One of the challenges in the automotive industry is to develop new vehicles and new technologies with minimal costs. In this context, modeling becomes an important tool for the design of future technologies by reducing the number of tests needed to develop a new exhaust system. With the emergence of future European standards [1], which are more restrictive on NOx and takes account of the differentiation between NO and NO2 emissions, European manufacturers have to describe precisely the formation and the behavior of NO2 in the after-treatment systems. The aim of this study is to improve the one dimensional after-treatment models developed by Renault by introducing the NO2 contribution from the engine to the tail pipe. The first part of this study focuses on the adaptation of after-treatment systems models in order to differentiate NO and NO2. Thus different global kinetics models for the Lean NOx-Trap System were studied. Different runs were carried out either at Renault or at the Laboratoire de Reactivite de Surface for understand the phenomena involved in NO2 formation and consumption. Then, the calibration of the model has been performed. Part of this paper focuses on the choice of an automated optimization method for this calibration step. By the end, the final model was validated versus engine bench tests performed on full size monoliths. The second part of the study describes more precisely the integration of the sulfur poisoning effect in the Lean NOx-Trap Model including the integration of NO2 in the model.