Carbon-to-oxygen ratios for gas-phase species as a function of evolution time steps (colours and markers) for each of the seven grain choices (the third x-axis category, 1em5, is the fiducial grain size of 0.1 μm). Left panel: situation at 1.5 AU, and the right panel at 5 AU. The y-axis scales are identical in both panels. The models for 20 and 30 AU are not plotted because at 20 AU, only CO gas remains in the gas phase, and at 30 AU, all carbon and oxygen carriers are in the ice.