Gas surface density (top) and solid surface density (bottom) of the main chemical species that contribute to the C/O ratio at 0.1, 1, and 5 Myr. We ran this simulation with M_∗ = 1 M_⊙ and α = 10⁻³. The total (dust or gas) surface density is represented as a dark solid line in each row. Vertical dashed lines indicate the sublimation lines of the displayed species in their respective colour, using T_sub,CO = 20 K, T_sub,CH4 = 30 K, T_sub,CO2 = 70 K, T_sub,C2H2 = 70 K, T_sub,H2O = 150 K, and T_{sub,refractory} = 350 K (see Table A.1).