The effect of particle shape on shortwave radiative forcing in stratospheric aerosol injection

Stratospheric aerosol injection (SAI) seeks to increase Earth’s planetary albedo by introducing particles into the lower stratosphere that efficiently backscatter solar radiation. Most optical optimization studies have focused on spherical particles, leaving the potential benefits of non-spherical morphologies largely unexplored. Here, we develop an analytical Legendre-moment method that uses closed-form expansion coefficients within the T-matrix framework to efficiently and accurately evaluate the isotropic upscatter fraction for randomly oriented non-spherical particles. This is used to quantify how particle shape and aspect ratio influences the change in Bond albedo, Δ A , within a single-scattering framework. Orientation-averaged extinction and scattering cross-sections and phase function moments are computed over the solar spectrum to obtain Δ A and global-mean radiative forcing per unit mass for candidate SAI materials (rutile, anatase, α -SiC, diamond, cubic ZrO 2 , α -Al 2 O 3 , calcite, and sulphate). We systematically explore spheroids with varying aspect ratio and superspheroids spanning a wide range of roundness/boxiness. Overall, once volume-equivalent radius and refractive index are optimized, Δ A is only weakly sensitive to particle morphology. Therefore, while non-spherical particles can improve Δ A , the resulting gains are modest at best, and Δ A remains controlled primarily by volume-equivalent radius and refractive index.