AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Phase Separation, Morphology, and Diffusivity of Secondary Organic Matter Determined using Aerosol Optical Tweezers
KYLE GORKOWSKI, Neil Donahue, Ryan Sullivan, Carnegie Mellon University
Abstract Number: 518 Working Group: Aerosol Chemistry
Abstract Atmospheric aerosols contain a wide variety of organic and inorganic components and can phase-separate into distinct liquid phases, resulting in either a core-shell or a partial-shell particle morphology. Understanding and predicting when each of these morphologies forms is critical to understanding gas-particle interactions and climate-forcing properties. We conducted experiments exploring phase-separation of droplets suspended using aerosol optical tweezers (AOT). The droplet levitation and the surface resonant whispering gallery modes (WGMs), retrieved in the cavity-enhanced Raman spectrum, provide a direct and real-time assessment of the droplet’s morphology. Analysis of the Raman spectra using our new algorithm allows us to independently observed changes in the size and refractive index of both the core and shell phases in core-shell droplet morphologies.
We performed the first optical tweezers experiment on droplets to which secondary organic matter (SOM) was added through in situ precursor ozonolysis directly in the tweezing chamber. The alpha-pinene SOM formed a composite droplet with separate phases when added to an aqueous salt or a squalane droplet, producing a shell of secondary organic matter. Using spreading coefficients, we can bound the surface tension of alpha-pinene SOM and conclude that the air-liquid surface tension of alpha-pinene SOM is less than or equal to that of squalane, 28 mN/m. This, in turn, helps to constrain the Kelvin diameter for condensation of alpha-pinene SOM onto ultrafine particles soon after nucleation in the atmosphere. On aqueous cores, we observed that the coating of SOM does not impede water equilibration timescales. We also observe a humidity-dependent phase separation of glycerol and alpha-pinene SOM, which is supported by the Hansen solubility parameterization.