Temporal evolution of the NH⁺ (left) and NH (right) abundances using gas and surface chemistry and a typical active non-thermal desorption efficiency a_RRK = 0.01 using four different sets of conditions (see Table 3). Typical diffuse cloud conditions in green, translucent cloud conditions in black, dense cloud conditions in red, and a special model for the Sgr B2 (M) molecular envelope in blue. The solid and dashed lines represent T_K = 30 K and 50 K (translucent model) and T_K = 30 K and 10 K (dense model), respectively. The observed upper limits from both species (Table 2) in dense gas (from W31C), diffuse gas (similar limits for both sight-lines), and in the Sgr B2 (M) molecular envelope are indicated by dashed horizontal lines following the respective colour code.