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 NO₂ may be somewhat unstable when stored in a tank at low mixing ratios, so that the use of a more stable NO standard mixture in N₂ may be oxidized to NO₂ by O₃ in a specified manner referred to as the NO₂ reference procedure.