Results overview for the WASP-121b NIRISS SOSS eclipse hybrid free retrieval including parameterised thermal dissociation. Top left: retrieved day-side vertical temperature structure (black line: median, grey contours: 1 and 2 σ bounds) compared to the average TP profile at mid-eclipse of a fiducial GCM model (Parmentier et al. 2018, yellow line). Top right: measured day-side brightness temperature of WASP-121b (black points) compared to the best fit atmospheric model (solid green line). Opacity contributions from individual species are shown in colour. Multiple H₂O emission bands can be seen, as well as contribution from CO around 2.4 µm. At shorter wavelengths, H⁻, VO, and TiO are necessary to match the rise in brightness temperature (note that an albedo of zero was assumed for this retrieval). Bottom: each panel shows the retrieved abundance profiles for an individual species fitted in the retrieval (solid black line: median, grey contours: 1 and 2 σ bounds). Average contribution functions (dotted black line) show the pressure level probed by each species. The abundance profiles predicted from an equilibrium chemistry model assuming the median TP profile and a stellar-like composition (Evans-Soma et al. 2025) are also shown for comparison (dashed lime green lines). Constrained abundances are obtained for H₂O, CO, VO, TiO, and H⁻, with only upper limits obtained for the other species. Error bars (1σ) or upper limits (2σ) are shown at the approximate average pressure probed (peak of the contribution function).