Dust surface densities as a function of orbital distance and grain size for the different α values used here, and for additional simulations with the lowest gas surface densities and with the highest dust-to-gas ratio that we have tried. When the turbulence is reduced, the maximum grain size increases (since less destructive collisions are expected). In addition to that, the reduced α-viscosity allows the discs to become cooler, so the opacity of the larger grains (~mm) becomes comparable or larger than that of the smaller (~μm) grains. For α = 10⁻⁴ we find thatthe grains grow up to a few centimeters in the discs with either one of the distributions. The dashed lines divide the grain sizes into two groups which contribute equally to the total opacity of the disc (see Figs. 8and 10).