Fractional abundance of HCO, H¹³CO⁺, C₂ H, c-C₃ H₂, H¹³ CO⁺, and HC¹⁵N relative to H₂ as a function of N(H₂). The orange indicates HII regions, while green and black symbols are IR bright and dark non-HII regions. Most upper left panel: the brown squares indicate nine cold dark clouds from Agúndez et al. (2015). The purple and blue star symbols are HCO abundances from a PDR and cold gas region, which are resolved by the observation, in the Horsehead nebula (Gerin et al. 2009). Top left panel: the gray dashed line represents a simple abundance jump model.