Mass loading of solid particles in the gas as a function of distance from the star (top plot) and the mass loading as a function of the Stokes number (bottom plot). Results are shown for and three different values of the pebble-to-gas flux ratio . We overplot the threshold metallicity for planetesimal formation by the streaming instability in the bottom plot, from two different sources (Yang et al. 2017; Li & Youdin 2021). An unrealistically large pebble flux of ? > 0.03 is needed to trigger the streaming instability directly for the considered values of St.