Surfactant aggregation biases aerosol-cloud climate effects

Organic surfactants are common constituents of atmospheric aerosols, and their internal organization has potentially important yet insufficiently understood implications for cloud droplet growth and global climate effects. Here, we demonstrate that advanced NMR relaxation and diffusion measurements, combined with state-of-the-art relaxation modeling, reveal aggregation behavior that departs markedly from conventional descriptions. Our NMR analysis shows that in aqueous sodium decanoate solutions, surfactants do not exist solely as monomers below the critical micelle concentration (CMC); instead, they form small clusters comprising a few molecules. Comprehensive modeling that incorporates dynamical and structural parameters from molecular dynamics (MD) simulations yields realistic estimates of micelle dimensions above the CMC, with diameters of approximately 5.5 nm. Water diffusion experiments further indicate the presence of small water-encapsulating aggregates at surfactant concentrations above 550 mM, and ultrafast diffusion exchange spectroscopy (UF-DEXSY) enables quantification of exchange rates between encapsulated and bulk water. Incorporating these aggregation phenomena into thermodynamic modeling of aerosol hygroscopic growth and cloud droplet activation reveals that hygroscopic properties typically measured at lower relative humidity, where aggregation is more pronounced, can significantly misrepresent aerosol cooling potential and introduce substantial biases into global climate predictions.