Date/Time
Date(s) - 26/08/2024
12:30 pm - 1:30 pm

Title: Surfactant Partitioning in Aerosol Droplets and Impact on Cloud Droplet Activation

Date: Monday August 26

Time: 12:30pm

Room: ABB B118

Host: Dr. Rodrigo Vargas-Hernandez

Abstract:

In the atmosphere, aerosol particles interact with climate by scattering and absorbing radiation (the direct effect) and interacting with cloud properties, (the indirect effect). Some field observations have measured larger concentrations of cloud condensation nuclei (CCN) than expected from model predictions. A common assumption in model predictions is that the surface tension of aerosol droplets is equal to that of pure water, despite sampled aerosol containing a variety of surface-active organics in large enough quantities to reduce the surface tension of macroscopic solutions. Surface tension lowering by surfactants has been suggested to explain discrepancies between CCN observations and predictions. A reduced aerosol surface tension would lower the barrier to cloud droplet activation and lead to higher CCN concentrations. However, 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. Furthermore, the repartitioning of surfactant impacts droplet surface tension and water activity, which have opposing effects on cloud droplet formation.

To understand the partitioning of surfactants in high surface-area-to-volume droplets, we use holographic optical tweezers to measure the surface tension of picolitre volume droplets suspended in air. We use these measurements to validate predictions made by two independent partitioning models for a range of surfactant systems.  Finally, we investigate how the partitioning of surfactants during hygroscopic growth impacts droplet surface tension, water activity, and ability to activate into a cloud droplet for aerosol of different dry diameters and organic fraction compositions.