page_title: Photoinitiator
section_title: Peroxides
hierarchical_section_title: Photoinitiator / Atmospheric photoinitiators / Peroxides
context_page_description: A photoinitiator is a molecule that creates reactive species when exposed to radiation. Synthetic photoinitiators are key components in photopolymers.
  Some small molecules in the atmosphere can also act as photoinitiators by decomposing to give free radicals. For instance, nitrogen dioxide is produced in large quantities by gasoline-burning internal combustion engines. NO₂ in the troposphere gives smog its brown coloration and catalyzes production of toxic ground-level ozone. Molecular oxygen also serves as a photoinitiator in the stratosphere, breaking down into atomic oxygen and combining with O₂ in order to form the ozone in the ozone layer.
context_section_description: Since molecular oxygen can abstract H atoms from certain radicals, the HOO· radical is easily created. This particular radical can further abstract H atoms, creating H₂O₂, or hydrogen peroxide; peroxides can further cleave photolytically into two hydroxyl radicals. More commonly, HOO can react with free oxygen atoms to yield a hydroxy radical (·OH) and oxygen gas. In both cases, the ·OH radicals formed can serve to oxidize organic compounds in the atmosphere.
  H₂O₂ → 2 ·OH
  HOO· + O → O₂ + ·OH
  ·OH + CH₄ → ·CH₃ + H₂O
caption: Hydrogen peroxide, the simplest peroxide
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