Photochemical reactions often dominate over collision-based equilibrium chemistry at atmospheric pressures less than 1 mbar, corresponding to about 0.1% of the Earth’s surface pressure and heights above 60 km for Earth-like rocky planets. These reactions occur when photons, usually at ultraviolet wavelengths, dissociate or ionize molecules leading to subsequent reactions that produce abundances of important species that are very different from equilibrium values in the lower atmosphere.