Left: ¹³CO-derived mass surface density, Σ_13CO, as a function of the Herschel-derived mass surface density, Σ_FIR, for all clouds in our sample (different colours). The dotted line corresponds to the one-to-one line. Right: normalised CO depletion factor, ${f^{\prime }}_{D,\text{CO}}$, as a function of the Herschel-derived mass surface density, Σ_FIR, for all clouds in our sample (different colours). The dotted horizontal line corresponds to ${f^{\prime }}_{D,\text{CO}}=1$. The black points represent the average uncertainties in the quantities.