Dependence between different observational and physical parameters derived from the BOND models (Vale Asari et al. 2016) produced with the Cloudy photoionization code (Ferland et al. 2017). Panel a: log([SIII]/[SII]) vs the logarithm of the ionization parameter, log(U), color-coded with the age of the ionizing star cluster. log([SIII]/[SII]) strongly correlates with the ionization parameter, with the secondary dependence on the age. Panel b: log([SIII]/[SII]) vs logarithm of the outer size of the H II region, log(R_out), color-coded by the logarithm of the hydrogen ionizing photons rate, log(Q⁰). At the fixed value of the ionizing flux, the observed log([SIII]/[SII]) depends on the size of the region. Panel c: Dependence of F_8μm/F_24μm, similar to R_PAH^*, on the log([SIII]/[SII]). The existing Cloudy models can only partially reproduce the trend seen in Fig. 5.