An internal combustion engine is described in conjunction with a method for dynamically determining a mass flow rate of nitrogen oxides (NOx) for its exhaust gas feedstream. The method includes determining a present engine operating point and determining a reference NOx content for a reference engine operating point. A combustion chemical reaction rate is determined based upon the present engine operating point and the reference engine operating point. A NOx content in the exhaust gas feedstream is dynamically determined during operation of the internal combustion engine based upon the reference NOx content, the combustion chemical reaction rate and a combustion mixing rate constant.