Sketch of the physical processes included in our model. Ice-covered particles (blue) grow by the deposition of vapour (red) in a region where the saturation ratio is unity. Ice particles crossing into a region where the saturation ratio is lower than unity sublimate and, if they stay in this region long enough, eventually leave behind their silicate cores (grey) in addition to the sublimated vapour. Bare dust grains cannot acquire a new ice mantle since the saturation ratio does not reach the critical saturation ratio required for heterogenous nucleation. Dynamical processes are represented by grey arrows, with the left-directed arrows representing the size-dependent radial drift, and the arrows pointing to the right representingthe outwards-directed part of the turbulent diffusion. Collisions are not included in this model.