This paper deals with the important issue of the gaseous pollutants dispersion in the environmental flow within urban atmospheres including the products of on-ground agriculture fires. The paper presents an accurate and efficient semi-implicit pressure-based algorithm developed for solving numerically the conservation equations governing weakly compressible turbulent environmental flow of multicomponent fluid. An in-house CFD code has been developed to implement the numerical solution of the present algorithm. The code is an integrated one consisting of three main elements which are the pre-processor, the solver and the post-processor. This in-house CFD code undergoes a standard validation process, using six generic test cases simulating a steady-state environmental flow associated with gas flow from a ground point source under different flow conditions. A prerequisite verification of the code has been considered for successful validation. The detailed standard validation process was performed according to AIAA Guide by comparing the predicted results with measurements especially conducted for the present work in the test section of the Atmospheric Boundary Layer Wind Tunnel facility of Assiut University. The results showed good qualitative and quantitative agreements for the present in-house CFD code with the corresponding flow measurements. These agreements together with the satisfaction of the prerequisite verification insure a satisfactory validation of the present in-house CFD code.