OPR of NH₂ computed as a function of kinetic temperature for a density of n_H = 2 × 10⁴ 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 at four different times for both models: 1 × 10⁴ yr (solid lines), 5 × 10⁵ yr (dashed lines), 10⁶ yr (dotted lines), and steady-state (dashed-dotted lines). The hatched boxes represent our best estimates of the average OPR, including formal error bars. In blue we show the W31C, W51, and G34.3 molecular envelopes, and in pink the filament connected to W51 at V_LSR ~ 68 km s^-1 and the W49N molecular envelope.