Scatter plots of the integrated C¹⁸O(2–1), [CI] and [CII] emission versus the molecular column density and extinction (in mag) derived from the Herschel far-infrared data. The color-coding separates between the different regions as marked in each panel. The integration regimes are the same as those mentioned in the previous corresponding figures (for G48.66 only the main spectral component and for IRDC 18454 only the 100 km s^-1 component are used). The dashed line in the middle panel shows the fit to the modeled [CI] data from Glover et al. (2014) who fitted their data in their approximately linear regime below 10 mag extinction. For IRDC 18454, we only use the 100 km s^-1 component that is clearly associated with the W43 complex.