Removal of gas phase lines and local continuum estimate. Top: contribution of gas phase SO₂ emission (yellow) and CH₄ absorption (red) lines estimated from the observed IRAS 18089 mm spectrum (gray). The corrected spectrum is shown in black. Bottom: corrected optical depth spectrum (black) and the local continuum interpolation (blue line). The red dots mark the guiding points used for the interpolation. The green dashed line shows the continuum interpolation based on a fifth-order polynomial (see Appendix E).