Conclusions. The results of our work point to the necessity of considering the stereochemistry of biogenically emitted VOCs and their subsequent oxidation products when determining the physical properties of SOAs and their role(s) in the climate system. The propensity of surfactant-like species to depress the surface tension of an aqueous interface can be described by two factors: the hydrophobicity of the compound and its influence on the interfacial hydrogen bonding network. Our surface tension measurements indicate that the surface activity of the α-pinene-derived dimers somewhat depends on the backbone stereochemistry of the compounds. Octanol-ammonium sulfate and octanol-water partitioning coefficient measurements show small differences between diastereomers, with the more surface-active dimers derived from a condensation of both (+)-and (-)-1 exhibiting greater hydrophobicity than those derived from a single enantiomer of a-pinene (i.e., (+)-1 or (-)-1). These small differences would suggest that solubility plays some role in the increased surface activity of the mixed chirality dimers, however the differences are not so large as to suggest that solubility is solely responsible. Due to changes in the 3D shape of the dimeric diastereomers, it is not unreasonable to imagine that their orientation at the interface is also different, thus influencing the degree to which they disrupt the hydrogen bonding network. Future work is needed to establish the absolute orientation of the dimers at the particle interface in order to further explain the observed differences in surface depression propensity. Such molecular studies require chemical bond selectivity, along with structural sensitivity and surface selectivity.