Sensitivity analysis of radiative effects of alumina particles from spacecraft re-entries

The rapid growth of satellite mega-constellations is expected to substantially increase spacecraft disposal and atmospheric reentry rates in the coming decades. As most spacecraft are composed primarily of aluminum, reentries are anticipated to release aluminum oxide (Al₂O₃, alumina) particles into the upper atmosphere. Alumina efficiently scatters solar radiation and has therefore also been discussed in potential solar radiation modification (SRM) scenarios. However, the respective climatic impact, and even the sign of the radiative forcing, remain highly uncertain due to limited constraints on particle size distributions and associated microphysical processes. Here, the radiative effects of alumina aerosols are investigated using sensitivity experiments with the radiative transfer model libRadtran, complemented by a simplified global climate model to estimate stratospherically adjusted radiative forcings. The analysis focuses on the influence of aerosol particle size, injection altitude, and background atmospheric conditions on radiative forcing and heating rates. Alumina distributions based on two scenarios from Maloney et al. (2025) are considered as reference cases and form the basis for the sensitivity studies: RS1, representing small particles with effective radii of approximately 10nm, and RS2, representing larger particles around 0.1μm. The results demonstrate a strong dependence of both the magnitude and sign of the radiative forcing on particle size and atmospheric background assumptions, particularly cloud configurations. Although the simulated forcings fall within the uncertainty range of Maloney et al. (2025), the RS1 scenario generally produces a positive radiative forcing, whereas the RS2 scenario leads to a negative forcing under most conditions, resulting in signs opposite to their reported best estimates. Potential reasons for these discrepancies are currently being investigated; however, the results generally emphasize the key role of aerosol microphysics and the large uncertainties in the climatic impact of alumina aerosols.