  The long-timescale behavior of adsorbed carbon monoxide on the surface of
amorphous water ice is studied under dense cloud conditions by means of
off-lattice, on-the-fly, kinetic Monte Carlo simula- tions. It is found that
the CO mobility is strongly influenced by the morphology of the ice substrate.
Nanopores on the surface provide strong binding sites which can effectively
immobilize the adsorbates at low coverage. As the coverage increases, these
strong binding sites are gradually occupied leav- ing a number of admolecules
with the ability to diffuse over the surface. Binding energies, and the energy
barrier for diffusion are extracted for various coverages. Additionally, the
mobility of CO is determined from isothermal desorption experiments. Reasonable
agreement on the diffusivity of CO is found with the simulations. Analysis of
the 2152 cm$^{-1}$, polar CO band supports the computational findings that the
pores in the water ice provide the strongest binding sites and dominate
diffusion at low temperatures.
