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We present a charge distribution based model that computes the infraredspectrum of polycyclic aromatic hydrocarbon (PAH) molecules using recentmeasurements or quantum chemical calculations of specific PAHs. The model isapplied to a sample of well-studied photodissociation regions (PDRs) withwell-determined physical conditions (the radiation field strength, $G_{0}$,electron density $n_{e}$, and the gas temperature, $T_{\rm gas}$).Specifically, we modelled the emission of five PAHs ranging in size from 18 to96 carbon atoms, over a range of physical conditions characterized by theionization parameter $\gamma = G_{0}\times T_{\rm gas}^{1/2}/n_{e}$. The anionsemerge as the dominant charge carriers in low $\gamma $ ($< 2\times 10^{2}$)environments, neutrals in the intermediate $\gamma$ ($10^{3} - 10^{4}$)environments, and cations in the high $\gamma$ ($ > 10^{5}$) environments.Furthermore, the PAH anions and cations exhibit similar spectralcharacteristics. The similarity in the cationic and anionic spectra translatesinto the interpretation of the 6.2/(11.0+11.2) band ratio, with high values ofthis ratio associated with large contributions from either cations or anions.The model's predicted values of 6.2/(11.0+11.2) and 3.3/6.2 compared well tothe observations in the PDRs NGC 7023, NGC 2023, the horsehead nebula, theOrion bar, and the diffuse ISM, demonstrating that changes in the charge statecan account for the variations in the observed PAH emission. We also reassessthe diagnostic potential of the 6.2/(11.0+11.2) vs 3.3/(11.0+11.2) ratios andshow that without any prior knowledge about $\gamma$, the 3.3/(11.0+11.2) canpredict the PAH size, but the 6.2/(11.0+11.2) cannot predict the $\gamma$ ofthe astrophysical environment.

Polycyclic Aromatic Hydrocarbon emission model in photodissociation regions I: Application to the 3.3, 6.2, and 11.2 $\mu$m bands