Stokes number (solid lines) and turbulent diffusion coefficient (dotted lines) for pebble growth limited by fragmentation and bouncing, shown for three different stages of protoplanetary disc evolution. The ratio of the scale height of the pebbles to the scale height of the gas is H_p ∕H_g = 0.1 in the top plot and H_p∕H_g = 0.2 in the bottom plot. The outer regions of the protoplanetary disc have pebble sizes set by fragmentation, yielding a relatively constant Stokes number and diffusion coefficient. The bouncing-limited growth closer to the star gives very low values of the Stokes number and turbulent diffusion coefficient. Overall, the approximately millimeter-sized pebbles obtained in our model agrees with the pebble sizes inferred for the disc around the young star HL Tau (Carrasco-González et al. 2019).