Left: initial placement of the dust grains at Δt = 0 at the base of the photoevaporative flow (black), slightly below the sonic surface. The grains are colored according to their size and the color scale (scaling with ) of the gas disk represents the extent of its local radial velocity (with − 2 ≲ v_r [km s⁻¹] ≲ 30). The gas map is mostly smooth in the region of interest, that is everywhere but close to the midplane at high R. Right: snapshot of the grain positions at Δt ≃ 100 yr, all else equal. The very low spread of the lines of individual grain sizes is due to the initial setup, placing particles directly on the disk surface without a spread in their initial velocities. For comments on this, see also Sect. 3 and Appendix B.