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

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 $1.4 \times 10^{-4}$ to $1.2 \times 10^{-3}$. Particle-scale electron energy-loss spectroscopy provided independent validation and revealed that the non-ideal absorption arises from oxygen vacancy defects in the alumina's crystal structure. Aerosol chemistry climate model simulations to evaluate shortwave absorption radiative effects revealed insignificant impacts on radiative forcing and stratospheric warming. Our findings indicate that alumina's shortwave absorption, previously reported as a source of uncertainty, is unlikely to affect SAI impact calculations.