Abstract. The process of formation of a large scale spot-like structure of the cumulative deposition pattern due to a powerful elevated finite lifetime source is studied using numerical solutions of a model diffusion problem. A possible cause of spottiness at regional scale, namely the diurnal cycle of the vertical turbulent mixing intensity in the atmospheric boundary layer, is investigated. It is shown that the atmospheric boundary layer diurnal cycle can cause the appearance of the spot-like structure when the source lifetime, t * , is much shorter than the diurnal period, T .