CO depletion factor as a function of radius, r, at 1, 2, 5, and 10 Myr for the model that includes grain-surface chemistry, reactive desorption, and fractal grains (GR+RD+FR; solid lines), and the same model but also scaling the ice mass by the dust-to-gas mass ratio (GR+RD+FR+D2G; dotted lines). The empirically derived CO depletion factors from J. K. Calahan et al. (2021) are overlaid in black stars. The left-hand panel shows the ratio of gas-phase CO to H₂ for the full vertical extent of the disk, whereas the right-hand plot shows that calculated for the molecular layer only (i.e., restricting the calculation to above the CO snow surface only).