Interstellar chemical differentiation across grain sizes
  In this work we investigate the effects of ion accretion and size-dependent
dust temperatures on the abundances of both gas-phase and grain-surface
species. While past work has assumed a constant areal density for icy species,
we show that this assumption is invalid and the chemical differentiation over
grain sizes are significant. We use a gas-grain chemical code to numerically
demonstrate this in two typical interstellar conditions: dark cloud (DC) and
cold neutral medium (CNM). It is shown that, although the grain size
distribution variation (but with the total grain surface area unchanged) has
little effect on the gas-phase abundances, it can alter the abundances of some
surface species by factors up to $\sim2-4$ orders of magnitude. The areal
densities of ice species are larger on smaller grains in the DC model as the
consequence of ion accretion. However, the surface areal density evolution
tracks are more complex in the CNM model due to the combined effects of ion
accretion and dust temperature variation. The surface areal density differences
between the smallest ($\sim 0.01\mu$m) and the biggest ($\sim 0.2\mu$m) grains
can reach $\sim$1 and $\sim$5 orders of magnitude in the DC and CNM models,
respectively.
