Schematic representation of the distribution of gas-phase CO (blue) and N₂H⁺ (red) in disks with either a steep vertical temperature profile, as for TW Hya (left), or a shallow vertical temperature profile (right). These differences can be due to different degrees of grain settling. To highlight the region around the CO snowline, the vertical direction depicts scale height, z/r. The dashed black contour represents the CO snow surface and the light blue area directly outside this contour shows that, at the snow surface, the gas phase abundance drops by 50%. The N₂H⁺ surface layer is indicated by dotted red lines. The predicted column density profiles are shown below. For N₂H⁺, the column density profile is shown with (dotted line) and without (solid line) the surface layer. The vertical dashed black line indicates the location of the midplane CO snowline.