Chemical family

A set of species interconverted rapidly relative to the production or removal of the set as a whole. Fast interconversion can justify a concentration ratio without establishing that the total family concentration is in steady state.

In a simple stratospheric model, hydroxyl and hydroperoxyl radicals interconvert rapidly. Their total abundance is controlled by slower sources and termination reactions. Atomic hydrogen can be included in a broader family definition when its chemistry is treated explicitly.

For $X\rightleftharpoons Y$ with forward and reverse first-order rates $a,b$, the closed-family partition relaxes at rate $a+b$. Adding losses changes the relaxation eigenvalues. If family abundance evolves much more slowly, a quasi-equilibrium ratio can be useful; setting the entire family derivative to zero needs a separate timescale comparison.