Evolution of the o/p-NH₃, o/p-NH₂D, and o/p-NHD₂ ratios and the fractionation ratios NH₂D/NH₃, NHD₂/NH₂D, and ND₃/NHD₂ according to the PH (a) and FS (b) models visualised in Fig. 5. The fractional abundances used for the ratios are calculated in the same way as those shown in Fig. 5. The horizontal straight dashed lines and semi-transparent thick lines represent the abundance ratios derived towards (0,0) from APEX observations (Sect. 3). Observational data for NH₃ and ND₃ are not available at the same resolution.