Simulated transmission spectra of HD 209458 b around the Na I D1 line, showing the RM+CLV effects along with atmospheric absorption. The spectra are computed in the planet rest frame and averaged between transit contact points as indicated in the top left or top right corner of each panel. The different colours correspond to the same atmospheric models represented in Fig. 2. The ESPRESSO data are the same as those reported in Fig. 5 of Casasayas-Barris et al. (2021).