Same as Fig. 8, but for the Na I D₁ and D₂ lines. The middle panel shows the same as the top panel but after masking the contaminated stellar line cores around ± 10 km s⁻¹ (white regions). The bottom panel shows the transmission spectrum obtained using different methodologies: using all data from the top panel (cyan), masking the centre of the stellar Na I lines core as in the middle panel (magenta), and correcting the telluric Na I emission contamination (black). The transmission spectrum is binned by 0.2Å for better visualisation. The best-fit Gaussian profile is shown in red.