CO and NO concentrations in chemical equilibrium as a function of gas temperature (with GGChem, Woitke et al. 2018) for high-CO₂ gas mixtures. The different cases considered are: 0% (solid lines), 0.9% (dashed), 5% (dash dotted), and 10% (dotted) of H₂. As H₂ is binding free oxygen into H₂O, increasing the H₂ concentration prevents the recombination of CO to CO₂ as well as the production of NO. CO (red) and NO (blue shaded area) measurements with uncertainties from wet 120 min experiments. Minimum freeze-out temperature estimates for CO (orange) and NO (green) with uncertainties.