STIS/G430L transmission spectrum of WASP-121 b as published by Evans et al. (2018) indicated with open circles. A model spectrum assuming chemical equilibrium, a temperature of 3000 K, 20× solar metallicity and SH computed as described in Sect. 2.3.2 and Table 2 is overplotted, both at the native resolution (light-blue) and binned to the resolution of STIS (dark-blue line and circles). The wavelength bin at 395 nm coincides with the Al I doublet, at which location the transit radius is strongly over-predicted compared to the STIS observations. This models predicts that the cores of the Al I doublet become optically thick at transit radii of ~ 5 × 10⁻³ above the local continuum, which corresponds to an altitude of ~0.3R_p above the planet. At such low pressures, photo-ionisation is expected to be important (e.g. Moses et al. 2011; Kitzmann et al. 2018). At the same time, Al I has one of the lowest ionisation thresholds of the elements, so we predict that equilibrium models may strongly over-predict the strength of the Al I doublet at this wavelength.