page_title: Photoinitiator
section_title: Nitrogen dioxide
hierarchical_section_title: Photoinitiator / Atmospheric photoinitiators / Nitrogen dioxide
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: Nitrogen dioxide can also be photolytically cleaved by photons of wavelength less than 400 nm producing atomic oxygen and nitric oxide.
  NO₂ → NO + O
  Atomic oxygen is a highly reactive species, and can abstract a H atom from anything, including water.
  O + H₂O → 2 ·OH
  Nitrogen dioxide can be regenerated through a reaction between certain peroxy-containing radicals and NO.
  ROO· + NO → NO₂ + RO·
caption: Nitrogen dioxide, a large contributor to the production of smog
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