¹³CO and C¹⁸O J = 3−2 line fluxes of a viscously evolving disk, shown in the top and bottom panel, respectively. Presented here are models with M_* = 0.32 M_⊙, α_visc = 10⁻³−10⁻⁴ that have an initial disk mass M_disk(t = 0) = 2 × 10⁻³−2 × 10⁻² M_⊙. A full overview of the model fluxes can be found in Fig. B.1. Solid lines show fluxes for models that include CO conversion through grain-surface chemistry. For comparison, dashed lines show fluxes for models without CO conversion as in Fig. 5. The red line highlights the age range of disks in the Lupus star-forming region.