10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Design and Application of Aerosol Optical Tweezers to Explore Temperature Effects on Phase Separation and Acidity of Organic Aerosol at Subzero Temperatures
HALLIE BOYER, Kyle Gorkowski, Neil Donahue, Ryan Sullivan, Carnegie Mellon University
Abstract Number: 1076 Working Group: Instrumentation
Abstract Phase and morphology in atmospheric aerosols are critical for understanding gas-particle and particle-cloud interactions, yet they remain hard to measure and predict due to multiple operative processes and properties. Further, there is currently a lack of data needed to train predictive models for explicit temperature dependence, especially data obtained from direct measurements on particles rather than bulk solutions. Phase separation is governed by ambient conditions and particle properties that are difficult to measure in situ, such as particle composition, interfacial tensions, and pH; thus, predictions frequently rely on a bulk perspective. Liquid-liquid phase separation can result in a core-shell or partially-engulfed morphology, where the aqueous phase is the core of the particle and the organic phase is present either as a complete shell or included lens, respectively. In the atmosphere, the morphology of the particle and the presence of an organic surficial layer can profoundly affect the particle’s ability to scatter light and interact with chemical vapors and water. We directly determined pH, surface tension, and morphology of optically levitated particles using cavity enhanced Raman spectroscopy while monitoring changes in ambient conditions, including chemical composition and temperatures ranging from -20 to 40 C.
We use an aerosol optical tweezers (AOT) platform with a new chamber design for the integration and real-time measurement of particle properties under relative humidity and temperature control. Previously, we determined particle morphology of secondary organic aerosol in the AOT and identified core-shell and homogenous morphologies in response to organic vapor condensation and relative humidity. In the new chamber, we have explored the effects of pH and surface tension on phase mixing of multicomponent aerosol particles and investigated sub-zero temperatures of each property. The AOT provides steerable, contactless traps for single droplets of ~5 micron radii. Particles trapped in the AOT can reach metastable states, such as sub-cooled liquid states or supersaturation of solute with respect to solvent or other solute species, enabling measurements where data are scarce due to inaccessibility in bulk experiments. We determine surface tension by analysis of elastic backscattered light induced by the coalescence of two droplets trapped in a holographic optical trap. Surface tensions are necessary for predictions of particle morphology using spreading coefficient analysis, to predict aerosol interfacial composition, as well as for determination of cloud activation through Köhler theory. Finally, measurements of surface tensions as a function of temperature are applied to an analytic surface thermodynamic model to produce accurate predictions of interfacial tensions where little or no experimental data are available.