Determining the Buffering Capacity and PH of Aerosols and Their Control of the Multiphase Chemical Evolution and Kinetics of Optically Levitated Particles

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With support from the Environmental Chemical Sciences (ECS) Program in the Chemistry Division, Ryan Sullivan at Carnegie Mellon University will study atmospheric aerosols utilizing optical tweezers. Aerosols are micron-sized liquid or solid particles suspended in the air. They have unique chemical properties and reactivity because of their high surface area-to-volume ratio. Atmospheric aerosols can act as chemical catalysts, accelerating chemical reactions and facilitating aqueous-phase chemistry that cannot occur in the gas-phase alone. The properties of the aerosol surfaces play an important role in controlling the chemical reactivity of aerosols and their many effects on atmospheric chemistry, air quality, health, cloud microphysics and climate. A tool called aerosol optical tweezers (AOT) will be used in this project to carry out experiments on levitated droplets to determine the evolution of various key chemical properties. Aerosol acidity is a critical chemical property that strongly influences the chemistry of the interface and chemical kinetics. The experiments are expected to produce new chemical understanding of variations in particle composition, acidity, and morphology, that change the reactivity of these complex particles. In addition to training graduate and undergraduate students, Dr. Sullivan will create and deploy hands-on experiments and curricular materials on environmental chemistry for grade 6-12 students in local public schools which serve significant numbers of members of underrepresented groups.Changes in acidity, water content, polarity, and composition can cause liquid–liquid phase separations that alter the properties at the interface of the particle. Thus, changes in aerosol composition can alter particle morphology which in turn can change the kinetics and reaction mechanisms of further gas–particle reactions. The evolution of a suite of key chemical properties including droplet pH will be determined as a particle undergoes aqueous reactions such as with two important trace gases: N2O5 and the epoxydiol IEPOX produced from isoprene oxidation. Physicochemical properties to be determined include phase separations and morphology, acidity, and diffusivity, as a function of relative humidity (RH), temperature, and droplet size. This research sets out to address outstanding questions regarding how the chemical evolution of atmospheric particles affects the extensive impacts of particulate matter on air quality, human health, ecosystems, acid deposition, radiative forcing, cloud microphysics, and climate change.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.