Time evolution of total CO abundance (ice and gas) as function of the assumed tunnelling barrier width (a_tunnel) and diffusion-to-binding energy ratio (f_diff). All these models have s⁻¹. The first rowof models uses T_gas = 13 K, n_gas = 10⁸ cm⁻³. The second row of models uses T_gas = 18 K and n_gas = 3 × 10¹¹ cm⁻³. The third row of models uses T_gas = 25 K and n_gas = 5 × 10¹¹ cm⁻³. The fourth row of models uses T_gas = 21 K and n_gas = 3 × 10⁶ cm⁻³. Significant reduction of the total CO abundance in less than 3 Myr is only possible if sCO + sH is efficient, which is at low barrier widths or if sOH preferably reacts with sCO, which is enhanced at low values of f_diff. The fiducial values are a_tunnel = 1 Å and f_diff = 0.3).