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| Environmental electron microscopy of aerosol particles |
We propose to study the effects of the nano-size regime on thermodynamic and kinetic properties of atmospheric nanoparticles. Particles having diameters between 1 and 100 nm are the ubiquitous and abundant precursors to the larger particles that strongly influence global climate. A governing property of atmospheric particles is their interaction with water vapor, which determines whether the particles are crystalline or aqueous. In the nano-size regime, physical state also depends in an unknown manner on particle diameter, which implies that phase diagrams for bulk systems are shifted in uncertain ways for nano-size systems. In the proposed work interdisciplinary progress will be made through integrated observational and first-principle thermodynamic investigations:
- The deliquescence relative humidity (DRH) of particles will be determined and modeled as a function of (nano-)size parameter x. Experimental work and improved thermodynamic models are required to reconcile conflicting calculations for nanoparticles, which differ both on the sign and magnitude of d(DRH)/dx in the nano-size regime.
- Hydrates having lower surface tensions become more favorable at smaller sizes, and the magnitude of this effect may be enough that a metastable hydrate at bulk dimensions transitions into the stable hydrate at nano-sizes. Shifts in the relative stabilities of known hydrates will be measured and modeled, and new hydrates may be discovered.
- Rates and mechanisms will be investigated for crystallization in the nano-size regime. Crystallization rates depend linearly on particle volume for bulk systems. Entering the nano-size regime, an open question is where and why this linear relationship falters. Hypothesized reasons are that the droplet may become smaller than the critical germ or that parallel processes such as nucleation at the droplet/air interface may begin to compete with volume nucleation.
Experimental investigations of the foregoing projects are currently technique limited. We propose to develop and refine two state-of-the-art instruments, an environmental (“in situ”) transmission electron microscope (ETEM) and a scanning polarization force microscope (SPFM), for the study of nanoparticle reactions.
Our proposed work will provide significant insights into the formation, growth, and stability of particles in the atmosphere, all of which have important implications for radiative, health, and visibility impacts of those particles. The fundamental understanding that we will develop for nano-size effects on phase transitions will provide a basis for other advances in nanoparticles, such as their application in materials and biotechnology fields, structuring nanodevices from particles, and particle-based imprinting technologies.
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