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Micro-Raman Spectroscopy as a Probe of the Composition, Ice Nucleation, and Low Temperature Water Uptake of Individual Substrate Deposited Aerosol Particles

Abstract

Atmospheric aerosols represent one of the largest sources of uncertainty in modeling the Earth’s climate and radiative budget. Aerosol indirect effects are a major source of this uncertainty, part of which stems from ice nucleating particles (INPs), which are a major driver of cloud formation. Ice nucleating particles are a unique subset of aerosols that promote heterogeneous ice formation. There is a large gap in our knowledge concerning the composition, morphology, and fundamental processes that that govern heterogeneous ice nucleation in the atmosphere by these unique INPs. Marine INPs are of particular interest and importance and have recently been shown to contribute significantly to cloud formation in the southern hemisphere. This dissertation investigates the mechanism, chemical composition, and fundamental phase state behavior of atmospherically relevant INPs. In an effort to further understand these particles, we developed instrumentation—a Raman spectrometer coupled with an environmental cell—to probe ice nucleation, water uptake, chemical composition, and the impacts of temperature and relative humidity on these particles. Following extensive characterization and validation, we probed the relationship between temperature and water uptake of atmospherically relevant biological INPs because water uptake is a requirement for immersion freezing. This work reveals an unexpected trend: water uptake of biological, atmospherically relevant INPs begins at lower RH at lower temperatures, the opposite of solubility trends for small soluble organics. Following this, we then used the instrumentation to study the relationship between atmospherically relevant INPs and the surrounding water as a function of temperature. This work used spectral characteristics of water in the presence of these different INPs. In an additional and more complex experiment, we used this instrumentation to investigate real samples from SeaSCAPE, a large-scale field study. We examined the connection between morphology and ice nucleation behavior with chemical composition in conjunction with a suite of other instruments and collaborators. Through this work, several spectral signatures were identified, but no clear correlation between morphology and composition was determined. Finally, marine INP were isolated from the wave channel and we attempted to (1) characterize the chemical composition of unique INP and (2) identify any trends in functional groups with IN behavior/progression of the bloom. This work is ongoing but has created a library of potential model marine INP and their ice nucleating behavior. The work presented in this dissertation improves our understanding of ice nucleating particles and their fundamental physical and chemical properties, ultimately contributing to improving implementation and accuracy of climate models.

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