Radial and vertical diffusion coefficients with respect to Stokes number for runs SI41-4-2-d and SI30-4-2-d. Here, D_x is approximately constant for small Stokes numbers and increases with increasing particle size as τ_s ≥ 10^−2.5 in the case of run SI41-4-2-d. The vertical diffusion coefficient increases with particle size until τ_s ≈ 10^−1.5. Above this value it decreases as the particles are less coupled to the gas. Both the radial and vertical diffusion coefficients are higher in the case of run SI30-4-2-d, since this model is more abundant in solids with τ_s ≳ 10⁻² which are responsible for triggering the streaming instability. The diffusion coefficients corresponding to species that saturate to scale heights similar to the maximum allowed scale height in the box are marked with a cross.