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

CO has long been thought to be the best tracer to measure gas masses as it is readily detected at (sub)mm wavelengths in many disks. Inferred gas masses from CO in recent ALMA observations of large samples of disks seem inconsistent with their inferred dust masses. The derived gas-to-dust mass ratios from CO are 1-2 orders of magnitude lower than $\sim$100 even if photodissociation and freeze-out are included. Herschel measurements of HD line emission imply gas masses in line with gas-to-dust mass ratios of 100. This suggests that at least one additional mechanism is removing gaseous CO. Here we test the suggestion that the bulk of the CO is chemically processed and that the carbon is sequestered 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 we perform a parameter exploration and follow the CO abundance evolution over a range of conditions representative of shielded disk midplanes. We find that no chemical processing of CO takes place on 1-3 Myr timescales for low cosmic-ray ionisation 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 the parts of the disk below 30 K. This order of magnitude destruction of CO is robust against the choice of grain-surface reaction rate parameters except for temperatures below 20 K, where there is a strong dependence on the H tunnelling barrier. The low temperatures needed for CO chemical processing indicate that the exact disk temperature structure is important, with warm disks around luminous Herbig stars expected to have little to no CO conversion. For cold disks around T Tauri stars, a large fraction of the emitting CO layer is affected unless the disks are young (< 1 Myr). This can lead to inferred gas masses that are up to two orders of magnitude too low.