¹³CO (top) and C¹⁸O (bottom) J = −2 line fluxes versus disk mass. Solid lines show models that include CO conversion through grain-surface chemistry. For comparison, dashed lines show the models without grain-surface chemistry. For the observations, shown in orange, we use M_gas ≃ 100 × M_dust as a proxy for the disk mass (see, e.g., Ansdell et al. 2016, 2018; Yen et al. 2018). Colors show models with different viscous alpha. Models with age between 1 and 3 Myr, the approximate age of Lupus, are highlighted in red. We note that in our models M_disk decreases with time, meaning that time runs right to left in this figure for our models. Observations for which we only have an upper limit on the ¹³ CO or C¹⁸ O line flux are shown in gray. Stacked non-detections are shown in black (cf. Sect. 3.3).