A Unified Monte Carlo Treatment of Gas-Grain Chemistry for Large
  Reaction Networks. II. A Multiphase Gas-Surface-Layered Bulk Model
  The observed gas-phase molecular inventory of hot cores is believed to be
significantly impacted by the products of chemistry in interstellar ices. In
this study, we report the construction of a full macroscopic Monte Carlo model
of both the gas-phase chemistry and the chemistry occurring in the icy mantles
of interstellar grains. Our model treats icy grain mantles in a layer-by-layer
manner, which incorporates laboratory data on ice desorption correctly. The ice
treatment includes a distinction between a reactive ice surface and an inert
bulk. The treatment also distinguishes between zeroth and first order
desorption, and includes the entrapment of volatile species in more refractory
ice mantles. We apply the model to the investigation of the chemistry in hot
cores, in which a thick ice mantle built up during the previous cold phase of
protostellar evolution undergoes surface reactions and is eventually
evaporated. For the first time, the impact of a detailed multilayer approach to
grain mantle formation on the warm-up chemistry is explored. The use of a
multilayer ice structure has a mixed impact on the abundances of organic
species formed during the warm-up phase. For example, the abundance of gaseous
HCOOCH3 is lower in the multilayer model than in previous grain models that do
not distinguish between layers (so-called "two phase" models). Other gaseous
organic species formed in the warm-up phase are affected slightly. Finally, we
find that the entrapment of volatile species in water ice can explain the
two-jump behavior of H2CO previously found in observations of protostars.
