, life would be impossible as a matter of fact. The 'bad' ozone in the lower atmosphere that we can thus inhale, can have detrimental effects (see here). The 'good' ozone in the ozone layer is chemically identical to the 'bad' ozone at ground level, yet the chemical reactions leading to formation of ozone in the ozone layer are different from those in the lower atmosphere.

Certain chemical compounds, such as CFC's (chlorofluorocarbons), bromine compounds, ... can disturb the reactions involved in ozone formation in the ozone layer, which will lead to a decreased concentration of ozone in the ozone layer (or in other words the ozone layer will grow thinner). Because of geographical and climatological reasons, this decrease is most spectacular above the South Pole.

Ozone-depleting substances were frequently used in the chemical industry, for example as coolant or as disinfectant in agriculture, and are very stable chemical compounds, allowing them to remain in the atmosphere for a long time. It can take up to 50 years before they reach the stratosphere and are eventually degraded; Most of these ozone-depleting compounds are forbidden by now, but because of their long lifespan it can still take many years before the ozone layer will regain its original thickness.

The ozone data published on this website are data about the lower atmosphere ('bad' ozone). Data on the stratospheric ('good') ozone cn be found on the KMI website.