Total vertical column density of HCO⁺ and N₂H⁺ for the models. Top row shows models Z1, Z01, and Z001 (solid, dashed, and dotted lines, respectively). The middle row shows the difference between the Z01 model (black dashed line) and the values by reducing the vertical column density of Z1 manually by a factor of 10 (red dashed line). The red show the cases where the vertical column density is higher than the simple scaled-down values. The bottom row shows the same plots as the middle row but for the Z001 model and the Z1 model reduced by a factor of 100.