CO underabundance vs. R_{90, gas}/R_{90, dust}. Middle panel: models with R_c = 20 AU. Bottom panel: models with R_c = 50 AU. The observed range of R_{90, gas}/R_{90, dust} from Ansdell et al. (2018) is shown in grey. Top panel: histogram of the gas-to-dust ratios measured in Lupus (Ansdell et al. 2016; Miotello et al. 2017). These have been converted into an effective CO underabundance using δ_{C,O, eff} = Δ_gd/100, where Δ_gd is the gas-to-dust mass ratio.