Evolution of the N₂D⁺/N₂H⁺ ratio when the grain size is multiplied by 3 (bottom row) or divided by 2 (top row) for the slow (right) and fast (left) models. The dashed lines trace the reference results (from Fig. 5). The dotted line traces the new versus reference model ratio for the last epoch (corresponding to a peak density of 2  ×  10⁶ cm^-3) in each frame.