Explosive volcanic eruptions are a major natural perturbation to the ocean-atmosphere system, injecting sulfur gases that form sulfate aerosols and modify Earth’s radiation budget. Volcanic forcing is commonly attributed to relatively long-lived sulfate aerosols in the stratosphere and their direct radiative effects. However, for some eruptions, a substantial fraction of volcanic aerosols may reside in the lowermost stratosphere (LMS) and troposphere. Observations of sustained effusive eruptions indicate that aerosol-rich plume can interact with tropospheric liquid clouds and alter their shortwave reflectivity. Such aerosol-cloud interactions, however, have not yet been established for short-lived explosive events.

Extending estimates of volcanic forcing beyond the satellite era relies heavily on sulfate deposition preserved in polar ice cores. Yet bulk sulfate concentration alone cannot resolve whether deposited sulfate reflects long-lived aerosol transport through the stratosphere or shorted-lived pathways through the LMS and troposphere. Using the 1912 Katmai eruption as a case study, we apply high-resolution sulfur isotope measurements (δ³⁴S and Δ³³S) to constrain the vertical partitioning of volcanic aerosol loadings. With sulfur isotope evidence for a substantial ozone-shielded aerosol component, we then use the UK Earth System Model (UKESM) to examine how these shorter-lived aerosols perturb cloud microphysics and related radiative properties. This combined observational-modelling framework enables us to evaluate not only the well-recognised stratospheric direct radiative forcing, but also possible indirect forcing associated with aerosol-cloud interactions.