Calculated vertical column densities (down to the midplane) of the radicals C₂ H and CN and some sulfur-bearing molecules as a function of radius r for the T Tauri (left panel) and Herbig Ae/Be (right panel) disks. HCN is also shown to allow for the visualization of the CN/HCN ratio. The ranges of column densities derived from observations are indicated by the vertical lines plotted in the outer disk (see Table 3).