UC Riverside

Abstract: Characterization of exoplanet atmospheres requires a close interplay between observations, modelling, and experimental data. The accuracy of input data used in atmospheric models is essential because it impacts our interpretation of planetary spectra with retrieval codes. Molecular absorption cross sections in the UV–visible range are fundamental input parameters, determining chemical kinetics, particularly on temperate terrestrial planets. However, several atmospheric species remain poorly, or even entirely, uncharacterized. This is the case for HSO, a radical with unconstrained photolysis cross sections, often approximated by hydroperoxyl (HO 2 ). Sulphur chemistry can strongly influence the composition of rocky exoplanets, particularly in anoxic environments where volcanic SO 2 —the main source of HSO—is more efficiently photolyzed, and sulphur aerosols such as S 8 can form. HSO photolysis contributes to the formation of such sulphur chains, which have been proposed as indirect signatures of volcanic outgassing. Assessing the sensitivity of photochemical models to different UV–visible cross section prescriptions for HSO is therefore important for guiding its prioritization among poorly characterized atmospheric species. Here, we derive an updated HSO cross-section prescription from simulated HSO 2 data, providing a more reliable representation of HSO photolysis than HO 2 . We compare results from these new cross sections to the default prescription for temperate terrestrial planets with Archean-like atmospheres. We find that our updated HSO cross-section prescription enhances aerosol scattering and absorption signatures in transmission, emission, and reflection spectra for planets orbiting G- and K-type stars.