The Effect of an Inert Solid Reservoir on Molecular Abundances in Dense
  Interstellar Clouds
  The question, what is the role of freeze-out of chemical species in
determining the molecular abundances in the interstellar gas is a matter of
debate. We investigate a theoretical case of a dense interstellar molecular
cloud core by time-dependent modelling of chemical kinetics, where grain
surface reactions deliberately are not included. That means, the gas-phase and
solid-phase abundances are influenced by gas reactions, accretion on grains,
and desorption, only. We compare the results to a reference model, where no
accretion occurs and only gas-phase reactions are included. We can trace that
the purely physical processes of molecule accretion and desorption have major
chemical consequences on the gas-phase chemistry. The main effect of
introduction of the gas-grain interaction is long-term molecule abundance
changes that come nowhere near an equilibrium in during the typical lifetime of
a prestellar core.
