Publications

Aerosol particles are known to exist in highly viscous amorphous states at a low relative humidity and temperature. The slow diffusion of molecules in viscous particles impacts the uptake and loss of volatile and semivolatile species and the rate of heterogeneous chemistry. Recent work has demonstrated that in particles containing organic molecules and salts, the formation of two-phase gel states is possible, leading to observations of rigid particles that resist coalescence. The way that molecules diffuse and transport in gel systems is not well-characterized. In this work, we use an electrodynamic balance to levitate sample particles containing a range of organic compounds in mixtures with calcium chloride and measure the rate of water diffusion. Particles of the pure organics have been shown to form viscous amorphous states, while in mixtures with divalent salts, coalescence measurements have revealed the apparent solidification of particles, consistent with the formation of a gel state facilitated by ion-molecule interactions. We report in several cases that water transport can actually be increased in the rigid gel state relative to the pure compound that forms a viscous state under similar conditions. These measurements reveal the limitations of using viscosity as a metric for predicting molecular diffusion and that the gel structure that forms is a much stronger controlling factor in the rate of diffusion. This underscores the need for diffusion measurements as well as a deeper understanding of noncovalent molecular assembly that leads to supramolecular structures in aerosol particles.

Single particle levitation methods are a powerful subset of aerosol instrumentation that allow a wide range of particle properties and processes to be explored. One of the most common forms of single particle levitation uses electric fields and is generally referred to as an electrodynamic balance (EDB). There are many different kinds of EDB's that have been designed with different applications in mind, and a corresponding array of analytical tools have been developed to characterize particles held in these traps. In this tutorial, we review the design and development of the EDB and discuss a range of analytical methods, including electrostatic analysis, light scattering, spectroscopy, and imaging, that allow for measurements of hygroscopic growth, volatility, surface tension and viscosity, diffusion, and phase and morphology. We go on to review recent advanced analytical methods using mass spectrometry to probe particle composition. This review is intended to provide readers with the basic knowledge to set up an EDB platform, design measurement protocols based on the available analytical tools, and run experiments to probe the fundamental properties of aerosol particles relevant to their role in the atmosphere, impacts on clouds and climate, effects on air quality, role in health and disease, and applications in industrial processes.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Mineral dust can indirectly impact climate by nucleation of atmospheric solids, for example, by heterogeneously nucleating ice in mixed-phase clouds or by impacting the phase of aerosols and clouds through contact nucleation. The effectiveness toward nucleation of individual components of mineral dust requires further study. Here, the nucleation behavior of metal oxide nanoparticle components of atmospheric mineral dust is investigated. A long-working-distance optical trap is used to study contact and immersion nucleation of ammonium sulfate by transition-metal oxides, and an environmental chamber is used to probe depositional ice nucleation on metal oxide particles. Previous theory dictates that ice nucleation and heterogeneous nucleation of atmospheric salts can be impacted by several factors including morphology, lattice match, and surface area. Here, we observe a correlation between the cationic oxidation states of the metal oxide heterogeneous nuclei and their effectiveness in causing nucleation in both contact efflorescence mode and depositional freezing mode. In contrast to the activity of contact efflorescence, the same metal oxide particles did not cause a significant increase in efflorescence relative humidity when immersed in the droplet. These experiments suggest that metal speciation, possibly as a result of cationic charge sites, may play a role in the effectiveness of nucleation that is initiated at particle surfaces.

Abstract not provided.

Abstract not provided.