Abstract Aqueous sulfuric acid is the benchmark sulfate aerosol for stratospheric aerosol injection (SAI), but H2SO4 ${mathrm{H} }{2}{text{SO} }{4}$/H2 ${mathrm{H} }{2}$O aerosol absorbs strongly in the infrared, producing longwave warming that significantly offsets its shortwave cooling. Here, we evaluate magnesium sulfate (MgSO4 ${text{MgSO} }{4}$) as a non‐acidic sulfate alternative. Single‐particle optical trapping measurements show that dry MgSO4 ${text{MgSO} }{4}$ aerosol remains in a gel phase during cooling to −50 ${-}50$° ${}^{circ}$C and has a visible real refractive index slightly larger than that of 70 wt.% H2SO4 ${mathrm{H} }{2}{text{SO} }{4}$. Measured infrared optical constants show that gel‐phase MgSO4 ${text{MgSO} }{4}$ absorbs far more weakly than H2SO4 ${mathrm{H} }{2}{text{SO} }{4}$ because it lacks the low‐pH HSO4− ${text{HSO} }{4}^{-}$/H3 ${mathrm{H} }{3}$O+ ${mathrm{O} }^{+}$ hydrogen‐bonding network responsible for the strong infrared bands of sulfuric acid. Radiative transfer calculations show that MgSO4 ${text{MgSO} }{4}$ and H2SO4 ${mathrm{H} }{2}{text{SO} }{4}$ have similar shortwave forcing, but gel‐phase MgSO4 ${text{MgSO} }{4}$ produces negligible longwave forcing across the particle‐size range relevant to SAI. Therefore, gel‐phase MgSO4 ${text{MgSO} }_{4}$ provides stronger net cooling per unit aerosol burden than sulfuric acid for submicron size distributions.