Chemical pathways to create OH and H₂O molecules without mixing (left) and with α_mix = 10⁻³ (right) at one selected point in the disc at r = 10 au located in the upper disc layers (see text). The green numbers represent the particle concentrations and the blue numbers represent the rates [cm⁻³ s⁻¹] in the time-independent case. The big red ‘IN’ and ‘OUT’ arrows on the right denote the in-mixing and out-mixing rates for H₂. Wiggled arrows indicate photo-dissociation. Light green arrows on the left mark the H₂ formation on grains, and the radiative association reactions O + H → OH + hv and OH + H → H₂O + hv. denotes electronically excited molecular hydrogen. The figure does not follow up the production of CH₂⁺, CO⁺, and HCO⁺ marked in blue. The notation a(−b) means a × 10^−b.