Average particle number per cell in the midplane as a function of time. Initially, the particle number is N_p = 70 per cell, the particle size range is τ_s = 10⁻³–10⁻¹ and the size distribution exponent is q = 3.5. As the particles sediment toward the midplane, the particle number and thus the dust-to-gas ratio increases here significantly. The particle number is around 300 per cell, even in the early stages of the simulation. This is much higher than the mean particle number per cell, since the scale height of the dust layer is much smaller than the vertical extent of the box.