Acetylene (C₂H₂) and its isotope emission over 12.8–14.8 μm. Black: observations. Colored curves: models. All models assume a terrestrial ¹²C/¹³C ratio. Top: LTE models (i.e,. using the T_kin profile) for various C₂H₂ profiles. Light blue: reference profile from Lavvas et al. (2021); red: vertically uniform C₂H₂ with 4 × 10⁻⁶ mixing ratio; green: slope profile from Fig. 8. Residuals for this latter model are shown. Bottom: Non-LTE models (adjusted T_vib) using the C₂H₂ profile from Lavvas et al. (2021). Green curve: nominal solution, using the dashed-dotted light blue T_vib(z) profile in Fig. 3. Observation minus model residuals are shown for this model, as well as for models using T_vib(z) + 1.5 K (dark blue) and T_vib(z) – 1.5 K (red). Insets: Zoom on the region of the ν₅ band of C₂HD at 14.75 μm. Models use either the C₂H₂ slope profile (top) or the T_vib solution profile (bottom), and D/H ratios in C₂H₂ equal to one, three, and five times the telluric value.