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CO is the most widely used gas tracer of protoplanetary disks. Its abundanceis usually assumed to be an interstellar ratio throughout the warm molecularlayer of the disk. But recent observations of low CO gas abundance in manyprotoplanetary disks challenge our understanding of physical and chemicalevolutions in disks. Here we investigate the CO abundance structures in fourwell-studied disks and compare their structures with predictions of chemicalprocessing of CO and transport of CO ice-coated dust grains in disks. We usespatially resolved CO isotopologue line observations and detailedthermo-chemical models to derive CO abundance structures. We find that the COabundance varies with radius by an order of magnitude in these disks. We showthat although chemical processes can efficiently reduce the total column of COgas within 1 Myr under an ISM level of cosmic-ray ionization rate, thedepletion mostly occurs at the deep region of a disk. Without sufficientvertical mixing, the surface layer is not depleted enough to reproduce weak COemissions observed. The radial profiles of CO depletion in three disks arequalitatively consistent with predictions of pebble formation, settling, anddrifting in disks. But the dust evolution alone cannot fully explain the highdepletion observed in some disks. These results suggest that dust evolution mayplay a significant role in transporting volatile materials and a coupledchemical-dynamical study is necessary to understand what raw materials areavailable for planet formation at different distances from the central star.

Systematic Variations of CO Gas Abundance with Radius in Gas-rich Protoplanetary Disks