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CO has long been thought to be the best tracer to measure gas masses as it isreadily detected at (sub)mm wavelengths in many disks. Inferred gas masses fromCO in recent ALMA observations of large samples of disks seem inconsistent withtheir inferred dust masses. The derived gas-to-dust mass ratios from CO are 1-2orders of magnitude lower than $\sim$100 even if photodissociation andfreeze-out are included. Herschel measurements of HD line emission imply gasmasses in line with gas-to-dust mass ratios of 100. This suggests that at leastone additional mechanism is removing gaseous CO. Here we test the suggestionthat the bulk of the CO is chemically processed and that the carbon issequestered into less volatile species such as CO2, CH3OH and CH4 in the dense,shielded midplane regions of the disk. Using our gas-grain chemical code weperform a parameter exploration and follow the CO abundance evolution over arange of conditions representative of shielded disk midplanes. We find that nochemical processing of CO takes place on 1-3 Myr timescales for low cosmic-rayionisation rates, $< 5\times 10^{-18}$ s$^{-1}$. Assuming an ionisation rate of$10^{-17}$ s$^{-1}$, more than 90% of the CO is destroyed, but only in theparts of the disk below 30 K. This order of magnitude destruction of CO isrobust against the choice of grain-surface reaction rate parameters except fortemperatures below 20 K, where there is a strong dependence on the H tunnellingbarrier. The low temperatures needed for CO chemical processing indicate thatthe exact disk temperature structure is important, with warm disks aroundluminous Herbig stars expected to have little to no CO conversion. For colddisks around T Tauri stars, a large fraction of the emitting CO layer isaffected unless the disks are young (< 1 Myr). This can lead to inferred gasmasses that are up to two orders of magnitude too low.

CO destruction in protoplanetary disk midplanes: inside versus outside the CO snow surface