PDR modelling (KOMSA-τ model; Röllig et al. 2006) of clumps with masses of M = 1 and 10 M_⊙ for surface volume hydrogen densities of n(H)= 10³ to 10⁵ cm^-3. The images on the left (A) and C)) illustrate the modelling for an ensemble of clumps. The images on the right (B) and D)) show the modelling for a single clump. For the single clump we anticipated a beam filling factor of Φ_b = 0.5. The images at the top (A) and B)) show the modelled [Cii] and ¹²CO(1–0) integrated line intensities. The black and grey lines show the modelled integrated line intensities of CO(1–0) and [C ii]. The style of the line marks the anticipated clump mass. The grey areas show the rough lower and/or upper limits of the CO(1–0) and [C ii] observations. The images at the bottom (C) and D)) illustrate the modelled CO(2–1)/CO(1–0) ratio for clumps with M = 1 and 10 M_⊙. The range of the observed CO(2–1)/CO(1–0) ratio (≃0.73 ± 0.16) is marked by the grey area.