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We study the effects of grain surface reactions on the chemistry ofprotoplanetary disks where gas, ice surface layers and icy mantles of dustgrains are considered as three distinct phases. Gas phase and grain surfacechemistry is found to be mainly driven by photo-reactions and dust temperaturegradients. The icy disk interior has three distinct chemical regions: (i) theinner midplane with low FUV fluxes and warm dust ($\gtrsim 15$K) that lead tothe formation of complex organic molecules, (ii) the outer midplane with higherFUV from the ISM and cold dust where hydrogenation reactions dominate and,(iii) a molecular layer above the midplane but below the water condensationfront where photodissociation of ices affects gas phase compositions. Somecommon radicals, e.g., CN and C$_2$H, exhibit a two-layered vertical structureand are abundant near the CO photodissociation front and near the watercondensation front. The 3-phase approximation in general leads to lowervertical column densities than 2-phase models for many gas-phase molecules dueto 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 speciesoriginate near the water condensation front; photo-processes determine theircolumn densities which do not vary significantly with key disk properties suchas mass and dust/gas ratio.

A three-phase approach to grain surface chemistry in protoplanetary disks: Gas, ice surfaces and ice mantles of dust grains