Line ratios as functions of outer abundance, inner abundance and gas to dust ratio (g/d_gas). One line ratio in the P branch (P(15):P(51)) (top panel), one line ratio in the R branch (R(7):R(39)) (middle panel) of the 01¹0 → 00⁰0 15 μm transition are shown together with the line ratio between the ¹³CO₂Q-branch and the neighbouring ¹²CO₂P(25) line. See the main text for more details.