A three-phase approach to grain surface chemistry in protoplanetary
  disks: Gas, ice surfaces and ice mantles of dust grains
  We study the effects of grain surface reactions on the chemistry of
protoplanetary disks where gas, ice surface layers and icy mantles of dust
grains are considered as three distinct phases. Gas phase and grain surface
chemistry is found to be mainly driven by photo-reactions and dust temperature
gradients. The icy disk interior has three distinct chemical regions: (i) the
inner midplane with low FUV fluxes and warm dust ($\gtrsim 15$K) that lead to
the formation of complex organic molecules, (ii) the outer midplane with higher
FUV from the ISM and cold dust where hydrogenation reactions dominate and,
(iii) a molecular layer above the midplane but below the water condensation
front where photodissociation of ices affects gas phase compositions. Some
common radicals, e.g., CN and C$_2$H, exhibit a two-layered vertical structure
and are abundant near the CO photodissociation front and near the water
condensation front. The 3-phase approximation in general leads to lower
vertical column densities than 2-phase models for many gas-phase molecules due
to reduced desorption, e.g., H$_2$O, CO$_2$, HCN and HCOOH decrease by $\sim$
two orders of magnitude. Finally, we find that many observed gas phase species
originate near the water condensation front; photo-processes determine their
column densities which do not vary significantly with key disk properties such
as mass and dust/gas ratio.
