Radial dependence of the dust grain size distribution, from the model ℳ₁ with M_⋆ = 0.4 M_⊙. Concentric shells of varying radii from 100 to 4000 au (as shown in the upper panels) are used to compute the mean dust grain size distributions shown in the lower panels. The dust grains size distribution is represented by the mass fraction distribution (see text for details). Here only the five first concentric shells with radius [100,200,300,400,500] au are shown in upper panel with the corresponding average dust grain size distribution weighted by the gas density (blue line) in the lower panel. The dust grain size distributions in each cell defining the shell are shown (light grey lines) in the lower panel.