Method for operating an exhaust gas purification system for an internal combustion engine

Method for operating an exhaust gas purification system for an internal combustion engine having a first catalyst unit that generates ammonia from corresponding exhaust gas components in rich exhaust gas composition, and one of the first downstream second catalytic converter unit, which temporarily stores the ammonia generated by the first catalyst unit in rich exhaust gas composition and contained in a lean exhaust gas composition in the exhaust gas subjecting nitrogen oxides reduction reaction using the intermediately stored ammonia as a reducing agent, wherein an intermediate the two catalyst units, third catalyst unit, the nitrogen oxides contained in the exhaust gas is oxidized during lean exhaust gas composition to an extent to nitrogen dioxide, the nitrogen oxides contained in the exhaust gas entering the second catalyst unit 25 to 75 vol .-% composed of nitrogen dioxide.