The Lagrangian approach by Lamb (1982) is used for particle dispersion. Sufficient enough particles to determine the mean concentration of the passive material from a point source are released in a quasi-stationary CBL developed by LES under weak winds. Then, we track each of particles through the CBL using the LES-calculated Eulerian velocity field for Lagrangian velocities. Simulations are performed for varying nondimensional wind speed in weak wind (e.g., U/W*=0, 0.2, 0.5, 1.0, 1.5, 2.0, where U is the mean wind speed and W* is the convective velocity scale) and nondimensional source height. The mean concentration fields and the statistics of the concentration fluctuations from LES are analyzed as a function of nondimensional upwind and downwind distance (X=x/h), crosswind distance (Y=y/h), and height above the surface (Z=z/h), where h is the CBL height. The results are compared with those from the theory (Arya, 1995) and the Gaussian plume model.