PDFs derived from Herschel column density maps (SED fit using only the Herschel wavelengths 160, 250, 350, and 500 μm) at 36′′ angular resolution. Left panel shows the PDFs of pixels comprising only the IRDC i.e. G18.82-0.28 (Cloud A), G28.53-0.25 (Cloud D), and G11.11-0.12. Right panel shows PDFs from associated GMC, including the IRDC. The left y-axis gives the normalised probability p(η), the right y-axis the number of pixels per log bin. The upper x-axis is the visual extinction and the lower x-axis the logarithm of the normalised column density. The dashed vertical line indicates the completeness level. The red line indicates a power-law fit to the high column density tail with the slope s together with its error and the reduced X² goodness-of-fit. The exponent α of an equivalent spherical density distribution ρ(r) ∝ r^{− α} is also indicated in the panel. The dashed line in the upper panel indicates the completeness level; the PDF left of this line is incomplete.