Unveiling the Shortwave Absorption Spectra of Alumina Aerosols: Radiative Implications for Solar Radiation Modification

Abstract Alumina is proposed for Stratospheric Aerosol Injection (SAI)‐based solar radiation modification due to its presumed ability to scatter sunlight strongly while absorbing weakly. Alumina is assigned negligible solar shortwave absorption in climate models; this assumption is not validated owing to technological challenges in quantifying its weak absorption signals. We report alumina's shortwave imaginary refractive index ( k ), a determinant of its absorption strength, using sensitive in situ photoacoustic spectrometry, finding values ranging from 1.4 × 10 −4 to 1.2 × 10 −3 . Particle‐scale electron energy‐loss spectroscopy provided independent validation and revealed that the non‐ideal absorption arises from oxygen vacancy defects in the alumina. Aerosol chemistry climate model simulations of an established scenario reveal that shortwave absorption has negligible effects on radiative forcing and stratospheric warming. While these results suggest that alumina's shortwave absorption is unlikely to significantly increase model uncertainty, further holistic assessments are required to confirm these impacts.