Modelled ratio between singly deuterated methanol (CH₂DOH) and non-deuterated methanol (CH₃OH) for the 1D-4 model for four different time steps ranging from 10⁵ yr (top left) to 10⁶ yr (bottom right). Additionally, we show two variations of this model: one with four layers in the chemically active surface phase instead of one (1D-4-1), and one with a two-phase model (1D-4-2; see also Table 2 and Sect. 3.2 for a more detailed explanation). The coloured lines show the column density ratio of the models, while the black line indicates the observed ratio (errors as grey-shaded areas).