2D representations of the results obtained including isotope-selective effects for a representative model (M_d = 10^-2M_⊙, T Tauri star, f_large = 10^-2). The CO isotopologue abundances normalized to the total gas density are presented. The black solid line indicates the layer where the dust temperature is equal to 20 K. For lower T_dust values, CO freeze-out may become important.