Date of Award
Program or Major
Master of Science
Margaret E. Greenslade
Aerosol can alter climate through the interaction with solar radiation by absorbing and scattering solar radiation. Absorption and scattering are governed by complex refractive index (CRI) dependent on the chemical composition and wavelength, the physical properties of the aerosol such as size, and other atmospheric parameters such as relative humidity (RH). CRI is commonly reported as the real portion at a single wavelength, or is unknown, which introduces uncertainty when those values are used as input for climate models. Two optical interrogation techniques, Aerosol Extinction Differential Absorption Spectroscopy (AE-DOAS) and Cavity Ring-Down Spectroscopy (CRD), allow for controlled measurements of laboratory generated aerosol.
The AE-DOAS has facilitated the retrieval of wavelength dependent CRI for polystyrene latex spheres (PSL). The instrument includes a white-type multi-pass gas cell coupled to a spectrometer designed to maximize sensitivity over the spectral range of 235-700 nm where particularly the UV region is not possible with most other aerosol instruments. Retrievals are achieved by iteratively minimizing the disagreement between Mie theory and measured extinction as a function of sphere size. The PSL retrieved values show agreement to previously published literature results while expanding the range of known values.
The CRD has allowed for precise measurement of extinction for montmorillonite aerosol. The instrument includes a laser source at 532 nm and two cavities allowing for a direct comparison of measured extinction at different RH. Montmorillonite is an alumina silicate clay composed of a layered structure allowing for generated aerosol particles to swell in the presence of increased RH. Studies were conducted comparing two different generation methods, wet and dry, for aerosol samples. Experimental results indicate that the difficulty of fully drying wet generated aerosol may significantly impact the measured amount of adsorbed water.
Galpin, Tyler, "OPTICAL PROPERTIES OF LABORATORY GENERATED AEROSOL: COMPLEX REFRACTIVE INDEX RETRIEVAL FOR POLYSTYRENE LATEX SPHERES, AND EXTINCTION ENHANCEMENT OF MONTMORILLONITE BY WATER SWELLING" (2018). Master's Theses and Capstones. 1243.