  We study the impact of dust evolution in a protoplanetary disk around a T
Tauri star on the disk chemical composition. For the first time we utilize a
comprehensive model of dust evolution which includes growth, fragmentation and
sedimentation. Specific attention is paid to the influence of grain evolution
on the penetration of the UV field in the disk. A chemical model that includes
a comprehensive set of gas phase and grain surface chemical reactions is used
to simulate the chemical structure of the disk. The main effect of the grain
evolution on the disk chemical composition comes from sedimentation, and, to a
lesser degree, from the reduction of the total grain surface area. The net
effect of grain growth is suppressed by the fragmentation process which
maintains a population of small grains, dominating the total grain surface
area. We consider three models of dust properties. In model GS both growth and
sedimentation are taken into account. In models A5 and A4 all grains are
assumed to have the same size (10(-5) cm and 10(-4) cm, respectively) with
constant gas-to-dust mass ratio of 100. Like in previous studies, the
"three-layer" pattern (midplane, molecular layer, hot atmosphere) in the disk
chemical structure is preserved in all models, but shifted closer to the
midplane in models with increased grain size (GS and A4). Unlike other similar
studies, we find that in models GS and A4 column densities of most gas-phase
species are enhanced by 1-3 orders of magnitude relative to those in a model
with pristine dust (A5), while column densities of their surface counterparts
are decreased. We show that column densities of certain species, like C2H,
HC(2n+1)N (n=0-3), H2O and some other molecules, as well as the C2H2/HCN
abundance ratio which are accessible with Herschel and ALMA can be used as
observational tracers of early stages of the grain evolution process in
protoplanetary disks.
