The combustion of fossil fuels, e.g. in engines of vehicles or in large-scale combustion plants, produces nitrogen monoxide (NO) and nitrogen dioxide (NO2), gases that are significant for air quality. In the atmosphere, NO and NO2 undergo chemical reactions that depend on meteorological conditions and on the trace gas composition of the ambient air. This standard describes an extended reaction system which accounts for the chemistry of nitrogen oxides with sufficient accuracy, in order to allow a quantitative immission forecast as part of a chemistry and transport model. It is suitable for evaluating procedures for reducing nitrogen oxide pollution with the aim of complying with statutory provisions.