HCO⁺ (left column) and N₂H⁺ (right column) radial column density profiles for models with different dust-size distributions: small grains (SG, blue), medium grains (MG, red), and large grains (LG, black). The dashed lines are for models where only the gas component is considered in the X-ray RT, where the solid lines show models where both X-ray gas and dust opacities are included. The ISM cosmic-ray models are shown in the top row and the low cosmic-ray models are shown in the bottom row. The grey shaded area marks a difference of a factor three in N with respect to the reference model (LG gas+dust).