Reduced Longwave Absorption of Magnesium Sulfate Aerosol as an Alternative to Sulfuric Acid for Stratospheric Aerosol Injection

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