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[Abridged] Observations of protoplanetary disks suggest that they aredepleted in gas-phase CO. It has been posed that gas-phase CO is chemicallyconsumed and converted into less volatile species through gas-grain processes.Observations of interstellar ices reveal a CO$_2$ component within H$_2$O icesuggesting co-formation. The aim of this work is to experimentally verify theinteraction of gas-phase CO with solid-state OH radicals above the sublimationtemperature of CO. Amorphous solid water (ASW) is deposited at 15 K andfollowed by vacuum-UV (VUV) irradiation to dissociate H$_2$O and create OHradicals. Gas-phase CO is simultaneously admitted and only adsorbs with a shortresidence time on the ASW. Products in the solid state are studied withinfrared spectroscopy and once released into the gas phase with massspectrometry. Results show that gas-phase CO is converted into CO$_2$, with anefficiency of 7-27%, when interacting with VUV irradiated ASW. Between 40 and90 K, CO$_2$ production is constant, above 90 K, O$_2$ production takes over.In the temperature range of 40-60 K, the CO$_2$ remains in the solid state,while at temperatures $\geq$ 70 K the formed CO$_2$ is released into the gasphase. We conclude that gas-phase CO reacts with solid-state OH radicals aboveits sublimation temperature. This gas-phase CO and solid-state OH radicalinteraction could explain the observed CO$_2$ embedded in water-rich ices. Itmay also contribute to the observed lack of gas-phase CO in planet-formingdisks, as previously suggested. Our experiments indicate a lower water icedissociation efficiency than originally adopted in model descriptions ofplanet-forming disks and molecular clouds. Incorporation of the reduced waterice dissociation and increased binding energy of CO on a water ice surfaces inthese models would allow investigation of this gas-grain interaction to itsfull extend.

The formation of CO$_2$ through consumption of gas-phase CO on vacuum-UV irradiated water ice