= 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.
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