NIRT: Nanoscale Processes in the Environment: Atmospheric Nanoparticles

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This proposal was received in response to Nanoscale Science and Engineering initiative, NSF 02-148, category NIRT (Nanoscience Interdisciplinary Research Teams). This award supports a collaboration of Prof. Scot Martin (Harvard University), Prof. Peter Buseck (Arizona State University), and Prof. Lynn Russell (Princeton University). It addresses 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. This interdisciplinary project will integrate observational and first-principle thermodynamic investigations. The following topics will be addressed:(1) The deliquescence relative humidity (DRH) of nanoparticles will be determined and modeled as a function of size. (2) Shifts in the relative stabilities of known hydrates will be measured and modeled. 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. (3) 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 these topics are currently technique limited. Two state-of-the-art instruments will be developed and refined for this project, an environmental transmission electron microscope and a scanning polarization force microscope. This 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. A nanogeosciences summer undergraduate program will integrate research, education, and diversity into the project.