First column: correlation between the observed I_3.3/I_PAH0 and I_3.4/I_PAH0 as a function of G₀. Second column: same plots for model maps of I_3.3/I_PAH0 and I_3.4/I_PAH0 computed using Eqs. (6) and (8) and the obtained values for I_PAH⁺/I_PAH⁰ (see text for details). Third column: semi-empirical model that in addition includes the dependency with G₀ on the intensity at 3.3 and 3.4 μm relative to all PAH⁰ bands (see text). In all panels, the linear fit to the data points (red line) is shown as well as its slope m.