Gas surface density of C₂H₂ (top) and gas-phase C/O ratio (bottom) at 1 Myr for various Schmidt numbers for a disc around a solar-mass star with α = 10⁻³. We ran an additional model with a lower initial abundance of CO₂ and Sc = 0.7 to show that it allows one to obtain C/O ratios greater than unity outside the water iceline despite a higher Schmidt number. In that model, the mass fraction removed from CO₂ is given to CO, meaning that more oxygen becomes available for H₂O (see Table A.1). Inside the water iceline, the lowered abundance of CO₂ is compensated by the increased abundance of H₂O such that the C/O ratio resembles the model with ${\chi }_{{\text{CO}}_2}=10\text{\%}$ and Sc = 0.7. The sublimation lines of the main species contributing to the C/O ratio are indicated with vertical lines.