Abundance ratios of the ammonia isotopologs as functions of time at two different densities. Solid lines correspond to the bulk ice model, while dashed lines correspond to the multilayer ice model. For reference, the shaded areas show the observed ammonia isotopolog ratios, including errors, toward LDN 134N given in Table 8 of Roueff et al. (2005). For ND₃/ NHD₂, the shaded area represents an upper limit of 0.032.