Defines a photometric procedure for determining the mass concentration nitrogen oxides in exhaust gases from ducts or chimneys. Applies to the determining of the mass concentration of nitrogen oxides in exhaust gases from combustion processes, processes for surface treatment of metals and from organic chemical reactions, if necessary after removal by washing, in-furnace reduction and/or catalytic denitration, before their dispersion into the atmosphere. Applies to the concentration range 5 mg/m3 to 1000 mg/m3 as NO2 for a sample gas volume of 1000 ml. Does not apply to the determination of dinitrogen monoxide (N2O).