Formation of ozone
Schematic representation of the photochemical ozone formation and destruction.
Ozone is a gas found in the atmosphere consisting of three oxygen atoms: O3. Ozone is formed in the atmosphere when energetic ultraviolet (UV) radiation dissociates molecules of oxygen, O2 , into separate oxygen atoms. Free oxygen atoms can recombine to form oxygen molecules but if a free oxygen atom collides with an oxygen molecule, it joins up, forming ozone. Ozone molecules can also be decomposed by ultraviolet radiation into a free atom and an oxygen molecule. Ozone is thus continuously created and destroyed in the atmosphere by UV radiation coming from the sun. This highly energetic UV radiation is called UVC (wavelength 280 nm) and is very harmful for human health. UVC is fully absorbed in the atmosphere by oxygen and ozone molecules. Ozone also absorbs UVB radiation, which is less energetic (wavelength 280 - 325 nm) but also harmful, before it reaches the surface of the Earth. In this creation/destruction process the amounts of ozone molecules created and destroyed are roughly equal, so that the total amount of ozone in the atmosphere is nearly constant. The absolute concentration of ozone in the atmosphere is very low: out of 10 million air molecules only 3 are ozone molecules.
However, reactions of ozone with other trace gases alter the ozone budget. These reactions are catalytic reactions. A catalyst is a substance, usually present in small amounts, that facilitates chemical reactions without itself being consumed by those reactions. In the catalytic ozone loss reactions, the ozone molecule is lost while the catalyst is reformed to potentially destroy another ozone molecule. The most important catalysts for ozone loss are species containing chlorine (Cl + ClO), nitrogen (NO+NO2), bromine (Br+BrO) or hydrogen (OH+HO2). For instance, over its lifetime in the stratosphere, an individual Cl atom can destroy about 100000 ozone molecules. Many of the ozone depleting substances involved in one of those catalytic cycles have an anthropogenic origin and were transported from the surface into the stratospheric by atmospheric circulation patterns.