Cation Identity, Phase State, and Density as Key Determinants of Radiative Forcing in Atmospheric Sulfate Systems

[Geoengineering News]

https://chemrxiv.org/doi/full/10.26434/chemrxiv.15002773/v1

Authors: Vahid Shahabadi, Yingshi Luo, Aaron M. Palmisano, Pedro De Allende, Alison Bain, James F. Davies, and Thomas C. Preston

06 May 2026

Abstract

Sulfate aerosols strongly influence climate through aerosol–radiation and aerosolcloud interactions and are widely considered for climate mitigation strategies. Despite this relevance, sulfate aerosols are commonly represented using simplified assumptions based on the binary H2SO4–H2O 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 (Li2SO4, Na2SO4, K2SO4, (NH4)2SO4, MgSO4, Al2(SO4)3, and MnSO4) 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 MgSO4, MnSO4, and Al2(SO4)3, exhibit pronounced inflection points and hysteresis consistent with transitions to highly viscous, gel-like phases below RH ≈ 35–40%. In this low-RH regime, these systems display humidity-buffered behavior, in which density, refractive index, and growth factor vary minimally with short-range humidity fluctuations, strongly suppressing water transport and heterogeneous reactivity. Elevated refractive indices at 589 nm, reaching ∼ 1.48 for MgSO4 and Al2(SO4)3 and ∼ 1.52 for MnSO4, 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.

Source: ChemRxiv