In this work, we investigate a diffusion model of substance from a point source in an urban area. One can define the plume at edge where the concentration falls to zero. The new in this work, taking the vertical wind speed consists of sum of power and logarithmic law, in neutral, stable and unstable conditions. The proposed concentrations model is compared with the observation's concentration data. Also, one gets the concentration at the ground of the Iodine-138 (I138) which is the same as the observed concentration value and adjusted its source strength when the wind speed is the sum of logarithmic and power law than previous models. The proposed model is a well perfect model because the ground corrected concentration (0.16Bq/m3) is the same as the observed concentration (0.16Bq/m3) than the previous ground concentrations in ref. (1) (0.18Bq/m3) and in ref. (2) (0.155Bq/m3). We conclude that the proposed model is well agreement than the two previous models.