Photochemical steady state

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.