Vertical column density profiles of the molecular discs in NGC 7331. The red and black thin solid lines are the observed vertical profiles for two different halves of the molecular disc. The thick blue dashed lines and the thick red solid lines represent the simulated vertical column density profiles for the single component and the two-component molecular discs, respectively. Different panels show the vertical profiles at different radii, as quoted in the top left corners of the respective panels. As can be seen, there is no detectable difference in the vertical profiles of the single-component and two-component molecular discs. See the text for more details.