Steady-state solutions for the solid-to-gas mixing ratio Z as a function of the Stokes number of solid particles τ_s for different values of the solid-to-dust-mass flux ratio ξ (as labeled), assuming a value of the turbulent viscosity α = 10^-3. The values of τ_s corresponding to expected pebble sizes are highlighted with larger symbols. The two solutions provided by Eq. (13)are indicated by filled and open symbols, respectively. The gray area highlights the region in which planetesimals should form by a streaming instability (Carrera et al. 2015).