The surface tension and CCN activation of sea spray aerosol particles

Abstract. In marine environments, sea spray aerosol (SSA) particles have been found to contain surface-active substances (surfactants) originating from the sea surface microlayer. These surfactants can lower the surface tension of the SSA particles, facilitating their activation to cloud droplets. This effect is not considered in classical Köhler theory, which assumes droplets to be homogeneous with a surface tension of pure water. In this study the CCN activity of SSA particles calculated with classical Köhler theory is compared to a more complex calculation that considers bulk–surface partitioning, surface tension lowering, and liquid–liquid phase separation. The model approach presented here combines the multi-component Eberhart model for surface tension with the Monolayer model and an activity model (AIOMFAC). This combination allows for the first time to calculate Köhler curves of surfactant-containing particles with a large number of compounds. In a sensitivity study we show that  organic compounds can be categorized into weak, intermediate, and strong surfactants for CCN activation based on their separation factor in water S_1i and their pure component surface tension σ_i. For a quaternary model system of SSA particles, it is shown that a high content of hydrophobic organic material (i.e., strong surfactants) in Aitken mode particles does not necessarily prevent good CCN activation, but rather facilitates effective activation via surface tension lowering. Since common climate models use parametrizations that are based on classical Köhler theory, these results suggest that the CCN activity of small SSA particles might be underestimated in climate models.