Cumulative emission lines profiles of some chosen tracers as a function of the visual extinction A_V, for two models drawn from the SFGX grid. Both models are computed with a density at the illuminated edge of 100 cm⁻², an ionization parameter at the illuminated edge U = 10⁻², an instantaneous stellar burst of 3 Myr computed with BPASS stellar atmospheres (Eldridge et al. 2017), and no X-ray source. The top row shows a solar metallicity model, while the bottom row shows amodel with Z = 1/10 Z_⊙ and Z_dust = 1/155 Z_dust,⊙, following the prescription from Sect. 3.1.4. The shaded areas mark the location of the PDR (light gray) and molecular zone (dark gray). The vertical dashed lines show the cuts considered for each model. We note that the C/CO transition is not visible in the first panel since it occurs after A_V = 10.