Schematic diagram of our two-population opacity model. Depending on the velocities of sedimenting pebbles, they can experience growth (coalescence), fragmentation, or erosion. In the growth-limited regime, the pebble size in the interior is regulated by the collision and sedimentation timescales (R_peb = R_coal). In the velocity-limited regime, fragmentation and/or erosion restrict the pebble size below the growth potential (R_peb = R_vlim < R_coal). In our model, dust grains of constant size are produced in collisions between pebbles or in high-velocity pebble-dust encounters (erosion), while they are lost by sticking encounters with slow-moving pebbles (dust sweep-up), leading to a local steady state in their abundance.