Investigating the Impact of Seawater Divalent Cations on the Surface Tension of Aerosol Containing Ionic Surfactants

ALISON BAIN, Kunal Gosh, Konstantin Tumashevich, Nønne L. Prisle, Bryan R. Bzdek, University of Bristol

     Abstract Number: 54
     Working Group: Aerosols, Clouds and Climate

Abstract
Quantifying the surface tension of aerosol is of great interest in part due to field observations identifying larger concentrations of cloud condensation nuclei (CCN) than expected from model predictions. A common assumption to these model predictions is that the surface tension of aerosol droplets is equal to that of pure water. However, using this assumption, the surface tension of surfactant-containing aerosol may be overpredicted. Reducing the surface tension of aerosol lowers the barrier to cloud droplet activation and leads to higher predicted CCN concentrations.

Sea spray aerosol, one of the largest aerosol sources by mass, is known to be a complex mixture of inorganic salts and surface-active organics. A reduction in aerosol surface tension due to surfactant adsorption has been suggested to explain discrepancies between CCN observations and predictions. Surface tension is typically measured using macroscopic samples, but in high surface-area-to-volume ratio droplets, surfactant partitioning to the interface can leave the bulk concentration depleted, thus requiring a larger total surfactant concentration to reduce the surface tension. Recent work has focused on surfactant-water or surfactant-NaCl solutions but divalent cations, which are present in seawater, are known to significantly impact the partitioning of ionic surfactants.

We previously showed the bulk surfactant depletion experimentally for nonionic surfactants in aerosol droplets containing glutaric acid or NaCl cosolutes. Here, we extend surface tension measurements of single, picolitre volume droplets to ionic surfactants and compare the results to two partitioning models. Additionally, we investigate the impact of a sea salt mixture containing divalent cations on the partitioning of ionic surfactants in aerosol droplets. These results are compared to our previous knowledge of nonionic surfactant-containing aerosol droplets.