Influence of the grain size distribution (carbonaceous grains with a 10 nm thick aromatic-rich carbon mantle and silicates with a 5 nm mantle). The yellow lines show the model with the standard size distribution of Jones et al. (2013) and the pink lines the case where the centre radius of the log-normal size distributions of both carbon and silicate grains have been divided by two. The radiation field is also varied with . See Sect. 2.2 for description of the black and white lines and symbols.