Fractional abundance as a function of depth into the cloud from PDR modeling and observations. The depth into the cloud is given as the equivalent visual extinction. The black dots are numbered according to the GMC they represent and error bars show the observed abundance towards that GMC, their position on the x-axis is arbitrary and does not represent the cloud sizes. The lines show UCL_PDR models with different parameters. The shaded band covers the abundance range of all GMC measurements. In the low cosmic-ray ionization rate models (left), the abundance decreases with distance into the cloud so the total column density is less than the observed column density despite the high abundances reached at low A_v.