Contribution to the midplane mean Rosseland opacity per grain size for the simulation with the BODon the top and the MRN distribution on the bottom, for the nominal disc parameters α = 5 × 10⁻³, Σ_g,0 = 1000 g cm⁻² and f_DG = 1%. The black line indicates the grain sizes that contribute the most to the opacity of the disc as a function of orbital distance. The BOD distribution causes a jump in these dominant grain sizes because of the dip in the dust surface density (see Fig. 3) in the transition between the two turbulence regimes. The total opacity at each radius is the sum of the contributionfrom each grain size, or in other words, it is the sum of the corresponding column. For each grain size, the maximum opacities can be found at around 6–6.5 AU, where the iceline is located. The red lines show the percentage of the contribution from the grains below the corresponding line. This clearly illustrates that the dominant grain size does not necessarily determine the whole opacity. At the same time, we can see that the smallest grain sizes (up to roughly 50 μm beyond 20 AU) contribute the least to the total opacity.