Our work indicates that nascent SSA are much more heterogeneous than traditional morphological models suggest, which may explain their high hygroscopicities. At the air/water interface, homogenous long chain saturated FA surfactants form a tightly-packed monolayer that is known to have high rigidity, low fluidity, and impermeability to water. However, the insertion of additional molecular species such as LPS and BCL as in our experiments, or other biogenic marine cosurfactants such as alcohols, cholesterols, or shorter chain FAs, results in a more porous, flexible, and fluid surface. Thus, despite having a significant mass percentage composed of organic material, the interface is not a thick organic coating as many models might suggest, but rather a porous surfactant film that facilitates water uptake and evaporation.