Close to the sources, the plume spreads over a domain which is smaller than the mesh sizes used in the Eulerien transport model. Without any special care, excessive artificial diffusion is produced by the transport model. In the case of reactive pollutants, the non-linearity of the chemical reactions can bring mistakes in calculation which propagate through the domain and which may be difficult to estimate. So fine representation of these emission zones is necessary in the framework of the modelling of reactive pollutant at regional scale. There are two ways to deal with such emission zones which can be point or line sources.
Local refinement is a numerical powerful tool.
Example of the traffic contribution to the pollution over Paris area will be shown. Some of the calculation have been done using a parallel computer. Problems, advantages and difficulties encountered in the parallel calculations using an Eulerien transport model will be discussed. Another way is to use a physical modelling of the plume spread inside the mesh until the spatial section of the plume is greater the mesh size.