N(H₂) and N(X)/N(H₂) ratio integrated over specific velocity intervals from top to bottom toward W3(OH), G29.96−0.02, G19.61−0.23, and G32.80+0.19. Different species are labeled with different symbols and colors as shown in the legend displayed in the right-hand corner of the top panel. The gray areas indicate the velocity ranges of the emission lines. In the upper panels, the dotted horizontal lines correspond to A_v of 1 and 5, respectively. In the lower panels, the black horizontal arrows indicate the velocity ranges in which CO emission features have been detected from archival data at its given sensitivity. All the other sources are displayed in Figs. E.1 and E.2.