Calculated OPR of NH₂ 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, Sect. 4.2); and (iii) with model b (red, corresponding to model a plus the NH₂ + H reaction, Sect. 4.3). The OPR is plotted for 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 our best estimates of the average OPR, including formal errors, for the translucent gas towards W31C at V_LSR ~ 22,28 and 40 km s^-1. The dotted horizontal lines with arrows mark the lower limits in the translucent gas towards W31C at V_LSR ~ 17 km s^-1 (blue) and W49N at V_LSR ~ 39 km s^-1 (pink).