OH, a molecule with an unpaired electron consisting of one hydrogen atom and one oxygen atom. This radical readily reacts with methane and other hydrocarbons in the atmosphere and can begin the process by which tropospheric oxygen is incorporated in a mechanism for the oxidation of NO to NO₂. Photolysis of NO₂ yields atomic O and quickly ozone at tropospheric pressures. In urban environments this is a major source of ozone:

OH + CH₃CH₃ ----> CH₃CH₂^. + H₂O

O₂ + CH₃CH₂^. ----> CH₃CH₂O₂^.

NO + CH₃CH₂O₂^. ----> CH₃CH₂O^. + NO₂

NO₂ + hv ----> NO + O

O₂ + O + M ----> O₃ + M

The source of the hydroxyl radical in the atmosphere occurs primarily through the 1) photolysis of hydrogen peroxide, heat, and light and 2) the attack on water of an excited oxygen radical (created by the photolysis of ozone).