Co-added lines of the HARPS transmission spectrum sodium doublet as a function of velocity. The line centre was set to zero. The data are shown in grey and binned × 10 in black. A Gaussian fit derived as in Sect. 4.2 is shown in red with its FWHM marked by the red dashed lines. The HARPS instrumental line spread function (FWHM = 2.7 km s⁻¹) is shown in orange and the escape velocity as green dotted-dashed lines. The FWHM velocity does not exceed the escape velocity at any point, making the atmospheric escape of sodium unlikely. The best Gaussian fit to the data is significantly wider than the instrumental response, but the corresponding velocity is well below the escape velocity of WASP-76b. Given that the conversion of relative depth to height depends on the choice of height at the continuum level (here: WASP-76b white light radius), the height is not an absolute height but an equivalence height (Pino et al. 2018b).