  The presence of dust can strongly affect the chemical composition of the
interstellar medium. We model the chemistry in photodissociation regions (PDRs)
using both gas-phase and dust-phase chemical reactions. Our aim is to determine
the chemical compositions of the interstellar medium (gas/dust/ice) in regions
with distinct (molecular) gas densities that are exposed to radiation fields
with different intensities. We have significantly improved the Meijerink PDR
code by including 3050 new gas-phase chemical reactions and also by
implementing surface chemistry. In particular, we have included 117 chemical
reactions occurring on grain surfaces covering different processes, such as
adsorption, thermal desorption, chemical desorption, two-body reactions, photo
processes, and cosmic-ray processes on dust grains. We obtain abundances for
different gas and solid species as a function of visual extinction, depending
on the density and radiation field. We also analyse the rates of the formation
of CO2 and H2O ices in different environments. In addition, we study how
chemistry is affected by the presence/absence of ice mantles (bare dust or icy
dust) and the impact of considering different desorption probabilities. The
type of substrate (bare dust or icy dust) and the probability of desorption can
significantly alter the chemistry occurring on grain surfaces, leading to
differences of several orders of magnitude in the abundances of gas-phase
species, such as CO, H2CO, and CH3OH. The type of substrate, together with the
density and intensity of the radiation field, also determine the threshold
extinction to form ices of CO2 and H2O. We also conclude that H2CO and CH3OH
are mainly released into the gas phase of low, far-ultraviolet illuminated PDRs
through chemical desorption upon two-body surface reactions, rather than
through photodesorption.
