Measuring Surface Tension of Atmospheric Secondary Organic Aerosols Using Atomic Force Microscopy

Atmospheric aerosols are particles on the nanoscale that are suspended in air. Aerosols play a major role in cloud development and thus are relevant for climate modeling. Specifically, certain atmospheric aerosols are capable of forming cloud condensation nuclei, or cloud droplets.  Models that predict particles’ capacities to become cloud condensation nuclei require knowledge of the surface tension of the particles. However, obtaining direct measurements of particle surface tension using traditional methods is challenging, because there is insufficient volume of liquid in aerosol particles to use in typical instruments that measure surface tension. Recently, a method of measuring the surface tension of droplets using atomic force microscopy has been developed. In this study, that method was utilized to directly measure the surface tension of liquid aerosols generated in the lab. The liquid aerosols were generated by exposing vapor-phase volatile organic compounds to ozone in an isolated and particle-free environment. The aerosols were collected via impaction, where the resulting film was probed with a specifically-designed tip in the atomic force microscope (AFM) to calculate its surface tension. As a result of this work, the surface tension of oxidized alpha-pinene aerosols was determined.