The Object-Oriented Paradigm (OOP) provides methodologies how to build flexible and reusable software. The OOP methodology of patterns and pattern languages was applied to construct the object oriented version of the large scale air pollution model known as the Danish Eulerian Model (DEM). The obtained framework is amenable to resolve new computational tasks (e.g. parallel local refinement simulations over Europe), and the design and analysis of new (for the framework) numerical methods. In the paper will be described the general design of the object-oriented DEM, the design of the different layers, and the documentation organization. It will be also discussed the advantages the embedding a computer algebra system in the framework.