  CO is the most widely used gas tracer of protoplanetary disks. Its abundance
is usually assumed to be an interstellar ratio throughout the warm molecular
layer of the disk. But recent observations of low CO gas abundance in many
protoplanetary disks challenge our understanding of physical and chemical
evolutions in disks. Here we investigate the CO abundance structures in four
well-studied disks and compare their structures with predictions of chemical
processing of CO and transport of CO ice-coated dust grains in disks. We use
spatially resolved CO isotopologue line observations and detailed
thermo-chemical models to derive CO abundance structures. We find that the CO
abundance varies with radius by an order of magnitude in these disks. We show
that although chemical processes can efficiently reduce the total column of CO
gas within 1 Myr under an ISM level of cosmic-ray ionization rate, the
depletion mostly occurs at the deep region of a disk. Without sufficient
vertical mixing, the surface layer is not depleted enough to reproduce weak CO
emissions observed. The radial profiles of CO depletion in three disks are
qualitatively consistent with predictions of pebble formation, settling, and
drifting in disks. But the dust evolution alone cannot fully explain the high
depletion observed in some disks. These results suggest that dust evolution may
play a significant role in transporting volatile materials and a coupled
chemical-dynamical study is necessary to understand what raw materials are
available for planet formation at different distances from the central star.
