Top panel: range of variations in the kernel spectra as a function of the number of PAH species considered in the mixture: 10 species (light grey) through 90 species (dark grey). In red is the range for 100 species and in black the range for 548 species. Bottom panel: evolution of the norm N_r which captures the variations in the kernel spectra (blue line) and the norm N_obs which captures the variations in the observations (see text for details).