CO abundance computed using the full chemical network in Bosman et al. (2018; see also Cridland et al. 2020) for the model with M_* = 0.32 M_⊙ and α_visc = 10⁻³ at 1 Myr. The chemistry was calculated using a ζ_cr = 10⁻¹⁶ s⁻¹ to show where CO can be transformed. Taken from the same model, but with the chemistry calculated within DALI, the dashed blue line shows where the CO outward column is 10¹⁶ cm⁻². The black contour shows where 5% of the total oxygen abundance is in OH(g) + O(g). The red contour shows where T_gas = 35 K. The hatched region below the three dashed lines shows where the CO abundance is recalculated using the approximate grain-surface chemistry.