Abundance relative to H₂ as a functionof the H₂ column density derived from the submillimeter CH observations at 532 or 536 GHz or from the reddening. The different symbols refer to different sight-lines as noted at top. HCO⁺ is shown in blue, HF in green, H₂O in purple, and CCH in gray. The CCH data were taken with the IRAM-30 m telescope (Gerin et al. 2011) as well as the HCO⁺ data for W49N and W51 (Godard et al. 2010).