Agreed method for determining one or more air quality characteristics where it is not practical to produce a reference material; the result obtained is defined as the measure of the air quality characteristic. For example, a gas such as $\ce{NO2}$ may be somewhat unstable when stored in a tank at low mixing ratios, so that the use of a more stable $\ce{NO}$ standard mixture in $\ce{N2}$ may be oxidized to $\ce{NO2}$ by $\ce{O3}$ in a specified manner referred to as the $\ce{NO2}$ reference procedure.