= 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.
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