Particle sizes at t = 1000 yr when starting from micrometre-sized and millimetre-sized icy particles. The micrometre-sized bare silicate grains are shown in black, and ice-covered particles in blue. Panel a: results from the full temperature-dependent nucleation model, in which thecritical saturation ratio for heterogeneous nucleation is higher than for vapour deposition. Although a large amount of dust is present, the suppression of heterogeneous nucleation allows for growth only of the ice-covered particles, which quickly reach centimetre sizes. Panel b: results from the simple model, in which we ignore that a higher critical saturation ratio is needed for heterogenous nucleation than for vapour deposition. For micrometre-sized particles, growth by vapour deposition dominates, whereas for millimetre-sized pebbles the dominating process is sublimation. Thus, the available vapour is shared between the large amount of dust grains present, and larger particles do not grow.