Ratio of total PAH collisional cross-section σ_PAH to the total dust collisional cross-section σ_dust for the standard Herbig disc model STD with ISM PAH abundance. As the surface area of the dust is dominated by the smallest grains, which are closely coupled to the gas, the ratio is almost constant σ_PAH/σ_dust ≈ 310 resp. log(σ_PAH/σ_dust) ≈ 2.5 at relevant coagulation altitudes (z < 3H). Only in the outer disc does this ratio change; however, the relevant coagulation height decreases as well (see Fig. 8). The gas pressure scale heights of z =1 H, 3 H, 5 H are indicated by dashed lines.