Cartoon of the effects of dust transport in disks for different types of disks. Grain growth, settling, and vertical mixing drain volatiles from the outer disk atmosphere and lock them in grains in the midplane, leading to changes in C/O depending on the relative location of the snow line and the pressure bump. The C₂H-emitting region is indicated, where purple indicates weak emission and magenta strong emission. Left: warm disk with low C/O (low C₂H). It could be either a compact dust disk without pressure bumps (and hence with efficient radial drift) or a transition disk with a dust ring (pressure bump) inside the CO snow line. In both cases the millimeter dust pebbles are not covered with CO ice. Right: cold disk with high C/O (high C₂H). It could be either a ring disk or a transition disk with a dust ring (pressure bump) outside the CO snow line. As such, there is a large CO-ice dust reservoir, which depletes the surface layers of volatiles.