Abstract
<jats:p> Sulfate aerosols strongly influence climate through aerosol–radiation and aerosol–cloud interactions and are widely considered for climate mitigation strategies. Despite this relevance, sulfate aerosols are commonly represented using simplified assumptions based on the binary H <jats:sub>2</jats:sub> SO <jats:sub>4</jats:sub> –H <jats:sub>2</jats:sub> O system and a composition-independent refractive index. Here, we show that cation identity, phase state, and density can substantially alter the optical properties of sulfate aerosols and the extent to which their evolution is governed by diffusion-limited mass transport. We combine bulk measurements of density, water activity, and refractive index with single-particle optical trapping and electrodynamic balance experiments to quantify droplet size, radial growth factors, and wavelength-dependent refractive indices. This approach is applied to seven inorganic sulfates (Li <jats:sub>2</jats:sub> SO <jats:sub>4</jats:sub> , Na <jats:sub>2</jats:sub> SO <jats:sub>4</jats:sub> , K <jats:sub>2</jats:sub> SO <jats:sub>4</jats:sub> , (NH <jats:sub>4</jats:sub> ) <jats:sub>2</jats:sub> SO <jats:sub>4</jats:sub> , MgSO <jats:sub>4</jats:sub> , Al <jats:sub>2</jats:sub> (SO <jats:sub>4</jats:sub> ) <jats:sub>3</jats:sub> , and MnSO <jats:sub>4</jats:sub> ) and two short-chain organosulfates. We further introduce a bulk–droplet equivalency framework that retrieves the droplet thermodynamic state (e.g., composition and water activity, including metastable and supersaturated regimes) directly from levitated-droplet measurements. Multivalent sulfates, specifically MgSO <jats:sub>4</jats:sub> , MnSO <jats:sub>4</jats:sub> , and Al <jats:sub>2</jats:sub> (SO <jats:sub>4</jats:sub> ) <jats:sub>3</jats:sub> , exhibit pronounced inflection points and hysteresis. These features are consistent with a transition to highly viscous, gel-like phases at relative humidity (RH) ≈ 35–40%. In this low-RH regime, these systems display humidity-buffered behavior, in which density, refractive index, and growth factor vary only weakly with further decreases in humidity, strongly suppressing water transport and likely reducing heterogeneous reactivity. Elevated refractive indices at 589 nm, reaching ~1.49 for MgSO <jats:sub>4</jats:sub> and Al <jats:sub>2</jats:sub> (SO <jats:sub>4</jats:sub> ) <jats:sub>3</jats:sub> and ~1.53 for MnSO <jats:sub>4</jats:sub> , yield enhanced shortwave radiative forcing efficiencies relative to conventional sulfate, as shown using Mie scattering and one-dimensional radiative transfer calculations (libRadtran/DISORT) for lognormal size distributions. These results highlight the importance of cation-specific phase behavior and optical properties for accurately representing sulfate aerosols in climate, atmospheric chemistry models, and geoengineering proposals. </jats:p>