text: Photolysis occurs in the atmosphere as part of a series of reactions by which primary pollutants such as hydrocarbons and nitrogen oxides react to form secondary pollutants such as peroxyacyl nitrates. See Photochemical smog.
  The two most important photodissociation reactions in the troposphere are firstly:
  which generates an excited oxygen atom which can react with water to give the hydroxyl radical:
  The hydroxyl radical is central to atmospheric chemistry as it initiates the oxidation of hydrocarbons in the atmosphere and so acts as a detergent.
  Secondly the reaction:
  is a key reaction in the formation of tropospheric ozone.
  The formation of the ozone layer is also caused by photodissociation. Ozone in the Earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (), splitting them into individual oxygen atoms (atomic oxygen). The atomic oxygen then combines with unbroken  to create ozone, . In addition, photolysis is the process by which CFCs are broken down in the upper atmosphere to form ozone-destroying chlorine free radicals.