Spectroscopy of Atmospheric Aerosol Particles

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Aerosols are liquid or solid particles suspended in a gas and are a key component in many atmospheric processes on Earth. Whether aerosols originate from natural or anthropogenic sources, they play an important role in the global climate as they greatly impact the Earth's radiation balance, act as cloud condensation nuclei, and serve as sites for chemical reactions. In many of these processes, however, the exact influence of aerosols is poorly understood. Optical tweezers are an analytical instrument at the frontier of high-precision single aerosol particle measurements as they allow for surface-free control of pico- and femtogram liquids and solids across phase states and timescales which are inaccessible via bulk measurements. With this tool, our group has been able to interrogate many important microphysical properties such as hygroscopic growth, surface tension, viscosity, liquid-liquid equilibrium, and atmospheric multiphase chemistry. The dual objectives of our research program are (i) the development of experimental platforms for spectroscopic measurements (e.g. cavity-enhanced Raman scattering) of single micron- and nanosized particles that can find application across a broad spectrum of disciplines and (ii) utilizing these platforms to address outstanding scientific questions within atmospheric aerosol science. In the coming grant period, we will build on our progress since 2014 and develop the next generation of optical trapping platforms and spectroscopic techniques to investigate the role that condensed-phase chemistry plays in Earth's atmosphere. Examples of phenomena that will be studied using our optical traps include multistep phase changes in supersaturated droplets, phase states of high viscosity organic aerosols (e.g. semisolid, glassy, and gel formation) and their effect on multicomponent mass transport, the role of surface tension in aerosol activation into cloud droplets, and measuring the optical properties of strongly absorbing particles (e.g. brown carbon). In addition to the new traps that will be designed and constructed to study these systems, we will simultaneously continue our development of the theoretical framework that is required to model light scattering, radiation pressure, multicomponent mass transport, and aerosol thermodynamics in our single particle experiments. These tools are important not only for the accurate retrieval of aerosol physical properties from spectroscopic measurements, but they also serve our overall goal of a detailed understanding of airborne particles. In summary, our program will continue to develop both state-of-the-art analytical instrumentation and models while studying systems of relevance to Earth's atmosphere. This is all in pursuit of improving our fundamental knowledge of the physical chemistry of atmospheric aerosol particles.