Surface Tension of Aerosol Droplets Containing Complex Mixtures of Organics

With support from the Environmental Chemical Sciences (ECS) program in the Division of Chemistry, Professor Alison Bain of Oregon State University, Corvallis, will investigate the effects of complex mixtures of organic solutes on the surface tension of aerosol droplets and their role in the activation of cloud droplets. The effect of complex organic mixtures on surface tension reduction of ambient aerosol during hygroscopic growth and eventual cloud droplet activation remains unknown because laboratory experiments have largely focused on simple mixtures, which do not mimic the chemical complexity of ambient aerosol. Professor Bain and her team will conduct detailed laboratory surface tension measurements of solutions and droplets containing complex mixtures of salts and organic species with a range of surface-active properties. Their studies could contribute to the fundamental understanding of the interactions between aerosol droplets and clouds by providing detailed information on how surfactants partition in complex mixtures. This could lead to improved predictions of the number of aerosol droplets that activate into cloud droplets, which greatly impacts cloud properties including reflectivity, lifetime, and precipitation. The research team will partner with Oregon State University’s Science & Math Investigative Learning Experiences (SMILE) program to develop a classroom activity introducing middle school students to atmospheric aerosol. Due to the large surface-area-to-volume ratio of aerosol droplets, the partitioning of surfactants cannot be described by the surface tension of a macroscopic solution alone, and a partitioning model must be employed. Partitioning models have largely been tested for aerosol systems containing only one or two organic molecules, which do not begin to describe the chemical complexity of ambient aerosols. This project will investigate the surface tension of complex organic mixtures, including surfactant mixtures and their mixtures with salt, in macroscopic aqueous solutions and picolitre volume droplets. These results will be used to test the ability of partitioning models to describe the surface tension of droplets containing complex mixtures. The goals of this project are to determine how different ratios of strong, moderate, and weak surface-active molecules impact aerosol surface tension, how the surface tension of ambient aerosol changes as the total organic fraction decreases during hygroscopic growth, and to identify the chemical complexity required to accurately represent the surface tension of ambient aerosol with a partitioning model. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.