OPR of NH₃ shown as a function of kinetic temperature for a density of n_H = 1 × 10³ cm^-3: (i) at thermal equilibrium (black solid line); (ii) with model a (green) (details in Sect. 4.2); and (iii) with model b (red) (corresponding to model a plus the NH₂ + H reaction, details in Sect. 4.3). The OPR is shown at four different times for both models: 1 × 10⁴ yr (solid lines), 5 × 10⁴ yr (dashed lines), 10⁵ yr (dotted lines), and steady-state (dashed-dotted lines). The hatched box represents the OPR of NH₃, including formal errors, in the translucent gas in the sight-lines towards W31C and W49N from (Persson et al. 2012).