Influence of the carbon abundance (for carbonaceous grains with a 10 nm thick aromatic mantle and silicates with a 5 nm aromatic mantle). The solid yellow lines show the model with the standard carbon abundance of Jones et al. (2013), the solid pink lines with an extra 60 ppm in both the small and big carbonaceous grains, and the solid turquoise lines with an extra 120 ppm in the big carbonaceous grains only. The solid green lines show the results for the case, where only 12 ppm of carbon are included in the big carbonaceous grains (see Sect. 4.3 for details). The radiation field is varied with . The dashed green lines present the results for the same carbon abundance as the solid green lines but for . See Sect. 2.2 for description of the black and white lines and symbols.