The production and the destruction of ozone in the upper atmosphere is a permanent and well-balanced phenomenon. This is the O3 - O2 cycle (ozone - oxygen cycle).
The particularity of this cycle lies in the absorption of two ultraviolet photons (UVB, UVC).
A first photon (λ < 240 nm) is absorbed to dissociate an oxygen molecule (O2) into two atoms of oxygen (O •).
O2 + ℎν → 2 O•
A second photon (λ> 240 nm) is absorbed by an ozone molecule (O3).
O3 + ℎν → O2 + O•
Given the harmfulness of UV radiation for animal and plant cells, we understand why the ozone layer is considered to be a protective "shield". The discovery of a hole in the ozone layer above the South Pole in the 1960s therefore caused great concern within the international community. Since its signing in 1985, the Montreal Protocol has imposed a total ban on certain ozone-depleting compounds, including CFCs (chlorofluorocarbons). This animation illustrates that a single chlorine atom can degrade two ozone molecules. In reality, hundreds of thousands of ozone molecules are potentially destroyed by a single chlorine atom.