Comparison of the ¹²CO₂ (green) and ¹³CO₂ (purple) Q branch shapes. Top: a zoom-in of the ¹²CO₂ Q branch with the best-fit slab model plotted in the green shaded region. A model with the derived ¹³CO₂ parameters is plotted in purple (see text). Middle: a zoom-in of the ¹³CO₂ Q branch on the same vertical scale as the top panel, where the emission from ¹²CO₂ and H₂O has been subtracted. The best-fit slab model is plotted in the purple shaded region. Bottom: a further zoomin of the middle panel where the best-fit model (180 K; purple shaded region) and slab models of 300 K (blue line) and 500 K (red line) are shown. The latter two models have their emitting radius scaled to produce the same peak flux. In green, a slab model with the derived ¹²CO₂ parameters (see text) is shown.