Fractional abundances (with respect to n_H) of selected gas-phase and ice species (labeled in the plots) as functions of time in a homogeneous model with T_gas = T_dust = 10 K. For those species that have spin states, the plotted abundances represent sums over the spin states. Solid lines correspond to the bulk ice model, while dashed lines correspond to the multilayer ice model. In the latter case, the ice abundances (right-hand panels) represent sums over the surface and mantle. Upper row: n_H = 10⁵ cm^-3. Lower row: n_H = 10⁶ cm^-3.