Physical conditions in the NW PDR of NGC 7023. The top panels show the variation of the densities and gas temperature inside the cloud. The H I to H₂ transition occurs at an A_V ~ 0.9, while the C II–C I transition occurs deeper in the cloud at an A_V ~ 4. The bottom panels show the physical parameters relevant for PAH hydrogenation and ionization evolution. The hydrogenation of PAHs is expected to be controlled by the ratio between the UV field intensity G₀ and the density of atomic hydrogen, which in our case only varies by about 2 orders of magnitude throughout the cloud. The ionization state of the PAH population on the other hand depends on the so-called ionization parameter, , which measures the ratio between ionization and electron recombination of the PAH species.