Atmospheric photochemistry is the study of light initiated chemical reactions which occur in the atmospheres of planetary bodies. These reactions play an important role in climate, and likely contributed to prebiotic organic synthesis.
Planetary atmospheric photochemistry occurs when incident radiation contains enough energy to dissociate bonds within the atmospheric molecules. Bond energies in atmospheric molecules are on the order of 102–103 kJ mol−1, and thus bond dissociation requires photons in the ultraviolet and visible region of the electromagnetic spectrum. A photochemical reaction occurs when the absorption of light triggers an electronic transition within the molecule, activating the reaction. Products of photo dissociation are typically highly reactive radicals.