Constant density gas-phase PDR models with G₀ = 100 and n_H = 5 × 10³ cm⁻³ (left) and 5 × 10⁴ cm⁻³ (right). These models adopt E_b = 1200 K for Reaction (8). Upper panels in (a) and (b): dashed curves show the depth-dependent column density ratios of selected species (left y-axis). The blue continuous curves in the upper panels of (a) and (b) show the HCN/HNC column density ratio adopting E_b = 200 K. Green continuous curves show the temperature structure as a function of A_V (right y-axis). Lower panels in (a) and (b): abundance profiles with respect to H nuclei. (c) and (d): Contribution (in percent) of the main formation and destruction reactions for HCN (continuous curves) and HNC (dashed curves).