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The gas-phase CO abundance (relative to hydrogen) in protoplanetary disksdecreases by up to 2 orders of magnitude from its ISM value ${\sim}10^{-4}$,even after accounting for freeze-out and photo-dissociation. Previous studieshave shown that while local chemical processing of CO and the sequestration ofCO ice on solids in the midplane can both contribute, neither of theseprocesses appears capable of consistently reaching the observed depletionfactors on the relevant timescale of $1{-}3\mathrm{~Myr}$. In this study, wemodel these processes simultaneously by including a compact chemical network(centered on carbon and oxygen) to 2D ($r+z$) simulations of the outer($r>20\mathrm{~au}$) disk regions that include turbulent diffusion, pebbleformation, and pebble dynamics. In general, we find that the CO/H$_2$ abundanceis a complex function of time and location. Focusing on CO in the warmmolecular layer, we find that only the most complete model (with chemistry andpebble evolution included) can reach depletion factors consistent withobservations. In the absence of pressure traps, highly-efficient planetesimalformation, or high cosmic ray ionization rates, this model also predicts aresurgence of CO vapor interior to the CO snowline. We show the impact ofphysical and chemical processes on the elemental (C/O) and (C/H) ratios (in thegas and ice phases), discuss the use of CO as a disk mass tracer, and, finally,connect our predicted pebble ice compositions to those of pristineplanetesimals as found in the Cold Classical Kuiper Belt and debris disks.

CO Depletion in Protoplanetary Disks: A Unified Picture Combining Physical Sequestration and Chemical Processing