Average absorption spectrum of MASCARA-1 b between contact times 2 and 3, computed with the out-of-transit spectrum as reference, and plotted in the planetary rest frame. Blue points are ESPRESSO data from Casasayas-Barris et al. (2022), red profiles show EVE simulations. The vertical grey line indicates the rest wavelength of the NaI Dl line. Upper panel: Computed without planetary atmosphere using literature parameters and a solar abundance for the star. Middle panel: Computed without planetary atmosphere (dashed red curve ) for a modified v_eq sin(i) and stellar Na I abundance, and with atmospheric sodium (solid red curve). The shaded area shows the atmospheric contribution to the absorption spectrum. Lower panel: Computed without atmosphere, for a modified v_eq sin(i), orbital inclination, and stellar Na I abundance yielding the best match to both the disc-integrated and absorption spectra.