 Atmospheric photochemistry

Atmospheric photochemistry is the network of chemical reactions initiated when absorbed photons dissociate or ionize atmospheric species. The initial fragments can react with other molecules, so a small abundance of short-lived products can control the chemistry of much more abundant gases. The reaction rates depend on the stellar spectrum, absorption and the amount of shielding above a layer.

The Chapman ozone equilibrium illustrates coupling between photodissociation, molecular formation and destruction. In an exoplanet atmosphere, such reactions can change spectral absorbers or supply material for atmospheric haze. A photochemical steady state balances production and loss while reactions continue; it need not be thermochemical equilibrium, because stellar radiation continuously drives the system.