Distributions of CO (top row), N₂ (middle row), and H₂O (bottom row) ice as functions of time at three different medium densities. The green solid lines correspond to model M1. The red andblue lines correspond to model M3; the red lines represent the abundances on each grain population, while the blue line represents the sum over the individual grain populations. The green and blue lines overlap almost perfectly for H₂O ice. The effective grain radius increases with bin label as indicated in the top right panel.