Predictions of the Meudon PDR code for the reference model. From top to bottom: physical conditions, volume emissivities of the H₂ S(1) and [S I] 25.249 µm lines, and chemical abundances relative to hydrogen nuclei as a function of the visual extinction from the HII region (${\text{A}}_V^{\text{IF}}$). The volume emissivity is defined as emi = n × f_u × E_u × A_ij(4 × π), where n is the gas density, ƒ_u is the fraction of particles in the upper level, and E_u and A_ij are the upper level energy and the Einstein spontaneous emission coefficient. The transition of S⁺/S takes place at a visual extinction, $A_V^{\text{IF}}\sim 3.0$ mag, where the gas temperature drops below 100 K and densities are ~10⁷ cm⁻³. The local volume emissivity of the [S I] 25.249 µm line remains high deeper into the molecular cloud, until $A_V^{\text{IF}}\sim 5$ mag. We indicate $A_V^{\text{IF}}=1,2$, and 5 mag with dashed vertical lines.