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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.

Let brackets denote number densities and impose a local photochemical steady state. The atomic-oxygen and ozone balances areSubtracting gives $J_1[{\mathrm{O}}_2]=K_4[\mathrm{O}][{\mathrm{O}}_3]$. If photodissociation is the dominant direct ozone loss, $J_3[{\mathrm{O}}_3]\gg K_4[\mathrm{O}][{\mathrm{O}}_3]$, the second balance becomes $K_2[\mathrm{O}][{\mathrm{O}}_2][\mathrm{M}]\simeq J_3[{\mathrm{O}}_3]$. Eliminating atomic oxygen yields the Chapman ozone equilibriumHigh in the atmosphere ultraviolet photons make $J_1$ large but the third-body density is small; low down, $[\mathrm{M}]$ is large but O2-dissociating ultraviolet radiation has been absorbed. Their product peaks at intermediate altitude, producing an ozone layer.

A photochemical steady state has approximately constant local abundances because production and loss balance for each species. If a molecule is produced at a rate $P$ per unit volume and removed with a first-order rate coefficient $k$, then $dn/dt=P-kn$ and the stationary number density is $n=P/k$. The relaxation time is $1/k$ when the coefficients stay fixed.

This state is useful for short-lived intermediates in atmospheric photochemistry. It does not mean that molecular reactions stop or that thermodynamic equilibrium has been reached. Transport adds another term to the abundance balance; if it is important, the local chemical production and loss rates need not balance each other by themselves.