Fraction of ionized species (PAHs and carbon clusters) along the Star-NW cut in the NW PDR as derived from the observations (yellow, from Pilleri et al. 2012) and as computed with our model (other colours). The total AIB flux is also plotted in black. The grey surface shows the uncertainty range on the ionized fraction for species (N = 0−19), which reflects the range of values we considered for the electron recombination rate. The PDR front is assumed to lie at 42″ from the star.