In the framework of land use planning, massive releases have to be modelled considering they generate important toxic effect distances. The common way to deal with such scenarios is currently based on Gaussian models or, more recently, CFD codes based on a RANS turbulence model. While the first does not enable taking account of the environment with a high level of precision, the second are not suitable for accounting for Atmospheric Boundary Layers (ABLs) anisotropy. Using LES CFD approaches then appears nowadays as promising to overcome those difficulties. However it is necessary to perform the best input parametrization among a large variety of method (recycling method, synthetic method, forcing method) to generate appropriate inflow boundary conditions. The objective of this paper is to present results obtained with the open source LES CFD code FDS by using synthetic eddy method. Parameterizations to generate inlet conditions were tested and compared to large scale INERIS ammonia releases. Comparison with the RANS CFD model Code_Saturne and the shallow layer SLAB model are also presented and discussed.